Write doubles with the shortest digits (Żmij), digits in registers

Write doubles with the conversion of Zmij by Victor Zverovich (MIT), ported to C++11 (detail/conversions/zmij.hpp). It finds the shortest decimal that reads back as the same double, and the closest one if there are several. Grisu2, used until now, is fast but not always shortest: it sometimes writes a 17th digit where 16 suffice, or a last digit that is not the closest. The layout is unchanged (1.5, 100.0, 1e+100, -0.0); float keeps Grisu2. Digits are converted eight at a time with the BCD conversion of Xiang JunBo, as in Zmij, and written with one byte swap per eight digits and fixed-size moves instead of per-digit loops. Leading and trailing zeros are counted from those bytes. to_chars() uses a local buffer when the caller's is shorter than the 41 bytes this may write. The powers of ten come from the number-parsing table, adjusted where it holds values rounded up, and extended with Zmij's compressed tables beyond 10^308. write_shortest() converts its 16 digits in one vector register (SSE2 on x86-64, NEON on 64-bit Arm, both baseline) and inserts the decimal point inside the register, avoiding a store-forwarding stall that cost about 25% of the time to write a double. dump() writes floats and integers straight into the serializer's write buffer instead of copying them from a member buffer, and small integers eight digits at a time. read_eight_bytes() and parse_eight_digits() are marked always-inline, which GCC had been calling out of line in the number-parsing loops. Of one million random doubles, about 0.14% are now written with different digits, always to a value that still reads back as the same double. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Speed up the lexer: own float parser, string scan, and \u table
2026-10-06 14:40:32 +00:00 · 2026-10-06 10:48:06 +02:00 · 2026-10-06 10:48:06 +02:00 · 2026-10-06 08:36:40 +02:00
27 changed files with 4976 additions and 1224 deletions
@@ -25,6 +25,7 @@ cc_library(
        "include/nlohmann/detail/conversions/from_json.hpp",
        "include/nlohmann/detail/conversions/to_chars.hpp",
        "include/nlohmann/detail/conversions/to_json.hpp",
+        "include/nlohmann/detail/conversions/zmij.hpp",
        "include/nlohmann/detail/exceptions.hpp",
        "include/nlohmann/detail/hash.hpp",
        "include/nlohmann/detail/input/binary_reader.hpp",
@@ -1391,9 +1391,10 @@ THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR I

 - The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed under the [MIT License](https://opensource.org/licenses/MIT) (see above). Copyright &copy; 2008-2009 [Björn Hoehrmann](https://bjoern.hoehrmann.de/) <bjoern@hoehrmann.de>
 - The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the [MIT License](https://opensource.org/licenses/MIT) (see above). Copyright &copy; 2009 [Florian Loitsch](https://florian.loitsch.com/)
+- The class contains a port of the shortest double-to-decimal conversion of [Żmij](https://github.com/vitaut/zmij) by Victor Zverovich, which is licensed under the [MIT License](https://opensource.org/licenses/MIT) (see above). Copyright &copy; 2025 [Victor Zverovich](https://github.com/vitaut)
 - The class contains a copy of [Hedley](https://nemequ.github.io/hedley/) from Evan Nemerson which is licensed as [CC0-1.0](https://creativecommons.org/publicdomain/zero/1.0/).
 - The class contains parts of [Google Abseil](https://github.com/abseil/abseil-cpp) which is licensed under the [Apache 2.0 License](https://opensource.org/licenses/Apache-2.0).
- The class contains an adapted version of the Eisel-Lemire algorithm and its table of powers of five from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors
+- The class contains an adapted version of the Eisel-Lemire algorithm, its table of powers of five, and its digit comparison for long numbers from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors

 <img align="right" src="https://git.fsfe.org/reuse/reuse-ci/raw/branch/master/reuse-horizontal.png" alt="REUSE Software">

@@ -62,6 +62,9 @@ Linear.

 ## Notes

+Floating-point numbers are written with the fewest digits that read back as the same value (for `#!cpp double`; see
+[number handling](../../features/types/number_handling.md#number-serialization)).
+
 Binary values are serialized as an object containing two keys:

 - "bytes": an array of bytes as integers
@@ -97,3 +100,5 @@ Binary values are serialized as an object containing two keys:
 - Error handlers added in version 3.4.0.
 - Serialization of binary values added in version 3.8.0.
 - Error handler `keep` added in version 3.13.0.
+- Doubles are written with the shortest digits (Żmij instead of Grisu2) since version 3.13.0; about 0.1% of doubles are
+  written differently, most of them with fewer digits.
@@ -23,9 +23,10 @@ type to use.
 ## Template parameters

 `NumberFloatType`
-:   the type to store floating-point numbers. Parsing and serialization are implemented in terms of
-    `#!cpp std::strtof`/`#!cpp std::strtod`/`#!cpp std::strtold` and `#!cpp std::snprintf`, so the type must be
-    `#!cpp float`, `#!cpp double`, or `#!cpp long double`. The
+:   the type to store floating-point numbers. The parser converts `#!cpp float`, `#!cpp double`, and a
+    `#!cpp long double` that is IEEE 754 binary64 itself and other `#!cpp long double` formats with
+    `#!cpp std::from_chars` or `#!cpp std::strtold`, and serialization falls back to `#!cpp std::snprintf`, so the
+    type must be `#!cpp float`, `#!cpp double`, or `#!cpp long double`. The
    [binary formats](../../features/binary_formats/index.md) additionally require `#!cpp float` or `#!cpp double`,
    because they have no encoding for `#!cpp long double`. See
    [Template Parameter Requirements](../../features/types/template_parameters.md#numberfloattype).
@@ -131,7 +131,6 @@ The library maps CBOR types to JSON value types as follows:
 | Byte string            | binary          | 0x59       |
 | Byte string            | binary          | 0x5A       |
 | Byte string            | binary          | 0x5B       |
-| Byte string            | binary          | 0x5F       |
 | UTF-8 string           | string          | 0x60..0x77 |
 | UTF-8 string           | string          | 0x78       |
 | UTF-8 string           | string          | 0x79       |
@@ -157,9 +156,6 @@ The library maps CBOR types to JSON value types as follows:
 | Single-Precision Float | number_float    | 0xFA       |
 | Double-Precision Float | number_float    | 0xFB       |

-Indefinite-length UTF-8 strings (0x7F) and byte strings (0x5F) are supported. Each chunk must be a definite-length
-string of the same major type, as required by [RFC 8949, Section 3.2.3](https://www.rfc-editor.org/rfc/rfc8949.html#section-3.2.3).
-
 !!! warning "Incomplete mapping"

    The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:
@@ -82,12 +82,13 @@ flowchart TD

    - Numbers with a decimal digit or scientific notation are always stored as `#!c double`.
    - The number types can be changed, see [Template number types](#template-number-types). 
-    - Integers are converted by the library's own digit parser. Floating-point numbers are converted with
-      [`std::from_chars`](https://en.cppreference.com/w/cpp/utility/from_chars) if the library is compiled with C++17
-      and the standard library supports it, then with an exact fast path for `#!c double` values with few significant
-      digits, and otherwise with the locale-aware
-      [`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof) (`std::strtof`/`std::strtold` for the
-      other floating-point types). Before version 3.13.0, the conversion was realized by
+    - The library converts integers and floating-point numbers itself, independent of the locale. Floating-point
+      numbers are correctly rounded (to nearest, ties to even). Only a `#!c long double` that is not IEEE 754 binary64
+      (e.g., the 80-bit x87 format) is converted with `#!cpp std::from_chars` where available, or else with
+      [`std::strtold`](https://en.cppreference.com/w/cpp/string/byte/strtof). For that call, the library temporarily
+      replaces the `.` with the decimal point of the current locale (which may be longer than one byte, e.g., in
+      `fa_IR.UTF-8`), so the result does not depend on the locale either. Changing the locale in another thread during
+      parsing is undefined behavior of the C library, though. Before version 3.13.0, the conversion was realized by
      [`std::strtoull`](https://en.cppreference.com/w/cpp/string/byte/strtoul),
      [`std::strtoll`](https://en.cppreference.com/w/cpp/string/byte/strtol), and `std::strtod`, respectively.

@@ -100,10 +101,10 @@ flowchart TD
 ### Number limits

 - Any 64-bit signed or unsigned integer can be stored without loss of precision.
- Numbers exceeding the limits of `#!c double` (i.e., numbers that after conversion via
-[`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof) are not satisfying
+- Numbers exceeding the limits of `#!c double` (i.e., numbers whose rounded value is not satisfying
 [`std::isfinite`](https://en.cppreference.com/w/cpp/numeric/math/isfinite) such as `#!c 1E400`) will throw exception
-[`json.exception.out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406) during parsing.
+[`json.exception.out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406) during parsing. Numbers too
+small for `#!c double` (such as `#!c 1E-400`) become zero, with the sign of the number.
 - Floating-point numbers are rounded to the next number representable as `double`. For instance
 `#!c 3.141592653589793238462643383279` is stored as [`0x400921fb54442d18`](https://float.exposed/0x400921fb54442d18).
 This is the same behavior as the code `#!c double x = 3.141592653589793238462643383279;`.
@@ -133,9 +134,10 @@ That is, `-0` is stored as a signed integer, but the serialization does not repr
 ### Number serialization

 - Integer numbers are serialized as is; that is, no scientific notation is used.
- Floating-point numbers are serialized as specified by the `#!c %g` printf modifier with 
-  [`std::numeric_limits<double>::max_digits10`](https://en.cppreference.com/w/cpp/types/numeric_limits/max_digits10)
-  significant digits. The rationale is to use the shortest representation while still allowing round-tripping.
+- Floating-point numbers are serialized with the fewest digits that read back as the same value (the closest such
+  digits if there are several), in the layout of the `#!c %g` printf modifier: `#!c 1.5`, `#!c 100.0`, `#!c 1e+100`.
+  Doubles are converted with the algorithm of [Żmij](https://github.com/vitaut/zmij), floats with Grisu2, which
+  can write more digits than necessary.

 !!! hint "Notes regarding precision of floating-point numbers"

@@ -26,9 +26,9 @@ Requirements are split into two groups:
    diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside
    the library. Four violations are not caught at compile time at all:

-    - A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and can silently misparse
-      floating-point numbers, because the lexer may hand the buffer to `#!cpp std::strtod`, which reads up to the
-      terminating null character.
+    - A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers
+      stored as a `#!cpp long double` that is not IEEE 754 binary64 (e.g., the 80-bit x87 format), because the lexer
+      hands the buffer to `#!cpp std::strtold`.
    - A stateful [`AllocatorType`](#allocatortype) compiles and silently ignores its state: allocation, deallocation,
      and [`get_allocator()`](../../api/basic_json/get_allocator.md) each use a different default-constructed instance.
    - The two [cross-specialization conversions](#cross-specialization-conversions) below. These abort on an assertion
@@ -537,16 +537,18 @@ therefore silently changes parse results rather than raising an error. See

 `NumberFloatType` must be one of `#!cpp float`, `#!cpp double`, or `#!cpp long double`:

- The [parser](../parsing/index.md) converts number literals with `#!cpp std::from_chars` or, as a fallback, with
-  `#!cpp std::strtof`, `#!cpp std::strtod`, or `#!cpp std::strtold`; the library provides overloads for exactly these
-  three types.
+- The [parser](../parsing/index.md) converts number literals to `#!cpp float`, `#!cpp double`, and a
+  `#!cpp long double` that is IEEE 754 binary64 itself; other `#!cpp long double` formats are converted with
+  `#!cpp std::from_chars` where available, or with `#!cpp std::strtold`. The library provides overloads for exactly
+  these three types.
 - [`dump`](../../api/basic_json/dump.md) falls back to `#!cpp std::snprintf` with the `%g` and `%Lg` conversion
  specifiers, for which the library likewise provides only `#!cpp double` and `#!cpp long double` overloads
  (`#!cpp float` is promoted to `#!cpp double`).

-If `#!cpp std::numeric_limits<NumberFloatType>` describes an IEEE 754 binary32 or binary64 number, `dump` uses the
-Grisu2 algorithm, which produces the shortest representation that round-trips. Otherwise the `snprintf` fallback with
-`max_digits10` digits is used.
+If `#!cpp std::numeric_limits<NumberFloatType>` describes an IEEE 754 binary64 number, `dump` uses the algorithm of
+Żmij, which produces the shortest representation that round-trips. For IEEE 754 binary32 numbers, it uses Grisu2,
+which produces a short representation that round-trips. Otherwise the `snprintf` fallback with `max_digits10` digits is
+used.

 ### Required for the binary formats

@@ -558,7 +560,7 @@ binary32 or binary64 field and have no encoding for `#!cpp long double`.

 | Type                     | Support                                                                                                               |
 |--------------------------|-----------------------------------------------------------------------------------------------------------------------|
-| `#!cpp double` (default) | full; short round-trip output through Grisu2                                                                          |
+| `#!cpp double` (default) | full; shortest round-trip output through Żmij                                                                         |
 | `#!cpp float`            | full; short round-trip output through Grisu2                                                                          |
 | `#!cpp long double`      | `dump` and `parse` only; the binary format writers do not compile, as they only handle IEEE 754 binary32 and binary64 |
 | any other type           | not usable                                                                                                            |
@@ -18,6 +18,8 @@ The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed und

 The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the [MIT License](https://opensource.org/licenses/MIT) (see above). Copyright &copy; 2009 [Florian Loitsch](https://florian.loitsch.com/)

+The class contains a port of the shortest double-to-decimal conversion of [Żmij](https://github.com/vitaut/zmij) by Victor Zverovich, which is licensed under the [MIT License](https://opensource.org/licenses/MIT) (see above). Copyright &copy; 2025 [Victor Zverovich](https://github.com/vitaut)
+
 The class contains a copy of [Hedley](https://nemequ.github.io/hedley/) from Evan Nemerson which is licensed as [CC0-1.0](https://creativecommons.org/publicdomain/zero/1.0/).

-The class contains an adapted version of the Eisel-Lemire algorithm and its table of powers of five from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors
+The class contains an adapted version of the Eisel-Lemire algorithm, its table of powers of five, and its digit comparison for long numbers from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors
@@ -39,6 +39,25 @@ inline int count_leading_zeros(std::uint64_t x) noexcept
 #endif
 }

+/// number of trailing zero bits of x (x != 0)
+inline int count_trailing_zeros(std::uint64_t x) noexcept
+{
+#if defined(__GNUC__) || defined(__clang__)
+    return __builtin_ctzll(x);
+#else
+    int n = 0;
+    for (int shift = 32; shift != 0; shift >>= 1)
+    {
+        if ((x << (64 - shift)) == 0)
+        {
+            n += shift;
+            x >>= shift;
+        }
+    }
+    return n;
+#endif
+}
+
 /// the 128-bit product of two 64-bit numbers
 struct uint128_parts
 {
@@ -67,15 +86,21 @@ inline uint128_parts full_multiplication(std::uint64_t a, std::uint64_t b) noexc
 }

 /// eight bytes as a little-endian word (compilers fold this into one load on
-/// little-endian targets)
-inline std::uint64_t read_eight_bytes(const char* p) noexcept
+/// little-endian targets; always inlined, as GCC otherwise calls it in the
+/// number loops)
+JSON_HEDLEY_ALWAYS_INLINE std::uint64_t read_eight_bytes(const unsigned char* b) noexcept
 {
-    const auto* b = reinterpret_cast<const unsigned char*>(p); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
    return static_cast<std::uint64_t>(b[0]) | (static_cast<std::uint64_t>(b[1]) << 8u)
           | (static_cast<std::uint64_t>(b[2]) << 16u) | (static_cast<std::uint64_t>(b[3]) << 24u)
           | (static_cast<std::uint64_t>(b[4]) << 32u) | (static_cast<std::uint64_t>(b[5]) << 40u)
           | (static_cast<std::uint64_t>(b[6]) << 48u) | (static_cast<std::uint64_t>(b[7]) << 56u);
 }

+/// eight bytes as a little-endian word
+JSON_HEDLEY_ALWAYS_INLINE std::uint64_t read_eight_bytes(const char* p) noexcept
+{
+    return read_eight_bytes(reinterpret_cast<const unsigned char*>(p)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+}
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
@@ -11,11 +11,32 @@

 #include <array> // array
 #include <cmath>   // signbit, isfinite
+#include <cstddef> // size_t
 #include <cstdint> // intN_t, uintN_t
 #include <cstring> // memcpy, memmove
 #include <limits> // numeric_limits
 #include <type_traits> // conditional

+#ifdef _MSC_VER
+    #include <cstdlib> // _byteswap_uint64
+#endif
+
+// SSE2 (every x86-64 CPU) and NEON (every 64-bit Arm CPU) convert the 16
+// digits of a double at once
+#if defined(__x86_64__) || (defined(_M_X64) && !defined(_M_ARM64EC))
+    #include <emmintrin.h>
+    #define JSON_DTOA_SSE2 1
+    #define JSON_DTOA_NEON 0
+#elif (defined(__aarch64__) || defined(_M_ARM64)) && !defined(_M_ARM64EC) && !defined(__ARM_BIG_ENDIAN)
+    #include <arm_neon.h>
+    #define JSON_DTOA_SSE2 0
+    #define JSON_DTOA_NEON 1
+#else
+    #define JSON_DTOA_SSE2 0
+    #define JSON_DTOA_NEON 0
+#endif
+
+#include <nlohmann/detail/conversions/zmij.hpp>
 #include <nlohmann/detail/macro_scope.hpp>

 NLOHMANN_JSON_NAMESPACE_BEGIN
@@ -918,6 +939,88 @@ void grisu2(char* buf, int& len, int& decimal_exponent, FloatType value)
    grisu2(buf, len, decimal_exponent, w.minus, w.w, w.plus);
 }

+/*!
+@brief the shortest digits of a positive finite float (other than double): Grisu2
+*/
+template<typename FloatType>
+JSON_HEDLEY_NON_NULL(1)
+void shortest_digits(char* buf, int& len, int& decimal_exponent, FloatType value)
+{
+    grisu2(buf, len, decimal_exponent, value);
+}
+
+/*!
+@brief the shortest digits of a positive finite double: the conversion of
+Zmij (see zmij.hpp), which always finds the shortest digits that read back as
+the same value (Grisu2 does not for about one double in a thousand), and the
+closest of them if there are several
+
+v = buf * 10^decimal_exponent, as for grisu2()
+*/
+JSON_HEDLEY_NON_NULL(1)
+inline void shortest_digits(char* buf, int& len, int& decimal_exponent, double value)
+{
+    static_assert(std::numeric_limits<double>::is_iec559 && std::numeric_limits<double>::digits == 53,
+                  "internal error: the conversion of Zmij needs IEEE 754 binary64 doubles");
+    JSON_ASSERT(std::isfinite(value));
+    JSON_ASSERT(value > 0);
+
+    std::uint64_t bits = 0;
+    std::memcpy(&bits, &value, sizeof(bits));
+    zmij::decimal d = zmij::to_decimal(bits);
+    // without trailing zeros (up to 16): 8, 4, 2, 1 at a time
+    while (d.significand % 100000000 == 0)
+    {
+        d.significand /= 100000000;
+        d.exponent += 8;
+    }
+    if (d.significand % 10000 == 0)
+    {
+        d.significand /= 10000;
+        d.exponent += 4;
+    }
+    if (d.significand % 100 == 0)
+    {
+        d.significand /= 100;
+        d.exponent += 2;
+    }
+    if (d.significand % 10 == 0)
+    {
+        d.significand /= 10;
+        d.exponent += 1;
+    }
+    // at most 17 digits, written from the back two at a time
+    static constexpr const char* pairs =
+        "00010203040506070809101112131415161718192021222324252627282930313233343536373839"
+        "40414243444546474849505152535455565758596061626364656667686970717273747576777879"
+        "8081828384858687888990919293949596979899";
+    std::array<char, 20> digits{};
+    std::size_t n = digits.size();
+    while (d.significand >= 100)
+    {
+        const std::uint64_t two_digits = d.significand % 100; // a variable: GCC calls a cast of the remainder useless where std::uint64_t is std::size_t
+        const auto i = static_cast<std::size_t>(two_digits) * 2;
+        d.significand /= 100;
+        n -= 2;
+        digits[n] = pairs[i];
+        digits[n + 1] = pairs[i + 1];
+    }
+    if (d.significand >= 10)
+    {
+        const auto i = static_cast<std::size_t>(d.significand) * 2;
+        n -= 2;
+        digits[n] = pairs[i];
+        digits[n + 1] = pairs[i + 1];
+    }
+    else
+    {
+        digits[--n] = static_cast<char>('0' + d.significand);
+    }
+    len = static_cast<int>(digits.size() - n);
+    std::memcpy(buf, digits.data() + n, static_cast<std::size_t>(len));
+    decimal_exponent = d.exponent;
+}
+
 /*!
@brief appends a decimal representation of e to buf
@return a pointer to the element following the exponent.
@@ -1047,6 +1150,374 @@ inline char* format_buffer(char* buf, int len, int decimal_exponent,
    return append_exponent(buf, n - 1);
 }

+/// eight decimal digits (a value below 10^8) as bytes 0..9, the first digit
+/// in the most significant byte: three steps that divide all lanes at once
+/// by a multiplication (the conversion of Xiang JunBo, as in Zmij)
+inline std::uint64_t eight_digit_bytes(std::uint64_t abcdefgh) noexcept
+{
+    const std::uint64_t abcd_efgh = abcdefgh + (((std::uint64_t{1} << 32u) - 10000u) * ((abcdefgh * (((std::uint64_t{1} << 40u) / 10000u) + 1u)) >> 40u));
+    const std::uint64_t ab_cd_ef_gh = abcd_efgh + (((std::uint64_t{1} << 16u) - 100u) * (((abcd_efgh * (((std::uint64_t{1} << 19u) / 100u) + 1u)) >> 19u) & 0x7F0000007Fu));
+    return ab_cd_ef_gh + (((std::uint64_t{1} << 8u) - 10u) * (((ab_cd_ef_gh * (((std::uint64_t{1} << 10u) / 10u) + 1u)) >> 10u) & 0x000F000F000F000Fu));
+}
+
+/// store the bytes of v, the most significant one first (one byte swap and
+/// one store where the byte order is known: compilers do not reliably merge
+/// the byte stores once this is inlined)
+inline void store_msb_first(char* p, std::uint64_t v) noexcept
+{
+#if defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+    v = __builtin_bswap64(v);
+    std::memcpy(p, &v, sizeof(v));
+#elif defined(__BYTE_ORDER__) && defined(__ORDER_BIG_ENDIAN__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+    std::memcpy(p, &v, sizeof(v));
+#elif defined(_MSC_VER) // (little-endian on all its targets)
+    v = _byteswap_uint64(v);
+    std::memcpy(p, &v, sizeof(v));
+#else
+    for (unsigned i = 0; i < 8; ++i)
+    {
+        p[i] = static_cast<char>(v >> (56u - (8u * i)));
+    }
+#endif
+}
+
+/*!
+@brief digits * 10^exp for a double, in the layout of format_buffer()
+
+The layout is that of format_buffer() with min_exp -4 and max_exp 15 (the
+digits10 of double). The digits are converted eight at a time and placed
+with fixed-size moves instead of per-digit loops and moves of the buffer.
+
+@param[in] digits  the digits (not 0, at most 17 digits; trailing zeros allowed)
+@param[in] exp     the decimal exponent of the last digit
+@return a pointer past the text; up to 41 bytes at @a first are written
+        (some beyond the returned end)
+*/
+JSON_HEDLEY_NON_NULL(1)
+JSON_HEDLEY_RETURNS_NON_NULL
+inline char* write_decimal(char* first, std::uint64_t digits, int exp) noexcept
+{
+    JSON_ASSERT(digits != 0 && digits < 100000000000000000u);
+    const std::uint64_t upper = digits / 100000000u;
+    const std::uint64_t b0 = upper / 100000000u; // (one digit: it is its own byte)
+    const std::uint64_t b1 = eight_digit_bytes(upper % 100000000u);
+    const std::uint64_t b2 = eight_digit_bytes(digits % 100000000u);
+    // leading and trailing zero digits: zero bytes, counted without division
+    int leading = 16;
+    int zeros = 16;
+    if (b0 != 0)
+    {
+        leading = count_leading_zeros(b0) / 8;
+    }
+    else if (b1 != 0)
+    {
+        leading = 8 + (count_leading_zeros(b1) / 8);
+    }
+    else
+    {
+        leading += count_leading_zeros(b2) / 8;
+    }
+    if (b2 != 0)
+    {
+        zeros = count_trailing_zeros(b2) / 8;
+    }
+    else if (b1 != 0)
+    {
+        zeros = 8 + (count_trailing_zeros(b1) / 8);
+    }
+    // (else: 16, b0 is the one digit that is not 0)
+    // the digits as text at text + leading, then '0's, so that fixed-size
+    // moves need not check how many digits there are
+    std::array<char, 64> text; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+    store_msb_first(text.data(), b0 + 0x3030303030303030u);
+    store_msb_first(text.data() + 8, b1 + 0x3030303030303030u);
+    store_msb_first(text.data() + 16, b2 + 0x3030303030303030u);
+    std::memset(text.data() + 24, '0', 40);
+    const int k = 24 - leading - zeros; // significant digits
+    const int n = k + exp + zeros;      // position of the decimal point after the first digit
+    const char* const s0 = text.data() + leading;
+
+    if (-4 < n && n <= 15)
+    {
+        // "0.[000]digits" (n <= 0) is the digits after 1 - n leading '0's
+        // with the point after the first; "digits[000].0" (n >= k) and
+        // "dig.its" put the point after n characters
+        const int pad = n <= 0 ? 1 - n : 0;
+        const char* const s = s0 - pad;
+        const int len = k + pad;
+        const int point = n + pad;
+        std::memcpy(first, s, 16);
+        std::memcpy(first + point + 1, s + point, 24);
+        first[point] = '.';
+        return first + (point >= len ? point + 2 : len + 1);
+    }
+
+    // d.igitse+XX, with at least two exponent digits (as append_exponent())
+    std::memcpy(first, s0, 16);
+    std::memcpy(first + 2, s0 + 1, 16);
+    first[1] = '.';
+    char* const end = first + (k == 1 ? 1 : k + 1);
+    const int e = n - 1;
+    const auto ea = static_cast<unsigned>(e < 0 ? -e : e);
+    const bool three = ea >= 100;
+    end[0] = 'e';
+    end[1] = e < 0 ? '-' : '+';
+    end[2] = static_cast<char>('0' + (three ? ea / 100 : (ea / 10) % 10));
+    end[3] = static_cast<char>('0' + (three ? (ea / 10) % 10 : ea % 10));
+    end[4] = static_cast<char>('0' + (ea % 10));
+    return end + (three ? 5 : 4);
+}
+
+/*!
+@brief the shortest decimal of a positive double (Zmij), as write_decimal()
+writes it
+
+For a normal double, the shorter candidate has 15 or 16 digits: they are
+converted at once (two halves of eight digits) and followed by the digit
+after them, if there is one, without the multiplication and division by 10
+that counting the digits of one number would take. The fixed layouts move
+the digits after the point by one byte.
+
+@return a pointer past the text; up to 41 bytes at @a first are written
+        (some beyond the returned end)
+*/
+JSON_HEDLEY_NON_NULL(1)
+JSON_HEDLEY_RETURNS_NON_NULL
+inline char* write_shortest(char* first, const zmij::shortest_decimal d) noexcept
+{
+    const std::uint64_t sig = d.integral;
+    if (JSON_HEDLEY_UNLIKELY(sig < 100000000000000u || sig >= 10000000000000000u))
+    {
+        // (subnormals)
+        return d.has_digit ? write_decimal(first, (sig * 10) + d.digit, d.exponent) : write_decimal(first, sig, d.exponent + 1);
+    }
+    const bool sixteen = sig >= 1000000000000000u; // (else 15 digits)
+    const int last = d.has_digit ? d.digit : 0;
+    const std::uint64_t upper = sig / 100000000u;
+#if JSON_DTOA_SSE2
+    // NOLINTBEGIN(portability-simd-intrinsics)
+    // the two halves in the 64-bit lanes, each as abcd * 2^32 + efgh, then as
+    // bytes (as eight_digit_bytes(), one lane each)
+    const __m128i x = _mm_set_epi64x(static_cast<long long>(sig - (upper * 100000000u)), static_cast<long long>(upper));
+    const __m128i abcd = _mm_srli_epi64(_mm_mul_epu32(x, _mm_set1_epi64x(109951163)), 40); // 2^40 / 10000 + 1
+    const __m128i abcd_efgh = _mm_add_epi64(x, _mm_mul_epu32(abcd, _mm_set1_epi64x(4294957296))); // 2^32 - 10000
+    // 32-bit lanes in the order of the text: abcd, efgh of both halves
+    const __m128i fours = _mm_shuffle_epi32(abcd_efgh, _MM_SHUFFLE(2, 3, 0, 1));
+    const __m128i ab = _mm_srli_epi16(_mm_mulhi_epu16(fours, _mm_set1_epi32(5243)), 3);
+    const __m128i ab_cd = _mm_or_si128(_mm_slli_epi32(_mm_sub_epi16(fours, _mm_mullo_epi16(ab, _mm_set1_epi32(100))), 16), ab);
+    // 16-bit lanes ab (< 100) -> bytes a, b: 256 * ab - 2559 * (ab / 10)
+    const __m128i bytes = _mm_sub_epi16(_mm_slli_epi16(ab_cd, 8), _mm_mullo_epi16(_mm_set1_epi16(2559), _mm_mulhi_epu16(ab_cd, _mm_set1_epi16(6554))));
+    // the last digit that is not 0 (sig is not 0)
+    const auto nonzero = static_cast<std::uint64_t>(_mm_movemask_epi8(_mm_cmpgt_epi8(bytes, _mm_setzero_si128())));
+    const int digits = 63 - count_leading_zeros(nonzero) + (sixteen ? 1 : 0); // without trailing zeros
+    const __m128i chars = _mm_add_epi8(bytes, _mm_set1_epi8('0'));
+    // the 16 characters from the first digit
+    const __m128i s = sixteen ? chars : _mm_or_si128(_mm_srli_si128(chars, 1), _mm_slli_si128(_mm_cvtsi32_si128('0' + last), 15));
+    const char s16 = static_cast<char>(sixteen ? '0' + last : '0'); // the 17th
+    const auto store_16 = [&s](char* p) noexcept
+    {
+        std::memcpy(p, &s, 16);
+    };
+    const char first_digit = static_cast<char>(_mm_cvtsi128_si32(s));
+    // NOLINTEND(portability-simd-intrinsics)
+#elif JSON_DTOA_NEON
+    // as with SSE2: the halves in 32-bit lanes, then abcd, efgh of both
+    const uint32x2_t halves = vcreate_u32(upper | ((sig - (upper * 100000000u)) << 32u));
+    const uint32x2_t abcd = vmovn_u64(vshrq_n_u64(vmull_n_u32(halves, static_cast<std::uint32_t>(((std::uint64_t{1} << 40u) / 10000u) + 1u)), 40));
+    const uint32x2_t efgh = vmls_n_u32(halves, abcd, 10000u);
+    const uint32x4_t fours = vcombine_u32(vzip1_u32(abcd, efgh), vzip2_u32(abcd, efgh));
+    const uint32x4_t ab = vshrq_n_u32(vmulq_n_u32(fours, 5243u), 19);
+    const uint16x8_t ab_cd = vreinterpretq_u16_u32(vorrq_u32(ab, vshlq_n_u32(vmlsq_n_u32(fours, ab, 100u), 16)));
+    const uint16x8_t tens = vshrq_n_u16(vmulq_n_u16(ab_cd, 103u), 10);
+    const uint8x16_t bytes = vreinterpretq_u8_u16(vorrq_u16(tens, vshlq_n_u16(vmlsq_n_u16(ab_cd, tens, 10u), 8)));
+    // the last digit that is not 0 (sig is not 0): a nibble per byte
+    const std::uint64_t nonzero = vget_lane_u64(vreinterpret_u64_u8(vshrn_n_u16(vreinterpretq_u16_u8(vtstq_u8(bytes, bytes)), 4)), 0);
+    const int digits = ((63 - count_leading_zeros(nonzero)) / 4) + (sixteen ? 1 : 0); // without trailing zeros
+    const uint8x16_t chars = vaddq_u8(bytes, vdupq_n_u8('0'));
+    // the 16 characters from the first digit
+    const uint8x16_t s = sixteen ? chars : vextq_u8(chars, vdupq_n_u8(static_cast<std::uint8_t>('0' + last)), 1);
+    const char s16 = static_cast<char>(sixteen ? '0' + last : '0'); // the 17th
+    const auto store_16 = [&s](char* p) noexcept
+    {
+        vst1q_u8(reinterpret_cast<std::uint8_t*>(p), s); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+    };
+    const auto first_digit = static_cast<char>(vgetq_lane_u8(s, 0));
+#else
+    const std::uint64_t hi = eight_digit_bytes(upper);
+    const std::uint64_t lo = eight_digit_bytes(sig - (upper * 100000000u));
+    // trailing zero digits: zero bytes (sig is not 0)
+    const int zeros = lo != 0 ? count_trailing_zeros(lo) / 8 : 8 + (count_trailing_zeros(hi) / 8);
+    const int digits = 15 - zeros + (sixteen ? 1 : 0); // without trailing zeros
+    // the 16 characters from the first digit
+    const std::uint64_t s_hi = (sixteen ? hi : (hi << 8u) | (lo >> 56u)) + 0x3030303030303030u;
+    const std::uint64_t s_lo = (sixteen ? lo : (lo << 8u) | static_cast<std::uint64_t>(last)) + 0x3030303030303030u;
+    const char s16 = static_cast<char>(sixteen ? '0' + last : '0'); // the 17th
+    const auto store_16 = [s_hi, s_lo](char* p) noexcept
+    {
+        store_msb_first(p, s_hi);
+        store_msb_first(p + 8, s_lo);
+    };
+    const auto first_digit = static_cast<char>(s_hi >> 56u);
+#endif
+    const int len = d.has_digit ? 16 + (sixteen ? 1 : 0) : digits; // significant digits
+    const int n = 16 + (sixteen ? 1 : 0) + d.exponent;           // digits before the point
+
+    if (JSON_HEDLEY_LIKELY(n >= 1 && n <= 15))
+    {
+        // "dig.its" and "digits[000].0": the digits after the point move by
+        // one byte ('0's follow the digits)
+#if JSON_DTOA_SSE2
+        // NOLINTBEGIN(portability-simd-intrinsics)
+        // (in the register: reading the digits back from memory right after
+        // storing them waits until the stores are done)
+        const __m128i at = _mm_set1_epi8(static_cast<char>(n));
+        const __m128i index = _mm_setr_epi8(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15);
+        const __m128i before = _mm_cmpgt_epi8(at, index);
+        const __m128i after = _mm_cmpgt_epi8(index, at);
+        const __m128i text = _mm_or_si128(_mm_or_si128(_mm_and_si128(s, before), _mm_and_si128(_mm_slli_si128(s, 1), after)),
+                                          _mm_andnot_si128(_mm_or_si128(before, after), _mm_set1_epi8('.')));
+        std::memcpy(first, &text, 16);
+        first[16] = static_cast<char>(_mm_extract_epi16(s, 7) >> 8);
+        first[17] = s16;
+        // NOLINTEND(portability-simd-intrinsics)
+#elif JSON_DTOA_NEON
+        const uint8x16_t index = vcombine_u8(vcreate_u8(0x0706050403020100u), vcreate_u8(0x0F0E0D0C0B0A0908u));
+        const uint8x16_t at = vdupq_n_u8(static_cast<std::uint8_t>(n));
+        const uint8x16_t after_point = vbslq_u8(vcgtq_u8(index, at), vextq_u8(vdupq_n_u8(0), s, 15), vdupq_n_u8('.'));
+        vst1q_u8(reinterpret_cast<std::uint8_t*>(first), vbslq_u8(vcltq_u8(index, at), s, after_point)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+        first[16] = static_cast<char>(vgetq_lane_u8(s, 15));
+        first[17] = s16;
+#else
+        store_16(first);
+        first[16] = s16;
+        std::uint64_t after_point[2]; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+        std::memcpy(after_point, first + n, 16);
+        std::memcpy(first + n + 1, after_point, 16);
+        first[n] = '.';
+#endif
+        return first + (n >= len ? n + 2 : len + 1);
+    }
+    if (n <= 0 && n > -4)
+    {
+        // "0.[000]digits"
+        std::memset(first, '0', 8);
+        first[1] = '.';
+        store_16(first + 2 - n);
+        first[18 - n] = s16;
+        return first + 2 - n + len;
+    }
+    // d.igitse+XX, with at least two exponent digits (as append_exponent())
+    store_16(first + 1);
+    first[17] = s16;
+    first[0] = first_digit;
+    first[1] = '.';
+    char* const end = first + (len == 1 ? 1 : len + 1);
+    const int e = n - 1;
+    const auto ea = static_cast<unsigned>(e < 0 ? -e : e);
+    const bool three = ea >= 100;
+    end[0] = 'e';
+    end[1] = e < 0 ? '-' : '+';
+    end[2] = static_cast<char>('0' + (three ? ea / 100 : (ea / 10) % 10));
+    end[3] = static_cast<char>('0' + (three ? (ea / 10) % 10 : ea % 10));
+    end[4] = static_cast<char>('0' + (ea % 10));
+    return end + (three ? 5 : 4);
+}
+
+/// the powers of ten up to 10^16
+inline const std::array<std::uint64_t, 17>& powers_of_ten_16() noexcept
+{
+    static const std::array<std::uint64_t, 17> powers =
+    {
+        {
+            1u, 10u, 100u, 1000u, 10000u, 100000u, 1000000u, 10000000u, 100000000u, 1000000000u, 10000000000u,
+            100000000000u, 1000000000000u, 10000000000000u, 100000000000000u, 1000000000000000u, 10000000000000000u
+        }
+    };
+    return powers;
+}
+
+/*!
+@brief digits * 10^exp, as write_decimal() writes it, for the digits of a
+double that need no conversion (count digits, at most 15, the first not 0;
+trailing zeros allowed): extended to 16 digits and written by write_shortest()
+
+@return a pointer past the text; up to 41 bytes at @a first are written
+        (some beyond the returned end)
+*/
+JSON_HEDLEY_NON_NULL(1)
+JSON_HEDLEY_RETURNS_NON_NULL
+inline char* write_short_decimal(char* first, std::uint64_t digits, int count, int exp) noexcept
+{
+    JSON_ASSERT(digits >= powers_of_ten_16()[static_cast<std::size_t>(count - 1)] && count <= 15);
+    const int scale = 16 - count;
+    return write_shortest(first, zmij::shortest_decimal{digits * powers_of_ten_16()[static_cast<std::size_t>(scale)], exp - scale - 1, 0, false});
+}
+
+/// as write_short_decimal(), counting the digits (not 0, less than 10^15)
+JSON_HEDLEY_NON_NULL(1)
+JSON_HEDLEY_RETURNS_NON_NULL
+inline char* write_short_decimal(char* first, std::uint64_t digits, int exp) noexcept
+{
+    JSON_ASSERT(digits != 0 && digits < 1000000000000000u);
+    // floor(log10(2^bits)) + 1 digits, or one less
+    const int log2_bound = ((64 - count_leading_zeros(digits)) * 1233) >> 12;
+    const int count = log2_bound + (digits >= powers_of_ten_16()[static_cast<std::size_t>(log2_bound)] ? 1 : 0);
+    return write_short_decimal(first, digits, count, exp);
+}
+
+/// a positive finite float (other than double): Grisu2 and format_buffer()
+template<typename FloatType>
+JSON_HEDLEY_NON_NULL(1, 2)
+JSON_HEDLEY_RETURNS_NON_NULL
+char* write_positive(char* first, const char* last, FloatType value)
+{
+    JSON_ASSERT(last - first >= std::numeric_limits<FloatType>::max_digits10);
+
+    // Compute v = buffer * 10^decimal_exponent.
+    // The decimal digits are stored in the buffer, which needs to be interpreted
+    // as an unsigned decimal integer.
+    // len is the length of the buffer, i.e., the number of decimal digits.
+    int len = 0;
+    int decimal_exponent = 0;
+    shortest_digits(first, len, decimal_exponent, value);
+
+    JSON_ASSERT(len <= std::numeric_limits<FloatType>::max_digits10);
+
+    // Format the buffer like printf("%.*g", prec, value)
+    constexpr int kMinExp = -4;
+    // Use digits10 here to increase compatibility with version 2.
+    constexpr int kMaxExp = std::numeric_limits<FloatType>::digits10;
+
+    JSON_ASSERT(last - first >= kMaxExp + 2);
+    JSON_ASSERT(last - first >= 2 + (-kMinExp - 1) + std::numeric_limits<FloatType>::max_digits10);
+    JSON_ASSERT(last - first >= std::numeric_limits<FloatType>::max_digits10 + 6);
+
+    return format_buffer(first, len, decimal_exponent, kMinExp, kMaxExp);
+}
+
+/// a positive finite double: the shortest digits (Zmij), laid out by
+/// write_shortest() (through a local buffer if [first, last) is shorter than
+/// the 41 bytes it may write)
+JSON_HEDLEY_NON_NULL(1, 2)
+JSON_HEDLEY_RETURNS_NON_NULL
+inline char* write_positive(char* first, const char* last, double value)
+{
+    static_assert(std::numeric_limits<double>::is_iec559 && std::numeric_limits<double>::digits == 53,
+                  "internal error: the conversion of Zmij needs IEEE 754 binary64 doubles");
+    std::uint64_t bits = 0;
+    std::memcpy(&bits, &value, sizeof(bits));
+    const zmij::shortest_decimal d = zmij::to_shortest(bits);
+    if (JSON_HEDLEY_LIKELY(last - first >= 41))
+    {
+        return write_shortest(first, d);
+    }
+    std::array<char, 64> buf; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+    const auto len = static_cast<std::size_t>(write_shortest(buf.data(), d) - buf.data());
+    JSON_ASSERT(static_cast<std::size_t>(last - first) >= len);
+    std::memcpy(first, buf.data(), len);
+    return first + len;
+}
+
 }  // namespace dtoa_impl

 /*!
@@ -1064,7 +1535,6 @@ JSON_HEDLEY_NON_NULL(1, 2)
 JSON_HEDLEY_RETURNS_NON_NULL
 char* to_chars(char* first, const char* last, FloatType value)
 {
-    static_cast<void>(last); // maybe unused - fix warning
    JSON_ASSERT(std::isfinite(value));

    // Use signbit(value) instead of (value < 0) since signbit works for -0.
@@ -1090,28 +1560,7 @@ char* to_chars(char* first, const char* last, FloatType value)
    JSON_HEDLEY_DIAGNOSTIC_POP
 #endif

-    JSON_ASSERT(last - first >= std::numeric_limits<FloatType>::max_digits10);
-
-    // Compute v = buffer * 10^decimal_exponent.
-    // The decimal digits are stored in the buffer, which needs to be interpreted
-    // as an unsigned decimal integer.
-    // len is the length of the buffer, i.e., the number of decimal digits.
-    int len = 0;
-    int decimal_exponent = 0;
-    dtoa_impl::grisu2(first, len, decimal_exponent, value);
-
-    JSON_ASSERT(len <= std::numeric_limits<FloatType>::max_digits10);
-
-    // Format the buffer like printf("%.*g", prec, value)
-    constexpr int kMinExp = -4;
-    // Use digits10 here to increase compatibility with version 2.
-    constexpr int kMaxExp = std::numeric_limits<FloatType>::digits10;
-
-    JSON_ASSERT(last - first >= kMaxExp + 2);
-    JSON_ASSERT(last - first >= 2 + (-kMinExp - 1) + std::numeric_limits<FloatType>::max_digits10);
-    JSON_ASSERT(last - first >= std::numeric_limits<FloatType>::max_digits10 + 6);
-
-    return dtoa_impl::format_buffer(first, len, decimal_exponent, kMinExp, kMaxExp);
+    return dtoa_impl::write_positive(first, last, value);
 }

 }  // namespace detail
@@ -0,0 +1,238 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2025 Victor Zverovich <https://github.com/vitaut/zmij>
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <array> // array
+#include <cstddef> // size_t
+#include <cstdint> // uint32_t, uint64_t
+
+#include <nlohmann/detail/abi_macros.hpp>
+#include <nlohmann/detail/bit_ops.hpp>
+#include <nlohmann/detail/input/pow5_table.hpp>
+#include <nlohmann/detail/macro_scope.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+
+/*!
+@brief the shortest decimal representation of a double
+
+A C++11 port of the conversion of Zmij by Victor Zverovich
+(https://github.com/vitaut/zmij, MIT license): the shortest decimal in the
+rounding interval of a double, the closest one if there are several. Zmij
+credits Xiang JunBo (producing the shorter candidate without a division) and
+Dougall Johnson (the compressed powers of ten). The powers of ten are taken
+from the table for number parsing (pow5_table.hpp) where it holds them, and
+computed from the compressed tables of Zmij beyond it.
+*/
+namespace zmij
+{
+
+/// significand * 10^exponent
+struct decimal
+{
+    std::uint64_t significand;
+    int exponent;
+};
+
+/// the compressed powers of ten of Zmij
+inline const std::array<std::uint64_t, 28>& pow10_minor() noexcept
+{
+    static const std::array<std::uint64_t, 28> table =
+    {
+        {
+            0x8000000000000000u, 0xa000000000000000u, 0xc800000000000000u, 0xfa00000000000000u, 0x9c40000000000000u,
+            0xc350000000000000u, 0xf424000000000000u, 0x9896800000000000u, 0xbebc200000000000u, 0xee6b280000000000u,
+            0x9502f90000000000u, 0xba43b74000000000u, 0xe8d4a51000000000u, 0x9184e72a00000000u, 0xb5e620f480000000u,
+            0xe35fa931a0000000u, 0x8e1bc9bf04000000u, 0xb1a2bc2ec5000000u, 0xde0b6b3a76400000u, 0x8ac7230489e80000u,
+            0xad78ebc5ac620000u, 0xd8d726b7177a8000u, 0x878678326eac9000u, 0xa968163f0a57b400u, 0xd3c21bcecceda100u,
+            0x84595161401484a0u, 0xa56fa5b99019a5c8u, 0xcecb8f27f4200f3au
+        }
+    };
+    return table;
+}
+
+/// (high, low) pairs
+inline const std::array<std::uint64_t, 50>& pow10_major() noexcept
+{
+    static const std::array<std::uint64_t, 50> table =
+    {
+        {
+            0xaddcb9e83c6b1793u, 0xdf4abe242a1bbf3eu, 0xaf8e5410288e1b6fu, 0x07ecf0ae5ee44ddau, 0xb1442798f49ffb4au, 0x99cd11cfdf41779du,
+            0xb2fe3f0b8599ef07u, 0x861fa7e6dcb4aa15u, 0xb4bca50b065abe63u, 0x0fed077a756b53aau, 0xb67f6455292cbf08u, 0x1a3bc84c17b1d543u,
+            0xb84687c269ef3bfbu, 0x3d5d514f40eea742u, 0xba121a4650e4ddebu, 0x92f34d62616ce413u, 0xbbe226efb628afeau, 0x890489f70a55368cu,
+            0xbdb6b8e905cb600fu, 0x5400e987bbc1c921u, 0xbf8fdb78849a5f96u, 0xde98520472bdd034u, 0xc16d9a0095928a27u, 0x75b7053c0f178294u,
+            0xc350000000000000u, 0x0000000000000000u, 0xc5371912364ce305u, 0x6c28000000000000u, 0xc722f0ef9d80aad6u, 0x424d3ad2b7b97ef6u,
+            0xc913936dd571c84cu, 0x03bc3a19cd1e38eau, 0xcb090c8001ab551cu, 0x5cadf5bfd3072cc6u, 0xcd036837130890a1u, 0x36dba887c37a8c10u,
+            0xcf02b2c21207ef2eu, 0x94f967e45e03f4bcu, 0xd106f86e69d785c7u, 0xe13336d701beba52u, 0xd31045a8341ca07cu, 0x1ede48111209a051u,
+            0xd51ea6fa85785631u, 0x552a74227f3ea566u, 0xd732290fbacaf133u, 0xa97c177947ad4096u, 0xd94ad8b1c7380874u, 0x18375281ae7822bdu,
+            0xdb68c2ca82ed2a05u, 0xa67398db9f6820e1u
+        }
+    };
+    return table;
+}
+
+/// one bit per power: whether the computed value is one unit too large
+inline const std::array<std::uint32_t, 21>& pow10_fixups() noexcept
+{
+    static const std::array<std::uint32_t, 21> table =
+    {
+        {
+            0x8d8fc810u, 0x06100293u, 0x19000000u, 0x00100000u, 0x00000908u, 0x00000000u, 0x04e00300u, 0x3807e0b2u, 0x3d83d793u, 0x0006f5ccu,
+            0x00000000u, 0xffff0000u, 0x8076337du, 0x4ff45ba0u, 0x09405033u, 0x034376d9u, 0x09000000u, 0x4e100501u, 0x076d14dcu, 0xf964f45eu,
+            0x0000003du
+        }
+    };
+    return table;
+}
+
+/// the 128-bit significand of 10^k, rounded down, for k in [-307, 341]
+/// (compute_pow10 of Zmij)
+inline uint128_parts compute_pow10(int k) noexcept
+{
+    const auto i = static_cast<unsigned>(k + 307);
+    const std::uint64_t m = pow10_minor()[(i + 24) % 28];
+    const std::size_t j = 2 * static_cast<std::size_t>((i + 24) / 28);
+    const std::uint64_t h_hi = pow10_major()[j];
+    const std::uint64_t h_lo = pow10_major()[j + 1];
+    const std::uint64_t h1 = full_multiplication(h_lo, m).high;
+    const std::uint64_t c0 = h_lo * m;
+    const std::uint64_t c1 = h1 + (h_hi * m);
+    const std::uint64_t c2 = (c1 < h1 ? 1u : 0u) + full_multiplication(h_hi, m).high;
+    uint128_parts r{};
+    if ((c2 >> 63u) != 0)
+    {
+        r.high = c2;
+        r.low = c1;
+    }
+    else
+    {
+        r.high = (c2 << 1u) | (c1 >> 63u);
+        r.low = (c1 << 1u) | (c0 >> 63u);
+    }
+    r.low -= (pow10_fixups()[i >> 5u] >> (i & 31u)) & 1u;
+    return r;
+}
+
+/// The 128-bit significand of 10^k, rounded down, for k in [-342, 341].
+/// Up to 10^308, the table for number parsing holds the same significands
+/// (those of 5^k), except for k in [-27, -1], where it holds them one unit
+/// larger (as the Eisel-Lemire algorithm needs them).
+inline uint128_parts pow10(int k) noexcept
+{
+    if (k > pow5_128_largest_power)
+    {
+        return compute_pow10(k); // (only for the smallest doubles)
+    }
+    const auto i = 2 * static_cast<std::size_t>(k - pow5_128_smallest_power);
+    uint128_parts r{pow5_128()[i + 1], pow5_128()[i]};
+    const std::uint64_t adjust = static_cast<unsigned>(k + 27) < 27u ? 1u : 0u;
+    r.high -= r.low < adjust ? 1u : 0u;
+    r.low -= adjust;
+    return r;
+}
+
+/// (x_hi * 2^64 + x_lo) * y >> 64, as 128 bits
+inline uint128_parts umul192_hi128(std::uint64_t x_hi, std::uint64_t x_lo, std::uint64_t y) noexcept
+{
+    const uint128_parts p = full_multiplication(x_hi, y);
+    uint128_parts r{};
+    r.low = p.low + full_multiplication(x_lo, y).high;
+    r.high = p.high + (r.low < p.low ? 1u : 0u);
+    return r;
+}
+
+/// (x * y + c) >> 64
+inline std::uint64_t umul128_add_hi64(std::uint64_t x, std::uint64_t y, std::uint64_t c) noexcept
+{
+    const uint128_parts p = full_multiplication(x, y);
+    return p.high + (p.low + c < p.low ? 1u : 0u);
+}
+
+/// the result of Zmij: the shorter candidate and, if that is outside the
+/// rounding interval, the digit after it (16 bytes: returned in registers)
+struct shortest_decimal
+{
+    std::uint64_t integral;  ///< the shorter candidate (15 or 16 digits for normal doubles)
+    int exponent;            ///< the decimal exponent of the digit after it
+    unsigned char digit;     ///< the digit after it (if has_digit)
+    bool has_digit;          ///< whether the shortest decimal is integral * 10 + digit
+};
+
+/// The shortest decimal in the rounding interval of a positive finite double
+/// given by its bits, the closest one if there are several (to_decimal of
+/// Zmij, which keeps the last digit apart: the 15 or 16 digits before it can be
+/// converted without a multiplication by 10 first). Always inlined: GCC
+/// otherwise calls it, and its result goes through memory.
+JSON_HEDLEY_ALWAYS_INLINE shortest_decimal to_shortest(std::uint64_t bits) noexcept
+{
+    constexpr int extra_shift = 9;
+    const auto raw_exp = static_cast<int>((bits >> 52u) & 0x7FFu);
+    std::uint64_t bin_sig = bits & ((std::uint64_t{1} << 52u) - 1);
+    // a power of two has a narrower interval below (except the smallest normal)
+    const bool regular = bin_sig != 0 || raw_exp <= 1;
+    const int bin_exp = (raw_exp == 0 ? 1 : raw_exp) - 1075;
+    if (raw_exp != 0)
+    {
+        bin_sig |= std::uint64_t{1} << 52u;
+    }
+    // floor(log10(2^bin_exp)), or floor(log10(3/4 * 2^bin_exp)) for the irregular case
+    const int dec_exp = ((bin_exp * 315653) - (regular ? 0 : 131072)) >> 20;
+    // scaled by 10^(-dec_exp - 1): the integral part is the shorter candidate
+    const int shift = bin_exp + ((-(dec_exp + 1) * 217707) >> 16) + 1 + extra_shift;
+    const uint128_parts p10 = pow10(-dec_exp - 1);
+    const uint128_parts p = umul192_hi128(p10.high, p10.low, bin_sig << static_cast<unsigned>(shift));
+    std::uint64_t integral = p.high >> static_cast<unsigned>(extra_shift);
+    const std::uint64_t fractional = (p.high << static_cast<unsigned>(64 - extra_shift)) | (p.low >> static_cast<unsigned>(extra_shift));
+    std::uint64_t digit = 0;
+    bool round_up = false;
+    bool round_down = false;
+    if (JSON_HEDLEY_LIKELY(regular))
+    {
+        const std::uint64_t half_ulp = (p10.high >> static_cast<unsigned>(extra_shift + 1 - shift)) + (1 - (bin_sig & 1u));
+        round_up = fractional + half_ulp < fractional;
+        round_down = half_ulp > fractional;
+        // the last digit of the longer candidate, rounded to nearest
+        digit = umul128_add_hi64(fractional, 10, (std::uint64_t{1} << 63u) + 6);
+        if (fractional == (std::uint64_t{1} << 62u))
+        {
+            digit = 2; // 2.5 rounds to 2
+        }
+    }
+    else
+    {
+        const std::uint64_t half_ulp = p10.high >> static_cast<unsigned>(extra_shift + 1 - shift);
+        round_up = half_ulp > ~std::uint64_t{0} - fractional;
+        round_down = (half_ulp >> 1u) > fractional;
+        digit = umul128_add_hi64(fractional, 10, (std::uint64_t{1} << 63u) - 1);
+        const std::uint64_t lowest = umul128_add_hi64(fractional - (half_ulp >> 1u), 10, ~std::uint64_t{0});
+        digit = digit < lowest ? lowest : digit;
+    }
+    integral += round_up ? 1u : 0u;
+    // if the shorter candidate is outside the rounding interval: one digit more
+    return shortest_decimal{integral, dec_exp, static_cast<unsigned char>(digit), !round_up && !round_down};
+}
+
+/// The shortest decimal in the rounding interval of a positive finite double
+/// given by its bits, as one number. The significand can end in zeros.
+inline decimal to_decimal(std::uint64_t bits) noexcept
+{
+    const shortest_decimal d = to_shortest(bits);
+    if (d.has_digit)
+    {
+        return decimal{(d.integral * 10) + d.digit, d.exponent};
+    }
+    return decimal{d.integral, d.exponent + 1};
+}
+
+}  // namespace zmij
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END
@@ -1072,20 +1072,6 @@ class binary_reader
        }
    }

-    /*!
-    @brief reports a nested indefinite-length CBOR string or byte array
-    @param[in] type_name  name of the rejected string type
-    @param[in] context  parsing context for the error message
-    @return whether the SAX consumer accepts the parse error
-    */
-    bool cbor_indefinite_string_error(const char* type_name, const char* context)
-    {
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(concat("indefinite-length ", type_name,
-                                        " is not allowed inside indefinite-length ", type_name, "; last byte: 0x", last_token), context), nullptr));
-    }
-
    /*!
    @brief reads a definite-length CBOR string

@@ -1095,13 +1081,12 @@ class binary_reader
    into the same string.

    @param[out] result  string the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string

    @return whether string creation completed

    @pre @a current is not EOF
    */
-    bool get_cbor_string_chunk(string_t& result, const bool inside_indefinite)
+    bool get_cbor_string_chunk(string_t& result)
    {
        switch (current)
        {
@@ -1162,7 +1147,7 @@ class binary_reader
            {
                auto last_token = get_token_string();
                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x60-0x7B)", inside_indefinite ? "" : " or indefinite string type (0x7F)", "; last byte: 0x", last_token), "string"), nullptr));
+                                        exception_message(concat("expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x", last_token), "string"), nullptr));
            }
        }
    }
@@ -1180,9 +1165,13 @@ class binary_reader
    */
    bool get_cbor_string(string_t& result, const char* context = "string")
    {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // number of indefinite-length strings that have been opened and not
+        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
+        // but this reader has always accepted it, so the open levels are
+        // counted instead of recursed through, which overflowed the stack for
+        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
+        // to the same result, so no per-level state is needed.
+        std::size_t open = 0;

        while (true)
        {
@@ -1193,28 +1182,29 @@ class binary_reader

            if (current == 0x7F) // UTF-8 string (indefinite length)
            {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("string", "string");
-                }
-                indefinite = true;
+                ++open;
                get();
                continue;
            }

-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length string;
+            // outside of one it is not a string and falls through to the error
+            if (open != 0 && current == 0xFF)
            {
-                return check_string_utf8(result, context);
+                if (--open == 0)
+                {
+                    return check_string_utf8(result, context);
+                }
+                get();
+                continue;
            }

-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
            {
                return false;
            }

-            if (!indefinite)
+            if (open == 0)
            {
                return check_string_utf8(result, context);
            }
@@ -1306,13 +1296,12 @@ class binary_reader
    read into the same byte array.

    @param[out] result  byte array the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string

    @return whether byte array creation completed

    @pre @a current is not EOF
    */
-    bool get_cbor_binary_chunk(binary_t& result, const bool inside_indefinite)
+    bool get_cbor_binary_chunk(binary_t& result)
    {
        switch (current)
        {
@@ -1377,7 +1366,7 @@ class binary_reader
            {
                auto last_token = get_token_string();
                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x40-0x5B)", inside_indefinite ? "" : " or indefinite binary array type (0x5F)", "; last byte: 0x", last_token), "binary"), nullptr));
+                                        exception_message(concat("expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x", last_token), "binary"), nullptr));
            }
        }
    }
@@ -1395,9 +1384,9 @@ class binary_reader
    */
    bool get_cbor_binary(binary_t& result)
    {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // the open indefinite-length byte arrays are counted rather than
+        // recursed through, for the reason given in @ref get_cbor_string
+        std::size_t open = 0;

        while (true)
        {
@@ -1408,28 +1397,29 @@ class binary_reader

            if (current == 0x5F) // Binary data (indefinite length)
            {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("binary array", "binary");
-                }
-                indefinite = true;
+                ++open;
                get();
                continue;
            }

-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length byte
+            // array; outside of one it falls through to the error below
+            if (open != 0 && current == 0xFF)
            {
-                return true;
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
            }

-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
            {
                return false;
            }

-            if (!indefinite)
+            if (open == 0)
            {
                return true;
            }
@@ -221,6 +221,44 @@ class lexer : public lexer_base<BasicJsonType>
    // scan functions
    /////////////////////

+    /// contiguous input: try to decode the 4 hex digits following `\\u`
+    /// directly from the input buffer via hex_codepoint(), instead of 4 calls
+    /// to get(). On success, advances the adapter and the position counters
+    /// exactly as those 4 get() calls would (a hex digit is never '\n', so
+    /// only the flat counters move) and leaves @a current holding the last of
+    /// the 4 digits, just as the last such get() would; the codepoint is
+    /// written to @a out. Makes no state change and returns false - for a
+    /// pending unget, fewer than 4 remaining bytes, or any of the 4 bytes not
+    /// being a hex digit - so the caller falls back unchanged to the
+    /// per-character loop, which then reports the same diagnostic (stopping
+    /// at the first invalid digit) as before this optimization.
+    bool get_codepoint_bulk(std::true_type /*bulk*/, int& out)
+    {
+        if (next_unget || ia.bulk_remaining() < 4)
+        {
+            return false;
+        }
+        const char_type* const raw = ia.bulk_data();
+        const int codepoint = hex_codepoint(reinterpret_cast<const unsigned char*>(raw));
+        if (codepoint < 0)
+        {
+            return false;
+        }
+        ia.bulk_skip(4);
+        // a hex digit is never a newline, so only the flat counters advance
+        position.chars_read_total += 4;
+        position.chars_read_current_line += 4;
+        current = char_traits<char_type>::to_int_type(raw[3]);
+        out = codepoint;
+        return true;
+    }
+
+    /// streaming input: no bulk fast path
+    bool get_codepoint_bulk(std::false_type /*bulk*/, int& /*out*/) const noexcept
+    {
+        return false;
+    }
+
    /*!
    @brief get codepoint from 4 hex characters following `\\u`

@@ -240,6 +278,14 @@ class lexer : public lexer_base<BasicJsonType>
    {
        // this function only makes sense after reading `\u`
        JSON_ASSERT(current == 'u');
+
+        // contiguous input: decode all 4 hex digits directly from the buffer
+        int fast_codepoint = 0;
+        if (get_codepoint_bulk(std::integral_constant<bool, bulk_scan> {}, fast_codepoint))
+        {
+            return fast_codepoint;
+        }
+
        int codepoint = 0;

        const auto factors = { 12u, 8u, 4u, 0u };
@@ -1044,9 +1090,11 @@ class lexer : public lexer_base<BasicJsonType>
            token_type::parse_error otherwise

    @note The scanner is independent of the current locale: token_buffer
-          always holds `.`. Only the std::strtod fallback of convert_number()
-          depends on the locale, and it looks up the decimal point right
-          before converting (see detail::convert_float_locale_aware()).
+          always holds `.`. The conversion of float and double does not use
+          the locale either. Only the std::strtold fallback of
+          convert_number() for long double formats other than binary64
+          depends on it, and it looks up the decimal point right before
+          converting (see detail::convert_float_locale_aware()).
    */
    token_type scan_number()  // lgtm [cpp/use-of-goto] `goto` is used in this function to implement the number-parsing state machine described above. By design, any finite input will eventually reach the "done" state or return token_type::parse_error. In each intermediate state, 1 byte of the input is appended to the token_buffer vector, and only the already initialized variables token_buffer, number_type, and error_message are manipulated.
    {
@@ -1059,7 +1107,7 @@ class lexer : public lexer_base<BasicJsonType>

        // offset just past the last mantissa byte in token_buffer (i.e. the
        // index of 'e'/'E', or the whole token when there is no exponent).
-        // convert_number() uses it to count significant digits; npos means
+        // convert_number() uses it to split the token; npos means
        // "not seen an exponent yet" and is resolved at scan_number_done
        std::size_t mantissa_end = std::string::npos;

@@ -1389,8 +1437,8 @@ scan_number_done:
    @param[in] mantissa_end  offset just past the last mantissa byte in
                             token_buffer (the index of 'e'/'E', or
                             token_buffer.size() when there is no exponent);
-                             used to skip Clinger's fast path when it cannot
-                             possibly succeed - see detail::mantissa_fits_clinger()
+                             with decimal_point_position, it locates the parts
+                             of a float token without scanning it again
    */
    token_type convert_number(token_type number_type, std::size_t mantissa_end)
    {
@@ -1444,10 +1492,11 @@ scan_number_done:
        }

        // this code is reached if we parse a floating-point number or if an
-        // integer conversion above overflowed. Prefer std::from_chars
-        // (Eisel-Lemire, locale-independent, correctly rounded) when available;
-        // otherwise the exact Clinger fast path (double only); otherwise the
-        // locale-aware strtof/strtod/strtold.
+        // integer conversion above overflowed. float and double (and long
+        // double where it is binary64) are converted by the library itself,
+        // correctly rounded and independent of the locale; other long double
+        // formats use std::from_chars when available, otherwise the
+        // locale-aware strtold.
        if (convert_float_fast(num_begin, num_end, decimal_point_position, mantissa_end, value_float))
        {
            return token_type::value_float;
@@ -2021,6 +2070,39 @@ scan_number_done:
        // read the next character and ignore whitespace
        skip_whitespace();

+        return scan_after_whitespace();
+    }
+
+    /*!
+    @brief scan the next token when the caller expects a separator (':' or
+           ',') most of the time
+
+    After an object key the next token is almost always ':', after a value
+    inside an object or array almost always ','. Testing for that character
+    first is a compare and a well-predicted branch, where the switch in
+    scan_after_whitespace() is an indirect jump through a table. Anything else
+    goes through the switch, so the result is the same as scan()'s.
+
+    May only be called after scan() has run once (the BOM check is skipped).
+    */
+    token_type scan_expecting(token_type expected_type)
+    {
+        JSON_ASSERT(expected_type == token_type::name_separator || expected_type == token_type::value_separator);
+        JSON_ASSERT(position.chars_read_total > 0);
+        const char_int_type expected_char = static_cast<unsigned char>((expected_type == token_type::name_separator) ? ':' : ',');
+        skip_whitespace();
+        if (JSON_HEDLEY_LIKELY(current == expected_char))
+        {
+            return expected_type;
+        }
+        return scan_after_whitespace();
+    }
+
+  private:
+    /// the part of scan() after the leading whitespace: skip comments and
+    /// scan the token that starts with current
+    token_type scan_after_whitespace()
+    {
        // ignore comments
        while (ignore_comments && current == '/')
        {
@@ -2100,7 +2182,6 @@ scan_number_done:
        }
    }

-  private:
    /// input adapter
    InputAdapterType ia;

@@ -260,7 +260,7 @@ class parser
                        }

                        // parse separator (:)
-                        if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                        if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                        {
                            return sax->parse_error(m_lexer.get_position(),
                                                    m_lexer.get_token_string(),
@@ -423,7 +423,7 @@ class parser
            {
                // comma -> next value
                // or end of array (ignore_trailing_commas = true)
-                if (get_token() == token_type::value_separator)
+                if (get_token_expecting(token_type::value_separator))
                {
                    // parse a new value
                    get_token();
@@ -463,7 +463,7 @@ class parser

            // comma -> next value
            // or end of object (ignore_trailing_commas = true)
-            if (get_token() == token_type::value_separator)
+            if (get_token_expecting(token_type::value_separator))
            {
                get_token();

@@ -484,7 +484,7 @@ class parser
                    }

                    // parse separator (:)
-                    if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                    if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                    {
                        return sax->parse_error(m_lexer.get_position(),
                                                m_lexer.get_token_string(),
@@ -528,6 +528,13 @@ class parser
        return last_token = m_lexer.scan();
    }

+    /// get next token from lexer; true if it is the separator @a expected_type
+    /// (name_separator or value_separator), which it usually is
+    bool get_token_expecting(token_type expected_type)
+    {
+        return (last_token = m_lexer.scan_expecting(expected_type)) == expected_type;
+    }
+
    std::string exception_message(const token_type expected, const std::string& context)
    {
        std::string error_msg = "syntax error ";
@@ -8,10 +8,12 @@

 #pragma once

+#include <array> // array
 #include <cstddef> // size_t
-#include <cstdint> // uint64_t
+#include <cstdint> // uint64_t, uint8_t
 #include <cstring> // memcpy

+#include <nlohmann/detail/bit_ops.hpp>
 #include <nlohmann/detail/macro_scope.hpp>

 // Optional SIMD backend for bulk UTF-8 validation. This is an opt-in external
@@ -69,18 +71,12 @@ inline std::size_t find_string_special(const unsigned char* data, std::size_t n)
    std::size_t i = 0;
    for (; i + 8 <= n; i += 8)
    {
-        std::uint64_t word = 0;
-        std::memcpy(&word, data + i, sizeof(word));
-        if (swar_string_special(word) != 0)
+        const std::uint64_t special = swar_string_special(read_eight_bytes(data + i));
+        if (special != 0)
        {
-            // a special byte is in this word; locate it (endian-agnostic)
-            for (std::size_t j = 0; j < 8; ++j)
-            {
-                if (is_string_special(data[i + j]))
-                {
-                    return i + j;
-                }
-            }
+            // the lowest flagged byte is the first special one: the borrows of
+            // the subtractions can only flag bytes above a true hit
+            return i + (static_cast<std::size_t>(count_trailing_zeros(special)) / 8);
        }
    }
    for (; i < n; ++i)
@@ -114,8 +110,7 @@ inline std::size_t find_ascii_copyable_run(const unsigned char* data, std::size_
    std::size_t i = 0;
    for (; i + 8 <= n; i += 8)
    {
-        std::uint64_t v = 0;
-        std::memcpy(&v, data + i, sizeof(v));
+        const std::uint64_t v = read_eight_bytes(data + i);
        const std::uint64_t q = v ^ 0x2222222222222222ull; // '"'  (0x22)
        const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\' (0x5C)
        const std::uint64_t d = v ^ 0x7F7F7F7F7F7F7F7Full; // DEL  (0x7F)
@@ -126,7 +121,9 @@ inline std::size_t find_ascii_copyable_run(const unsigned char* data, std::size_
                                   | (v & high);               // >= 0x80
        if (stop != 0)
        {
-            break;
+            // the lowest flagged byte is the first one to stop at (see
+            // find_string_special())
+            return i + (static_cast<std::size_t>(count_trailing_zeros(stop)) / 8);
        }
    }
    for (; i < n; ++i)
@@ -253,12 +250,18 @@ inline std::size_t scalar_string_bulk_run(const unsigned char* data, std::size_t
        {
            break; // end of buffer, or a quote/escape/control byte
        }
-        const std::size_t seq = validate_one_utf8(data + pos, n - pos);
-        if (seq == 0)
+        // a run of multi-byte sequences (e.g. CJK text) is validated sequence
+        // by sequence without searching for the next special byte in between
+        do
        {
-            break; // ill-formed or truncated: let the byte path diagnose it
+            const std::size_t seq = validate_one_utf8(data + pos, n - pos);
+            if (seq == 0)
+            {
+                return pos; // ill-formed or truncated: let the byte path diagnose it
+            }
+            pos += seq;
        }
-        pos += seq;
+        while (pos < n && data[pos] >= 0x80u);
    }
    return pos;
 }
@@ -273,8 +276,7 @@ inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t
    std::size_t i = 0;
    for (; i + 8 <= n; i += 8)
    {
-        std::uint64_t v = 0;
-        std::memcpy(&v, data + i, sizeof(v));
+        const std::uint64_t v = read_eight_bytes(data + i);
        const std::uint64_t q = v ^ 0x2222222222222222ull;
        const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull;
        const std::uint64_t hit = ((q - ones) & ~q & high)
@@ -282,14 +284,8 @@ inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t
                                  | ((v - 0x2020202020202020ull) & ~v & high);
        if (hit != 0)
        {
-            for (std::size_t j = 0; j < 8; ++j)
-            {
-                const unsigned char c = data[i + j];
-                if (c == '\"' || c == '\\' || c < 0x20u)
-                {
-                    return i + j;
-                }
-            }
+            // the lowest flagged byte is the first delimiter (see find_string_special())
+            return i + (static_cast<std::size_t>(count_trailing_zeros(hit)) / 8);
        }
    }
    for (; i < n; ++i)
@@ -320,5 +316,50 @@ inline std::size_t string_bulk_run(const unsigned char* data, std::size_t n) noe
    return scalar_string_bulk_run(data, n);
 }

+// Decode the 4 hex digits at [data, data+4) - the digits following a `\u`
+// escape - into a codepoint 0x0000..0xFFFF via one table lookup per byte
+// (after yyjson's read_hex_u16), or return -1 if any of the 4 bytes is not a
+// hex digit ('0'..'9', 'A'..'F', 'a'..'f'). The caller must already have
+// checked that 4 bytes are available; used by lexer::get_codepoint()'s
+// contiguous fast path. On -1 it falls back to the byte-at-a-time loop, which
+// stops at the first invalid digit, so the reported error and position are
+// unaffected by this fast path.
+inline int hex_codepoint(const unsigned char* data) noexcept
+{
+    static const std::array<std::uint8_t, 256> hex_digit_table = // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
+    {
+        {
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 00..0F
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 10..1F
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 20..2F
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 30..3F ('0'..'9')
+            0xFF, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 40..4F ('A'..'F')
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 50..5F
+            0xFF, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 60..6F ('a'..'f')
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 70..7F
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 80..8F
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 90..9F
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // A0..AF
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // B0..BF
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // C0..CF
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // D0..DF
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // E0..EF
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF  // F0..FF
+        }
+    };
+
+    const std::uint8_t d0 = hex_digit_table[data[0]];
+    const std::uint8_t d1 = hex_digit_table[data[1]];
+    const std::uint8_t d2 = hex_digit_table[data[2]];
+    const std::uint8_t d3 = hex_digit_table[data[3]];
+    // every valid digit is <= 0xF; the combined OR only exceeds it if at
+    // least one of the four bytes was not a hex digit (looked up as 0xFF)
+    if ((d0 | d1 | d2 | d3) > 0x0F)
+    {
+        return -1;
+    }
+    return (d0 << 12) | (d1 << 8) | (d2 << 4) | d3;
+}
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
@@ -21,6 +21,8 @@
 #undef JSON_NO_UNIQUE_ADDRESS
 #undef JSON_DISABLE_ENUM_SERIALIZATION
 #undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
+#undef JSON_DTOA_SSE2
+#undef JSON_DTOA_NEON

 #ifndef JSON_TEST_KEEP_MACROS
    #undef JSON_CATCH
@@ -1366,8 +1366,9 @@ class serializer
    /*!
    @brief dump an integer

-    Dump a given integer, appending it to @ref write_buffer. Works internally with
-    @a number_buffer.
+    Dump a given integer, appending it to @ref write_buffer (directly: copying
+    the digits from another buffer right after writing them waits until the
+    stores are done).

    @param[in] x  integer number (signed or unsigned) to dump
    @tparam NumberType either @a number_integer_t or @a number_unsigned_t
@@ -1402,33 +1403,57 @@ class serializer
            return;
        }

-        // use a pointer to fill the buffer
-        auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto)
+        // use a pointer to fill the buffer (room for as much as number_buffer holds)
+        if (JSON_HEDLEY_UNLIKELY(write_buffer_pos + number_buffer.size() > write_buffer.size()))
+        {
+            flush();
+        }
+        auto* buffer_ptr = write_buffer.data() + write_buffer_pos;

        number_unsigned_t abs_value;

-        unsigned int n_chars{};
+        // one byte for the minus sign
+        unsigned int n_chars = 0;

        if (is_negative_number(x))
        {
            *buffer_ptr = '-';
            abs_value = remove_sign(static_cast<number_integer_t>(x));
-
-            // account one more byte for the minus sign
-            n_chars = 1 + count_digits(abs_value);
+            n_chars = 1;
        }
        else
        {
            abs_value = static_cast<number_unsigned_t>(x);
-            n_chars = count_digits(abs_value);
        }

+        // up to 16 digits: eight at a time (as the digits of floats), written
+        // without leading zeros
+        if (abs_value < 10000000000000000u)
+        {
+            const std::uint64_t value = abs_value;
+            const std::uint64_t upper = value / 100000000u;
+            const std::uint64_t first = dtoa_impl::eight_digit_bytes(upper != 0 ? upper : value);
+            const auto leading = static_cast<unsigned>(count_leading_zeros(first) / 8); // (first is not 0)
+            char* const p = buffer_ptr + n_chars;
+            dtoa_impl::store_msb_first(p, (first << (8 * leading)) + 0x3030303030303030u);
+            n_chars += 8 - leading;
+            if (upper != 0)
+            {
+                dtoa_impl::store_msb_first(p + 8 - leading, dtoa_impl::eight_digit_bytes(value - (upper * 100000000u)) + 0x3030303030303030u);
+                n_chars += 8;
+            }
+            write_buffer_pos += n_chars;
+            return;
+        }
+
+        n_chars += count_digits(abs_value);
+
        // spare 1 byte for '\0'
        JSON_ASSERT(n_chars < number_buffer.size() - 1);

        // jump to the end to generate the string from backward,
        // so we later avoid reversing the result
-        buffer_ptr += static_cast<typename decltype(number_buffer)::difference_type>(n_chars);
+        buffer_ptr += n_chars;

        // Fast int2ascii implementation inspired by "Fastware" talk by Andrei Alexandrescu
        // See: https://www.youtube.com/watch?v=o4-CwDo2zpg
@@ -1451,14 +1476,13 @@ class serializer
            *(--buffer_ptr) = static_cast<char>('0' + abs_value);
        }

-        put_buffer(number_buffer, n_chars);
+        write_buffer_pos += n_chars;
    }

    /*!
    @brief dump a floating-point number

-    Dump a given floating-point number, appending it to @ref write_buffer. Works internally
-    with @a number_buffer.
+    Dump a given floating-point number, appending it to @ref write_buffer.

    @param[in] x  floating-point number to dump
    */
@@ -1485,10 +1509,15 @@ class serializer

    void dump_float(number_float_t x, std::true_type /*is_ieee_single_or_double*/)
    {
-        auto* begin = number_buffer.data();
+        // directly into the write buffer: copying the text from number_buffer
+        // right after to_chars() wrote it waits until its stores are done
+        if (JSON_HEDLEY_UNLIKELY(write_buffer_pos + number_buffer.size() > write_buffer.size()))
+        {
+            flush();
+        }
+        auto* begin = write_buffer.data() + write_buffer_pos;
        auto* end = ::nlohmann::detail::to_chars(begin, begin + number_buffer.size(), x);
-
-        put_buffer(number_buffer, static_cast<std::size_t>(end - begin));
+        write_buffer_pos += static_cast<std::size_t>(end - begin);
    }

    JSON_HEDLEY_NON_NULL(1)
@@ -0,0 +1,599 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <array> // array
+#include <cstdint> // uint32_t, uint64_t
+
+// Number tokens that are hard to round correctly, with the IEEE-754 binary64
+// and binary32 bits of their correctly rounded values (ties to even; infinity
+// for an overflow, a signed zero for an underflow).
+//
+// For doubles and floats around 0, the smallest normal number, 1, 2^24, 2^53,
+// 0.1, and the largest finite number, and for random ones, the exact midpoint
+// m to the next number gives: m, m with one unit more and less in the last
+// digit, m with "01" and "0...01" appended, m with trailing zeros, and m cut
+// after 17 to 30 digits (rounded down and up, so that the rounding is decided
+// after the 19th digit), in fixed and exponent notation, 30% of them negative.
+// Tokens longer than 80 characters are left out, except for four of 700 digits
+// and more. Zeros, underflow, overflow, huge exponents, and integers beyond 64
+// bits complete the set. Of the 508 tokens, 134 (as double) and 150 (as
+// float) need the exact comparison with the midpoint (detail::digit_comparison()).
+//
+// The expected bits were computed with exact rational arithmetic in Python
+// (fractions.Fraction) and cross-checked with Python's float(); strtod_l and
+// strtof_l of Apple's libc and of glibc agree. Generated by
+// compact_hard_cases.py 5 (with hard_cases.py), see the pull request that
+// added this file.
+
+namespace float_hard_cases
+{
+
+struct hard_case
+{
+    const char* token;
+    std::uint64_t bits64;
+    std::uint32_t bits32;
+};
+
+inline const std::array<hard_case, 508>& cases()
+{
+    static const std::array<hard_case, 508> table =
+    {
+        {
+            {"-2.4703282292062327e-324", 0x8000000000000000u, 0x80000000u},
+            {"24703282292062328e-340", 0x0000000000000001u, 0x00000000u},
+            {"247032822920623272e-341", 0x0000000000000000u, 0x00000000u},
+            {"-0.2470328229206232721e-323", 0x8000000000000001u, 0x80000000u},
+            {"-0.24703282292062327208e-323", 0x8000000000000000u, 0x80000000u},
+            {"-2.4703282292062327209e-324", 0x8000000000000001u, 0x80000000u},
+            {"2.47032822920623272088e-324", 0x0000000000000000u, 0x00000000u},
+            {"247032822920623272089e-344", 0x0000000000000001u, 0x00000000u},
+            {"-247032822920623272088284396434e-353", 0x8000000000000000u, 0x80000000u},
+            {"0.247032822920623272088284396435e-323", 0x0000000000000001u, 0x00000000u},
+            {"-74109846876186981e-340", 0x8000000000000001u, 0x80000000u},
+            {"0.74109846876186982e-323", 0x0000000000000002u, 0x00000000u},
+            {"-0.7410984687618698162e-323", 0x8000000000000001u, 0x80000000u},
+            {"-7.410984687618698163e-324", 0x8000000000000002u, 0x80000000u},
+            {"7.4109846876186981626e-324", 0x0000000000000001u, 0x00000000u},
+            {"-74109846876186981627e-343", 0x8000000000000002u, 0x80000000u},
+            {"-741098468761869816264e-344", 0x8000000000000001u, 0x80000000u},
+            {"0.741098468761869816265e-323", 0x0000000000000002u, 0x00000000u},
+            {"0.741098468761869816264853189302e-323", 0x0000000000000001u, 0x00000000u},
+            {"-7.41098468761869816264853189303e-324", 0x8000000000000002u, 0x80000000u},
+            {"0.22250738585072006e-307", 0x000FFFFFFFFFFFFEu, 0x00000000u},
+            {"2.2250738585072007e-308", 0x000FFFFFFFFFFFFFu, 0x00000000u},
+            {"2.225073858507200641e-308", 0x000FFFFFFFFFFFFEu, 0x00000000u},
+            {"-2225073858507200642e-326", 0x800FFFFFFFFFFFFFu, 0x80000000u},
+            {"22250738585072006419e-327", 0x000FFFFFFFFFFFFEu, 0x00000000u},
+            {"0.2225073858507200642e-307", 0x000FFFFFFFFFFFFFu, 0x00000000u},
+            {"0.222507385850720064199e-307", 0x000FFFFFFFFFFFFEu, 0x00000000u},
+            {"2.225073858507200642e-308", 0x000FFFFFFFFFFFFFu, 0x00000000u},
+            {"-2.22507385850720064199176395546e-308", 0x800FFFFFFFFFFFFEu, 0x80000000u},
+            {"222507385850720064199176395547e-337", 0x000FFFFFFFFFFFFFu, 0x00000000u},
+            {"-2.2250738585072011e-308", 0x800FFFFFFFFFFFFFu, 0x80000000u},
+            {"-22250738585072012e-324", 0x8010000000000000u, 0x80000000u},
+            {"-2225073858507201136e-326", 0x800FFFFFFFFFFFFFu, 0x80000000u},
+            {"0.2225073858507201137e-307", 0x0010000000000000u, 0x00000000u},
+            {"0.2225073858507201136e-307", 0x000FFFFFFFFFFFFFu, 0x00000000u},
+            {"-2.2250738585072011361e-308", 0x8010000000000000u, 0x80000000u},
+            {"2.22507385850720113605e-308", 0x000FFFFFFFFFFFFFu, 0x00000000u},
+            {"222507385850720113606e-328", 0x0010000000000000u, 0x00000000u},
+            {"22250738585072011360574097967e-336", 0x000FFFFFFFFFFFFFu, 0x00000000u},
+            {"0.222507385850720113605740979671e-307", 0x0010000000000000u, 0x00000000u},
+            {"22250738585072016e-324", 0x0010000000000000u, 0x00000000u},
+            {"0.22250738585072017e-307", 0x0010000000000001u, 0x00000000u},
+            {"0.222507385850720163e-307", 0x0010000000000000u, 0x00000000u},
+            {"2.225073858507201631e-308", 0x0010000000000001u, 0x00000000u},
+            {"-2.2250738585072016301e-308", 0x8010000000000000u, 0x80000000u},
+            {"22250738585072016302e-327", 0x0010000000000001u, 0x00000000u},
+            {"-222507385850720163012e-328", 0x8010000000000000u, 0x80000000u},
+            {"0.222507385850720163013e-307", 0x0010000000000001u, 0x00000000u},
+            {"0.222507385850720163012305563795e-307", 0x0010000000000000u, 0x00000000u},
+            {"-2.22507385850720163012305563796e-308", 0x8010000000000001u, 0x80000000u},
+            {"0.17976931348623156E+309", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
+            {"1.7976931348623157e308", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
+            {"1.797693134862315608e308", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
+            {"-1797693134862315609e290", 0xFFEFFFFFFFFFFFFFu, 0xFF800000u},
+            {"-17976931348623156083e289", 0xFFEFFFFFFFFFFFFEu, 0xFF800000u},
+            {"-0.17976931348623156084E+309", 0xFFEFFFFFFFFFFFFFu, 0xFF800000u},
+            {"0.179769313486231560835E+309", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
+            {"-1.79769313486231560836e308", 0xFFEFFFFFFFFFFFFFu, 0xFF800000u},
+            {"1.79769313486231560835325876058e308", 0x7FEFFFFFFFFFFFFEu, 0x7F800000u},
+            {"179769313486231560835325876059e279", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
+            {"1.7976931348623158e308", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
+            {"17976931348623159e292", 0x7FF0000000000000u, 0x7F800000u},
+            {"1797693134862315807e290", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
+            {"0.1797693134862315808E+309", 0x7FF0000000000000u, 0x7F800000u},
+            {"0.17976931348623158079E+309", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
+            {"-1.797693134862315808e308", 0xFFF0000000000000u, 0xFF800000u},
+            {"1.79769313486231580793e308", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
+            {"179769313486231580794e288", 0x7FF0000000000000u, 0x7F800000u},
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+                "00000000000000000000000000000000000000000000000000000000000000000e-37", 0x380FFFFFE0000000u, 0x00800000u
+            },
+            {
+                "1175494280757364291727882991035766513322858992758990427682963118425003064965173038558532425668090581"
+                "8939208984375000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+                "00000000000001e-751", 0x380FFFFFE0000000u, 0x00800000u
+            },
+            {"0", 0x0000000000000000u, 0x00000000u},
+            {"-0", 0x8000000000000000u, 0x80000000u},
+            {"0.0", 0x0000000000000000u, 0x00000000u},
+            {"-0.0", 0x8000000000000000u, 0x80000000u},
+            {"0e999999999999999999999", 0x0000000000000000u, 0x00000000u},
+            {"-0.000e-99999", 0x8000000000000000u, 0x80000000u},
+            {"1e-400", 0x0000000000000000u, 0x00000000u},
+            {"-1e-400", 0x8000000000000000u, 0x80000000u},
+            {"1e400", 0x7FF0000000000000u, 0x7F800000u},
+            {"-1e400", 0xFFF0000000000000u, 0xFF800000u},
+            {"1e-50", 0x358DEE7A4AD4B81Fu, 0x00000000u},
+            {"-1e-50", 0xB58DEE7A4AD4B81Fu, 0x80000000u},
+            {"1e39", 0x48078287F49C4A1Du, 0x7F800000u},
+            {"-1e39", 0xC8078287F49C4A1Du, 0xFF800000u},
+            {"1e99999999999999999999999999", 0x7FF0000000000000u, 0x7F800000u},
+            {"1e-99999999999999999999999999", 0x0000000000000000u, 0x00000000u},
+            {"1e0000000000000000000000000000000000000000308", 0x7FE1CCF385EBC8A0u, 0x7F800000u},
+            {"123456789012345678901234567890e-30", 0x3FBF9ADD3746F65Fu, 0x3DFCD6EAu},
+            {"18446744073709551615", 0x43F0000000000000u, 0x5F800000u},
+            {"18446744073709551616", 0x43F0000000000000u, 0x5F800000u},
+            {"-9223372036854775808", 0xC3E0000000000000u, 0xDF000000u},
+            {"-9223372036854775809", 0xC3E0000000000000u, 0xDF000000u},
+        }
+    };
+    return table;
+}
+
+} // namespace float_hard_cases
@@ -38,7 +38,7 @@ TEST_CASE("Binary Formats" * doctest::skip())
        const auto ubjson_2_size = json::to_ubjson(j, true).size();
        const auto ubjson_3_size = json::to_ubjson(j, true, true).size();

-        CHECK(json_size == 2090303);
+        CHECK(json_size == 2090234);
        CHECK(bjdata_1_size == 1112030);
        CHECK(bjdata_2_size == 1224148);
        CHECK(bjdata_3_size == 1224148);
@@ -51,16 +51,16 @@ TEST_CASE("Binary Formats" * doctest::skip())
        CHECK(ubjson_3_size == 1169069);

        CHECK((100.0 * double(json_size) / double(json_size)) == Approx(100.0));
-        CHECK((100.0 * double(bjdata_1_size) / double(json_size)) == Approx(53.199));
-        CHECK((100.0 * double(bjdata_2_size) / double(json_size)) == Approx(58.563));
-        CHECK((100.0 * double(bjdata_3_size) / double(json_size)) == Approx(58.563));
-        CHECK((100.0 * double(bon8_size) / double(json_size)) == Approx(50.509));
-        CHECK((100.0 * double(bson_size) / double(json_size)) == Approx(85.849));
-        CHECK((100.0 * double(cbor_size) / double(json_size)) == Approx(50.497));
-        CHECK((100.0 * double(msgpack_size) / double(json_size)) == Approx(50.526));
-        CHECK((100.0 * double(ubjson_1_size) / double(json_size)) == Approx(53.199));
-        CHECK((100.0 * double(ubjson_2_size) / double(json_size)) == Approx(58.563));
-        CHECK((100.0 * double(ubjson_3_size) / double(json_size)) == Approx(55.928));
+        CHECK((100.0 * double(bjdata_1_size) / double(json_size)) == Approx(53.201));
+        CHECK((100.0 * double(bjdata_2_size) / double(json_size)) == Approx(58.565));
+        CHECK((100.0 * double(bjdata_3_size) / double(json_size)) == Approx(58.565));
+        CHECK((100.0 * double(bon8_size) / double(json_size)) == Approx(50.511));
+        CHECK((100.0 * double(bson_size) / double(json_size)) == Approx(85.853));
+        CHECK((100.0 * double(cbor_size) / double(json_size)) == Approx(50.499));
+        CHECK((100.0 * double(msgpack_size) / double(json_size)) == Approx(50.528));
+        CHECK((100.0 * double(ubjson_1_size) / double(json_size)) == Approx(53.201));
+        CHECK((100.0 * double(ubjson_2_size) / double(json_size)) == Approx(58.565));
+        CHECK((100.0 * double(ubjson_3_size) / double(json_size)) == Approx(55.930));
    }

    SECTION("twitter.json")
@@ -1699,7 +1699,7 @@ TEST_CASE("CBOR")
            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xBF, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
-            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x41})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);

            CHECK(json::from_cbor(std::vector<uint8_t>({0x18}), true, false).is_discarded());
@@ -2305,21 +2305,22 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
 {
    // Reading an indefinite-length string or byte array used to call itself
    // once per chunk, so a payload of repeated 0x7F (or 0x5F) bytes exhausted
-    // the call stack before any of the input was rejected. Nested indefinite
-    // chunks are now rejected at the second byte, without recursing.
+    // the call stack before any of the input was rejected. The open levels are
+    // counted now, and the levels below prove the reader still reads the same
+    // values and reports the same errors at the same byte offsets.
    json _;

-    SECTION("nested levels are rejected, not crashed on")
+    SECTION("many open levels are reported, not crashed on")
    {
        const std::vector<uint8_t> input(200000, 0x7F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR string: unexpected end of input", json::parse_error&);
        CHECK(json::from_cbor(input, true, false).is_discarded());
    }

-    SECTION("nested levels are rejected, not crashed on (binary)")
+    SECTION("many open levels are reported, not crashed on (binary)")
    {
        const std::vector<uint8_t> input(200000, 0x5F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
        CHECK(json::from_cbor(input, true, false).is_discarded());
    }

@@ -2327,22 +2328,22 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
    {
        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0xFF})) == json(""));
        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 0x61, 0xFF})) == json("a"));
-        // empty and nonempty definite-length chunks concatenate in order
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 'a', 0x60, 0x61, 'b', 0x61, 'c', 0xFF})) == json("abc"));
+        // nested indefinite-length strings are concatenated across levels
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0x61, 0x62, 0xFF})) == json("ab"));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x7F, 0x61, 0x7A, 0xFF, 0xFF, 0xFF})) == json("z"));
        CHECK(json::from_cbor(std::vector<uint8_t>({0xA1, 0x7F, 0x61, 0x61, 0xFF, 0x01})) == json({{"a", 1}}));
    }

    SECTION("chunks are still concatenated (binary)")
    {
        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0xFF})) == json::binary({0x61}));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0xFF})) == json::binary({}));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0x40, 0x41, 0x62, 0x41, 0x63, 0xFF})) == json::binary({0x61, 0x62, 0x63}));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0x41, 0x62, 0xFF})) == json::binary({0x61, 0x62}));
    }

    SECTION("a chunk that is not a string is still rejected")
    {
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B); last byte: 0x00", json::parse_error&);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
    }

    SECTION("a break marker outside an indefinite-length string is not a string")
@@ -2895,17 +2896,9 @@ TEST_CASE("examples from RFC 8949 Appendix A")
    {
        const auto packed = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.cbor");
        json j;
-        // the fixture's tail contains nested indefinite-length byte strings.
-        CHECK_THROWS_WITH_AS(j = json::from_cbor(packed), "[json.exception.parse_error.113] parse error at byte 513: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
+        CHECK_NOTHROW(j = json::from_cbor(packed));

-        // keep the byte-for-byte decoding check for its valid prefix: the first
-        // 512 encoded bytes contain 468 payload bytes in definite-length chunks.
-        auto valid_prefix = packed;
-        valid_prefix.resize(512);
-        valid_prefix.push_back(0xFF);
-        auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
-        expected.resize(468);
-        CHECK_NOTHROW(j = json::from_cbor(valid_prefix));
+        const auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
        CHECK(j == json::binary(expected));

        // 0xd8
@@ -13,15 +13,19 @@
 using nlohmann::json;

 #include <array> // array
-#include <cfloat> // FLT_EVAL_METHOD
 #include <cstdint> // uint32_t, uint64_t
+#include <cstdio> // snprintf
 #include <cstdlib> // strtod
 #include <cstring> // memcpy
+#include <map> // map
+#include <random> // mt19937
 #include <sstream> // stringstream
 #include <string> // string
 #include <utility> // pair
 #include <vector> // vector

+#include "float_hard_cases.hpp"
+
 namespace
 {
 // shortcut to scan a string literal
@@ -257,7 +261,7 @@ TEST_CASE("lexer number fast path")
            "123456789012345678901234567890",  // huge -> float
            "0.30000000000000004", "2.2250738585072014e-308", "1e308",
            // high-precision / wide-exponent values that exercise the
-            // std::from_chars (Eisel-Lemire) path beyond the Clinger subset
+            // Eisel-Lemire path beyond the Clinger subset
            "1.7976931348623157e308", "1.2345678901234567e-250",
            "9007199254740993", "5e-324", "1e-320"
        };
@@ -279,20 +283,18 @@ TEST_CASE("lexer number fast path")
        }
    }

-    SECTION("significant-digit gate for the Clinger fast path")
+    SECTION("significant digits around Clinger's fast path")
    {
-        // Clinger's fast path needs a significand below 2^53, so it cannot
-        // succeed once the mantissa has 17 or more significant digits (the
-        // significand would be at least 10^16). The lexer skips the attempt
-        // there. That is only allowed to save work: every value must still come
-        // out bit-exactly, and both scanners must agree. In particular the gate
-        // must not fire for tokens whose leading zeros merely look like extra
-        // digits - "0.1234567890123456" has 16 significant digits, not 17.
+        // Clinger's fast path needs a significand of at most 2^53, which
+        // tokens with 17 or more significant digits exceed. The conversion
+        // splits the token at the positions the scanners recorded, so leading
+        // zeros must not count as digits - "0.1234567890123456" has 16
+        // significant digits, not 17 - and both scanners must agree.
        const std::vector<std::string> numbers =
        {
            "1234567890123456",                  // 16 significant digits
-            "12345678901234567",                 // 17 -> attempt skipped
-            "123456789012345678",                // 18 -> attempt skipped
+            "12345678901234567",                 // 17
+            "123456789012345678",                // 18
            "0.1234567890123456",                // 16: the leading "0" is not significant
            "0.12345678901234567",               // 17
            "0.00000000000000001",               // 1, in a long token
@@ -663,46 +665,323 @@ TEST_CASE("lexer string fast path")
    }
 }

-TEST_CASE("parse_float_fast declines what it cannot convert exactly")
+TEST_CASE("lexer escape fast path")
 {
-    // The lexer only hands well-formed numbers to parse_float_fast, so the
-    // malformed ones below can only be passed to it directly. Declining is
-    // always safe: the caller then falls back to a slower, exact conversion.
-    const auto fast = [](const std::string & s, double & out)
+    // json::accept() never throws, so this section stays covered without
+    // exceptions; it pins which of the cases below are valid/invalid and
+    // checks the contiguous and streaming paths agree on that classification.
+    SECTION("accept() parity")
    {
-        return nlohmann::detail::parse_float_fast(s.data(), s.data() + s.size(), out);
+        const std::vector<std::pair<std::string, bool>> cases =
+        {
+            {"\\u0041", true}, {"\\u00e4", true}, {"\\u00E4", true},
+            {"\\uD83D\\uDE00", true},
+            {"\\u12", false}, {"\\u12G4", false}, {"\\uXYZW", false},
+            {"\\uD800", false}, {"\\uD800A", false}, {"\\uD800\\u0041", false},
+            {"\\uDC00", false}, {"\\u", false}
+        };
+
+        for (const auto& c : cases)
+        {
+            for (const std::size_t offset :
+                    {
+                        std::size_t{0}, std::size_t{9}
+                    })
+            {
+                const std::string doc = "[\"" + std::string(offset, 'a') + c.first + "\"]";
+                CAPTURE(doc)
+                CHECK(json::accept(doc) == c.second);
+                std::stringstream ss(doc);
+                CHECK(json::accept(ss) == c.second);
+            }
+        }
+    }
+
+#if !defined(JSON_NOEXCEPTION)
+    // the full outcome of parsing @a doc: the parsed value, or the exact
+    // error message, so a mismatch in either is caught
+    const auto outcome = [](const std::string & doc, bool streaming) -> std::string
+    {
+        try
+        {
+            if (streaming)
+            {
+                std::stringstream ss(doc);
+                const json j = json::parse(ss);
+                return j.dump();
+            }
+            const json j = json::parse(doc);
+            return j.dump();
+        }
+        catch (const json::exception& e)
+        {
+            return {e.what()};
+        }
    };
-    double out = 0;

-#if defined(FLT_EVAL_METHOD) && FLT_EVAL_METHOD != 0
-    // without true double precision, the fast path declines everything
-    CHECK_FALSE(fast("1.5", out));
-#else
-    CHECK(fast("1.5", out));
-    CHECK(out == 1.5);
-    CHECK(fast("+2.5e1", out));
-    CHECK(out == 25.0);
-    CHECK(fast("-25E-1", out));
-    CHECK(out == -2.5);
-    CHECK(fast("1e", out));
-    CHECK(out == 1.0);
+    SECTION("contiguous vs streaming parity")
+    {
+        const std::vector<std::string> escapes =
+        {
+            "\\u0041",             // "A"
+            "\\u00e4",             // "ä" (lowercase hex)
+            "\\u00E4",             // "ä" (uppercase hex)
+            "\\uD83D\\uDE00",      // valid surrogate pair (an emoji)
+            "\\u12",               // truncated: only 2 hex digits before the closing quote
+            "\\u12G4",             // invalid hex digit at the 3rd position
+            "\\uXYZW",             // all 4 bytes invalid
+            "\\uD800",             // lone high surrogate, string ends right after
+            "\\uD800A",            // high surrogate not followed by another \u escape
+            "\\uD800\\u0041",      // high surrogate followed by \u, but not a low surrogate
+            "\\uDC00",             // lone low surrogate
+            "\\u",                 // '\u' with nothing after (closing quote right away)
+        };
+
+        // once at the start of the string and once past the first 8-byte SWAR
+        // word of the outer string_bulk_run, so the escape is reached both
+        // right after the opening quote and mid-run
+        for (const auto& escape : escapes)
+        {
+            for (const std::size_t offset :
+                    {
+                        std::size_t{0}, std::size_t{9}
+                    })
+            {
+                const std::string doc = "[\"" + std::string(offset, 'a') + escape + "\"]";
+                CAPTURE(doc)
+                CHECK(outcome(doc, false) == outcome(doc, true));
+            }
+
+            // the escape is the last thing before end of input: no closing
+            // quote at all
+            const std::string truncated_doc = "[\"" + escape;
+            CAPTURE(truncated_doc)
+            CHECK(outcome(truncated_doc, false) == outcome(truncated_doc, true));
+        }
+    }
+
+    SECTION("truncated \\u escape at every distance from the end of input")
+    {
+        // ia.bulk_remaining() must correctly report fewer than 4 bytes for
+        // every possible count of trailing hex-looking bytes (0, 1, 2, or 3)
+        // before end of input, so the fast path declines and the byte path
+        // alone reports the "must be followed by 4 hex digits" error, at the
+        // same position, in every case
+        for (const std::string& tail :
+                {
+                    std::string{}, std::string("1"), std::string("12"), std::string("123")
+                })
+        {
+            const std::string doc = "[\"\\u" + tail;
+            CAPTURE(doc)
+            CHECK(outcome(doc, false) == outcome(doc, true));
+            CHECK(outcome(doc, false).find("must be followed by 4 hex digits") != std::string::npos);
+        }
+    }
+
+    SECTION("invalid hex digit at every position of the 4")
+    {
+        // the fast path must decline for *any* invalid byte among the 4, not
+        // just the first, and the byte path must then stop at exactly that
+        // position - same as it always has
+        for (std::size_t bad_pos = 0; bad_pos < 4; ++bad_pos)
+        {
+            std::string digits = "1234";
+            digits[bad_pos] = 'g'; // not a hex digit
+            const std::string doc = "[\"\\u" + digits + "\"]";
+            CAPTURE(doc)
+            CHECK(outcome(doc, false) == outcome(doc, true));
+            CHECK(outcome(doc, false).find("must be followed by 4 hex digits") != std::string::npos);
+        }
+    }
+
+    SECTION("random escapes")
+    {
+        // A seeded PRNG builds the 4 bytes following `\u` from a mix of hex
+        // digits and non-hex bytes, at varying distances from the start of
+        // the string, to compare the two scanners on many more shapes than
+        // are practical to enumerate by hand.
+        std::mt19937 gen(7654321); // NOLINT(cert-msc32-c,cert-msc51-cpp)
+        const std::string hex_alphabet = "0123456789AaBbCcDdEeFf";
+        std::uniform_int_distribution<std::size_t> pick_hex(0, hex_alphabet.size() - 1);
+        std::uniform_int_distribution<int> pick_byte(1, 255); // never NUL
+        std::uniform_int_distribution<int> pick_is_hex(0, 4); // 4-in-5 chance of a hex digit
+        std::uniform_int_distribution<std::size_t> pick_offset(0, 12);
+
+        std::vector<std::string> mismatches;
+        for (int iter = 0; iter < 3000; ++iter)
+        {
+            std::string digits;
+            for (int i = 0; i < 4; ++i)
+            {
+                if (pick_is_hex(gen) != 0)
+                {
+                    digits += hex_alphabet[pick_hex(gen)];
+                }
+                else
+                {
+                    char c = static_cast<char>(pick_byte(gen));
+                    if (c == '"' || c == '\\')
+                    {
+                        // keep the string well-formed apart from the escape
+                        // itself, so any mismatch is attributable to the \u
+                        // handling and not to an unrelated quote/escape
+                        c = 'z';
+                    }
+                    digits += c;
+                }
+            }
+            const std::string doc = "[\"" + std::string(pick_offset(gen), 'a') + "\\u" + digits + "\"]";
+            if (outcome(doc, false) != outcome(doc, true))
+            {
+                mismatches.push_back(doc);
+            }
+        }
+        CAPTURE(mismatches)
+        CHECK(mismatches.empty());
+    }
 #endif
+}

-    // not a number
-    CHECK_FALSE(fast("", out));
-    CHECK_FALSE(fast("-", out));
-    CHECK_FALSE(fast(".", out));
-    CHECK_FALSE(fast("1.2.3", out));
-    CHECK_FALSE(fast("1x", out));
-    CHECK_FALSE(fast("1e+", out));
-    CHECK_FALSE(fast("1e1x", out));
+namespace
+{
+// the index of the decimal point (or npos) and of the end of the mantissa of a
+// number token, which the lexer records while scanning it
+std::pair<std::size_t, std::size_t> float_token_layout(const std::string& s)
+{
+    std::size_t dot = std::string::npos;
+    std::size_t mantissa_end = s.size();
+    for (std::size_t i = 0; i < s.size(); ++i)
+    {
+        if (s[i] == '.')
+        {
+            dot = i;
+        }
+        else if (s[i] == 'e' || s[i] == 'E')
+        {
+            mantissa_end = i;
+            break;
+        }
+    }
+    return {dot, mantissa_end};
+}

-    // numbers that are not represented exactly on the fast path
-    CHECK_FALSE(fast("12345678901234567890", out));
-    CHECK_FALSE(fast("1e10000", out));
-    CHECK_FALSE(fast("9007199254740993", out));
-    CHECK_FALSE(fast("1e23", out));
-    CHECK_FALSE(fast("1e-23", out));
+template<typename FloatType>
+FloatType parse_native(const std::string& s)
+{
+    const auto layout = float_token_layout(s);
+    return nlohmann::detail::parse_float_native<FloatType>(s.data(), s.data() + s.size(), layout.first, layout.second);
+}
+
+std::uint64_t bits_of(double d)
+{
+    std::uint64_t b = 0;
+    std::memcpy(&b, &d, sizeof(b));
+    return b;
+}
+
+std::uint32_t bits_of(float f)
+{
+    std::uint32_t b = 0;
+    std::memcpy(&b, &f, sizeof(b));
+    return b;
+}
+
+std::uint64_t native_bits64(const std::string& s)
+{
+    return bits_of(parse_native<double>(s));
+}
+
+std::uint32_t native_bits32(const std::string& s)
+{
+    return bits_of(parse_native<float>(s));
+}
+} // namespace
+
+TEST_CASE("parse_float_native rounds correctly")
+{
+    SECTION("double")
+    {
+        CHECK(native_bits64("1.5") == 0x3FF8000000000000u);
+        CHECK(native_bits64("0.1") == 0x3FB999999999999Au);
+        CHECK(native_bits64("-0.0") == 0x8000000000000000u);
+        CHECK(native_bits64("0e999999999999999999999") == 0u);
+        // 2^53 + 1 is exactly between two doubles: ties to even, unless more digits follow
+        CHECK(native_bits64("9007199254740993") == 0x4340000000000000u);
+        CHECK(native_bits64("9007199254740993.0000000000000000001") == 0x4340000000000001u);
+        CHECK(native_bits64("9007199254740992.9999999999999999999") == 0x4340000000000000u);
+        // 1 + 2^-53 exactly (a tie), and one unit in the 55th digit around it
+        CHECK(native_bits64("1.00000000000000011102230246251565404236316680908203125") == 0x3FF0000000000000u);
+        CHECK(native_bits64("1.00000000000000011102230246251565404236316680908203126") == 0x3FF0000000000001u);
+        CHECK(native_bits64("1.00000000000000011102230246251565404236316680908203124") == 0x3FF0000000000000u);
+        // subnormal and overflow boundaries
+        CHECK(native_bits64("2.4703282292062327e-324") == 0u);
+        CHECK(native_bits64("2.4703282292062328e-324") == 1u);
+        CHECK(native_bits64("2.2250738585072011e-308") == 0x000FFFFFFFFFFFFFu);
+        CHECK(native_bits64("2.2250738585072012e-308") == 0x0010000000000000u);
+        CHECK(native_bits64("1.7976931348623157e308") == 0x7FEFFFFFFFFFFFFFu);
+        CHECK(native_bits64("1.7976931348623159e308") == 0x7FF0000000000000u);
+        CHECK(native_bits64("-1e400") == 0xFFF0000000000000u);
+        CHECK(native_bits64("-1e-400") == 0x8000000000000000u);
+        // exponents and zeros far beyond the range cancel out
+        CHECK(native_bits64("0." + std::string(1000, '0') + "1e1001") == 0x3FF0000000000000u);
+        CHECK(native_bits64("1" + std::string(1000, '0') + "e-1000") == 0x3FF0000000000000u);
+        CHECK(native_bits64("1e-99999999999999999999999") == 0u);
+        CHECK(native_bits64("1E+99999999999999999999999") == 0x7FF0000000000000u);
+        // more digits than any midpoint has (769): only whether a nonzero digit follows matters
+        const std::string tie = "1.00000000000000011102230246251565404236316680908203125";
+        CHECK(native_bits64(tie + std::string(800, '0')) == 0x3FF0000000000000u);
+        CHECK(native_bits64(tie + std::string(800, '0') + "1") == 0x3FF0000000000001u);
+    }
+
+    SECTION("float")
+    {
+        CHECK(native_bits32("1.5") == 0x3FC00000u);
+        CHECK(native_bits32("0.1") == 0x3DCCCCCDu);
+        CHECK(native_bits32("-0.0") == 0x80000000u);
+        // 2^24 + 1 is exactly between two floats
+        CHECK(native_bits32("16777217") == 0x4B800000u);
+        CHECK(native_bits32("16777217.000000000000000000001") == 0x4B800001u);
+        CHECK(native_bits32("16777218.999999999999999999999") == 0x4B800001u);
+        CHECK(native_bits32("16777219") == 0x4B800002u);
+        // subnormal and overflow boundaries
+        CHECK(native_bits32("3.4028235677973366e38") == 0x7F7FFFFFu);
+        CHECK(native_bits32("3.4028235677973367e38") == 0x7F800000u);
+        CHECK(native_bits32("7.006492321624085e-46") == 0u);
+        CHECK(native_bits32("7.006492321624086e-46") == 1u);
+        CHECK(native_bits32("1.1754942e-38") == 0x007FFFFFu);
+        CHECK(native_bits32("-1.17549435e-38") == 0x80800000u);
+        CHECK(native_bits32("1e39") == 0x7F800000u);
+        CHECK(native_bits32("-1e-50") == 0x80000000u);
+        // not rounded through double: its double would round to another float
+        CHECK(native_bits32("1.00000005960464477539062500000000001") == 0x3F800001u);
+        CHECK(native_bits32("9007199254740993") == 0x5A000000u);
+    }
+
+    SECTION("the conversion shared with other parsers")
+    {
+        // convert_float() gives the lexer's results, for every type
+        const std::vector<std::string> tokens =
+        {
+            "0", "-0.0", "1.5", "0.1", "1e-400", "-2.5E+3", "123456789012345678901234567890",
+            "9007199254740993.0000000000000000001", "4.9406564584124654e-324"
+        };
+        using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
+        using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
+        for (const auto& t : tokens)
+        {
+            CAPTURE(t)
+            const auto layout = float_token_layout(t);
+            const char* const first = t.data();
+            const char* const last = first + t.size();
+            const auto d = nlohmann::detail::convert_float<double>(first, last, layout.first, layout.second);
+            const auto f = nlohmann::detail::convert_float<float>(first, last, layout.first, layout.second);
+            const auto ld = nlohmann::detail::convert_float<long double>(first, last, layout.first, layout.second);
+            CHECK(bits_of(d) == bits_of(json::parse(t).get<double>()));
+            CHECK(bits_of(f) == bits_of(float_json::parse(t).get<float>()));
+            CHECK(ld == long_double_json::parse(t).get<long double>());
+        }
+    }
 }

 namespace
@@ -806,40 +1085,6 @@ std::size_t big_bit_length(const big_uint& a)
    }
    return n;
 }
-
-std::uint64_t bits_of(double d)
-{
-    std::uint64_t b = 0;
-    std::memcpy(&b, &d, sizeof(b));
-    return b;
-}
-
-bool eisel_lemire(const std::string& s, double& out)
-{
-    return nlohmann::detail::parse_float_eisel_lemire(s.data(), s.data() + s.size(), out);
-}
-
-// significant digits of a token, without trailing zeros
-std::size_t significant_digits(const std::string& s)
-{
-    std::string digits;
-    for (const char c : s)
-    {
-        if (c == 'e' || c == 'E')
-        {
-            break;
-        }
-        if (c >= '0' && c <= '9' && !(digits.empty() && c == '0'))
-        {
-            digits += c;
-        }
-    }
-    while (!digits.empty() && digits.back() == '0')
-    {
-        digits.pop_back();
-    }
-    return digits.size();
-}
 } // namespace

 TEST_CASE("Eisel-Lemire float conversion")
@@ -1237,26 +1482,33 @@ TEST_CASE("Eisel-Lemire float conversion")
        for (const auto& c : known)
        {
            CAPTURE(c.first)
-            double out = 0;
-            if (eisel_lemire(c.first, out))
-            {
-                CHECK(bits_of(out) == c.second);
-            }
-            else
-            {
-                // only tokens with more than 19 significant digits are left to
-                // strtod: those whose value lies too close to a tie
-                CHECK(significant_digits(c.first) > 19);
-            }
+            CHECK(native_bits64(c.first) == c.second);
        }
    }

+    SECTION("binary32")
+    {
+        using binary32 = nlohmann::detail::ieee_binary_format<24>;
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(0, 1) == 0x3F800000u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(-1, 1) == 0x3DCCCCCDu);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(-1, 15) == 0x3FC00000u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(0, 16777217) == 0x4B800000u); // tie, to even
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(0, 16777219) == 0x4B800002u); // tie, to even
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(-45, 1) == 0x00000001u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(-46, 7) == 0x00000000u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(-46, 8) == 0x00000001u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(-65, 9999999999999999999u) == 0x00000000u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(20, 3402823466385288598u) == 0x7F7FFFFFu);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(20, 3402823669209384635u) == 0x7F800000u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(39, 1) == 0x7F800000u);
+        CHECK(nlohmann::detail::eisel_lemire<binary32>(-5, 0) == 0x00000000u);
+    }
+
    SECTION("round trip")
    {
-        // every double written by to_chars and read back, also with trailing
-        // digits that make the token longer than 19 digits
+        // every double written by to_chars and read back, and its 17-digit
+        // form with trailing digits that make the token longer than 19 digits
        std::uint64_t state = 5295;
-        std::size_t declined = 0;
        for (int i = 0; i < 200000; ++i)
        {
            state ^= state << 13u;
@@ -1278,30 +1530,51 @@ TEST_CASE("Eisel-Lemire float conversion")
            const char* end = nlohmann::detail::to_chars(buffer.data(), buffer.data() + buffer.size(), d);
            const std::string token(buffer.data(), static_cast<std::size_t>(end - buffer.data()));
            CAPTURE(token)
-            double out = 0;
-            REQUIRE(eisel_lemire(token, out));
-            CHECK(bits_of(out) == b);
+            CHECK(native_bits64(token) == b);

-            // insert digits before the exponent: the value moves by far less
-            // than the distance to the rounding boundary, so it must not change
-            std::string longer = token;
+            // insert digits before the exponent of the 17-digit form: that
+            // form lies strictly inside the rounding interval of the double
+            // (the shortest one may lie on its boundary), and the digits move
+            // it by far less than the distance to the boundary, so the value
+            // must not change
+            std::array<char, 64> digits17{};
+            static_cast<void>(std::snprintf(digits17.data(), digits17.size(), "%.17g", d)); // NOLINT(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
+            std::string longer = digits17.data();
            const std::size_t e = longer.find('e');
            const std::size_t dot = longer.find('.');
            const std::string extra = dot == std::string::npos ? ".000000000000000000001" : "000000000000000000001";
            longer.insert(e == std::string::npos ? longer.size() : e, extra);
            CAPTURE(longer)
-            if (eisel_lemire(longer, out))
+            CHECK(native_bits64(longer) == b);
+        }
+    }
+
+    SECTION("round trip, binary32")
+    {
+        std::uint32_t state = 5295;
+        for (int i = 0; i < 100000; ++i)
+        {
+            state ^= state << 13u;
+            state ^= state >> 17u;
+            state ^= state << 5u;
+            std::uint32_t b = state;
+            if ((b & 0x7F800000u) == 0x7F800000u)
            {
-                CHECK(bits_of(out) == b);
+                continue; // infinity or NaN
            }
-            else
+            if (i % 4 == 0)
            {
-                // w and w + 1 round differently: only when the value is very
-                // close to a rounding boundary
-                ++declined;
+                b &= 0x807FFFFFu; // subnormals
            }
+            float f = 0;
+            std::memcpy(&f, &b, sizeof(f));
+
+            std::array<char, 64> buffer{};
+            const char* end = nlohmann::detail::to_chars(buffer.data(), buffer.data() + buffer.size(), f);
+            const std::string token(buffer.data(), static_cast<std::size_t>(end - buffer.data()));
+            CAPTURE(token)
+            CHECK(native_bits32(token) == b);
        }
-        CHECK(declined < 1000); // 107 of the 200,000
    }

    SECTION("used by the lexer")
@@ -1315,3 +1588,177 @@ TEST_CASE("Eisel-Lemire float conversion")
                             "[json.exception.out_of_range.406] number overflow parsing '1.7976931348623159e308'", json::out_of_range&);
    }
 }
+
+namespace
+{
+using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
+
+// the bits of the float that parse() gives for a token, via both scanners;
+// the value must be the same for both
+template<typename Json, typename Bits>
+void check_parse(const std::string& token, Bits expected, Bits infinity)
+{
+    std::stringstream stream(token);
+    if ((expected & ~(Bits{1} << (8 * sizeof(Bits) - 1))) == infinity)
+    {
+        Json _;
+        CHECK_THROWS_WITH_AS(_ = Json::parse(token), ("[json.exception.out_of_range.406] number overflow parsing '" + token + "'").c_str(), typename Json::out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = Json::parse(stream), ("[json.exception.out_of_range.406] number overflow parsing '" + token + "'").c_str(), typename Json::out_of_range&);
+        return;
+    }
+    const Json contiguous = Json::parse(token);
+    const Json streamed = Json::parse(stream);
+    if (contiguous.is_number_float()) // not an integer that fits
+    {
+        CHECK(bits_of(contiguous.template get<typename Json::number_float_t>()) == expected);
+        CHECK(bits_of(streamed.template get<typename Json::number_float_t>()) == expected);
+    }
+    else
+    {
+        CHECK(streamed.is_number_integer());
+    }
+}
+} // namespace
+
+TEST_CASE("float conversion of hard cases")
+{
+    // see float_hard_cases.hpp
+    for (const auto& c : float_hard_cases::cases())
+    {
+        const std::string token = c.token;
+        CAPTURE(token)
+        CHECK(native_bits64(token) == c.bits64);
+        CHECK(native_bits32(token) == c.bits32);
+        check_parse<json>(token, c.bits64, std::uint64_t{0x7FF0000000000000u});
+        check_parse<float_json>(token, c.bits32, std::uint32_t{0x7F800000u});
+    }
+}
+
+TEST_CASE("float overflow and underflow in the parser")
+{
+    SECTION("double")
+    {
+        check_parse<json>("1.7976931348623157e308", std::uint64_t{0x7FEFFFFFFFFFFFFFu}, std::uint64_t{0x7FF0000000000000u});
+        check_parse<json>("1.7976931348623159e308", std::uint64_t{0x7FF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
+        check_parse<json>("-1e309", std::uint64_t{0xFFF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
+        check_parse<json>("1" + std::string(400, '0'), std::uint64_t{0x7FF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
+        check_parse<json>("1e99999999999999999999", std::uint64_t{0x7FF0000000000000u}, std::uint64_t{0x7FF0000000000000u});
+        // an underflow gives a zero with the sign of the token
+        check_parse<json>("1e-400", std::uint64_t{0}, std::uint64_t{0x7FF0000000000000u});
+        check_parse<json>("-1e-400", std::uint64_t{0x8000000000000000u}, std::uint64_t{0x7FF0000000000000u});
+        check_parse<json>("-2.4703282292062327e-324", std::uint64_t{0x8000000000000000u}, std::uint64_t{0x7FF0000000000000u});
+        check_parse<json>("0." + std::string(400, '0') + "1", std::uint64_t{0}, std::uint64_t{0x7FF0000000000000u});
+    }
+
+    SECTION("float")
+    {
+        check_parse<float_json>("3.4028234e38", std::uint32_t{0x7F7FFFFFu}, std::uint32_t{0x7F800000u});
+        check_parse<float_json>("3.4028236e38", std::uint32_t{0x7F800000u}, std::uint32_t{0x7F800000u});
+        check_parse<float_json>("-1e39", std::uint32_t{0xFF800000u}, std::uint32_t{0x7F800000u});
+        check_parse<float_json>("1e-46", std::uint32_t{0}, std::uint32_t{0x7F800000u});
+        check_parse<float_json>("-1e-46", std::uint32_t{0x80000000u}, std::uint32_t{0x7F800000u});
+        check_parse<float_json>("-7.006492321624085e-46", std::uint32_t{0x80000000u}, std::uint32_t{0x7F800000u});
+        check_parse<float_json>("-7.006492321624086e-46", std::uint32_t{0x80000001u}, std::uint32_t{0x7F800000u});
+    }
+}
+
+TEST_CASE("string scanning kernels")
+{
+    // the word-at-a-time kernels must stop exactly where a byte-by-byte scan
+    // stops, for any content, length, and alignment
+    const auto reference_special = [](const unsigned char* data, std::size_t n)
+    {
+        std::size_t i = 0;
+        while (i < n && !nlohmann::detail::is_string_special(data[i]))
+        {
+            ++i;
+        }
+        return i;
+    };
+    const auto reference_copyable = [](const unsigned char* data, std::size_t n)
+    {
+        std::size_t i = 0;
+        while (i < n && nlohmann::detail::is_ascii_copyable(data[i]))
+        {
+            ++i;
+        }
+        return i;
+    };
+    const auto reference_bulk_run = [](const unsigned char* data, std::size_t n)
+    {
+        std::size_t i = 0;
+        while (i < n)
+        {
+            if (data[i] < 0x80u)
+            {
+                if (nlohmann::detail::is_string_special(data[i]))
+                {
+                    break;
+                }
+                ++i;
+                continue;
+            }
+            const std::size_t seq = nlohmann::detail::validate_one_utf8(data + i, n - i);
+            if (seq == 0)
+            {
+                break;
+            }
+            i += seq;
+        }
+        return i;
+    };
+
+    // pieces: ordinary ASCII, stops, DEL, well-formed sequences of every
+    // length, and ill-formed or truncated ones
+    const std::vector<std::string> pieces =
+    {
+        "a", "Z", " ", "~", "0123456789", "\"", "\\", std::string(1, '\0'), "\n", "\x1F", "\x7F",
+        "\xC3\xA4", "\xE2\x82\xAC", "\xE6\x97\xA5\xE6\x9C\xAC", "\xF0\x9F\x98\x80", "\xED\x9F\xBF",
+        "\x80", "\xC0\x80", "\xC3", "\xE2\x82", "\xED\xA0\x80", "\xF4\x90\x80\x80", "\xFF",
+    };
+    std::uint64_t state = 5295;
+    const auto next = [&state]()
+    {
+        state ^= state << 13u;
+        state ^= state >> 7u;
+        state ^= state << 17u;
+        return state;
+    };
+    // the upper half as a 32-bit value: converts to std::size_t implicitly on
+    // every platform (a cast of std::uint64_t is useless where both are the
+    // same type, and required where std::size_t is 32 bits wide)
+    const auto next_small = [&next]()
+    {
+        return static_cast<std::uint32_t>(next() >> 32u);
+    };
+    for (int round = 0; round < 100000; ++round)
+    {
+        // mostly ordinary text, so that runs span several words
+        std::string text(next_small() % 8u, '.');
+        const std::size_t count = next_small() % 12u;
+        for (std::size_t k = 0; k < count; ++k)
+        {
+            const std::size_t p = (next() % 4 == 0) ? next_small() % pieces.size() : 0;
+            text += pieces[p];
+            text += std::string(next_small() % 10u, 'x');
+        }
+        const auto* data = reinterpret_cast<const unsigned char*>(text.data()); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+        for (std::size_t offset = 0; offset < 3 && offset <= text.size(); ++offset)
+        {
+            const std::size_t n = text.size() - offset;
+            CAPTURE(text)
+            CAPTURE(offset)
+            CHECK(nlohmann::detail::find_string_special(data + offset, n) == reference_special(data + offset, n));
+            CHECK(nlohmann::detail::find_ascii_copyable_run(data + offset, n) == reference_copyable(data + offset, n));
+            CHECK(nlohmann::detail::scalar_string_bulk_run(data + offset, n) == reference_bulk_run(data + offset, n));
+        }
+    }
+
+    // the trailing-zero count, whichever implementation the compiler gets
+    for (int k = 0; k < 64; ++k)
+    {
+        const std::uint64_t bit = std::uint64_t{1} << k;
+        CHECK(nlohmann::detail::count_trailing_zeros(bit) == k);
+        CHECK(nlohmann::detail::count_trailing_zeros(bit | (bit << 1u) | 0x8000000000000000u) == k);
+    }
+}
@@ -2317,6 +2317,58 @@ TEST_CASE("parser class")
 #endif
    }

+    SECTION("comments before separators")
+    {
+        // The parser first checks for the expected ':' or ',' and only then
+        // falls back to the full token switch, which skips comments. A comment
+        // directly before a separator takes that fallback.
+        json _;
+
+        SECTION("ignored")
+        {
+            const std::vector<std::pair<std::string, json>> inputs =
+            {
+                {"{\"a\" /* c */ : 1}", {{"a", 1}}},
+                {"{\"a\" // c\n: 1}", {{"a", 1}}},
+                {R"({"a": 1, "b" /* c */ : 2})", {{"a", 1}, {"b", 2}}},
+                {R"({"a": 1 /* c */ , "b": 2})", {{"a", 1}, {"b", 2}}},
+                {"{\"a\": 1 // c\n, \"b\": 2}", {{"a", 1}, {"b", 2}}},
+                {"[1 /* c */ , 2]", {1, 2}},
+                {"[1 // c\n, 2]", {1, 2}},
+                {"{\"a\" /* c */ /* d */ : [1 // c\n , 2 /**/ ] /**/ , \"b\" : 3}", {{"a", {1, 2}}, {"b", 3}}}
+            };
+            for (const auto& input : inputs)
+            {
+                CAPTURE(input.first)
+                CHECK(json::parse(input.first, nullptr, true, true) == input.second);
+                CHECK(json::accept(input.first, true));
+            }
+        }
+
+        SECTION("ignored, with trailing commas")
+        {
+            CHECK(json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true, true) == json({1}));
+            CHECK(json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true, true) == json({{"a", 1}}));
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 19: syntax error while parsing object key - unexpected '}'; expected string literal", json::parse_error);
+        }
+
+        SECTION("not ignored")
+        {
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\" /* c */ : 1}")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing object separator - invalid literal; last read: '\"a\" /'; expected ':'", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1, \"b\" /* c */ : 2}")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing object separator - invalid literal; last read: '\"b\" /'; expected ':'", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , \"b\": 2}")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 9: syntax error while parsing object - invalid literal; last read: '1 /'; expected '}'", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , 2]")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing array - invalid literal; last read: '1 /'; expected ']'", json::parse_error);
+            CHECK(!json::accept(std::string("[1 /* c */ , 2]")));
+        }
+    }
+
 #if JSON_DIAGNOSTIC_POSITIONS
    // Macro for all test cases for start_pos and end_pos
 #define SETUP_TESTCASES() \
@@ -260,10 +260,11 @@ struct LocaleSwitchingSax final: public nlohmann::json_sax<json>

 TEST_CASE("locale changes between lexer construction and number conversion (#5198)")
 {
-    // The numbers are chosen so that the conversion also takes the strtod
-    // fallback, which honors the locale that is current at conversion time:
-    // too many significant digits for Clinger's fast path, an underflow that
-    // std::from_chars rejects, and a plain value.
+    // float and double are converted without the locale. A long double that
+    // is not binary64 can take the strtold fallback, which honors the locale
+    // that is current at conversion time. The numbers are chosen so that it
+    // does: too many significant digits for Clinger's fast path, an underflow
+    // that std::from_chars rejects, and a plain value.
    const std::vector<std::string> numbers = {"3.14159265358979323846", "1.5e-400", "12.34", "-0.000123456789012345678"};
    std::string text = "[";
    for (const auto& n : numbers)
@@ -327,7 +328,8 @@ TEST_CASE("locale changes between lexer construction and number conversion (#519
            }
        }

-        // a long double goes through std::strtold unless std::from_chars supports it
+        // a long double goes through std::strtold unless it is binary64 or
+        // std::from_chars supports it
        {
            bool switched = false;
            const auto cb = [&](int /*depth*/, long_double_json::parse_event_t event, long_double_json& /*parsed*/) noexcept
@@ -353,8 +355,15 @@ TEST_CASE("locale with a multi-byte decimal point")
 {
    // Some locales use a decimal point that is not a single character, e.g.
    // U+066B ARABIC DECIMAL SEPARATOR (two bytes in UTF-8). It cannot be
-    // substituted in place for '.', so the strtod fallback stops early. The
-    // conversion must still terminate rather than retry forever.
+    // substituted in place for '.', so the strtold fallback (only for long
+    // double formats other than binary64) converts a copy of the token with
+    // the whole decimal point instead (#5660). The values must be those of the
+    // "C" locale.
+    using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
+    const char* const long_double_numbers = "[3.14159265358979323846, 1.5e-400, -0.000123456789012345678]";
+    REQUIRE(std::setlocale(LC_NUMERIC, "C") != nullptr);
+    const long_double_json expected_long_double = long_double_json::parse(long_double_numbers);
+
    const std::array<const char*, 6> names = {{"ar_EG.UTF-8", "ar_SA.UTF-8", "fa_IR.UTF-8", "ps_AF.UTF-8", "ar_EG", "fa_IR"}};
    bool tested = false;
    for (const char* name : names)
@@ -372,12 +381,20 @@ TEST_CASE("locale with a multi-byte decimal point")
        tested = true;

        // too many significant digits for Clinger's fast path, and an underflow
-        // that std::from_chars rejects: both reach the strtod fallback
+        // that std::from_chars rejects: double does not depend on the locale
        json j;
        CHECK_NOTHROW(j = json::parse("[3.14159265358979323846, 1.5e-400, -0.000123456789012345678]"));
        CHECK(j.is_array());
+        CHECK(j[0] == 3.14159265358979323846);
+        CHECK(j[1] == 0.0);
+        CHECK(j[2] == -0.000123456789012345678);
        CHECK(json::accept("3.14159265358979323846"));

+        // a long double that reaches the strtold fallback is not truncated
+        long_double_json ld;
+        CHECK_NOTHROW(ld = long_double_json::parse(long_double_numbers));
+        CHECK(ld == expected_long_double);
+
        // a value the locale-independent paths convert is not affected
        CHECK(json::parse("12.5") == 12.5);
    }
@@ -920,12 +920,4 @@ TEST_CASE("regression test #5476 - array type without reserve()")
    }
 }

-TEST_CASE("issue #5317 - nested indefinite-length CBOR string chunks are rejected")
-{
-    json _;
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0xFF})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0xFF})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0xA1, 0x7F, 0x7F, 0xFF, 0xFF, 0x01})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
-}
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP
@@ -15,6 +15,23 @@
 #include <nlohmann/json.hpp>
 using nlohmann::detail::dtoa_impl::reinterpret_bits;

+#include <array>
+#include <cmath>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <iomanip>
+#include <limits>
+#include <locale>
+#include <random>
+#include <sstream>
+#include <string>
+#include <utility>
+#include <vector>
+#if defined(JSON_HAS_CPP_17)
+    #include <charconv>
+#endif
+
 namespace
 {
 float make_float(uint32_t sign_bit, uint32_t biased_exponent, uint32_t significand)
@@ -450,7 +467,7 @@ TEST_CASE("formatting")
        check_double(  1.2345e+18,   "1.2345e+18"             ); //  1.2345e+18                1.2345e+18                1.2345e18
        check_double(  1.2345e+19,   "1.2345e+19"             ); //  1.2345e+19                1.2345e+19                1.2345e19
        check_double(  1.2345e+20,   "1.2345e+20"             ); //  1.2345e+20                1.2345e+20                1.2345e20
-        check_double(  1.2345e+21,   "1.2344999999999999e+21" ); //  1.2345e+21                1.2344999999999999e+21    1.2345e21
+        check_double(  1.2345e+21,   "1.2345e+21"             ); //  1.2345e+21                1.2344999999999999e+21    1.2345e21
        check_double(  1.2345e+22,   "1.2345e+22"             ); //  1.2345e+22                1.2345e+22                1.2345e22
    }

@@ -514,3 +531,246 @@ TEST_CASE("formatting")
        check_integer(1000000000000000000LL, "1000000000000000000");
    }
 }
+
+namespace
+{
+// a small unsigned big integer (32-bit limbs, least significant first), to
+// recompute the powers of ten of the shortest double conversion
+using big = std::vector<std::uint32_t>;
+
+void big_mul_small(big& x, std::uint32_t m)
+{
+    std::uint64_t carry = 0;
+    for (auto& limb : x)
+    {
+        const std::uint64_t v = (static_cast<std::uint64_t>(limb) * m) + carry;
+        limb = static_cast<std::uint32_t>(v);
+        carry = v >> 32u;
+    }
+    if (carry != 0)
+    {
+        x.push_back(static_cast<std::uint32_t>(carry));
+    }
+}
+
+void big_div_small(big& x, std::uint32_t d)
+{
+    std::uint64_t rest = 0;
+    for (std::size_t i = x.size(); i-- > 0;)
+    {
+        const std::uint64_t v = (rest << 32u) | x[i];
+        x[i] = static_cast<std::uint32_t>(v / d);
+        rest = v % d;
+    }
+    while (!x.empty() && x.back() == 0)
+    {
+        x.pop_back();
+    }
+}
+
+std::size_t big_bit_length(const big& x)
+{
+    std::size_t n = 32 * x.size();
+    for (std::uint32_t top = x.back(); (top & 0x80000000u) == 0; top <<= 1u)
+    {
+        --n;
+    }
+    return n;
+}
+
+bool big_bit(const big& x, std::size_t i)
+{
+    return ((x[i / 32] >> (i % 32)) & 1u) != 0;
+}
+
+/// the 128 most significant bits of x (floor), shifted left if x has fewer bits
+std::pair<std::uint64_t, std::uint64_t> big_top128(const big& x)
+{
+    const std::size_t n = big_bit_length(x);
+    std::uint64_t high = 0;
+    std::uint64_t low = 0;
+    for (std::size_t k = 0; k < 128; ++k)
+    {
+        const bool bit = k < n && big_bit(x, n - 1 - k);
+        if (k < 64)
+        {
+            high = (high << 1u) | (bit ? 1u : 0u);
+        }
+        else
+        {
+            low = (low << 1u) | (bit ? 1u : 0u);
+        }
+    }
+    return {high, low};
+}
+
+/// the digits (without trailing zeros) and the decimal exponent of a
+/// representation "[-]d[.ddd][e[+-]x]"
+std::pair<std::string, int> digits_and_exponent(const std::string& s)
+{
+    std::string digits;
+    int point = -1;
+    int exponent = 0;
+    for (std::size_t i = 0; i < s.size(); ++i)
+    {
+        const char c = s[i];
+        if (c >= '0' && c <= '9')
+        {
+            digits += c;
+        }
+        else if (c == '.')
+        {
+            point = static_cast<int>(digits.size());
+        }
+        else if (c == 'e' || c == 'E')
+        {
+            exponent = std::stoi(s.substr(i + 1));
+            break;
+        }
+    }
+    int e = exponent + (point < 0 ? static_cast<int>(digits.size()) : point) - static_cast<int>(digits.size());
+    const std::size_t first = digits.find_first_not_of('0');
+    digits = first == std::string::npos ? "0" : digits.substr(first);
+    while (digits.size() > 1 && digits.back() == '0')
+    {
+        digits.pop_back();
+        ++e;
+    }
+    return {digits, e};
+}
+
+/// whether the decimal digits * 10^e reads back as v
+bool reads_back(const std::string& digits, int e, double v)
+{
+    const std::string text = digits + "e" + std::to_string(e);
+    // (compared bit for bit: v is positive and finite, and -Wfloat-equal)
+    return reinterpret_bits<std::uint64_t>(std::strtod(text.c_str(), nullptr)) == reinterpret_bits<std::uint64_t>(v);
+}
+
+/// Check the representation of a positive finite double: it reads back as
+/// the same value, and no representation with fewer digits does.
+void check_shortest(double v)
+{
+    std::array<char, 33> buf{};
+    char* end = nlohmann::detail::to_chars(buf.data(), buf.data() + 32, v);
+    const std::string text(buf.data(), end);
+    CAPTURE(text)
+    CHECK(std::strtod(text.c_str(), nullptr) == v);
+    // the layout is that of format_buffer() for the same digits
+    std::array<char, 64> reference{};
+    int len = 0;
+    int exponent = 0;
+    nlohmann::detail::dtoa_impl::shortest_digits(reference.data(), len, exponent, v);
+    const char* const reference_end = nlohmann::detail::dtoa_impl::format_buffer(reference.data(), len, exponent, -4, 15);
+    CHECK(text == std::string(reference.data(), static_cast<std::size_t>(reference_end - reference.data())));
+    const auto de = digits_and_exponent(text);
+    const std::string& digits = de.first;
+    if (digits.size() > 1)
+    {
+        // the decimals of one digit fewer next to the value
+        // (a stream rather than snprintf("%.*e"), whose output GCC cannot bound)
+        std::ostringstream shorter;
+        shorter.imbue(std::locale::classic());
+        shorter << std::scientific << std::setprecision(static_cast<int>(digits.size()) - 2) << v;
+        const auto near = digits_and_exponent(shorter.str());
+        // as an integer with digits.size() - 1 digits
+        std::string m = near.first;
+        int e = near.second;
+        while (m.size() < digits.size() - 1)
+        {
+            m += '0';
+            --e;
+        }
+        const std::uint64_t mid = std::stoull(m);
+        for (const std::uint64_t candidate :
+                {
+                    mid - 1, mid, mid + 1
+                })
+        {
+            CAPTURE(candidate)
+            CHECK(!reads_back(std::to_string(candidate), e, v));
+        }
+    }
+#if defined(JSON_HAS_CPP_17) && defined(__cpp_lib_to_chars)
+    // the closest of the shortest representations, as std::to_chars finds it
+    std::array<char, 64> std_text{};
+    const auto r = std::to_chars(std_text.data(), std_text.data() + std_text.size(), v, std::chars_format::scientific);
+    CHECK(digits_and_exponent(std::string(std_text.data(), r.ptr)) == de);
+#endif
+}
+} // namespace
+
+TEST_CASE("shortest digits of doubles")
+{
+    SECTION("powers of ten")
+    {
+        // the 128-bit significands of 10^k, rounded down, recomputed
+        for (int k = -342; k <= 341; ++k)
+        {
+            CAPTURE(k)
+            big x{1};
+            if (k >= 0)
+            {
+                for (int i = 0; i < k; ++i)
+                {
+                    big_mul_small(x, 10);
+                }
+            }
+            else
+            {
+                // floor(2^b / 10^-k) for a b that leaves more than 128 bits
+                const int b = 128 + 64 + (4 * -k);
+                x.assign(static_cast<std::size_t>(b / 32) + 1, 0);
+                x.back() = 1u << (b % 32);
+                for (int i = 0; i < -k; ++i)
+                {
+                    big_div_small(x, 10);
+                }
+            }
+            const auto expected = big_top128(x);
+            const auto actual = nlohmann::detail::zmij::pow10(k);
+            CHECK(actual.high == expected.first);
+            CHECK(actual.low == expected.second);
+        }
+    }
+
+    SECTION("boundary values")
+    {
+        for (const double v :
+                {
+                    std::numeric_limits<double>::min(), std::numeric_limits<double>::max(), std::numeric_limits<double>::denorm_min(),
+                    std::nextafter(std::numeric_limits<double>::min(), 0.0), 1.0, 2.0, 0.1, 0.3, 1e21, 1e22, 1e23, 5e-324, 9007199254740993.0,
+                    1.2345e+21, 2.2250738585072014e-308, 1.7976931348623157e308, 4.9406564584124654e-324, 123456789012345680.0
+                })
+        {
+            check_shortest(v);
+        }
+        // all powers of two (their rounding interval is narrower below)
+        for (int e = -1074; e <= 1023; ++e)
+        {
+            check_shortest(std::ldexp(1.0, e));
+        }
+        // powers of ten and their neighbors
+        for (int e = -323; e <= 308; ++e)
+        {
+            const double p = std::strtod(("1e" + std::to_string(e)).c_str(), nullptr);
+            check_shortest(p);
+            check_shortest(std::nextafter(p, 0.0));
+            check_shortest(std::nextafter(p, std::numeric_limits<double>::infinity()));
+        }
+    }
+
+    SECTION("random doubles")
+    {
+        std::mt19937_64 rng(5295); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed): reproducible
+        for (int i = 0; i < 100000; ++i)
+        {
+            const std::uint64_t bits = rng() & 0x7FFFFFFFFFFFFFFFu;
+            const auto v = reinterpret_bits<double>(bits);
+            if (std::isfinite(v) && bits != 0)
+            {
+                check_shortest(v);
+            }
+        }
+    }
+}