Fix destroy() for ObjectTypes without reverse iteration

The non-recursive destroy walk from #5762 picked an object's last child
via object_t::rbegin() and std::prev(end()). Neither is available for
every ObjectType: no_key_compare_map in unit-custom-object-type.cpp has
no rbegin(), so develop no longer compiles that test, and hash maps such
as std::unordered_map only have forward iterators.

The walk can take an object's children in any order, as long as it
finds the same child again while the object is not modified in between.
So objects with bidirectional iterators keep using their last child
(O(1) to remove from vector-based maps like ordered_map), and objects
with forward-only iterators use begin() instead. No reverse iteration
or rbegin() is needed any more, and the walk stays allocation-free.

Adds a forward-only ObjectType to the tests, destroyed both with mixed
nesting and 100000 levels deep.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

README.md

@@ -1393,7 +1393,7 @@ THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR I
 - 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 © 2009 [Florian Loitsch](https://florian.loitsch.com/)
 - 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, 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 © 2021 The fast_float authors
+- 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 © 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">
 

docs/mkdocs/docs/api/basic_json/number_float_t.md

@@ -23,10 +23,9 @@ type to use.
 ## Template parameters
 
 `NumberFloatType`
-:   the type to store floating-point numbers. The parser converts `#!cpp float`, `#!cpp double`, and a
+:   the type to store floating-point numbers. Parsing and serialization are implemented in terms of
-    `#!cpp long double` that is IEEE 754 binary64 itself and other `#!cpp long double` formats with
+    `#!cpp std::strtof`/`#!cpp std::strtod`/`#!cpp std::strtold` and `#!cpp std::snprintf`, so the type must be
-    `#!cpp std::from_chars` or `#!cpp std::strtold`, and serialization falls back to `#!cpp std::snprintf`, so the
+    `#!cpp float`, `#!cpp double`, or `#!cpp long double`. 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).

docs/mkdocs/docs/features/types/number_handling.md

@@ -82,13 +82,12 @@ 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). 
-    - The library converts integers and floating-point numbers itself, independent of the locale. Floating-point
+    - Integers are converted by the library's own digit parser. Floating-point numbers are converted with
-      numbers are correctly rounded (to nearest, ties to even). Only a `#!c long double` that is not IEEE 754 binary64
+      [`std::from_chars`](https://en.cppreference.com/w/cpp/utility/from_chars) if the library is compiled with C++17
-      (e.g., the 80-bit x87 format) is converted with `#!cpp std::from_chars` where available, or else with
+      and the standard library supports it, then with an exact fast path for `#!c double` values with few significant
-      [`std::strtold`](https://en.cppreference.com/w/cpp/string/byte/strtof). For that call, the library temporarily
+      digits, and otherwise with the locale-aware
-      replaces the `.` with the decimal point of the current locale (which may be longer than one byte, e.g., in
+      [`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof) (`std::strtof`/`std::strtold` for the
-      `fa_IR.UTF-8`), so the result does not depend on the locale either. Changing the locale in another thread during
+      other floating-point types). Before version 3.13.0, the conversion was realized by
-      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.
 
@@ -101,10 +100,10 @@ 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 whose rounded value is not satisfying
+- 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
 [`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. Numbers too
+[`json.exception.out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406) during parsing.
-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;`.

docs/mkdocs/docs/features/types/template_parameters.md

@@ -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 silently misparses numbers
+    - A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and can silently misparse
-      stored as a `#!cpp long double` that is not IEEE 754 binary64 (e.g., the 80-bit x87 format), because the lexer
+      floating-point numbers, because the lexer may hand the buffer to `#!cpp std::strtod`, which reads up to the
-      hands the buffer to `#!cpp std::strtold`.
+      terminating null character.
     - 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
@@ -537,10 +537,9 @@ 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 to `#!cpp float`, `#!cpp double`, and a
+- The [parser](../parsing/index.md) converts number literals with `#!cpp std::from_chars` or, as a fallback, with
-  `#!cpp long double` that is IEEE 754 binary64 itself; other `#!cpp long double` formats are converted with
+  `#!cpp std::strtof`, `#!cpp std::strtod`, or `#!cpp std::strtold`; the library provides overloads for exactly these
-  `#!cpp std::from_chars` where available, or with `#!cpp std::strtold`. The library provides overloads for exactly
+  three types.
-  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`).

docs/mkdocs/docs/home/license.md

@@ -20,4 +20,4 @@ The class contains a slightly modified version of the Grisu2 algorithm from Flor
 
 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, 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 © 2021 The fast_float authors
+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 © 2021 The fast_float authors

include/nlohmann/detail/bit_ops.hpp

@@ -39,25 +39,6 @@ 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
 {
@@ -87,19 +68,14 @@ 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 unsigned char* b) noexcept
+inline std::uint64_t read_eight_bytes(const char* p) 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
-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

include/nlohmann/detail/input/lexer.hpp

@@ -221,44 +221,6 @@ 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`
 
@@ -278,14 +240,6 @@ 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 };
@@ -1090,11 +1044,9 @@ class lexer : public lexer_base<BasicJsonType>
             token_type::parse_error otherwise
 
     @note The scanner is independent of the current locale: token_buffer
-          always holds `.`. The conversion of float and double does not use
+          always holds `.`. Only the std::strtod fallback of convert_number()
-          the locale either. Only the std::strtold fallback of
+          depends on the locale, and it looks up the decimal point right
-          convert_number() for long double formats other than binary64
+          before converting (see detail::convert_float_locale_aware()).
-          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.
     {
@@ -1107,7 +1059,7 @@ 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 split the token; npos means
+        // convert_number() uses it to count significant digits; npos means
         // "not seen an exponent yet" and is resolved at scan_number_done
         std::size_t mantissa_end = std::string::npos;
 
@@ -1437,8 +1389,8 @@ 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);
-                             with decimal_point_position, it locates the parts
+                             used to skip Clinger's fast path when it cannot
-                             of a float token without scanning it again
+                             possibly succeed - see detail::mantissa_fits_clinger()
     */
     token_type convert_number(token_type number_type, std::size_t mantissa_end)
     {
@@ -1492,11 +1444,10 @@ scan_number_done:
         }
 
         // this code is reached if we parse a floating-point number or if an
-        // integer conversion above overflowed. float and double (and long
+        // integer conversion above overflowed. Prefer std::from_chars
-        // double where it is binary64) are converted by the library itself,
+        // (Eisel-Lemire, locale-independent, correctly rounded) when available;
-        // correctly rounded and independent of the locale; other long double
+        // otherwise the exact Clinger fast path (double only); otherwise the
-        // formats use std::from_chars when available, otherwise the
+        // locale-aware strtof/strtod/strtold.
-        // locale-aware strtold.
         if (convert_float_fast(num_begin, num_end, decimal_point_position, mantissa_end, value_float))
         {
             return token_type::value_float;

include/nlohmann/detail/input/string_scan.hpp

@@ -8,12 +8,10 @@
 
 #pragma once
 
-#include <array> // array
 #include <cstddef> // size_t
-#include <cstdint> // uint64_t, uint8_t
+#include <cstdint> // uint64_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
@@ -71,12 +69,18 @@ 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)
     {
-        const std::uint64_t special = swar_string_special(read_eight_bytes(data + i));
+        std::uint64_t word = 0;
-        if (special != 0)
+        std::memcpy(&word, data + i, sizeof(word));
+        if (swar_string_special(word) != 0)
         {
-            // the lowest flagged byte is the first special one: the borrows of
+            // a special byte is in this word; locate it (endian-agnostic)
-            // the subtractions can only flag bytes above a true hit
+            for (std::size_t j = 0; j < 8; ++j)
-            return i + (static_cast<std::size_t>(count_trailing_zeros(special)) / 8);
+            {
+                if (is_string_special(data[i + j]))
+                {
+                    return i + j;
+                }
+            }
         }
     }
     for (; i < n; ++i)
@@ -110,7 +114,8 @@ 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)
     {
-        const std::uint64_t v = read_eight_bytes(data + i);
+        std::uint64_t v = 0;
+        std::memcpy(&v, data + i, sizeof(v));
         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)
@@ -121,9 +126,7 @@ inline std::size_t find_ascii_copyable_run(const unsigned char* data, std::size_
                                    | (v & high);               // >= 0x80
         if (stop != 0)
         {
-            // the lowest flagged byte is the first one to stop at (see
+            break;
-            // find_string_special())
-            return i + (static_cast<std::size_t>(count_trailing_zeros(stop)) / 8);
         }
     }
     for (; i < n; ++i)
@@ -250,18 +253,12 @@ 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
         }
-        // a run of multi-byte sequences (e.g. CJK text) is validated sequence
+        const std::size_t seq = validate_one_utf8(data + pos, n - pos);
-        // by sequence without searching for the next special byte in between
+        if (seq == 0)
-        do
         {
-            const std::size_t seq = validate_one_utf8(data + pos, n - pos);
+            break; // ill-formed or truncated: let the byte path diagnose it
-            if (seq == 0)
-            {
-                return pos; // ill-formed or truncated: let the byte path diagnose it
-            }
-            pos += seq;
         }
-        while (pos < n && data[pos] >= 0x80u);
+        pos += seq;
     }
     return pos;
 }
@@ -276,7 +273,8 @@ 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)
     {
-        const std::uint64_t v = read_eight_bytes(data + i);
+        std::uint64_t v = 0;
+        std::memcpy(&v, data + i, sizeof(v));
         const std::uint64_t q = v ^ 0x2222222222222222ull;
         const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull;
         const std::uint64_t hit = ((q - ones) & ~q & high)
@@ -284,8 +282,14 @@ inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t
                                   | ((v - 0x2020202020202020ull) & ~v & high);
         if (hit != 0)
         {
-            // the lowest flagged byte is the first delimiter (see find_string_special())
+            for (std::size_t j = 0; j < 8; ++j)
-            return i + (static_cast<std::size_t>(count_trailing_zeros(hit)) / 8);
+            {
+                const unsigned char c = data[i + j];
+                if (c == '\"' || c == '\\' || c < 0x20u)
+                {
+                    return i + j;
+                }
+            }
         }
     }
     for (; i < n; ++i)
@@ -316,50 +320,5 @@ 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

include/nlohmann/json.hpp

@@ -638,21 +638,49 @@ private:
             }
         }
 
-        static basic_json& last_child(basic_json& v)
+        // The walk in destroy_container() may take the children of a
+        // container in any order, as long as it picks the same child again
+        // while that container is not modified in between. Arrays and
+        // objects with bidirectional iterators (std::map, ordered_map, ...)
+        // use their last child, which a vector-based container can remove
+        // in O(1). ObjectType only needs forward iterators, though (e.g.
+        // std::unordered_map), so other objects use their first child.
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o, std::bidirectional_iterator_tag /*unused*/)
+        {
+            return std::prev(o.end());
+        }
+
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o, std::forward_iterator_tag /*unused*/)
+        {
+            return o.begin();
+        }
+
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o)
+        {
+            JSON_ASSERT(!o.empty());
+            return walk_child_it(o, typename std::iterator_traits<typename ObjectType_::iterator>::iterator_category());
+        }
+
+        // the child of a non-empty array/object v that the walk in
+        // destroy_container() continues with (see walk_child_it() above)
+        static basic_json& walk_child(basic_json& v)
         {
             if (v.m_data.m_type == value_t::array)
             {
                 return v.m_data.m_value.array->back();
             }
             JSON_ASSERT(v.m_data.m_type == value_t::object);
-            return v.m_data.m_value.object->rbegin()->second;
+            return walk_child_it(*v.m_data.m_value.object)->second;
         }
 
-        // removes the last child of a non-empty array/object v; this never
+        // removes walk_child(v) from a non-empty array/object v; this never
         // allocates, and since it is only ever called when that child is a
         // scalar or an already-empty array/object, destroying it never
         // recurses more than one level deep (see destroy() below)
-        static void pop_last_child(basic_json& v)
+        static void pop_walk_child(basic_json& v)
         {
             if (v.m_data.m_type == value_t::array)
             {
@@ -661,9 +689,7 @@ private:
             else
             {
                 JSON_ASSERT(v.m_data.m_type == value_t::object);
-                // erase() needs a forward iterator, so std::prev(end()) is
+                v.m_data.m_value.object->erase(walk_child_it(*v.m_data.m_value.object));
-                // used here rather than rbegin() (see last_child() above)
-                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
             }
         }
 
@@ -734,14 +760,17 @@ public:
             // bad_alloc, which would escape this noexcept destructor and
             // terminate the program (#5135).
             //
-            // Instead, walk down the "last child" chain, reversing links
+            // Instead, walk down a chain of children (always the one
-            // as we go: cur is the container currently being emptied,
+            // walk_child() picks), reversing links as we go: cur is the
-            // and prev is its parent (value_t::null when there is none).
+            // container currently being emptied, and prev is its parent
-            // Each parent's last child slot doubles as storage for that
+            // (value_t::null when there is none). Each parent's
-            // parent's own parent link while we are below it, so no
+            // walk_child() slot doubles as storage for that parent's own
-            // extra memory is needed. We only ever remove a child once
+            // parent link while we are below it, so no extra memory is
-            // it is a scalar or an empty array/object, which neither
+            // needed; the parent is not modified meanwhile, so
-            // allocates nor recurses more than one level deep.
+            // walk_child() finds that same slot again on the way up. We
+            // only ever remove a child once it is a scalar or an empty
+            // array/object, which neither allocates nor recurses more
+            // than one level deep.
             //
             // This json_value is not itself a basic_json, so the
             // top-level container is first moved into a local stand-in
@@ -767,11 +796,11 @@ public:
                     }
 
                     // ascend: detach the grandparent link from prev's
-                    // last slot, drop that (now null) slot, free cur
+                    // walk_child() slot, drop that (now null) slot, free cur
                     // (it is empty), then move up one level
                     basic_json gp;
-                    take(gp, last_child(prev));
+                    take(gp, walk_child(prev));
-                    pop_last_child(prev);
+                    pop_walk_child(prev);
 
                     free_container(cur);
 
@@ -780,21 +809,21 @@ public:
                     continue;
                 }
 
-                basic_json& cur_last_ref = last_child(cur);
+                basic_json& cur_child_ref = walk_child(cur);
 
-                if (has_no_children(cur_last_ref))
+                if (has_no_children(cur_child_ref))
                 {
                     // scalar, or already-empty array/object
-                    pop_last_child(cur);
+                    pop_walk_child(cur);
                     continue;
                 }
 
-                // descend into the non-empty last child, reversing the
+                // descend into the non-empty child, reversing the
                 // link: its slot takes over prev, and the child becomes
                 // the new cur
                 basic_json tmp;
-                take(tmp, cur_last_ref);
+                take(tmp, cur_child_ref);
-                take(cur_last_ref, prev);
+                take(cur_child_ref, prev);
                 take(prev, cur);
                 take(cur, tmp);
             }

tests/src/float_hard_cases.hpp

@@ -1,599 +0,0 @@
-//     __ _____ _____ _____
-//  __|  |   __|     |   | |  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},
-            {"179769313486231580793728971405e279", 0x7FEFFFFFFFFFFFFFu, 0x7F800000u},
-            {"-0.179769313486231580793728971406E+309", 0xFFF0000000000000u, 0xFF800000u},
-            {"100000000000000011102230246251565404236316680908203125e-53", 0x3FF0000000000000u, 0x3F800000u},
-            {"-1.00000000000000011102230246251565404236316680908203126", 0xBFF0000000000001u, 0xBF800000u},
-            {"1.00000000000000011102230246251565404236316680908203124e0", 0x3FF0000000000000u, 0x3F800000u},
-            {"10000000000000001110223024625156540423631668090820312501e-55", 0x3FF0000000000001u, 0x3F800000u},
-            {"1.00000000000000011102230246251565404236316680908203125000000000000000000001", 0x3FF0000000000001u, 0x3F800000u},
-            {"10000000000000001e-16", 0x3FF0000000000000u, 0x3F800000u},
-            {"1.0000000000000002", 0x3FF0000000000001u, 0x3F800000u},
-            {"1.000000000000000111", 0x3FF0000000000000u, 0x3F800000u},
-            {"1.000000000000000112e0", 0x3FF0000000000001u, 0x3F800000u},
-            {"1.000000000000000111e0", 0x3FF0000000000000u, 0x3F800000u},
-            {"-10000000000000001111e-19", 0xBFF0000000000001u, 0xBF800000u},
-            {"-100000000000000011102e-20", 0xBFF0000000000000u, 0xBF800000u},
-            {"-1.00000000000000011103", 0xBFF0000000000001u, 0xBF800000u},
-            {"1.00000000000000011102230246251", 0x3FF0000000000000u, 0x3F800000u},
-            {"1.00000000000000011102230246252e0", 0x3FF0000000000001u, 0x3F800000u},
-            {"-0.999999999999999944488848768742172978818416595458984375", 0xBFF0000000000000u, 0xBF800000u},
-            {"-9.99999999999999944488848768742172978818416595458984376e-1", 0xBFF0000000000000u, 0xBF800000u},
-            {"999999999999999944488848768742172978818416595458984374e-54", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"0.99999999999999994448884876874217297881841659545898437501", 0x3FF0000000000000u, 0x3F800000u},
-            {"9.99999999999999944488848768742172978818416595458984375000000000000000000001e-1", 0x3FF0000000000000u, 0x3F800000u},
-            {"-0.99999999999999994", 0xBFEFFFFFFFFFFFFFu, 0xBF800000u},
-            {"9.9999999999999995e-1", 0x3FF0000000000000u, 0x3F800000u},
-            {"9.999999999999999444e-1", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"9999999999999999445e-19", 0x3FF0000000000000u, 0x3F800000u},
-            {"99999999999999994448e-20", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"-0.99999999999999994449", 0xBFF0000000000000u, 0xBF800000u},
-            {"0.999999999999999944488", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"-9.99999999999999944489e-1", 0xBFF0000000000000u, 0xBF800000u},
-            {"9.99999999999999944488848768742e-1", 0x3FEFFFFFFFFFFFFFu, 0x3F800000u},
-            {"999999999999999944488848768743e-30", 0x3FF0000000000000u, 0x3F800000u},
-            {"-9.007199254740993e15", 0xC340000000000000u, 0xDA000000u},
-            {"9007199254740994e0", 0x4340000000000001u, 0x5A000000u},
-            {"9007199254740992", 0x4340000000000000u, 0x5A000000u},
-            {"9.00719925474099301e15", 0x4340000000000001u, 0x5A000000u},
-            {"9007199254740993000000000000000000001e-21", 0x4340000000000001u, 0x5A000000u},
-            {"-9007199254740993.000000000000000000000000000000", 0xC340000000000000u, 0xDA000000u},
-            {"90071992547409915e-1", 0x4340000000000000u, 0x5A000000u},
-            {"-9007199254740991.6", 0xC340000000000000u, 0xDA000000u},
-            {"9.0071992547409914e15", 0x433FFFFFFFFFFFFFu, 0x5A000000u},
-            {"-9007199254740991501e-3", 0xC340000000000000u, 0xDA000000u},
-            {"9007199254740991.5000000000000000000001", 0x4340000000000000u, 0x5A000000u},
-            {"9.0071992547409915000000000000000000000000000000e15", 0x4340000000000000u, 0x5A000000u},
-            {"0.100000000000000012490009027033011079765856266021728515625", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"1.00000000000000012490009027033011079765856266021728515626e-1", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"100000000000000012490009027033011079765856266021728515624e-57", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"0.10000000000000001249000902703301107976585626602172851562501", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"0.10000000000000001", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"1.0000000000000002e-1", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"-1.000000000000000124e-1", 0xBFB999999999999Au, 0xBDCCCCCDu},
-            {"1000000000000000125e-19", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"-10000000000000001249e-20", 0xBFB999999999999Au, 0xBDCCCCCDu},
-            {"0.1000000000000000125", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"0.10000000000000001249", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"1.00000000000000012491e-1", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"1.00000000000000012490009027033e-1", 0x3FB999999999999Au, 0x3DCCCCCDu},
-            {"100000000000000012490009027034e-30", 0x3FB999999999999Bu, 0x3DCCCCCDu},
-            {"2.45134755833537796875e14", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"-245134755833537796876e-6", 0xC2EBDE5C4164D83Au, 0xD75EF2E2u},
-            {"-245134755833537.796874", 0xC2EBDE5C4164D839u, 0xD75EF2E2u},
-            {"2.4513475583353779687501e14", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"245134755833537796875000000000000000000001e-27", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"245134755833537.796875000000000000000000000000000000", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"2.4513475583353779e14", 0x42EBDE5C4164D839u, 0x575EF2E2u},
-            {"2451347558335378e-1", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"2451347558335377968e-4", 0x42EBDE5C4164D839u, 0x575EF2E2u},
-            {"245134755833537.7969", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"245134755833537.79687", 0x42EBDE5C4164D839u, 0x575EF2E2u},
-            {"2.4513475583353779688e14", 0x42EBDE5C4164D83Au, 0x575EF2E2u},
-            {"181510327827821147441864013671875e-23", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"-1815103278.27821147441864013671876", 0xC1DB0C11CB91CE38u, 0xCED8608Eu},
-            {"1.81510327827821147441864013671874e9", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"18151032782782114744186401367187501e-25", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"1815103278.27821147441864013671875000000000000000000001", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"-1.81510327827821147441864013671875000000000000000000000000000000e9", 0xC1DB0C11CB91CE38u, 0xCED8608Eu},
-            {"18151032782782114e-7", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"1815103278.2782115", 0x41DB0C11CB91CE38u, 0x4ED8608Eu},
-            {"1815103278.278211474", 0x41DB0C11CB91CE37u, 0x4ED8608Eu},
-            {"-1.815103278278211475e9", 0xC1DB0C11CB91CE38u, 0xCED8608Eu},
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-            {"5.429001220703125e3", 0x40B5350050000000u, 0x45A9A802u},
-            {"-5429001220703126e-12", 0xC0B5350050000001u, 0xC5A9A803u},
-            {"-5429.001220703124", 0xC0B535004FFFFFFFu, 0xC5A9A802u},
-            {"5.42900122070312501e3", 0x40B5350050000000u, 0x45A9A803u},
-            {"5429001220703125000000000000000000001e-33", 0x40B5350050000000u, 0x45A9A803u},
-            {"-5429.001220703125000000000000000000000000000000", 0xC0B5350050000000u, 0xC5A9A802u},
-            {"503719056e0", 0x41BE062490000000u, 0x4DF03124u},
-            {"503719057", 0x41BE062491000000u, 0x4DF03125u},
-            {"5.03719055e8", 0x41BE06248F000000u, 0x4DF03124u},
-            {"50371905601e-2", 0x41BE062490028F5Cu, 0x4DF03125u},
-            {"503719056.000000000000000000001", 0x41BE062490000000u, 0x4DF03125u},
-            {"5.03719056000000000000000000000000000000e8", 0x41BE062490000000u, 0x4DF03124u},
-            {"-92331620", 0xC196037990000000u, 0xCCB01BCCu},
-            {"9.233163e7", 0x41960379B8000000u, 0x4CB01BCEu},
-            {"9233161e1", 0x4196037968000000u, 0x4CB01BCBu},
-            {"92331620.1", 0x4196037990666666u, 0x4CB01BCDu},
-            {"9.233162000000000000000000001e7", 0x4196037990000000u, 0x4CB01BCDu},
-            {"9233162000000000000000000000000000000e-29", 0x4196037990000000u, 0x4CB01BCCu},
-            {"3.002458625e6", 0x4146E82D50000000u, 0x4A37416Au},
-            {"3002458626e-3", 0x4146E82D5020C49Cu, 0x4A37416Bu},
-            {"3002458.624", 0x4146E82D4FDF3B64u, 0x4A37416Au},
-            {"-3.00245862501e6", 0xC146E82D500053E3u, 0xCA37416Bu},
-            {"3002458625000000000000000000001e-24", 0x4146E82D50000000u, 0x4A37416Bu},
-            {"3002458.625000000000000000000000000000000", 0x4146E82D50000000u, 0x4A37416Au},
-            {"-1095485584696182596504479582065262592e1", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"10954855846961825965044795820652625930", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"1.095485584696182596504479582065262591e37", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"109548558469618259650447958206526259201e-1", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"-10954855846961825965044795820652625920.00000000000000000001", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"1.095485584696182596504479582065262592000000000000000000000000000000e37", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"10954855846961825e21", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"10954855846961826000000000000000000000", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"10954855846961825960000000000000000000", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"1.095485584696182597e37", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"1.0954855846961825965e37", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"-10954855846961825966e18", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"10954855846961825965e18", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"10954855846961825965100000000000000000", 0x47A07BA830000000u, 0x7D03DD42u},
-            {"10954855846961825965044795820600000000", 0x47A07BA830000000u, 0x7D03DD41u},
-            {"-1.09548558469618259650447958207e37", 0xC7A07BA830000000u, 0xFD03DD42u},
-            {"1.6449216019182103706535606608388384863861375606575165875256061553955078126e-21", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"-16449216019182103706535606608388384863861375606575165875256061553955078124e-94", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"-0.0000000000000000000016449216019182103", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"-1.6449216019182104e-21", 0xBB9F125A50000000u, 0x9CF892D3u},
-            {"-1.64492160191821037e-21", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"-1644921601918210371e-39", 0xBB9F125A50000000u, 0x9CF892D3u},
-            {"-16449216019182103706e-40", 0xBB9F125A50000000u, 0x9CF892D2u},
-            {"0.0000000000000000000016449216019182103707", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"0.00000000000000000000164492160191821037065", 0x3B9F125A50000000u, 0x1CF892D2u},
-            {"1.64492160191821037066e-21", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"1.64492160191821037065356066083e-21", 0x3B9F125A50000000u, 0x1CF892D2u},
-            {"164492160191821037065356066084e-50", 0x3B9F125A50000000u, 0x1CF892D3u},
-            {"6.565061509609222412109375e-1", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"6565061509609222412109376e-25", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"0.6565061509609222412109374", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"6.56506150960922241210937501e-1", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"6565061509609222412109375000000000000000000001e-46", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"-0.6565061509609222412109375000000000000000000000000000000", 0xBFE5021930000000u, 0xBF2810CAu},
-            {"6.5650615096092224e-1", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"-65650615096092225e-17", 0xBFE5021930000000u, 0xBF2810CAu},
-            {"6565061509609222412e-19", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"0.6565061509609222413", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"0.65650615096092224121", 0x3FE5021930000000u, 0x3F2810C9u},
-            {"6.5650615096092224122e-1", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"-6.5650615096092224121e-1", 0xBFE5021930000000u, 0xBF2810C9u},
-            {"656506150960922241211e-21", 0x3FE5021930000000u, 0x3F2810CAu},
-            {"18014627239033005156980393746124491372029297053813934326171875e-77", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"0.00000000000000018014627239033005156980393746124491372029297053813934326171876", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"-1.8014627239033005156980393746124491372029297053813934326171874e-16", 0xBCA9F63970000000u, 0xA54FB1CBu},
-            {"1801462723903300515698039374612449137202929705381393432617187501e-79", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"18014627239033005e-32", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"0.00000000000000018014627239033006", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"0.0000000000000001801462723903300515", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"-1.801462723903300516e-16", 0xBCA9F63970000000u, 0xA54FB1CCu},
-            {"-1.8014627239033005156e-16", 0xBCA9F63970000000u, 0xA54FB1CBu},
-            {"18014627239033005157e-35", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"180146272390330051569e-36", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"-0.00000000000000018014627239033005157", 0xBCA9F63970000000u, 0xA54FB1CCu},
-            {"0.000000000000000180146272390330051569803937461", 0x3CA9F63970000000u, 0x254FB1CBu},
-            {"1.80146272390330051569803937462e-16", 0x3CA9F63970000000u, 0x254FB1CCu},
-            {"0.05534819327294826507568359375", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"-5.534819327294826507568359376e-2", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"5534819327294826507568359374e-29", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"-0.0553481932729482650756835937501", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"5.534819327294826507568359375000000000000000000001e-2", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"5534819327294826507568359375000000000000000000000000000000e-59", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"0.055348193272948265", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"5.5348193272948266e-2", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"5.534819327294826507e-2", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"5534819327294826508e-20", 0x3FAC569930000000u, 0x3D62B4CAu},
-            {"55348193272948265075e-21", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"-0.055348193272948265076", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"0.0553481932729482650756", 0x3FAC569930000000u, 0x3D62B4C9u},
-            {"-5.53481932729482650757e-2", 0xBFAC569930000000u, 0xBD62B4CAu},
-            {"5.179692133247783258005389047985340416e36", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"5179692133247783258005389047985340417e0", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"5179692133247783258005389047985340415", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"-5.17969213324778325800538904798534041601e36", 0xC78F2C9450000000u, 0xFC7964A3u},
-            {"-5179692133247783258005389047985340416000000000000000000001e-21", 0xC78F2C9450000000u, 0xFC7964A3u},
-            {"-5179692133247783258005389047985340416.000000000000000000000000000000", 0xC78F2C9450000000u, 0xFC7964A2u},
-            {"5.1796921332477832e36", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"51796921332477833e20", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"-5179692133247783258e18", 0xC78F2C9450000000u, 0xFC7964A2u},
-            {"5179692133247783259000000000000000000", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"5179692133247783258000000000000000000", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"5.1796921332477832581e36", 0x478F2C9450000000u, 0x7C7964A3u},
-            {"-5.179692133247783258e36", 0xC78F2C9450000000u, 0xFC7964A2u},
-            {"-517969213324778325801e16", 0xC78F2C9450000000u, 0xFC7964A3u},
-            {"517969213324778325800538904798e7", 0x478F2C9450000000u, 0x7C7964A2u},
-            {"5179692133247783258005389047990000000", 0x478F2C9450000000u, 0x7C7964A3u},
-            {
-                "0.22250738585072011360574097967091319759348195463516456480234261097248222220210769455165295239081350"
-                "8791414915891303962110687008643869459464552765720740782062174337998814106326732925355228688137214901"
-                "2981122451451889849057222307285255133155755015914397476397983411801999323962548289017107081850690630"
-                "6666559949382757725720157630626906633326475653000092458883164330377797918696120494973903778297049050"
-                "5108060994073026293712895895000358379996720725430436028407889577179615094551674824347103070260914462"
-                "1572289880258182545180325707018860872113128079512233426288368622321503775666622503982534335974568884"
-                "4239002654981983854879482922068947216898310996983658468140228542433306603398508864458040010349339704"
-                "2756718644338377048603786162277173854562306587467901408672332763671875e-307", 0x0010000000000000u, 0x00000000u
-            },
-            {
-                "2.22507385850720113605740979670913197593481954635164564802342610972482222202107694551652952390813508"
-                "7914149158913039621106870086438694594645527657207407820621743379988141063267329253552286881372149012"
-                "9811224514518898490572223072852551331557550159143974763979834118019993239625482890171070818506906306"
-                "6665599493827577257201576306269066333264756530000924588831643303777979186961204949739037782970490505"
-                "1080609940730262937128958950003583799967207254304360284078895771796150945516748243471030702609144621"
-                "5722898802581825451803257070188608721131280795122334262883686223215037756666225039825343359745688844"
-                "2390026549819838548794829220689472168983109969836584681402285424333066033985088644580400103493397042"
-                "756718644338377048603786162277173854562306587467901408672332763671875000000000000000000001e-308", 0x0010000000000000u, 0x00000000u
-            },
-            {
-                "0.11754942807573642917278829910357665133228589927589904276829631184250030649651730385585324256680905"
-                "8189392089843750000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
-                "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

tests/src/unit-class_lexer.cpp

@@ -13,19 +13,15 @@
 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
@@ -261,7 +257,7 @@ TEST_CASE("lexer number fast path")
             "123456789012345678901234567890",  // huge -> float
             "0.30000000000000004", "2.2250738585072014e-308", "1e308",
             // high-precision / wide-exponent values that exercise the
-            // Eisel-Lemire path beyond the Clinger subset
+            // std::from_chars (Eisel-Lemire) path beyond the Clinger subset
             "1.7976931348623157e308", "1.2345678901234567e-250",
             "9007199254740993", "5e-324", "1e-320"
         };
@@ -283,18 +279,20 @@ TEST_CASE("lexer number fast path")
         }
     }
 
-    SECTION("significant digits around Clinger's fast path")
+    SECTION("significant-digit gate for the Clinger fast path")
     {
-        // Clinger's fast path needs a significand of at most 2^53, which
+        // Clinger's fast path needs a significand below 2^53, so it cannot
-        // tokens with 17 or more significant digits exceed. The conversion
+        // succeed once the mantissa has 17 or more significant digits (the
-        // splits the token at the positions the scanners recorded, so leading
+        // significand would be at least 10^16). The lexer skips the attempt
-        // zeros must not count as digits - "0.1234567890123456" has 16
+        // there. That is only allowed to save work: every value must still come
-        // significant digits, not 17 - and both scanners must agree.
+        // 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.
         const std::vector<std::string> numbers =
         {
             "1234567890123456",                  // 16 significant digits
-            "12345678901234567",                 // 17
+            "12345678901234567",                 // 17 -> attempt skipped
-            "123456789012345678",                // 18
+            "123456789012345678",                // 18 -> attempt skipped
             "0.1234567890123456",                // 16: the leading "0" is not significant
             "0.12345678901234567",               // 17
             "0.00000000000000001",               // 1, in a long token
@@ -665,323 +663,46 @@ TEST_CASE("lexer string fast path")
     }
 }
 
-TEST_CASE("lexer escape fast path")
+TEST_CASE("parse_float_fast declines what it cannot convert exactly")
 {
-    // json::accept() never throws, so this section stays covered without
+    // The lexer only hands well-formed numbers to parse_float_fast, so the
-    // exceptions; it pins which of the cases below are valid/invalid and
+    // malformed ones below can only be passed to it directly. Declining is
-    // checks the contiguous and streaming paths agree on that classification.
+    // always safe: the caller then falls back to a slower, exact conversion.
-    SECTION("accept() parity")
+    const auto fast = [](const std::string & s, double & out)
     {
-        const std::vector<std::pair<std::string, bool>> cases =
+        return nlohmann::detail::parse_float_fast(s.data(), s.data() + s.size(), out);
-        {
-            {"\\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;
 
-    SECTION("contiguous vs streaming parity")
+#if defined(FLT_EVAL_METHOD) && FLT_EVAL_METHOD != 0
-    {
+    // without true double precision, the fast path declines everything
-        const std::vector<std::string> escapes =
+    CHECK_FALSE(fast("1.5", out));
-        {
+#else
-            "\\u0041",             // "A"
+    CHECK(fast("1.5", out));
-            "\\u00e4",             // "ä" (lowercase hex)
+    CHECK(out == 1.5);
-            "\\u00E4",             // "ä" (uppercase hex)
+    CHECK(fast("+2.5e1", out));
-            "\\uD83D\\uDE00",      // valid surrogate pair (an emoji)
+    CHECK(out == 25.0);
-            "\\u12",               // truncated: only 2 hex digits before the closing quote
+    CHECK(fast("-25E-1", out));
-            "\\u12G4",             // invalid hex digit at the 3rd position
+    CHECK(out == -2.5);
-            "\\uXYZW",             // all 4 bytes invalid
+    CHECK(fast("1e", out));
-            "\\uD800",             // lone high surrogate, string ends right after
+    CHECK(out == 1.0);
-            "\\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
-}
 
-namespace
+    // not a number
-{
+    CHECK_FALSE(fast("", out));
-// the index of the decimal point (or npos) and of the end of the mantissa of a
+    CHECK_FALSE(fast("-", out));
-// number token, which the lexer records while scanning it
+    CHECK_FALSE(fast(".", out));
-std::pair<std::size_t, std::size_t> float_token_layout(const std::string& s)
+    CHECK_FALSE(fast("1.2.3", out));
-{
+    CHECK_FALSE(fast("1x", out));
-    std::size_t dot = std::string::npos;
+    CHECK_FALSE(fast("1e+", out));
-    std::size_t mantissa_end = s.size();
+    CHECK_FALSE(fast("1e1x", out));
-    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};
-}
 
-template<typename FloatType>
+    // numbers that are not represented exactly on the fast path
-FloatType parse_native(const std::string& s)
+    CHECK_FALSE(fast("12345678901234567890", out));
-{
+    CHECK_FALSE(fast("1e10000", out));
-    const auto layout = float_token_layout(s);
+    CHECK_FALSE(fast("9007199254740993", out));
-    return nlohmann::detail::parse_float_native<FloatType>(s.data(), s.data() + s.size(), layout.first, layout.second);
+    CHECK_FALSE(fast("1e23", out));
-}
+    CHECK_FALSE(fast("1e-23", out));
-
-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
@@ -1085,6 +806,40 @@ 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")
@@ -1482,33 +1237,26 @@ TEST_CASE("Eisel-Lemire float conversion")
         for (const auto& c : known)
         {
             CAPTURE(c.first)
-            CHECK(native_bits64(c.first) == c.second);
+            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);
+            }
         }
     }
 
-    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, and its 17-digit
+        // every double written by to_chars and read back, also with trailing
-        // form with trailing digits that make the token longer than 19 digits
+        // 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;
@@ -1530,51 +1278,30 @@ 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)
-            CHECK(native_bits64(token) == b);
+            double out = 0;
+            REQUIRE(eisel_lemire(token, out));
+            CHECK(bits_of(out) == b);
 
-            // insert digits before the exponent of the 17-digit form: that
+            // insert digits before the exponent: the value moves by far less
-            // form lies strictly inside the rounding interval of the double
+            // than the distance to the rounding boundary, so it must not change
-            // (the shortest one may lie on its boundary), and the digits move
+            std::string longer = token;
-            // 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)
-            CHECK(native_bits64(longer) == b);
+            if (eisel_lemire(longer, out))
-        }
-    }
-
-    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)
             {
-                continue; // infinity or NaN
+                CHECK(bits_of(out) == b);
             }
-            if (i % 4 == 0)
+            else
             {
-                b &= 0x807FFFFFu; // subnormals
+                // w and w + 1 round differently: only when the value is very
+                // close to a rounding boundary
+                ++declined;
             }
-            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")
@@ -1588,177 +1315,3 @@ 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);
-    }
-}

tests/src/unit-custom-object-type.cpp

@@ -10,7 +10,9 @@
 
 #include <nlohmann/json.hpp>
 
+#include <cstddef>
 #include <cstdint>
+#include <iterator>
 #include <map>
 #include <string>
 #include <type_traits>
@@ -196,6 +198,198 @@ struct void_erase_map : std::map<Key, T, Compare, Allocator>
 
 using void_erase_json = nlohmann::basic_json<void_erase_map>;
 
+// wraps an iterator, but only offers the LegacyForwardIterator operations,
+// like the iterators of std::unordered_map and other hash maps
+template<class BaseIterator>
+class forward_only_iterator
+{
+    BaseIterator m_it{};
+
+  public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = typename std::iterator_traits<BaseIterator>::value_type;
+    using difference_type = typename std::iterator_traits<BaseIterator>::difference_type;
+    using pointer = typename std::iterator_traits<BaseIterator>::pointer;
+    using reference = typename std::iterator_traits<BaseIterator>::reference;
+
+    forward_only_iterator() = default;
+    explicit forward_only_iterator(BaseIterator it) : m_it(it) {}
+
+    BaseIterator base() const
+    {
+        return m_it;
+    }
+
+    reference operator*() const
+    {
+        return *m_it;
+    }
+    pointer operator->() const
+    {
+        return &*m_it;
+    }
+    forward_only_iterator& operator++()
+    {
+        ++m_it;
+        return *this;
+    }
+    forward_only_iterator operator++(int)
+    {
+        auto result = *this;
+        ++m_it;
+        return result;
+    }
+
+    friend bool operator==(const forward_only_iterator& lhs, const forward_only_iterator& rhs)
+    {
+        return lhs.m_it == rhs.m_it;
+    }
+    friend bool operator!=(const forward_only_iterator& lhs, const forward_only_iterator& rhs)
+    {
+        return lhs.m_it != rhs.m_it;
+    }
+};
+
+// An ObjectType whose iterators are forward-only, as those of hash maps are;
+// it has no rbegin() and its iterators no operator--. A hash map is not used
+// directly for the same reason as in no_key_compare_map above.
+template<class Key, class T, class Compare, class Allocator>
+class forward_only_map
+{
+    using map_t = std::map<Key, T, Compare, Allocator>;
+    map_t data;
+
+  public:
+    using key_type = typename map_t::key_type;
+    using mapped_type = typename map_t::mapped_type;
+    using value_type = typename map_t::value_type;
+    using size_type = typename map_t::size_type;
+    using allocator_type = typename map_t::allocator_type;
+    using iterator = forward_only_iterator<typename map_t::iterator>;
+    using const_iterator = forward_only_iterator<typename map_t::const_iterator>;
+
+    forward_only_map() noexcept(std::is_nothrow_default_constructible<map_t>::value) : data() {}
+
+    template<class InputIt>
+    forward_only_map(InputIt first, InputIt last) : data(first, last) {}
+
+    iterator begin() noexcept
+    {
+        return iterator(data.begin());
+    }
+    iterator end() noexcept
+    {
+        return iterator(data.end());
+    }
+    const_iterator begin() const noexcept
+    {
+        return const_iterator(data.begin());
+    }
+    const_iterator end() const noexcept
+    {
+        return const_iterator(data.end());
+    }
+    const_iterator cbegin() const noexcept
+    {
+        return const_iterator(data.cbegin());
+    }
+    const_iterator cend() const noexcept
+    {
+        return const_iterator(data.cend());
+    }
+
+    bool empty() const noexcept
+    {
+        return data.empty();
+    }
+    size_type size() const noexcept
+    {
+        return data.size();
+    }
+    size_type max_size() const noexcept
+    {
+        return data.max_size();
+    }
+    void clear() noexcept
+    {
+        data.clear();
+    }
+
+    iterator find(const key_type& key)
+    {
+        return iterator(data.find(key));
+    }
+    const_iterator find(const key_type& key) const
+    {
+        return const_iterator(data.find(key));
+    }
+    size_type count(const key_type& key) const
+    {
+        return data.count(key);
+    }
+
+    std::pair<iterator, bool> emplace(const key_type& key, const mapped_type& value)
+    {
+        const auto result = data.emplace(key, value);
+        return {iterator(result.first), result.second};
+    }
+
+    std::pair<iterator, bool> insert(const value_type& value)
+    {
+        const auto result = data.insert(value);
+        return {iterator(result.first), result.second};
+    }
+
+    template<class InputIt>
+    void insert(InputIt first, InputIt last)
+    {
+        data.insert(first, last);
+    }
+
+    mapped_type& operator[](const key_type& key)
+    {
+        return data[key];
+    }
+
+    mapped_type& at(const key_type& key)
+    {
+        return data.at(key);
+    }
+    const mapped_type& at(const key_type& key) const
+    {
+        return data.at(key);
+    }
+
+    iterator erase(iterator pos)
+    {
+        return iterator(data.erase(pos.base()));
+    }
+    iterator erase(iterator first, iterator last)
+    {
+        return iterator(data.erase(first.base(), last.base()));
+    }
+    size_type erase(const key_type& key)
+    {
+        return data.erase(key);
+    }
+
+    void swap(forward_only_map& other) noexcept(noexcept(data.swap(other.data)))
+    {
+        data.swap(other.data);
+    }
+
+    friend bool operator==(const forward_only_map& lhs, const forward_only_map& rhs)
+    {
+        return lhs.data == rhs.data;
+    }
+    friend bool operator<(const forward_only_map& lhs, const forward_only_map& rhs)
+    {
+        return lhs.data < rhs.data;
+    }
+};
+
+using forward_only_json = nlohmann::basic_json<forward_only_map>;
+
 } // namespace
 
 TEST_CASE("object type whose erase() returns void")
@@ -322,3 +516,46 @@ TEST_CASE("object type without key_compare")
     }
 }
 
+
+TEST_CASE("object type with forward-only iterators")
+{
+    CHECK(std::is_same<std::iterator_traits<forward_only_json::object_t::iterator>::iterator_category,
+          std::forward_iterator_tag>::value);
+
+    SECTION("destroying nested objects and arrays")
+    {
+        forward_only_json j;
+        j["a"] = 1;
+        j["b"]["c"] = "x";
+        j["b"]["d"] = forward_only_json::array();
+        j["b"]["d"].push_back(forward_only_json::object());
+        j["b"]["d"].push_back(true);
+        j["b"]["e"]["f"]["g"] = nullptr;
+        j["h"] = forward_only_json::object();
+        j["i"]["j"] = 2;
+
+        CHECK(j.size() == 4);
+        CHECK(j["b"].size() == 3);
+        CHECK(j["b"]["d"].size() == 2);
+        CHECK(j["b"]["e"]["f"]["g"].is_null());
+
+        CHECK(j.erase("b") == 1);
+        CHECK(j.size() == 3);
+        j = 42;
+        CHECK(j == 42);
+    }
+
+    SECTION("destroying a deeply nested object")
+    {
+        constexpr std::size_t depth = 100000;
+        forward_only_json j;
+        forward_only_json* cur = &j;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            (*cur)["s"] = i;
+            cur = &(*cur)["o"];
+        }
+        CHECK(j["o"]["o"]["s"] == 2);
+        // destroyed at the end of scope without recursing per level
+    }
+}

tests/src/unit-locale-cpp.cpp

@@ -260,11 +260,10 @@ struct LocaleSwitchingSax final: public nlohmann::json_sax<json>
 
 TEST_CASE("locale changes between lexer construction and number conversion (#5198)")
 {
-    // float and double are converted without the locale. A long double that
+    // The numbers are chosen so that the conversion also takes the strtod
-    // is not binary64 can take the strtold fallback, which honors the locale
+    // fallback, which honors the locale that is current at conversion time:
-    // that is current at conversion time. The numbers are chosen so that it
+    // too many significant digits for Clinger's fast path, an underflow that
-    // does: too many significant digits for Clinger's fast path, an underflow
+    // std::from_chars rejects, and a plain value.
-    // 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)
@@ -328,8 +327,7 @@ TEST_CASE("locale changes between lexer construction and number conversion (#519
             }
         }
 
-        // a long double goes through std::strtold unless it is binary64 or
+        // a long double goes through std::strtold unless std::from_chars supports it
-        // 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
@@ -355,15 +353,8 @@ 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 strtold fallback (only for long
+    // substituted in place for '.', so the strtod fallback stops early. The
-    // double formats other than binary64) converts a copy of the token with
+    // conversion must still terminate rather than retry forever.
-    // 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)
@@ -381,20 +372,12 @@ 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: double does not depend on the locale
+        // that std::from_chars rejects: both reach the strtod fallback
         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);
     }