* Write doubles with the shortest digits (Żmij), digits in registers
Write doubles with the conversion of Zmij by Victor Zverovich (MIT),
ported to C++11 (detail/conversions/zmij.hpp). It finds the
shortest decimal that reads back as the same double, and the
closest one if there are several. Grisu2, used until now, is fast
but not always shortest: it sometimes writes a 17th digit where 16
suffice, or a last digit that is not the closest. The layout is
unchanged (1.5, 100.0, 1e+100, -0.0); float keeps Grisu2.
Digits are converted eight at a time with the BCD conversion of
Xiang JunBo, as in Zmij, and written with one byte swap per eight
digits and fixed-size moves instead of per-digit loops. Leading and
trailing zeros are counted from those bytes. to_chars() uses a
local buffer when the caller's is shorter than the 41 bytes this
may write. The powers of ten come from the number-parsing table,
adjusted where it holds values rounded up, and extended with Zmij's
compressed tables beyond 10^308.
write_shortest() converts its 16 digits in one vector register
(SSE2 on x86-64, NEON on 64-bit Arm, both baseline) and inserts the
decimal point inside the register, avoiding a store-forwarding
stall that cost about 25% of the time to write a double. dump()
writes floats and integers straight into the serializer's write
buffer instead of copying them from a member buffer, and small
integers eight digits at a time. read_eight_bytes() and
parse_eight_digits() are marked always-inline, which GCC had been
calling out of line in the number-parsing loops.
Of one million random doubles, about 0.14% are now written with
different digits, always to a value that still reads back as the
same double.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Fix CI findings in the Zmij writer and its tests
- bit_ops.hpp: include macro_scope.hpp (JSON_HEDLEY_ALWAYS_INLINE); fixes IWYU
- to_chars.hpp: C4100 for the unused parameter in release builds, clang-tidy
sign comparison, cpplint runtime/int, GCC -Wstrict-overflow (unsigned abs)
- unit-to_chars.cpp: parse with the library instead of strtod (MinGW's strtod
rounds some 16/17 digit inputs wrongly); no floating-point std::to_chars
with icpc
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Select the Zmij conversion by a trait, not by the double overload
The non-template overloads for double asserted binary64 doubles wherever json.hpp was included, so the library no longer compiled where double is not IEEE 754 binary64 (AVR, -fshort-double). A trait now picks Zmij for any binary64 type, including a long double of that format (MSVC, Apple Arm), and Grisu2 for the others. Remove the unused write_short_decimal, powers_of_ten_16, zmij::decimal, zmij::to_decimal, and shortest_digits(double).
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Credit Zmij's authors in to_chars.hpp, the README, and the license page
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Silence MSVC C4127 for a platform-constant check in the to_chars test
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Regenerate the amalgamated headers
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---------
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Speed up the lexer: own float parser, string scan, and \u table
Give the library its own correctly rounded float converter for
binary32 and binary64 (IEEE 754), and speed up the lexer's string
and escape scanning.
The converter splits a number token into sign, significand, and
decimal exponent, then tries Clinger's fast path, then a templated
Eisel-Lemire step, and falls back to an exact big-integer digit
comparison for tokens with more than 19 significant digits whose two
candidate values round differently. This replaces std::from_chars
and strtod/strtof for both formats, so parsed values no longer
depend on the C/C++ library or the current locale. The strtold
fallback kept for other long double formats (x87, binary128) now
also copies a multi-byte decimal point correctly, fixing #5660.
eisel_lemire() and decimal_to_float() are always inlined so callers
keep the whole conversion in their hot loop.
The string-scanning kernels in string_scan.hpp find a stop byte with
the trailing-zero count of the SWAR mask instead of a byte loop, and
scalar_string_bulk_run() validates a run of multi-byte UTF-8
sequences one after another instead of re-searching after each one.
get_codepoint() decodes a contiguous \uXXXX escape with one table
lookup per byte instead of four range-checked get() calls; the
streaming path and all error positions are unchanged.
Adds 508 generated hard float-parsing cases with expected binary32
and binary64 bits, and kernel-comparison tests for the string scans
and the escape table against byte-by-byte references.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Fix CI warnings in own float parser
- pow5_table.hpp: pow5_128_largest_power was unused in this branch's
own code (GCC -Werror=unused-const-variable); tie it to the table
size with a static_assert instead of removing it, since a later
branch in the stack (json-view/23-zmij) uses it.
- number_parse.hpp: rename the local variable `copy` to `buffer` to
satisfy cpplint's build/include_what_you_use check.
- unit-class_lexer.cpp: extend the NOLINT list on the seeded mt19937
with bugprone-random-generator-seed, and parenthesize
`8 * sizeof(Bits) - 1` for clang-tidy.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Load string scan words with memcpy and use MSVC bit-scan intrinsics
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Reword float conversion notes for the string type and number_float_t docs
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Fix clang-tidy 22 findings in unit-class_lexer.cpp
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Mark the unsigned long of the MSVC bit scans for cpplint
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Regenerate the amalgamated headers
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Drop the position updates in the bulk \u path, which #5809 made unused
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Address review: std::any_of in has_nonzero_digit, comment punctuation
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Use data() instead of c_str() in the multi-byte decimal point path
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---------
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Pretty-printed JSON indents every line with a run of spaces, which
skip_whitespace() read one character at a time. For contiguous input
(bulk_scan) the lexer now skips the spaces after a newline 8 bytes at a
time directly in the input buffer, including the leading spaces of the
last, partial word. This bypasses get(), which is safe because
bulk_scan implies the lazy position from the previous commit: nothing
is counted per character.
Compared to lazy positions alone, pretty-printed input (twitter,
citm_catalog with indent 2 and 4) parses up to 13% faster and accept()
gets up to 30% faster (GCC 13 and clang 18 on x86-64, Apple clang 21 on
arm64). Minified input is unchanged within noise.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Convert floats with Eisel-Lemire when std::from_chars is unavailable
Float tokens that Clinger's fast path cannot convert (e.g. the 17-digit
coordinates of canada.json) went to strtod unless std::from_chars was
available. It is not used in C++11/14, and not with libc++, which does not
define __cpp_lib_to_chars. The Eisel-Lemire algorithm (after fast_float's
compute_float) now converts them with integer arithmetic, correctly rounded
for any token with at most 19 significant digits. Longer tokens are
truncated; the result is used if w and w + 1 round alike, else strtod
decides as before. Overflow still yields infinity (out_of_range.406).
The table of powers of five (fast_float's) lives in pow5_table.hpp; a unit
test recomputes every entry with big-integer arithmetic. Further tests:
known values generated with Python (whose float() is correctly rounded),
200,000 round trips through to_chars, and the 128-bit multiplication and
leading-zero count against big-integer references (both with and without a
128-bit type). Checked against strtod on 6.5 million tokens, among them
60,000 exact halfway cases: no difference.
json::parse on canada.json: -8.6% (C++11), -7.6% (C++17, Apple clang);
other files unchanged. Compile time of a TU including json.hpp: +0.7%.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Fix CI: unused parse result and find() == npos in the Eisel-Lemire tests
GCC (-Werror=unused-result) rejected CHECK_THROWS_WITH_AS(json::parse(...))
because parse() is [[nodiscard]]; assign the result to a dummy json as the
other tests do. clang-tidy flagged longer.find('.') == npos with
abseil-string-find-str-contains; store the position in a variable first.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---------
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
Suyog Verma