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Convert floats with Eisel-Lemire when std::from_chars is unavailable (#5617)

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* 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>
This commit is contained in:
Niels Lohmann
2026-09-30 20:06:26 +02:00
committed by GitHub
parent 5b9ffae8bb
commit d268eaa693
8 changed files with 2008 additions and 8 deletions
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@@ -1395,6 +1395,7 @@ 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 &copy; 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 and its table of powers of five from [fast_float](https://github.com/fastfloat/fast_float) by Daniel Lemire and contributors, which is available under the [MIT License](https://opensource.org/licenses/MIT) (used here), the Apache 2.0 License, and the Boost Software License. Copyright &copy; 2021 The fast_float authors
<img align="right" src="https://git.fsfe.org/reuse/reuse-ci/raw/branch/master/reuse-horizontal.png" alt="REUSE Software">