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Use std::from_chars (Eisel-Lemire) for float conversion when available

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The Clinger fast path is exact only for the "easy" subset (<=19 significant
digits, |exp10| <= 22); high-precision and scientific floats fall through to
strtod, where the failed Clinger attempt actually makes parsing a net loss.
std::from_chars implements the Eisel-Lemire algorithm in modern standard
libraries: locale-independent, correctly rounded, and fast over the whole
value range.

convert_number() now tries parse_float_from_chars() first (guarded by
__cpp_lib_to_chars, so C++11 and libc++-without-float-support keep the
Clinger + strtod path unchanged), then Clinger, then strtof. from_chars is
used only when it consumes the entire token; a partial parse means a non-'.'
locale decimal point, and an under-/overflow (result_out_of_range) also
declines - in both cases the existing strtod fallback supplies the exact
value and the well-defined +/-inf/0 the parser expects, side-stepping the
P4168 divergence between implementations. float and long double now get the
fast path too (Clinger was double-only).

Measured, C++17, g++ 13 -O3, json::parse/accept:
  - canada-style floats: ~unchanged (Clinger already covered them)
  - high-precision (17 digits):  parse 2.1x, accept 2.5x
  - scientific (17 digits + exp): parse 3.6x, accept 4.1x

Verified: C++11 (Clinger/strtod) and C++17 (from_chars) parse every value -
including subnormals, boundary values, and 1e9999/1e-9999 over-/underflow -
to bit-identical results; 2M number-fuzz clean; conversions/deserialization/
locale/number-fast-path suites pass in both C++11 and C++17; clang-tidy
clean; warning-clean on g++ and clang in C++11/17/20.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXcDtEma2PjxgmPS9cQGzA
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-20 19:43:31 +00:00
co-authored by Claude Opus 4.8
parent 4b9fc0ad76
commit 2aa570f8f3
4 changed files with 89 additions and 5 deletions
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tests/src/unit-class_lexer.cpp
+5 -1
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@@ -249,7 +249,11 @@ TEST_CASE("lexer number fast path")
"-9223372036854775808", // INT64_MIN -> integer
"-9223372036854775809", // INT64_MIN - 1 -> float
"123456789012345678901234567890", // huge -> float
"0.30000000000000004", "2.2250738585072014e-308", "1e308"
"0.30000000000000004", "2.2250738585072014e-308", "1e308",
// high-precision / wide-exponent values that exercise the
// std::from_chars (Eisel-Lemire) path beyond the Clinger subset
"1.7976931348623157e308", "1.2345678901234567e-250",
"9007199254740993", "5e-324", "1e-320"
};
for (const auto& n : numbers)