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Niels Lohmann
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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>