Niels LohmannandClaude Opus 4.8 7f88e8f3a1 Speed up number parsing in the lexer (fast paths from the fast_float/simdjson world) ...

The number scanner converted its already-validated digit buffer with
std::strtoull/std::strtoll/std::strtod. Those pull in locale and errno
machinery and dominate number-heavy parsing (strtod runs at ~6 M/s).

Replace them with dedicated parsers over the validated buffer:

- parse_integer_unsigned / parse_integer_signed: accumulate digits with
  overflow detection, falling back to the float path on overflow exactly
  as the strtoull/strtoll round-trip check did. Overflow behavior is
  unchanged for narrower or wider custom number types.

- parse_float_fast: Clinger's exact fast path for `double` (<=19 significant
  digits, |exp10| <= 22, significand < 2^53), where significand * 10^exp is
  exact under IEEE round-to-nearest. This is the same fast path used by
  fast_float/simdjson. It is bit-identical to strtod on this subset and
  declines (falling back to strtod) otherwise. Only `double` uses it; float
  and long double keep std::strtof/std::strtold via a templated overload.

Measured on representative data (g++ 13, -O3):
  - integers:  DOM parse +11%, SAX +25-34%
  - floats:    DOM parse +37%, SAX +70%  (clang: float DOM ~1.9x)

No dependencies added; header-only and C++11-clean. Existing parser,
lexer, conversion and deserialization unit tests pass unchanged; a
3M-value random-double fuzz matches strtod bit-for-bit.

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>

2026-07-20 09:51:16 +00:00

..

nlohmann

Speed up number parsing in the lexer (fast paths from the fast_float/simdjson world)

2026-07-20 09:51:16 +00:00