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Speed up the lexer: own float parser, string scan, and \u table

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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>
This commit is contained in:
Niels Lohmann
2026-10-06 10:48:06 +02:00
parent 5ecb704f6b
commit 953d74ddcd
13 changed files with 2929 additions and 984 deletions
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33
tests/src/unit-locale-cpp.cpp
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@@ -260,10 +260,11 @@ struct LocaleSwitchingSax final: public nlohmann::json_sax<json>
TEST_CASE("locale changes between lexer construction and number conversion (#5198)")
{
// The numbers are chosen so that the conversion also takes the strtod
// fallback, which honors the locale that is current at conversion time:
// too many significant digits for Clinger's fast path, an underflow that
// std::from_chars rejects, and a plain value.
// float and double are converted without the locale. A long double that
// is not binary64 can take the strtold fallback, which honors the locale
// that is current at conversion time. The numbers are chosen so that it
// does: too many significant digits for Clinger's fast path, an underflow
// that std::from_chars rejects, and a plain value.
const std::vector<std::string> numbers = {"3.14159265358979323846", "1.5e-400", "12.34", "-0.000123456789012345678"};
std::string text = "[";
for (const auto& n : numbers)
@@ -327,7 +328,8 @@ TEST_CASE("locale changes between lexer construction and number conversion (#519
}
}
// a long double goes through std::strtold unless std::from_chars supports it
// a long double goes through std::strtold unless it is binary64 or
// std::from_chars supports it
{
bool switched = false;
const auto cb = [&](int /*depth*/, long_double_json::parse_event_t event, long_double_json& /*parsed*/) noexcept
@@ -353,8 +355,15 @@ TEST_CASE("locale with a multi-byte decimal point")
{
// Some locales use a decimal point that is not a single character, e.g.
// U+066B ARABIC DECIMAL SEPARATOR (two bytes in UTF-8). It cannot be
// substituted in place for '.', so the strtod fallback stops early. The
// conversion must still terminate rather than retry forever.
// substituted in place for '.', so the strtold fallback (only for long
// double formats other than binary64) converts a copy of the token with
// the whole decimal point instead (#5660). The values must be those of the
// "C" locale.
using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
const char* const long_double_numbers = "[3.14159265358979323846, 1.5e-400, -0.000123456789012345678]";
REQUIRE(std::setlocale(LC_NUMERIC, "C") != nullptr);
const long_double_json expected_long_double = long_double_json::parse(long_double_numbers);
const std::array<const char*, 6> names = {{"ar_EG.UTF-8", "ar_SA.UTF-8", "fa_IR.UTF-8", "ps_AF.UTF-8", "ar_EG", "fa_IR"}};
bool tested = false;
for (const char* name : names)
@@ -372,12 +381,20 @@ TEST_CASE("locale with a multi-byte decimal point")
tested = true;
// too many significant digits for Clinger's fast path, and an underflow
// that std::from_chars rejects: both reach the strtod fallback
// that std::from_chars rejects: double does not depend on the locale
json j;
CHECK_NOTHROW(j = json::parse("[3.14159265358979323846, 1.5e-400, -0.000123456789012345678]"));
CHECK(j.is_array());
CHECK(j[0] == 3.14159265358979323846);
CHECK(j[1] == 0.0);
CHECK(j[2] == -0.000123456789012345678);
CHECK(json::accept("3.14159265358979323846"));
// a long double that reaches the strtold fallback is not truncated
long_double_json ld;
CHECK_NOTHROW(ld = long_double_json::parse(long_double_numbers));
CHECK(ld == expected_long_double);
// a value the locale-independent paths convert is not affected
CHECK(json::parse("12.5") == 12.5);
}