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Convert the floats of json_view from the digit layout

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The parser records where the integer digits, the fraction digits, and the
exponent of a float token are. For doubles with at most 19 digits, the
value is now read from that layout: the digits eight at a time, without
scanning the token, and rounded with Clinger's fast path where both
operands are exact, else with the Eisel-Lemire algorithm (which needs no
fallback for up to 19 digits). Both round correctly, so the values are
those of parse(); other tokens and types keep the library's conversion.

get<double>(), materialize(), dump(), and comparisons use it. Traversing
canada.json (111,000 floats, every number converted): 0.53 -> 0.86 GB/s.

Tests add tokens around the limits (19 and 20 digits, 2^53, 10^22) to the
bit-for-bit comparison with parse().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-29 14:03:02 +02:00
parent 1c7041908d
commit bf6b5b4719
6 changed files with 208 additions and 7 deletions
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include/nlohmann/detail/view/value.hpp
+1 -1
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@@ -49,7 +49,7 @@ NLOHMANN_VIEW_ALWAYS_INLINE T arithmetic_value(const document_data& d, const nod
case value_t::number_integer:
return static_cast<T>(static_cast<typename BasicJsonType::number_integer_t>(static_cast<std::int64_t>(integer_bits(n))));
case value_t::number_float:
return static_cast<T>(float_value<typename BasicJsonType::number_float_t>(d.str(n), n));
return static_cast<T>(float_value<typename BasicJsonType::number_float_t>(d, n));
case value_t::boolean:
return static_cast<T>((n.flags & node_flags::is_true) != 0);
case value_t::null: