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>
Niels Lohmann
bf6b5b4719