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Adding an explanation.

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Daniel Lemire 2020-10-08 10:58:42 -04:00
parent 4a3e34600a
commit ef3f98f82d
1 changed files with 18 additions and 0 deletions
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include/fast_double_parser.h
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@ -68,6 +68,24 @@ double cygwin_strtod_l(const char* start, char** end) {
namespace fast_double_parser {
/**
* The smallest non-zero float (binary64) is 2^−1074.
* We take as input numbers of the form w x 10^q where w < 2^64.
* We have that w * 10^-343 < 2^(64-344) 5^-343 < 2^-1076.
* However, we have that
* (2^64-1) * 10^-342 = (2^64-1) * 2^-342 * 5^-342 > 2^−1074.
* Thus it is possible for a number of the form w * 10^-342 where
* w is a 64-bit value to be a non-zero floating-point number.
*********
* If we are solely interested in the *normal* numbers then the
* smallest value is 2^-1022. We can generate a value larger
* than 2^-1022 with expressions of the form w * 10^-326.
* Thus we need to pick FASTFLOAT_SMALLEST_POWER >= -326.
*********
* Any number of form w * 10^309 where w>= 1 is going to be
* infinite in binary64 so we never need to worry about powers
* of 5 greater than 308.
*/
#define FASTFLOAT_SMALLEST_POWER -325
#define FASTFLOAT_LARGEST_POWER 308