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Merge pull request #426 from sahvx655-wq/power-negative-shift

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Avoid right-shifting a negative value in detail::power()
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Daniel Lemire 2026-10-17 08:54:58 -04:00 committed by GitHub
commit 638a8acd6d
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1 changed files with 7 additions and 1 deletions
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8
include/fast_float/decimal_to_binary.h
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@ -61,9 +61,15 @@ namespace detail {
* -ceil(p) + q
* where
* p = log(5**-q)/log(2) = -q * log(5)/log(2)
*
* The shift must floor towards negative infinity, as an arithmetic right
* shift does, but right-shifting a negative value is implementation-defined
* before C++20. Biasing the product by 2048 * 65536 keeps the shifted value
* non-negative without changing the result: the bias is a multiple of 2^16
* and is removed again after the shift.
*/
constexpr fastfloat_really_inline int32_t power(int32_t q) noexcept {
return (((152170 + 65536) * q) >> 16) + 63;
return (((152170 + 65536) * q + (2048 * 65536)) >> 16) - 2048 + 63;
}
} // namespace detail