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Cleanup digit count

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This commit is contained in:
Victor Zverovich 2021-06-07 07:42:22 -07:00
parent 1de80f5b22
commit 0e36681b8e
1 changed files with 11 additions and 12 deletions
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23
include/fmt/format.h
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@ -897,10 +897,8 @@ FMT_CONSTEXPR inline auto count_digits(uint128_t n) -> int {
// Returns the number of decimal digits in n. Leading zeros are not counted
// except for n == 0 in which case count_digits returns 1.
FMT_CONSTEXPR20 inline auto count_digits(uint64_t n) -> int {
if (is_constant_evaluated()) {
return count_digits_fallback(n);
}
#ifdef FMT_BUILTIN_CLZLL
if (!is_constant_evaluated()) {
// https://github.com/fmtlib/format-benchmark/blob/master/digits10
// Maps bsr(n) to ceil(log10(pow(2, bsr(n) + 1) - 1)).
FMT_STATIC_CONSTEXPR uint16_t bsr2log10[] = {
@ -913,9 +911,9 @@ FMT_CONSTEXPR20 inline auto count_digits(uint64_t n) -> int {
0, 0, FMT_POWERS_OF_10(1U), FMT_POWERS_OF_10(1000000000ULL),
10000000000000000000ULL};
return t - (n < zero_or_powers_of_10[t]);
#else
return count_digits_fallback(n);
}
#endif
return count_digits_fallback(n);
}
// Counts the number of digits in n. BITS = log2(radix).
@ -934,14 +932,12 @@ FMT_CONSTEXPR auto count_digits(UInt n) -> int {
template <> auto count_digits<4>(detail::fallback_uintptr n) -> int;
#ifdef FMT_BUILTIN_CLZ
// It is a separate function rather than a part of count_digits to workaround
// the lack of static constexpr in constexpr functions.
FMT_INLINE uint64_t count_digits_inc(int n) {
// An optimization by Kendall Willets from https://bit.ly/3uOIQrB.
// This increments the upper 32 bits (log10(T) - 1) when >= T is added.
# define FMT_INC(T) (((sizeof(# T) - 1ull) << 32) - T)
#define FMT_INC(T) (((sizeof(#T) - 1ull) << 32) - T)
static constexpr uint64_t table[] = {
FMT_INC(0), FMT_INC(0), FMT_INC(0), // 8
FMT_INC(10), FMT_INC(10), FMT_INC(10), // 64
@ -960,11 +956,14 @@ FMT_INLINE uint64_t count_digits_inc(int n) {
// Optional version of count_digits for better performance on 32-bit platforms.
FMT_CONSTEXPR20 inline auto count_digits(uint32_t n) -> int {
if (is_constant_evaluated()) return count_digits_fallback(n);
auto inc = count_digits_inc(FMT_BUILTIN_CLZ(n | 1) ^ 31);
return static_cast<int>((n + inc) >> 32);
}
#ifdef FMT_BUILTIN_CLZ
if (!is_constant_evaluated()) {
auto inc = count_digits_inc(FMT_BUILTIN_CLZ(n | 1) ^ 31);
return static_cast<int>((n + inc) >> 32);
}
#endif
return count_digits_fallback(n);
}
template <typename Int> constexpr auto digits10() FMT_NOEXCEPT -> int {
return std::numeric_limits<Int>::digits10;