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Optimize count_digits
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@ -263,6 +263,13 @@ const typename basic_data<T>::digit_pair basic_data<T>::digits[] = {
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template <typename T>
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const char basic_data<T>::hex_digits[] = "0123456789abcdef";
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template <typename T>
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const uint16_t basic_data<T>::bsr2log10[] = {
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1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5,
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6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 9, 9, 9, 10, 10, 10,
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10, 11, 11, 11, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 15, 15,
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15, 16, 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19, 19, 20};
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#define FMT_POWERS_OF_10(factor) \
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factor * 10, (factor)*100, (factor)*1000, (factor)*10000, (factor)*100000, \
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(factor)*1000000, (factor)*10000000, (factor)*100000000, \
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@ -770,6 +770,8 @@ using uint32_or_64_or_128_t =
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// Static data is placed in this class template for the header-only config.
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template <typename T = void> struct FMT_EXTERN_TEMPLATE_API basic_data {
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// Maps the result of bsr(n) to ceil(log10(n)).
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static const uint16_t bsr2log10[];
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static const uint64_t powers_of_10_64[];
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static const uint32_t zero_or_powers_of_10_32[];
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static const uint64_t zero_or_powers_of_10_64[];
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@ -799,10 +801,9 @@ struct data : basic_data<> {};
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// Returns the number of decimal digits in n. Leading zeros are not counted
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// except for n == 0 in which case count_digits returns 1.
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inline int count_digits(uint64_t n) {
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// Based on http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog10
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// and the benchmark https://github.com/localvoid/cxx-benchmark-count-digits.
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int t = ((FMT_BUILTIN_CLZLL(n | 1) ^ 63) + 1) * 1233 >> 12;
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return t - (n < data::zero_or_powers_of_10_64[t]) + 1;
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// https://github.com/fmtlib/format-benchmark/blob/master/digits10
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auto t = data::bsr2log10[FMT_BUILTIN_CLZLL(n | 1) ^ 63];
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return t - (n < data::zero_or_powers_of_10_64[t - 1]);
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}
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#else
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// Fallback version of count_digits used when __builtin_clz is not available.
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@ -859,8 +860,8 @@ template <> int count_digits<4>(detail::fallback_uintptr n);
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#ifdef FMT_BUILTIN_CLZ
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// Optional version of count_digits for better performance on 32-bit platforms.
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inline int count_digits(uint32_t n) {
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int t = ((FMT_BUILTIN_CLZ(n | 1) ^ 31) + 1) * 1233 >> 12;
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return t - (n < data::zero_or_powers_of_10_32[t]) + 1;
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auto t = data::bsr2log10[FMT_BUILTIN_CLZ(n | 1) ^ 31];
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return t - (n < data::zero_or_powers_of_10_32[t - 1]);
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}
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#endif
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