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Optimize format_decimal

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This commit is contained in:
vitaut 2015-08-05 07:50:29 -07:00
parent ecdc7ec1f1
commit 58fcf0c0da
1 changed files with 6 additions and 7 deletions
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13
format.h
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@ -712,24 +712,23 @@ inline unsigned count_digits(uint32_t n) {
// Formats a decimal unsigned integer value writing into buffer.
template <typename UInt, typename Char>
inline void format_decimal(Char *buffer, UInt value, unsigned num_digits) {
--num_digits;
buffer += num_digits;
while (value >= 100) {
// Integer division is slow so do it for a group of two digits instead
// of for every digit. The idea comes from the talk by Alexandrescu
// "Three Optimization Tips for C++". See speed-test for a comparison.
unsigned index = (value % 100) * 2;
value /= 100;
buffer[num_digits] = Data::DIGITS[index + 1];
buffer[num_digits - 1] = Data::DIGITS[index];
num_digits -= 2;
*--buffer = Data::DIGITS[index + 1];
*--buffer = Data::DIGITS[index];
}
if (value < 10) {
*buffer = static_cast<char>('0' + value);
*--buffer = static_cast<char>('0' + value);
return;
}
unsigned index = static_cast<unsigned>(value * 2);
buffer[1] = Data::DIGITS[index + 1];
buffer[0] = Data::DIGITS[index];
*--buffer = Data::DIGITS[index + 1];
*--buffer = Data::DIGITS[index];
}
#ifndef _WIN32