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Use "table look-up" method for reversing byte

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This method takes constant time and is less prone to (CPU) pipeline stalling due to less
computation. Also it is slightly faster than the previous method.
This commit is contained in:
Chocobo1 2022-02-09 18:30:05 +08:00
parent cf96e6c642
commit 4cb075b168
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1 changed files with 12 additions and 2 deletions
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14
src/base/bittorrent/ltqbitarray.cpp
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@ -37,8 +37,18 @@ namespace
{
unsigned char reverseByte(const unsigned char byte)
{
// https://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith64Bits
return (((byte * 0x80200802ULL) & 0x0884422110ULL) * 0x0101010101ULL) >> 32;
// https://graphics.stanford.edu/~seander/bithacks.html#BitReverseTable
static const unsigned char table[] =
{
#define R2(n) n, (n + (2 * 64)), (n + 64), (n + (3 * 64))
#define R4(n) R2(n), R2(n + (2 * 16)), R2(n + 16), R2(n + (3 * 16))
#define R6(n) R4(n), R4(n + (2 * 4)), R4(n + 4), R4(n + (3 * 4))
R6(0), R6(2), R6(1), R6(3)
#undef R6
#undef R4
#undef R2
};
return table[byte];
}
}