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Codechange: replace hand written function to find first/last bit with C++ variant

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This commit is contained in:
Rubidium
2024-01-18 18:40:52 +01:00
committed by rubidium42
parent 903119115b
commit 8faaedeff9
4 changed files with 31 additions and 60 deletions
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56
src/core/bitmath_func.cpp
View File

@@ -22,59 +22,3 @@ const uint8_t _ffb_64[64] = {
4, 0, 1, 0, 2, 0, 1, 0,
3, 0, 1, 0, 2, 0, 1, 0,
};
/**
* Search the first set bit in a 64 bit variable.
*
* This algorithm is a static implementation of a log
* congruence search algorithm. It checks the first half
* if there is a bit set search there further. And this
* way further. If no bit is set return 0.
*
* @param x The value to search
* @return The position of the first bit set
*/
uint8_t FindFirstBit(uint64_t x)
{
if (x == 0) return 0;
/* The macro FIND_FIRST_BIT is better to use when your x is
not more than 128. */
uint8_t pos = 0;
if ((x & 0xffffffffULL) == 0) { x >>= 32; pos += 32; }
if ((x & 0x0000ffffULL) == 0) { x >>= 16; pos += 16; }
if ((x & 0x000000ffULL) == 0) { x >>= 8; pos += 8; }
if ((x & 0x0000000fULL) == 0) { x >>= 4; pos += 4; }
if ((x & 0x00000003ULL) == 0) { x >>= 2; pos += 2; }
if ((x & 0x00000001ULL) == 0) { pos += 1; }
return pos;
}
/**
* Search the last set bit in a 64 bit variable.
*
* This algorithm is a static implementation of a log
* congruence search algorithm. It checks the second half
* if there is a bit set search there further. And this
* way further. If no bit is set return 0.
*
* @param x The value to search
* @return The position of the last bit set
*/
uint8_t FindLastBit(uint64_t x)
{
if (x == 0) return 0;
uint8_t pos = 0;
if ((x & 0xffffffff00000000ULL) != 0) { x >>= 32; pos += 32; }
if ((x & 0x00000000ffff0000ULL) != 0) { x >>= 16; pos += 16; }
if ((x & 0x000000000000ff00ULL) != 0) { x >>= 8; pos += 8; }
if ((x & 0x00000000000000f0ULL) != 0) { x >>= 4; pos += 4; }
if ((x & 0x000000000000000cULL) != 0) { x >>= 2; pos += 2; }
if ((x & 0x0000000000000002ULL) != 0) { pos += 1; }
return pos;
}

31
src/core/bitmath_func.hpp
View File

@@ -222,8 +222,35 @@ inline uint8_t FindFirstBit2x64(const int value)
}
}
uint8_t FindFirstBit(uint64_t x);
uint8_t FindLastBit(uint64_t x);
/**
* Search the first set bit in a value.
* When no bit is set, it returns 0.
*
* @param x The value to search.
* @return The position of the first bit set.
*/
template <typename T>
constexpr uint8_t FindFirstBit(T x)
{
if (x == 0) return 0;
return std::countr_zero(x);
}
/**
* Search the last set bit in a value.
* When no bit is set, it returns 0.
*
* @param x The value to search.
* @return The position of the last bit set.
*/
template <typename T>
constexpr uint8_t FindLastBit(T x)
{
if (x == 0) return 0;
return std::numeric_limits<T>::digits - std::countl_zero(x) - 1;
}
/**
* Clear the first bit in an integer.