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Document De Bruijn sequences

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Insted of raw magic numbers.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2012-08-19 14:12:39 +01:00
parent 2f4a9a140a
commit 0c6ed5929c
1 changed files with 10 additions and 6 deletions
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16
src/bitboard.cpp
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@ -55,6 +55,10 @@ int SquareDistance[64][64];
namespace {
// De Bruijn sequences. See chessprogramming.wikispaces.com/BitScan
const uint64_t DeBruijn_64 = 0x218A392CD3D5DBFULL;
const uint32_t DeBruijn_32 = 0x783A9B23;
CACHE_LINE_ALIGNMENT
int BSFTable[64];
@ -77,11 +81,11 @@ namespace {
Square lsb(Bitboard b) {
if (Is64Bit)
return Square(BSFTable[((b & -b) * 0x218A392CD3D5DBFULL) >> 58]);
return Square(BSFTable[((b & -b) * DeBruijn_64) >> 58]);
b ^= (b - 1);
uint32_t fold = unsigned(b) ^ unsigned(b >> 32);
return Square(BSFTable[(fold * 0x783A9B23) >> 26]);
return Square(BSFTable[(fold * DeBruijn_32) >> 26]);
}
Square pop_lsb(Bitboard* b) {
@ -90,11 +94,11 @@ Square pop_lsb(Bitboard* b) {
*b = bb & (bb - 1);
if (Is64Bit)
return Square(BSFTable[((bb & -bb) * 0x218A392CD3D5DBFULL) >> 58]);
return Square(BSFTable[((bb & -bb) * DeBruijn_64) >> 58]);
bb ^= (bb - 1);
uint32_t fold = unsigned(bb) ^ unsigned(bb >> 32);
return Square(BSFTable[(fold * 0x783A9B23) >> 26]);
return Square(BSFTable[(fold * DeBruijn_32) >> 26]);
}
Square msb(Bitboard b) {
@ -203,10 +207,10 @@ void Bitboards::init() {
Bitboard b = 1ULL << i;
b ^= b - 1;
b ^= b >> 32;
BSFTable[(uint32_t)(b * 0x783A9B23) >> 26] = i;
BSFTable[(uint32_t)(b * DeBruijn_32) >> 26] = i;
}
else
BSFTable[((1ULL << i) * 0x218A392CD3D5DBFULL) >> 58] = i;
BSFTable[((1ULL << i) * DeBruijn_64) >> 58] = i;
int steps[][9] = { {}, { 7, 9 }, { 17, 15, 10, 6, -6, -10, -15, -17 },
{}, {}, {}, { 9, 7, -7, -9, 8, 1, -1, -8 } };