From 0c6ed5929cb875e1507569424ab13036c5214f9b Mon Sep 17 00:00:00 2001 From: Marco Costalba Date: Sun, 19 Aug 2012 14:12:39 +0100 Subject: [PATCH] Document De Bruijn sequences Insted of raw magic numbers. No functional change. Signed-off-by: Marco Costalba --- src/bitboard.cpp | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/src/bitboard.cpp b/src/bitboard.cpp index 945169c3..e3fc93cc 100644 --- a/src/bitboard.cpp +++ b/src/bitboard.cpp @@ -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 } }; -- 2.39.2