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Document magics bitboards code

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Add comments and rename stuff to better clarify what the
magic bitboard initialization code does.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2011-10-31 15:37:46 +01:00
parent 8a89b12641
commit 90890844ad
2 changed files with 73 additions and 49 deletions
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98
src/bitboard.cpp
View file

@ -27,15 +27,15 @@
// Global bitboards definitions with static storage duration are
// automatically set to zero before enter main().
Bitboard RMask[64];
Bitboard RMult[64];
Bitboard RMasks[64];
Bitboard RMagics[64];
Bitboard* RAttacks[64];
int RShift[64];
int RShifts[64];
Bitboard BMask[64];
Bitboard BMult[64];
Bitboard BMasks[64];
Bitboard BMagics[64];
Bitboard* BAttacks[64];
int BShift[64];
int BShifts[64];
Bitboard SetMaskBB[65];
Bitboard ClearMaskBB[65];
@ -64,11 +64,11 @@ namespace {
CACHE_LINE_ALIGNMENT
int BSFTable[64];
Bitboard RAttacksTable[0x19000];
Bitboard BAttacksTable[0x1480];
Bitboard RookTable[0x19000]; // Storage space for rook attacks
Bitboard BishopTable[0x1480]; // Storage space for bishop attacks
void init_sliding_attacks(Bitboard magic[], Bitboard* attack[], Bitboard attTable[],
Bitboard mask[], int shift[], Square delta[]);
void init_magic_bitboards(Bitboard* attacks[], Bitboard magics[],
Bitboard masks[], int shifts[], Square deltas[]);
}
@ -228,11 +228,14 @@ void init_bitboards() {
set_bit(&StepAttacksBB[make_piece(c, pt)][s], to);
}
Square RDelta[] = { DELTA_N, DELTA_E, DELTA_S, DELTA_W };
Square BDelta[] = { DELTA_NE, DELTA_SE, DELTA_SW, DELTA_NW };
Square RDeltas[] = { DELTA_N, DELTA_E, DELTA_S, DELTA_W };
Square BDeltas[] = { DELTA_NE, DELTA_SE, DELTA_SW, DELTA_NW };
init_sliding_attacks(BMult, BAttacks, BAttacksTable, BMask, BShift, BDelta);
init_sliding_attacks(RMult, RAttacks, RAttacksTable, RMask, RShift, RDelta);
RAttacks[0] = RookTable;
BAttacks[0] = BishopTable;
init_magic_bitboards(RAttacks, RMagics, RMasks, RShifts, RDeltas);
init_magic_bitboards(BAttacks, BMagics, BMasks, BShifts, BDeltas);
for (Square s = SQ_A1; s <= SQ_H8; s++)
{
@ -258,28 +261,28 @@ void init_bitboards() {
namespace {
Bitboard sliding_attacks(Square sq, Bitboard occupied, Square delta[]) {
Bitboard sliding_attacks(Square sq, Bitboard occupied, Square deltas[]) {
Bitboard attacks = 0;
for (int i = 0; i < 4; i++)
{
Square s = sq + delta[i];
Square s = sq + deltas[i];
while (square_is_ok(s) && square_distance(s, s - delta[i]) == 1)
while (square_is_ok(s) && square_distance(s, s - deltas[i]) == 1)
{
set_bit(&attacks, s);
if (bit_is_set(occupied, s))
break;
s += delta[i];
s += deltas[i];
}
}
return attacks;
}
Bitboard pick_magic(Bitboard mask, RKISS& rk, int booster) {
Bitboard pick_random(Bitboard mask, RKISS& rk, int booster) {
Bitboard magic;
@ -300,48 +303,69 @@ namespace {
}
}
void init_sliding_attacks(Bitboard magic[], Bitboard* attack[], Bitboard attTable[],
Bitboard mask[], int shift[], Square delta[]) {
// init_magic_bitboards() computes all rook and bishop magics at startup.
// Magic bitboards are used to look up attacks of sliding pieces. As reference
// see chessprogramming.wikispaces.com/Magic+Bitboards. In particular, here we
// use the so called "fancy" approach.
void init_magic_bitboards(Bitboard* attacks[], Bitboard magics[],
Bitboard masks[], int shifts[], Square deltas[]) {
const int MagicBoosters[][8] = { { 3191, 2184, 1310, 3618, 2091, 1308, 2452, 3996 },
{ 1059, 3608, 605, 3234, 3326, 38, 2029, 3043 } };
RKISS rk;
Bitboard occupancy[4096], reference[4096], edges, b;
int key, maxKey, index, booster, offset = 0;
int key, maxKey, index, booster;
for (Square s = SQ_A1; s <= SQ_H8; s++)
{
// Board edges are not considered in the relevant occupancies
edges = ((Rank1BB | Rank8BB) & ~rank_bb(s)) | ((FileABB | FileHBB) & ~file_bb(s));
attack[s] = &attTable[offset];
mask[s] = sliding_attacks(s, EmptyBoardBB, delta) & ~edges;
shift[s] = (CpuIs64Bit ? 64 : 32) - count_1s<CNT32_MAX15>(mask[s]);
// Given a square 's', the mask is the bitboard of sliding attacks from
// 's' computed on an empty board. The index must be big enough to contain
// all the attacks for each possible subset of the mask and so is 2 power
// the number of 1s of the mask. Hence we deduce the size of the shift to
// apply to the 64 or 32 bits word to get the index.
masks[s] = sliding_attacks(s, EmptyBoardBB, deltas) & ~edges;
shifts[s] = (CpuIs64Bit ? 64 : 32) - count_1s<CNT32_MAX15>(masks[s]);
// Use Carry-Rippler trick to enumerate all subsets of mask[s]
// Use Carry-Rippler trick to enumerate all subsets of masks[s] and
// store the corresponding sliding attacks in reference[].
b = maxKey = 0;
do {
occupancy[maxKey] = b;
reference[maxKey++] = sliding_attacks(s, b, delta);
b = (b - mask[s]) & mask[s];
reference[maxKey++] = sliding_attacks(s, b, deltas);
b = (b - masks[s]) & masks[s];
} while (b);
offset += maxKey;
// Set the offset for the table of the next square. We have individual
// table sizes for each square with "Fancy Magic Bitboards".
if (s < SQ_H8)
attacks[s + 1] = attacks[s] + maxKey;
booster = MagicBoosters[CpuIs64Bit][rank_of(s)];
// Then find a possible magic and the corresponding attacks
// Find a magic for square 's' picking up an (almost) random number
// until we find the one that passes the verification test.
do {
magic[s] = pick_magic(mask[s], rk, booster);
memset(attack[s], 0, maxKey * sizeof(Bitboard));
magics[s] = pick_random(masks[s], rk, booster);
memset(attacks[s], 0, maxKey * sizeof(Bitboard));
// A good magic must map every possible occupancy to an index that
// looks up the correct sliding attack in the attacks[s] database.
// Note that we build up the database for square 's' as a side
// effect of verifying the magic.
for (key = 0; key < maxKey; key++)
{
index = CpuIs64Bit ? unsigned((occupancy[key] * magic[s]) >> shift[s])
: unsigned(occupancy[key] * magic[s] ^ (occupancy[key] >> 32) * (magic[s] >> 32)) >> shift[s];
index = CpuIs64Bit ? unsigned((occupancy[key] * magics[s]) >> shifts[s])
: unsigned(occupancy[key] * magics[s] ^ (occupancy[key] >> 32) * (magics[s] >> 32)) >> shifts[s];
if (!attack[s][index])
attack[s][index] = reference[key];
if (!attacks[s][index])
attacks[s][index] = reference[key];
else if (attack[s][index] != reference[key])
else if (attacks[s][index] != reference[key])
break;
}
} while (key != maxKey);

24
src/bitboard.h
View file

@ -60,14 +60,14 @@ extern Bitboard SquaresInFrontMask[2][64];
extern Bitboard PassedPawnMask[2][64];
extern Bitboard AttackSpanMask[2][64];
extern uint64_t RMult[64];
extern int RShift[64];
extern Bitboard RMask[64];
extern uint64_t RMagics[64];
extern int RShifts[64];
extern Bitboard RMasks[64];
extern Bitboard* RAttacks[64];
extern uint64_t BMult[64];
extern int BShift[64];
extern Bitboard BMask[64];
extern uint64_t BMagics[64];
extern int BShifts[64];
extern Bitboard BMasks[64];
extern Bitboard* BAttacks[64];
extern Bitboard BishopPseudoAttacks[64];
@ -172,25 +172,25 @@ inline Bitboard in_front_bb(Color c, Square s) {
#if defined(IS_64BIT)
inline Bitboard rook_attacks_bb(Square s, Bitboard occ) {
return RAttacks[s][((occ & RMask[s]) * RMult[s]) >> RShift[s]];
return RAttacks[s][((occ & RMasks[s]) * RMagics[s]) >> RShifts[s]];
}
inline Bitboard bishop_attacks_bb(Square s, Bitboard occ) {
return BAttacks[s][((occ & BMask[s]) * BMult[s]) >> BShift[s]];
return BAttacks[s][((occ & BMasks[s]) * BMagics[s]) >> BShifts[s]];
}
#else // if !defined(IS_64BIT)
inline Bitboard rook_attacks_bb(Square s, Bitboard occ) {
Bitboard b = occ & RMask[s];
Bitboard b = occ & RMasks[s];
return RAttacks[s]
[unsigned(int(b) * int(RMult[s]) ^ int(b >> 32) * int(RMult[s] >> 32)) >> RShift[s]];
[unsigned(int(b) * int(RMagics[s]) ^ int(b >> 32) * int(RMagics[s] >> 32)) >> RShifts[s]];
}
inline Bitboard bishop_attacks_bb(Square s, Bitboard occ) {
Bitboard b = occ & BMask[s];
Bitboard b = occ & BMasks[s];
return BAttacks[s]
[unsigned(int(b) * int(BMult[s]) ^ int(b >> 32) * int(BMult[s] >> 32)) >> BShift[s]];
[unsigned(int(b) * int(BMagics[s]) ^ int(b >> 32) * int(BMagics[s] >> 32)) >> BShifts[s]];
}
#endif