int alpha_beta(app_t *app, cnodeptr_t parent, int alpha, int beta, int depth)
{
int palpha = alpha;
int i, score = -MATE, highest = -MATE;
node_t node;
move_t cutoff = 0;
piece_t p;
init_node(&node);
assert(app);
app->search.nodes++;
node.depth = depth;
/* max depth */
if (app->game.board->ply > (SEARCH_MAXDEPTH - 1)) {
/* return evaluate(app->board); */
return quiescent(app, parent, alpha, beta);
}
/* recursive base */
if (depth == 0) {
return evaluate(app->game.board);
}
/* draws */
if (repetitions(app) || (app->game.board->half >= 100)) {
return 0;
}
/* if we are checked, set the nodes checked flag */
if (check(app->game.board, app->game.board->side)) {
node.flags |= NODE_CHECK;
/* extend our search by 1 depth if we are in check */
/* NOTES: we may want to NOT extend our search here if the parent
is in check, because the means we already extended once */
depth++;
}
/* TODO:
- NULL moves
- Late-move reduction
- Tactical extensions (pins & forks -> depth++)
*/
/* probe our table */
if (probe_hash(&app->hash, app->game.board, &cutoff, &score, depth, alpha, beta) == TRUE) {
app->hash.cut++;
return score;
}
/* generate moves */
generate_moves(app->game.board, &node.ml, &node.ml);
/* reset score */
score = -MATE;
/* try to match our hash hit move */
if (cutoff != 0) {
for (i = 0; i < node.ml.count; i++) {
if (node.ml.moves[i] == cutoff) {
node.ml.scores[i] = 20000;
break;
}
}
}
/* search negamax */
for (i = 0; i < node.ml.count; i++) {
/* get the next move ordered */
next_move(i, &node.ml);
if (!(do_move(app->game.board, &app->game.undo, node.ml.moves[i])))
continue;
score = -alpha_beta(app, &node, -beta, -alpha, depth - 1);
node.made++;
undo_move(app->game.board, &app->game.undo);
/* score whatever is best so far */
if (score > highest) {
node.best = node.ml.moves[i];
highest = score;
/* update alpha */
if (score > alpha) {
if (score >= beta) {
/* non-captures causing beta cutoffs (killers) */
if (!is_capture(node.ml.moves[i])) {
app->game.board->killers[1][app->game.board->ply] =
app->game.board->killers[0][app->game.board->ply];
app->game.board->killers[0][app->game.board->ply] = node.ml.moves[i];
}
/* store this beta in our transposition table */
store_hash(&app->hash, app->game.board, node.best, beta, HASH_BETA, depth);
return beta;
}
/* update alpha */
alpha = score;
/* update our history */
if (!is_capture(node.best)) {
p = app->game.board->pos.squares[move_from(node.best)];
app->game.board->history[piece_color(p)][piece_type(p)][move_to(node.best)] += depth;
}
}
}
}
/* check for checkmate or stalemate */
if (!node.made) {
if (node.flags & NODE_CHECK) {
return -MATE + app->game.board->ply;
} else {
return 0;
}
}
if (alpha != palpha) {
/* store this as an exact, since we beat alpha */
store_hash(&app->hash, app->game.board, node.best, highest, HASH_EXACT, depth);
} else {
/* store the current alpha */
store_hash(&app->hash, app->game.board, node.best, alpha, HASH_ALPHA, depth);
}
return alpha;
}
uint64 perft(int depth)
{
int i,move_count;
move_t ms[MOVE_STACK];
uint64 nodes;
uint64 val;
#ifdef EVASIONS
char strbuff[256];
move_t ms_test[MOVE_STACK];
#endif
#ifdef BITS
bitboard_t bb;
#endif
if(depth == 0) return 1;
if((val = probe_hash(depth)) != 0)
return val;
nodes = 0;
val = 0;
#ifdef BITS
bb = 0;
for(i = PLS(WP); i <= H8; i = PLN(i))
bitset(&bb, rsz[i]);
if(bb != board->bb_pawns[W])
printf("pawn_error\n");
bb = 0;
for(i = PLS(BP); i <= H8; i = PLN(i))
bitset(&bb, rsz[i]);
if(bb != board->bb_pawns[B])
printf("pawn_error\n");
#endif
#ifdef EVASIONS
if(is_in_check(board->side))
{ move_count = move_gen_evasions(&ms[0]);
if(move_count != move_gen_legal(&ms_test[0]))
{ board_to_fen(&strbuff[0]);
printf("error: \n %s \n",strbuff);
}
}
else
{ move_count = move_gen(&ms[0]);
}
#else
move_count = move_gen(&ms[0]);
#endif
for(i = 0; i < move_count; i++)
{ if(!move_make(ms[i])) continue;
if(depth == 1)
nodes++;
else
nodes += perft(depth - 1);
move_undo();
}
record_hash(depth,nodes);
return (nodes);
}
