int ZWS(int beta, int depth, int can_reduce)
{
if(is_draw_by_repetition_or_50_moves()) return DRAW;
Entry *entry = hash_get_entry();
if(entry != NULL && entry->depth >= depth)
{
int eval = entry->eval, flag = entry->flag;
if(flag != LESS_THAN_ALPHA && eval >= beta)
return beta;
if(flag != MORE_THAN_BETA && eval < beta)
return beta - 1;
}
if(depth == 0) return quiescence(beta - 1, beta);
/*if(depth <= 6 && evaluate(beta - 1, beta) >= beta)
{
return beta;
}*/
if(depth > 2 && can_reduce && !in_check(turn_to_move))
{
int R = depth > 6 ? 3: 2;
make_null_move();
int score = -ZWS(-beta + 1, depth - R - 1, 0);
unmake_null_move();
if(score >= beta) return beta;
}
Move movelist[256];
int n = generate_moves(movelist);
if(n == 0)
{
if(!in_check(turn_to_move))
return DRAW;
return LOSING + ply - begin_ply;
}
sorting_moves(movelist, n);
for(int i = 0; i < n; i += 1)
{
Move i_move = movelist[i];
make_move(i_move);
int score = -ZWS(-beta + 1, depth - 1, can_reduce);
unmake_move(i_move);
if(score >= beta)
{
hash_save_entry(depth, beta, i_move, MORE_THAN_BETA);
return beta;
}
}
hash_save_entry(depth, beta - 1, 0, LESS_THAN_ALPHA);
return beta - 1;
}
t_chess_value alphabeta(struct t_board *board, int ply, int depth, t_chess_value alpha, t_chess_value beta, BOOL early_cutoff, struct t_move_record *exclude_move) {
//-- Should we call qsearch?
if (depth <= 0 && !board->in_check)
return qsearch_plus(board, ply, depth, alpha, beta);
//-- Increment the nodes
nodes++;
//-- see if we need to update stats */
if ((nodes & message_update_mask) == 0)
uci_check_status(board, ply);
//-- Local Principle Variation variable
struct t_pv_data *pv = &(board->pv_data[ply]);
//-- Has the maximum depth been reached
if (ply >= MAXPLY || uci.stop) {
pv->best_line_length = ply;
assert(pv->eval->static_score >= -CHECKMATE && pv->eval->static_score <= CHECKMATE);
return pv->eval->static_score;
}
/* check to see if this is a repeated position or draw by 50 moves */
if (repetition_draw(board)) {
pv->best_line_length = ply;
return 0;
}
//-- Mate Distance Pruning
if (CHECKMATE - ply <= alpha) {
return alpha;
}
else if (-CHECKMATE + ply >= beta) {
return beta;
}
else if ((!board->in_check) && (-CHECKMATE + ply + 2 >= beta)) {
return beta;
}
//-- Declare local variables
struct t_pv_data *next_pv = pv->next_pv;
struct t_pv_data *previous_pv = pv->previous_pv;
t_chess_value best_score = -CHECKMATE;
t_chess_value a = alpha;
t_chess_value b = beta;
//-- Determine what type of node it is
if (beta > alpha + 1)
pv->node_type = node_pv;
else if (previous_pv->node_type == node_pv)
pv->node_type = node_lite_all;
else if (previous_pv->node_type == node_cut)
pv->node_type = node_all;
else
pv->node_type = node_cut;
//-- Probe Hash
struct t_move_record *hash_move = NULL;
struct t_hash_record *hash_record = probe(board->hash);
//-- Has there been a match?
if (hash_record != NULL) {
//-- Get the score from the hash table
t_chess_value hash_score = get_hash_score(hash_record, ply);
//-- Could it make a cut-off?
if (early_cutoff && hash_record->depth >= depth) {
//-- Score in hash table is at least as good as beta
if (hash_record->bound != HASH_UPPER && hash_score >= beta) {
hash_record->age = hash_age;
assert(hash_score >= -CHECKMATE && hash_score <= CHECKMATE);
return hash_score;
}
//-- Score is worse than alpha
if (hash_record->bound != HASH_LOWER && hash_score <= alpha) {
hash_record->age = hash_age;
assert(hash_score >= -CHECKMATE && hash_score <= CHECKMATE);
return hash_score;
}
//-- Score is more accurate
if (hash_record->bound == HASH_EXACT) {
hash_record->age = hash_age;
pv->best_line_length = ply;
update_best_line_from_hash(board, ply);
assert(hash_score >= -CHECKMATE && hash_score <= CHECKMATE);
return hash_score;
}
}
//-- Store the hash move for further use!
hash_move = hash_record->move;
//-- Use the hash score to refine the node type
if (hash_record->bound != HASH_UPPER && hash_score >= beta)
pv->node_type = node_super_cut;
else if (hash_record->bound != HASH_LOWER && hash_score <= alpha)
pv->node_type = node_super_all;
else if (hash_record->bound == HASH_EXACT && pv->node_type == node_all)
pv->node_type = node_lite_all;
}
//-- Beta pruning
if (early_cutoff && depth <= 4 && pv->node_type != node_pv && beta < MAX_CHECKMATE && beta > -MAX_CHECKMATE && !board->in_check) {
int pessimistic_score = pv->eval->static_score - depth * 50 - 100;
if (pessimistic_score >= beta)
return pessimistic_score;
}
//-- Razoring.
t_chess_value e;
if (early_cutoff && (depth <= 4) && pv->node_type != node_pv && !board->in_check){
t_chess_value razor_margin = depth * 50 + 50;
if (pv->eval->static_score + razor_margin <= alpha){
t_chess_value razor_alpha = alpha - razor_margin;
e = qsearch_plus(board, ply, depth, razor_alpha, razor_alpha + 1);
if (e <= razor_alpha)
return e;
}
}
//-- Null Move
t_undo undo[1];
pv->mate_threat = 0;
pv->null_refutation = NULL;
pv->extension = FALSE;
if (early_cutoff && can_do_null_move(board, pv, ply, alpha, beta)) {
//-- Calculate Reduction
//int r = (800 + 70 * depth) / 256 + min(3, (pv->eval->static_score - beta) / 128);
int r = min(4, 2 + (25 * depth) / 128 + (pv->eval->static_score - beta) / 128);
//int r = 3;
//-- Make the changes on the board
make_null_move(board, undo);
//-- Store the move in the PV data
pv->current_move = NULL;
//-- Clear the Killer +2
if (ply + 2 <= MAXPLY){
board->pv_data[ply + 2].killer1 = NULL;
board->pv_data[ply + 2].killer2 = NULL;
}
//-- Evaluate the new board position
evaluate(board, next_pv->eval);
//-- Find the new score
e = -alphabeta(board, ply + 1, depth - r - 1 , -beta, -beta + 1, TRUE, NULL);
//-- undo the null move
unmake_null_move(board, undo);
//-- is it good enough for a cut-off?
if (e >= beta) {
if (e > MAX_CHECKMATE)
e = beta;
poke(board->hash, e, ply, depth, HASH_LOWER, NULL);
return e;
}
//-- Is there a Mate Threat after a super-reduced move - if so then exit?
if (e < -MAX_CHECKMATE){
if (pv->previous_pv->reduction > 1)
return alpha;
pv->mate_threat = e;
}
//-- Record the move which refuted the NULL move
if (ply < MAXPLY)
pv->null_refutation = board->pv_data[ply + 1].current_move;
}
//-- Internal Iterative Deepening!
if (hash_move == NULL && !uci.stop){
//-- PV Nodes - we *really* need a good move
if (pv->node_type == node_pv && depth > 2) {
//-- Search with reduced depth
e = alphabeta(board, ply, depth - 2, alpha, beta, FALSE, NULL);
//-- If still no move then search with -INFINITY bound
if (e <= alpha)
e = alphabeta(board, ply, depth - 2, -CHESS_INFINITY, beta, FALSE, NULL);
//-- Probe the hash
hash_record = probe(board->hash);
//-- Set the hash move
if (hash_record != NULL)
hash_move = hash_record->move;
}
//-- Fail high nodes
//else if ((pv->node_type == node_cut || pv->node_type == node_super_cut) && (depth >= 7) && alpha > -MAX_CHECKMATE && beta < MAX_CHECKMATE){
// //-- Search with reduced depth
// e = alphabeta(board, ply, depth / 2, alpha, beta);
// //-- If still no move then search with -INFINITY bound
// if (e <= alpha)
// e = alphabeta(board, ply, depth / 2, -CHESS_INFINITY, beta);
// //-- Probe the hash
// hash_record = probe(board->hash);
// //-- Set the hash move
// if (hash_record != NULL)
// hash_move = hash_record->move;
//}
}
//-- Generate All Moves
struct t_move_list moves[1];
moves->hash_move = hash_move;
if (board->in_check) {
generate_evade_check(board, moves);
// Are we in checkmate?
if (moves->count == 0) {
pv->best_line_length = ply;
e = -CHECKMATE + ply;
return e;
}
order_evade_check(board, moves, ply);
}
else {
generate_moves(board, moves);
order_moves(board, moves, ply);
}
//-- Enhanced Transposition Cutoff?
t_chess_color to_move = board->to_move;
if (early_cutoff && (depth > 4) && pv->node_type != node_pv && beta < MAX_CHECKMATE && alpha > -MAX_CHECKMATE && !uci.stop) {
BOOL fail_low;
while (simple_make_next_move(board, moves, undo)) {
//-- Calculate Reduction Conservatively i.e. assume minimum reduction
if (board->in_check)
pv->reduction = 0;
else if (PIECETYPE(moves->current_move->piece) == PAWN && COLOR_RANK(to_move, moves->current_move->to_square) >= 6)
pv->reduction = 0;
else
pv->reduction = 1;
//-- Simple version of alpha_beta for tips of search
e = -alphabeta_tip(board, ply + 1, depth - pv->reduction, -beta, &fail_low);
//-- Take the move back
unmake_move(board, undo);
//-- Is it good enough for a cutoff?
if (e >= beta) {
poke(board->hash, e, ply, depth, HASH_LOWER, moves->current_move);
assert(e >= -CHECKMATE && e <= CHECKMATE);
return e;
}
//-- Is it going to enhance the move ordering?
if (fail_low) {
moves->value[moves->imove] += MOVE_ORDER_ETC;
assert(moves->move[moves->imove] == moves->current_move);
}
}
//-- Reset the real move count for the "proper" search
moves->imove = moves->count;
}
////-- No Hash Move for Cut Nodes with score below beta and no good capture and no ETC
//if (hash_move == NULL && (any_fail_low == FALSE) && ((pv->node_type == node_cut) || (pv->node_type == node_super_all)) && (depth >= 3) && (beta > pv->eval->static_score) && no_good_captures(board, moves)){
// //-- Search with reduced depth
// e = alphabeta(board, ply, depth - 3, alpha, beta);
// //-- If still no move then search with -INFINITY bound
// if (e <= alpha)
// e = alphabeta(board, ply, depth - 3, -CHESS_INFINITY, beta);
// //-- Probe the hash
// hash_record = probe(board->hash);
// //-- Set the hash move
// if (hash_record != NULL)
// hash_move = hash_record->move;
//}
//-- Create the list of "bad" captures
struct t_move_list bad_moves[1];
bad_moves->count = 0;
bad_moves->imove = 0;
//-- Reset the move count (must be after IID)
pv->legal_moves_played = 0;
//-- Variables used to calculate the reduction
t_chess_color opponent = OPPONENT(to_move);
BOOL in_check = board->in_check;
struct t_move_record *last_move = NULL;
if (ply > 1)
last_move = board->pv_data[ply - 2].current_move;
//-- Play moves
while (!uci.stop && make_next_move(board, moves, bad_moves, undo)) {
//-- Increment the "legal_moves_played" counter
pv->legal_moves_played++;
pv->current_move = moves->current_move;
//========================================//
// Futility Pruning
//========================================//
if (uci.options.futility_pruning && is_futile(pv, next_pv, depth, a, b)){
unmake_move(board, undo);
continue;
}
//-- Clear the Killer +2
if (ply + 2 <= MAXPLY){
board->pv_data[ply + 2].killer1 = NULL;
board->pv_data[ply + 2].killer2 = NULL;
}
//-- Evaluate the new board position
evaluate(board, next_pv->eval);
////========================================//
//// See if Extension is Necessary
////========================================//
//pv->extension = FALSE;
//if (pv->mate_threat){
// e = -alphabeta(board, ply + 1, depth - 1, -CHECKMATE, pv->mate_threat + 2);
// pv->extension = (e > pv->mate_threat);
// if (e <= pv->mate_threat && e <= a)
// {
// unmake_move(board, undo);
// continue;
// }
//}
//========================================//
// Calculate reduction
//========================================//
struct t_move_record *current_move = pv->current_move;
//-- In Check?
if (board->in_check){
if (see_safe(board, current_move->to_square, 0))
pv->reduction = 0;
else if (ply > 3 && board->pv_data[ply - 1].in_check && board->pv_data[ply - 3].in_check)
pv->reduction = 0;
else
pv->reduction = 1;
}
//-- Pawn push to 7th
else if (PIECETYPE(current_move->piece) == PAWN && COLOR_RANK(to_move, current_move->to_square) >= 6){
//-- Pawn Promotion
if (current_move->promote_to){
if (pv->legal_moves_played == 1 || PIECETYPE(current_move->promote_to) == QUEEN){
//--Extend if it's a safe pawn promotion or first move
if ((pv->current_move->captured && moves->current_move_see_positive) || see_safe(board, current_move->to_square, 0))
pv->reduction = 0;
else
pv->reduction = 1;
}
// Reduce Heavily if not a queen promotion
else
pv->reduction = 5;
}
//-- Push to the 7th
else if (pv->legal_moves_played == 1 || (pv->current_move->captured && moves->current_move_see_positive) || see_safe(board, current_move->to_square, 0))
pv->reduction = 0;
else
pv->reduction = 1;
}
//-- First Move?
else if (pv->legal_moves_played == 1)
pv->reduction = 1;
////-- Under Threat of Mate
//else if (pv->mate_threat)
// pv->reduction = 1;
//-- Good Capture?
else if (pv->current_move->captured && moves->current_move_see_positive){
pv->reduction = 1;
}
//-- Is this getting out of check?
else if (in_check){
if (pv->current_move == pv->check_killer1)
pv->reduction = 1;
else if (PIECETYPE(current_move->piece) == KING){
if (CAN_CASTLE(to_move, board->castling))
pv->reduction = 4;
else
pv->reduction = 1;
}
else if (current_move->captured) /* must be a bad capture */
pv->reduction = 2;
else if (see_safe(board, current_move->to_square, 0))
pv->reduction = 1;
else
pv->reduction = 2;
}
//-- Don't reduce Killers!
else if (pv->current_move == pv->killer1){
pv->reduction = 1;
}
//-- Does it move a threatened piece?
else if (pv->null_refutation != NULL && pv->null_refutation->to_square == pv->current_move->from_square){
if (see_safe(board, current_move->to_square, 0))
pv->reduction = 1;
else
pv->reduction = 3;
}
//-- Candidate for serious reductions
else{
switch (pv->node_type)
{
case node_cut:
pv->reduction = 3;
if (pv->current_move->captured)
pv->reduction += 1;
break;
case node_super_cut:
pv->reduction = 4;
if (pv->current_move->captured)
pv->reduction += 1;
break;
case node_pv:
if (pv->legal_moves_played > 2)
pv->reduction = 2;
else
pv->reduction = 1;
break;
case node_lite_all:
if (pv->legal_moves_played > 2)
pv->reduction = 2;
else
pv->reduction = 1;
if (pv->current_move->captured)
pv->reduction += 1;
break;
case node_super_all:
if (current_move->captured){
if (pv->legal_moves_played < 4)
pv->reduction = 3;
else
pv->reduction = 4;
}
else if (!see_safe(board, current_move->to_square, 0)){
if (pv->legal_moves_played < 4)
pv->reduction = 4;
else if (pv->legal_moves_played < 12)
pv->reduction = 5;
else
pv->reduction = 6;
}
else if (pv->legal_moves_played < 4)
pv->reduction = 2;
else if (pv->legal_moves_played < 12)
pv->reduction = 3;
else
pv->reduction = 4;
break;
case node_all:
if (current_move->captured){
if (pv->legal_moves_played < 4)
pv->reduction = 3;
else
pv->reduction = 4;
}
else if (!see_safe(board, current_move->to_square, 0)){
if (pv->legal_moves_played < 4)
pv->reduction = 4;
else
pv->reduction = 5;
}
else if (pv->legal_moves_played < 4)
pv->reduction = 2;
else if (pv->legal_moves_played < 18)
pv->reduction = 3;
else
pv->reduction = 4;
break;
}
}
//-- Search the next ply at reduced depth
e = -alphabeta(board, ply + 1, depth - pv->reduction, -b, -a, TRUE, NULL);
//-- Fail high on a super-reduced move?
if (e > a && pv->reduction > 1) {
pv->reduction = 1;
//-- Search again using the full width
e = -alphabeta(board, ply + 1, depth - 1, -beta, -a, TRUE, NULL);
}
//-- Is a research required?
else if (alpha + 1 != beta && e > a && a + 1 == b)
e = -alphabeta(board, ply + 1, depth - pv->reduction, -beta, -a, TRUE, NULL);
unmake_move(board, undo);
//-- Is it good enough to cut-off?
if (e >= beta) {
if (board->in_check)
update_check_killers(pv, depth);
else
update_killers(pv, depth);
//-- Record the cutoff
cutoffs++;
if (pv->legal_moves_played == 1)
first_move_cutoffs++;
//-- Store in the hash table
poke(board->hash, e, ply, depth, HASH_LOWER, pv->current_move);
return e;
}
//-- Is it the best so far?
if (e > best_score) {
best_score = e;
//-- Does it improve upon alpha (i.e. is it part of the PV)?
if (e > a) {
a = e;
//-- Update the Principle Variation
update_best_line(board, ply);
}
}
// Reset the zero width window
b = a + 1;
//-- Was this a fail low at a node which should have failed high?
//if (pv->node_type == )
}
//-- Is it a draw
if (pv->legal_moves_played == 0) {
pv->best_line_length = ply;
return 0;
}
//-- Update Hash
if (best_score > alpha)
poke(board->hash, best_score, ply, depth, HASH_EXACT, pv->best_line[ply]);
else
poke(board->hash, best_score, ply, depth, HASH_UPPER, NULL);
// Return Best Score found
assert(best_score >= -CHECKMATE && best_score <= CHECKMATE);
return best_score;
}
