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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "chess.h"
#include "data.h"
#include "epdglue.h"
/* last modified 10/18/99 */
/*
********************************************************************************
* *
* Search() is the recursive routine used to implement the alpha/beta *
* negamax search (similar to minimax but simpler to code.) Search() is *
* called whenever there is "depth" remaining so that all moves are subject *
* to searching, or when the side to move is in check, to make sure that this *
* side isn't mated. Search() recursively calls itself until depth is ex- *
* hausted, at which time it calls Quiesce() instead. *
* *
********************************************************************************
*/
int Search(TREE *tree, int alpha, int beta, int wtm, int depth,
int ply, int do_null) {
register int moves_searched=0;
register int o_alpha, value=0;
register int extensions, pieces;
int threat=0;
register int full_extension=ply<=2*iteration_depth;
/*
----------------------------------------------------------
| |
| check to see if we have searched enough nodes that it |
| is time to peek at how much time has been used, or if |
| is time to check for operator keyboard input. this is |
| usually enough nodes to force a time/input check about |
| once per second, except when the target time per move |
| is very small, in which case we try to check the time |
| at least 10 times during the search. |
| |
----------------------------------------------------------
*/
tree->nodes_searched++;
if (--next_time_check <= 0) {
next_time_check=nodes_between_time_checks;
if (CheckInput()) Interrupt(ply);
if (TimeCheck(0)) {
time_abort++;
abort_search=1;
return(0);
}
}
if (ply >= MAXPLY-1) return(beta);
/*
----------------------------------------------------------
| |
| check for draw by repetition. |
| |
----------------------------------------------------------
*/
if (RepetitionCheck(tree,ply,wtm)) {
value=DrawScore(ply&1);
if (value < beta) SavePV(tree,ply,value,0);
#if defined(TRACE)
if(ply <= trace_level) printf("draw by repetition detected, ply=%d.\n",ply);
#endif
return(value);
}
/*
----------------------------------------------------------
| |
| now call HashProbe() to see if this position has been |
| searched before. if so, we may get a real score, |
| produce a cutoff, or get nothing more than a good move |
| to try first. there are four cases to handle: |
| |
| 1. HashProbe() returned "EXACT" if this score is |
| greater than beta, return beta. otherwise, return the |
| score. In either case, no further searching is needed |
| from this position. note that lookup verified that |
| the table position has sufficient "draft" to meet the |
| requirements of the current search depth remaining. |
| |
| 2. HashProbe() returned "UPPER" which means that |
| when this position was searched previously, every move |
| was "refuted" by one of its descendents. as a result, |
| when the search was completed, we returned alpha at |
| that point. we simply return alpha here as well. |
| |
| 3. HashProbe() returned "LOWER" which means that |
| when we encountered this position before, we searched |
| one branch (probably) which promptly refuted the move |
| at the previous ply. |
| |
| 4. HashProbe() returned "AVOID_NULL_MOVE" which means |
| the hashed score/bound was no good, but it indicated |
| that trying a null-move in this position would be a |
| waste of time. |
| |
----------------------------------------------------------
*/
switch (HashProbe(tree,ply,depth,wtm,&alpha,&beta,&threat)) {
case EXACT:
if(alpha < beta) SavePV(tree,ply,alpha,1);
return(alpha);
case LOWER:
return(beta);
case UPPER:
return(alpha);
case AVOID_NULL_MOVE:
do_null=0;
}
/*
----------------------------------------------------------
| |
| now it's time to try a probe into the endgame table- |
| base files. this is done if we notice that there are |
| 5 or fewer pieces left on the board. EGTB_use tells |
| us how many pieces to probe on. note that this can be |
| zero when trying to swindle the opponent, so that no |
| probes are done since we know it is a draw. |
| |
----------------------------------------------------------
*/
if (ply<=iteration_depth && TotalPieces<=EGTB_use &&
(CaptureOrPromote(tree->current_move[ply-1]) || ply<3)) {
int egtb_value;
tree->egtb_probes++;
if (EGTBProbe(tree, ply, wtm, &egtb_value)) {
tree->egtb_probes_successful++;
alpha=egtb_value;
if (abs(alpha) > MATE-300) alpha+=(alpha > 0) ? -ply+1 : ply;
else if (alpha == 0) alpha=DrawScore(ply&1);
if(alpha < beta) SavePV(tree,ply,alpha,2);
return(alpha);
}
}
/*
----------------------------------------------------------
| |
| initialize. |
| |
----------------------------------------------------------
*/
tree->in_check[ply+1]=0;
tree->extended_reason[ply+1]=no_extension;
o_alpha=alpha;
tree->last[ply]=tree->last[ply-1];
/*
----------------------------------------------------------
| |
| first, we try a null move to see if we can get a quick |
| cutoff with only a little work. this operates as |
| follows. instead of making a legal move, the side on |
| move 'passes' and does nothing. the resulting position |
| is searched to a shallower depth than normal (usually |
| one ply less but settable by the operator) this should |
| result in a cutoff or at least should set the lower |
| bound better since anything should be better than not |
| doing anything. |
| |
| this is skipped for any of the following reasons: |
| |
| 1. the side on move is in check. the null move |
| results in an illegal position. |
| 2. no more than one null move can appear in succession |
| or else the search will degenerate into nothing. |
| 3. the side on move has little material left making |
| zugzwang positions more likely. |
| |
| the null-move search is also used to detect certain |
| types of threats. the original idea of using the value |
| returned by the null-move search was reported by C. |
| Donninger, but was modified by Bruce Moreland (Ferret) |
| in the following way: if the null-move search returns |
| a score that says "mated in N" then this position is a |
| dangerous one, because not moving gets the side to move |
| mated. we extend the search one ply in this case, al- |
| though, as always, no more than one ply of extensions |
| are allowed at any one level in the tree. note also |
| that this "threat" condition is hashed so that later, |
| if the hash table says "don't try the null move because |
| it likely will fail low, we still know that this is a |
| threat position and that it should be extended. |
| |
----------------------------------------------------------
*/
# if defined(NULL_MOVE_DEPTH)
pieces=(wtm) ? TotalWhitePieces : TotalBlackPieces;
if (do_null && !tree->in_check[ply] && pieces && (pieces>5 || depth<421)) {
register BITBOARD save_hash_key;
int null_depth;
tree->current_move[ply]=0;
tree->phase[ply]=NULL_MOVE;
#if defined(TRACE)
if (ply <= trace_level)
SearchTrace(tree,ply,depth,wtm,beta-1,beta,"Search",0);
#endif
tree->position[ply+1]=tree->position[ply];
Rule50Moves(ply+1)++;
save_hash_key=HashKey;
if (EnPassant(ply)) {
HashEP(EnPassant(ply+1),HashKey);
EnPassant(ply+1)=0;
}
null_depth=(depth > 6*INCPLY) ? 4*INCPLY : 3*INCPLY;
if (depth-null_depth >= INCPLY)
value=-Search(tree,-beta,1-beta,ChangeSide(wtm),
depth-null_depth,ply+1,NO_NULL);
else
value=-Quiesce(tree,-beta,1-beta,ChangeSide(wtm),ply+1);
HashKey=save_hash_key;
if (abort_search || tree->stop) return(0);
if (value >= beta) {
HashStore(tree,ply,depth,wtm,LOWER,value,threat);
return(value);
}
if (value < -MATE+300) threat=1;
}
# endif
/*
----------------------------------------------------------
| |
| if there is no best move from the hash table, and this |
| is a PV node, then we need a good move to search |
| first. while killers and history moves are good, they |
| are not "good enough". the simplest action is to try |
| a shallow search (depth-2) to get a move. note that |
| when we call Search() with depth-2, it, too, will |
| not have a hash move, and will therefore recursively |
| continue this process, hence the name "internal |
| iterative deepening." |
| |
----------------------------------------------------------
*/
tree->next_status[ply].phase=HASH_MOVE;
if (tree->hash_move[ply]==0 && do_null && depth>=3*INCPLY) do {
if (ply & 1) {
if (alpha!=root_alpha || beta!=root_beta) break;
}
else {
if (alpha!=-root_beta || beta!=-root_alpha) break;
}
tree->current_move[ply]=0;
if (depth-2*INCPLY >= INCPLY)
value=Search(tree,alpha,beta,wtm,depth-2*INCPLY,ply,DO_NULL);
else
value=Quiesce(tree,alpha,beta,wtm,ply);
if (abort_search || tree->stop) return(0);
if (value <= alpha) {
if (depth-2*INCPLY >= INCPLY)
value=Search(tree,-MATE,beta,wtm,depth-2*INCPLY,ply,DO_NULL);
else
value=Quiesce(tree,-MATE,beta,wtm,ply);
if (abort_search || tree->stop) return(0);
}
else if (value < beta) {
if ((int) tree->pv[ply-1].pathl >= ply)
tree->hash_move[ply]=tree->pv[ply-1].path[ply];
}
else tree->hash_move[ply]=tree->current_move[ply];
tree->last[ply]=tree->last[ply-1];
tree->next_status[ply].phase=HASH_MOVE;
} while(0);
/*
----------------------------------------------------------
| |
| now iterate through the move list and search the |
| resulting positions. note that Search() culls any |
| move that is not legal by using Check(). the special |
| case is that we must find one legal move to search to |
| confirm that it's not a mate or draw. |
| |
----------------------------------------------------------
*/
while ((tree->phase[ply]=(tree->in_check[ply]) ?
NextEvasion(tree,ply,wtm) : NextMove(tree,ply,wtm))) {
tree->extended_reason[ply]&=check_extension;
#if defined(TRACE)
if (ply <= trace_level)
SearchTrace(tree,ply,depth,wtm,alpha,beta,"Search",tree->phase[ply]);
#endif
/*
----------------------------------------------------------
| |
| if the null move found that the side on move gets |
| mated by not moving, then there must be some strong |
| threat at this position. extend the search to make |
| sure it is analyzed carefully. |
| |
----------------------------------------------------------
*/
extensions=-60;
if (threat) {
extensions+=(full_extension) ? threat_depth : threat_depth>>1;
tree->threat_extensions_done++;
}
/*
----------------------------------------------------------
| |
| if two successive moves are capture / re-capture so |
| that the material score is restored, extend the search |
| by one ply on the re-capture since it is pretty much |
| forced and easy to analyze. |
| |
----------------------------------------------------------
*/
if (Captured(tree->current_move[ply]) &&
Captured(tree->current_move[ply-1]) &&
To(tree->current_move[ply-1]) == To(tree->current_move[ply]) &&
(p_values[Captured(tree->current_move[ply-1])+7] ==
p_values[Captured(tree->current_move[ply])+7] ||
Promote(tree->current_move[ply-1])) &&
!(tree->extended_reason[ply-1]&recapture_extension)) {
tree->extended_reason[ply]|=recapture_extension;
tree->recapture_extensions_done++;
extensions+=(full_extension) ? recap_depth : recap_depth>>1;
}
/*
----------------------------------------------------------
| |
| if we push a passed pawn, we need to look deeper to |
| see if it is a legitimate threat. |
| |
----------------------------------------------------------
*/
if (Piece(tree->current_move[ply])==pawn &&
push_extensions[To(tree->current_move[ply])]) {
tree->extended_reason[ply]|=passed_pawn_extension;
tree->passed_pawn_extensions_done++;
extensions+=(full_extension) ? pushpp_depth : pushpp_depth>>1;
}
/*
----------------------------------------------------------
| |
| now make the move and search the resulting position. |
| if we are in check, the current move must be legal |
| since NextEvasion ensures this, otherwise we have to |
| make sure the side-on-move is not in check after the |
| move to weed out illegal moves and save time. |
| |
----------------------------------------------------------
*/
MakeMove(tree,ply,tree->current_move[ply],wtm);
if (tree->in_check[ply] || !Check(wtm)) {
/*
----------------------------------------------------------
| |
| if the move to be made checks the opponent, then we |
| need to remember that he's in check and also extend |
| the depth by one ply for him to get out. |
| |
----------------------------------------------------------
*/
if (Check(ChangeSide(wtm))) {
tree->in_check[ply+1]=1;
tree->extended_reason[ply+1]=check_extension;
tree->check_extensions_done++;
extensions+=(full_extension) ? incheck_depth : incheck_depth>>1;
}
else {
tree->in_check[ply+1]=0;
tree->extended_reason[ply+1]=no_extension;
}
/*
----------------------------------------------------------
| |
| if there's only one legal move, extend the search one |
| additional ply since this node is very easy to search. |
| |
----------------------------------------------------------
*/
if (!moves_searched) {
if (tree->in_check[ply] && tree->last[ply]-tree->last[ply-1] == 1) {
tree->extended_reason[ply]|=one_reply_extension;
tree->one_reply_extensions_done++;
extensions+=(full_extension) ? onerep_depth : onerep_depth>>1;
}
/*
----------------------------------------------------------
| |
| now it is time to call Search()/Quiesce to find out if |
| this move is reasonable or not. |
| |
----------------------------------------------------------
*/
extensions=Min(extensions,0);
if (depth+extensions >= INCPLY)
value=-Search(tree,-beta,-alpha,ChangeSide(wtm),
depth+extensions,ply+1,DO_NULL);
else
value=-Quiesce(tree,-beta,-alpha,ChangeSide(wtm),ply+1);
if (abort_search || tree->stop) {
UnMakeMove(tree,ply,tree->current_move[ply],wtm);
return(0);
}
}
else {
extensions=Min(extensions,0);
if (depth+extensions >= INCPLY)
value=-Search(tree,-alpha-1,-alpha,ChangeSide(wtm),
depth+extensions,ply+1,DO_NULL);
else
value=-Quiesce(tree,-alpha-1,-alpha,ChangeSide(wtm),ply+1);
if (abort_search || tree->stop) {
UnMakeMove(tree,ply,tree->current_move[ply],wtm);
return(0);
}
if (value>alpha && value<beta) {
if (depth+extensions >= INCPLY)
value=-Search(tree,-beta,-alpha,ChangeSide(wtm),
depth+extensions,ply+1,DO_NULL);
else
value=-Quiesce(tree,-beta,-alpha,ChangeSide(wtm),ply+1);
if (abort_search || tree->stop) {
UnMakeMove(tree,ply,tree->current_move[ply],wtm);
return(0);
}
}
}
if (value > alpha) {
if(value >= beta) {
History(tree,ply,depth,wtm,tree->current_move[ply]);
UnMakeMove(tree,ply,tree->current_move[ply],wtm);
HashStore(tree,ply,depth,wtm,LOWER,value,threat);
tree->fail_high++;
if (!moves_searched) tree->fail_high_first++;
return(value);
}
alpha=value;
}
moves_searched++;
} else tree->nodes_searched++;
UnMakeMove(tree,ply,tree->current_move[ply],wtm);
#if defined(SMP)
if (smp_idle && moves_searched && min_thread_depth<=depth) {
tree->alpha=alpha;
tree->beta=beta;
tree->value=alpha;
tree->wtm=wtm;
tree->ply=ply;
tree->depth=depth;
tree->threat=threat;
if(Thread(tree)) {
if (abort_search || tree->stop) return(0);
if (CheckInput()) Interrupt(ply);
value=tree->search_value;
if (value > alpha) {
if(value >= beta) {
History(tree,ply,depth,wtm,tree->current_move[ply]);
HashStore(tree,ply,depth,wtm,LOWER,value,threat);
tree->fail_high++;
return(value);
}
alpha=value;
break;
}
}
}
#endif
}
/*
----------------------------------------------------------
| |
| all moves have been searched. if none were legal, |
| return either MATE or DRAW depending on whether the |
| side to move is in check or not. |
| |
----------------------------------------------------------
*/
if (moves_searched == 0) {
value=(Check(wtm)) ? -(MATE-ply) : DrawScore(ply&1);
if (value>=alpha && value<beta) {
SavePV(tree,ply,value,0);
#if defined(TRACE)
if (ply <= trace_level) printf("Search() no moves! ply=%d\n",ply);
#endif
}
return(value);
}
else {
if (alpha != o_alpha) {
memcpy(&tree->pv[ply-1].path[ply],&tree->pv[ply].path[ply],(tree->pv[ply].pathl-ply+1)*4);
memcpy(&tree->pv[ply-1].pathh,&tree->pv[ply].pathh,3);
tree->pv[ply-1].path[ply-1]=tree->current_move[ply-1];
History(tree,ply,depth,wtm,tree->pv[ply].path[ply]);
}
HashStore(tree,ply,depth,wtm,(alpha==o_alpha)?UPPER:EXACT,alpha,threat);
return(alpha);
}
}
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