static int negascout(struct SearchData *sd,
int alpha,
int beta,
const int depth,
int node_type
#if MP
,int exclusiveP
#endif /* MP */
) {
struct Position *p = sd->position;
struct SearchStatus *st;
int best = -INF;
int bestm = M_NONE;
int tmp;
int talpha;
int incheck;
int lmove;
int move;
int extend = 0;
int threat = FALSE;
int reduce_extensions;
int next_type;
int was_futile = FALSE;
#if FUTILITY
int is_futile;
int optimistic = 0;
#endif
#if MP
int deferred_cnt = 0;
int deferred_list[MAX_DEFERRED];
int deferred_depth[MAX_DEFERRED];
#endif
EnterNode(sd);
Nodes++;
/* check for search termination */
if (sd->master && TerminateSearch(sd)) {
AbortSearch = TRUE;
goto EXIT;
}
/* max search depth reached */
if (sd->ply >= MaxDepth) goto EXIT;
/*
* Check for insufficent material or theoretical draw.
*/
if ( /* InsufMat(p) || CheckDraw(p) || */ Repeated(p, FALSE)) {
best = 0;
goto EXIT;
}
/*
* check extension
*/
incheck = InCheck(p, p->turn);
if (incheck && p->material[p->turn] > 0) {
extend += CheckExtend(p);
ChkExt++;
}
/*
* Check the hashtable
*/
st = sd->current;
HTry++;
#if MP
switch (ProbeHT(p->hkey, &tmp, depth, &(st->st_hashmove), &threat, sd->ply,
exclusiveP, sd->localHashTable))
#else
switch (ProbeHT(p->hkey, &tmp, depth, &(st->st_hashmove), &threat, sd->ply))
#endif /* MP */
{
case ExactScore:
HHit++;
best = tmp;
goto EXIT;
case UpperBound:
if (tmp <= alpha) {
HHit++;
best = tmp;
goto EXIT;
}
break;
case LowerBound:
if (tmp >= beta) {
HHit++;
best = tmp;
goto EXIT;
}
break;
case Useless:
threat = !incheck && MateThreat(p, OPP(p->turn));
break;
#if MP
case OnEvaluation:
best = -ON_EVALUATION;
goto EXIT;
#endif
}
/*
* Probe EGTB
*/
if (depth > EGTBDepth && ProbeEGTB(p, &tmp, sd->ply)) {
best = tmp;
goto EXIT;
}
/*
* Probe recognizers
*/
switch (ProbeRecognizer(p, &tmp)) {
case ExactScore:
best = tmp;
goto EXIT;
case LowerBound:
if (tmp >= beta) {
best = tmp;
goto EXIT;
}
break;
case UpperBound:
if (tmp <= alpha) {
best = tmp;
goto EXIT;
}
break;
}
#if NULLMOVE
/*
* Null move search.
* See Christian Donninger, "Null Move and Deep Search"
* ICCA Journal Volume 16, No. 3, pp. 137-143
*/
if (!incheck && node_type == CutNode && !threat) {
int next_depth;
int nms;
next_depth = depth - ReduceNullMove;
if (next_depth > 0) {
next_depth = depth - ReduceNullMoveDeep;
}
DoNull(p);
if (next_depth < 0) {
nms = -quies(sd, -beta, -beta+1, 0);
} else {
#if MP
nms = -negascout(sd, -beta, -beta+1, next_depth, AllNode, 0);
#else
nms = -negascout(sd, -beta, -beta+1, next_depth, AllNode);
#endif
}
UndoNull(p);
if (AbortSearch) goto EXIT;
if (nms >= beta) {
if (p->nonPawn[p->turn] >= Value[Queen]) {
best = nms;
goto EXIT;
} else {
if (next_depth < 0) {
nms = quies(sd, beta-1, beta, 0);
} else {
#if MP
nms = negascout(sd, beta-1, beta, next_depth, CutNodeNoNull,
0);
#else
nms = negascout(sd, beta-1, beta, next_depth,
CutNodeNoNull);
#endif
}
if (nms >= beta) {
best = nms;
goto EXIT;
} else {
extend += ExtendZugzwang;
ZZExt++;
}
}
} else if (nms <= -CMLIMIT) {
threat = TRUE;
}
}
#endif /* NULLMOVE */
lmove = (p->actLog-1)->gl_Move;
reduce_extensions = (sd->ply > 2*sd->depth);
talpha = alpha;
switch (node_type) {
case AllNode:
next_type = CutNode;
break;
case CutNode:
case CutNodeNoNull:
next_type = AllNode;
break;
default:
next_type = PVNode;
break;
}
#if FUTILITY
is_futile = !incheck && !threat && alpha < CMLIMIT && alpha > -CMLIMIT;
if (is_futile) {
if (p->turn == White) {
optimistic = MaterialBalance(p) + MaxPos;
} else {
optimistic = -MaterialBalance(p) + MaxPos;
}
}
#endif /* FUTILITY */
/*
* Internal iterative deepening. If we do not have a move, we try
* a shallow search to find a good candidate.
*/
if (depth > 2*OnePly && ((alpha + 1) != beta) && !LegalMove(p, st->st_hashmove)) {
int useless;
#if MP
useless = negascout(sd, alpha, beta, depth-2*OnePly, PVNode, 0);
#else
useless = negascout(sd, alpha, beta, depth-2*OnePly, PVNode);
#endif
st->st_hashmove = sd->pv_save[sd->ply+1];
}
/*
* Search all legal moves
*/
while ((move = incheck ? NextEvasion(sd) : NextMove(sd)) != M_NONE) {
int next_depth = extend;
if (move & M_CANY && !MayCastle(p, move)) continue;
/*
* recapture extension
*/
if ((move & M_CAPTURE) && (lmove & M_CAPTURE) &&
M_TO(move) == M_TO(lmove) &&
IsRecapture(p->piece[M_TO(move)], (p->actLog-1)->gl_Piece)) {
RCExt += 1;
next_depth += ExtendRecapture[TYPE(p->piece[M_TO(move)])];
}
/*
* passed pawn push extension
*/
if (TYPE(p->piece[M_FROM(move)]) == Pawn &&
p->nonPawn[OPP(p->turn)] <= Value[Queen]) {
int to = M_TO(move);
if (((p->turn == White && to >= a7)
|| (p->turn == Black && to <= h2))
&& IsPassed(p, to, p->turn) && SwapOff(p, move) >= 0) {
next_depth += ExtendPassedPawn;
PPExt += 1;
}
}
/*
* limit extensions to sensible range.
*/
if (reduce_extensions) next_depth /= 2;
next_depth += depth - OnePly;
#if FUTILITY
/*
* Futility cutoffs
*/
if (is_futile) {
if (next_depth < 0 && !IsCheckingMove(p, move)) {
tmp = optimistic + ScoreMove(p, move);
if (tmp <= alpha) {
if (tmp > best) {
best = tmp;
bestm = move;
was_futile = TRUE;
}
continue;
}
}
#if EXTENDED_FUTILITY
/*
* Extended futility cutoffs and limited razoring.
* See Ernst A. Heinz, "Extended Futility Pruning"
* ICCA Journal Volume 21, No. 2, pp 75-83
*/
else if (next_depth >= 0 && next_depth < OnePly
&& !IsCheckingMove(p, move)) {
tmp = optimistic + ScoreMove(p, move) + (3*Value[Pawn]);
if (tmp <= alpha) {
if (tmp > best) {
best = tmp;
bestm = move;
was_futile = TRUE;
}
continue;
}
}
#if RAZORING
else if (next_depth >= OnePly && next_depth < 2*OnePly
&& !IsCheckingMove(p, move)) {
tmp = optimistic + ScoreMove(p, move) + (6*Value[Pawn]);
if (tmp <= alpha) {
next_depth -= OnePly;
}
}
#endif /* RAZORING */
#endif /* EXTENDED_FUTILITY */
}
#endif /* FUTILITY */
DoMove(p, move);
if (InCheck(p, OPP(p->turn))) {
UndoMove(p, move);
} else {
/*
* Check extension
*/
if (p->material[p->turn] > 0 && InCheck(p, p->turn)) {
next_depth += (reduce_extensions) ?
ExtendInCheck>>1 : ExtendInCheck;
}
/*
* Recursively search this position. If depth is exhausted, use
* quies, otherwise use negascout.
*/
if (next_depth < 0) {
tmp = -quies(sd, -beta, -talpha, 0);
} else if (bestm != M_NONE && !was_futile) {
#if MP
tmp = -negascout(sd, -talpha-1, -talpha, next_depth, next_type,
bestm != M_NONE);
if (tmp != ON_EVALUATION && tmp > talpha && tmp < beta) {
tmp = -negascout(sd, -beta, -tmp, next_depth,
node_type == PVNode ? PVNode : AllNode,
bestm != M_NONE);
}
#else
tmp = -negascout(sd, -talpha-1, -talpha, next_depth, next_type);
if (tmp > talpha && tmp < beta) {
tmp = -negascout(sd, -beta, -tmp, next_depth,
node_type == PVNode ? PVNode : AllNode);
}
#endif /* MP */
} else {
#if MP
tmp = -negascout(sd, -beta, -talpha, next_depth, next_type,
bestm != M_NONE);
#else
tmp = -negascout(sd, -beta, -talpha, next_depth, next_type);
#endif /* MP */
}
UndoMove(p, move);
if (AbortSearch) goto EXIT;
#if MP
if (tmp == ON_EVALUATION) {
/*
* This child is ON_EVALUATION. Remember move and
* depth.
*/
deferred_list[deferred_cnt] = move;
deferred_depth[deferred_cnt] = next_depth;
deferred_cnt++;
} else {
#endif /* MP */
/*
* beta cutoff, enter move in Killer/Countermove table
*/
if (tmp >= beta) {
if (!(move & M_TACTICAL)) {
PutKiller(sd, move);
sd->counterTab[p->turn][lmove & 4095] = move;
}
StoreResult(sd, tmp, alpha, beta, move, depth, threat);
best = tmp;
goto EXIT;
}
/*
* Improvement on best move to date
*/
if (tmp > best) {
best = tmp;
bestm = move;
was_futile = FALSE;
if (best > talpha) {
talpha = best;
}
}
next_type = CutNode;
#if MP
}
#endif /* MP */
}
}
/*
********************************************************************************
* *
* 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(int alpha, int beta, int wtm, int depth, int ply, int do_null)
{
register int first_move=1;
register BITBOARD save_hash_key;
register int initial_alpha, value;
register int extensions;
/*
----------------------------------------------------------
| |
| 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. |
| |
----------------------------------------------------------
*/
if (ply >= MAXPLY-2) return(beta);
nodes_searched++;
if (--next_time_check <= 0) {
next_time_check=nodes_between_time_checks;
if (CheckInput()) Interrupt(ply);
time_abort+=TimeCheck(0);
if (time_abort) {
abort_search=1;
return(0);
}
}
/*
----------------------------------------------------------
| |
| check for draw by repetition. |
| |
----------------------------------------------------------
*/
if (RepetitionCheck(ply,wtm)) {
value=(wtm==root_wtm) ? DrawScore() : -DrawScore();
if (value < beta) SavePV(ply,value,0);
#if !defined(FAST)
if(ply <= trace_level) printf("draw by repetition detected, ply=%d.\n",ply);
#endif
return(value);
}
/*
----------------------------------------------------------
| |
| now call LookUp() 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. LookUp() returned "EXACT_SCORE" 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. LookUp() returned "LOWER_BOUND" 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. LookUp() returned "UPPER_BOUND" which means that |
| when we encountered this position before, we searched |
| one branch (probably) which promptly refuted the move |
| at the previous ply. |
| |
| 4. LookUp() returned "AVOID_NULL_MOVE" which means |
| the hashed score/bound was no good, but it indicated |
| that trying a null-move in this position will be a |
| waste of time. |
| |
----------------------------------------------------------
*/
switch (LookUp(ply,depth,wtm,&alpha,beta)) {
case EXACT_SCORE:
if(alpha >= beta) return(beta);
else {
SavePV(ply,alpha,1);
return(alpha);
}
case LOWER_BOUND:
return(alpha);
case UPPER_BOUND:
return(beta);
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 (a) the previous move was |
| a capture or promotion, unless we are at very shallow |
| plies (<4) in the search; (b) there are less than 5 |
| pieces left (currently all interesting 4 piece endings |
| are available.) |
| |
----------------------------------------------------------
*/
#if defined(TABLEBASES)
if (TotalPieces < 5) do {
register int wpawn, bpawn;
int tb_value;
if (TotalWhitePawns && TotalBlackPawns) {
wpawn=FirstOne(WhitePawns);
bpawn=FirstOne(BlackPawns);
if (FileDistance(wpawn,bpawn) == 1) {
if(((Rank(wpawn)==1) && (Rank(bpawn)>2)) ||
((Rank(bpawn)==6) && (Rank(wpawn)<5)) ||
EnPassant(ply)) break;
}
}
tb_probes++;
if (EGTBScore(ply, wtm, &tb_value)) {
tb_probes_successful++;
alpha=tb_value;
if (abs(alpha) > MATE-100) alpha+=(alpha > 0) ? -(ply-1) : +(ply-1);
else if (alpha == 0) alpha=(wtm==root_wtm) ? DrawScore() : -DrawScore();
if(alpha >= beta) return(beta);
else {
SavePV(ply,alpha,2);
return(alpha);
}
}
} while(0);
# endif
/*
----------------------------------------------------------
| |
| initialize. |
| |
----------------------------------------------------------
*/
in_check[ply+1]=0;
extended_reason[ply+1]=no_extension;
initial_alpha=alpha;
last[ply]=last[ply-1];
killer_count1[ply+1]=0;
killer_count2[ply+1]=0;
/*
----------------------------------------------------------
| |
| 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. |
| |
----------------------------------------------------------
*/
# if defined(NULL_MOVE_DEPTH)
if (do_null && !in_check[ply] &&
((wtm) ? TotalWhitePieces : TotalBlackPieces)>2) {
current_move[ply]=0;
current_phase[ply]=NULL_MOVE;
#if !defined(FAST)
if (ply <= trace_level)
SearchTrace(ply,depth,wtm,alpha,beta,"Search",0);
#endif
position[ply+1]=position[ply];
Rule50Moves(ply+1)++;
save_hash_key=HashKey;
if (EnPassant(ply)) {
HashEP(EnPassant(ply+1),HashKey);
EnPassant(ply+1)=0;
}
if ((depth-NULL_MOVE_DEPTH-INCREMENT_PLY) >= INCREMENT_PLY)
value=-Search(-beta,-alpha,ChangeSide(wtm),depth-NULL_MOVE_DEPTH-INCREMENT_PLY,ply+1,NO_NULL);
else
value=-Quiesce(-beta,-alpha,ChangeSide(wtm),ply+1);
HashKey=save_hash_key;
if (abort_search) return(0);
if (value >= beta) {
StoreRefutation(ply,depth,wtm,beta);
return(beta);
}
}
# 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." |
| |
----------------------------------------------------------
*/
next_status[ply].phase=FIRST_PHASE;
if (hash_move[ply]==0 && (depth > 2*INCREMENT_PLY) &&
(((ply & 1) && alpha == root_alpha && beta == root_beta) ||
(!(ply & 1) && alpha == -root_beta && beta == -root_alpha))) {
current_move[ply]=0;
value=Search(alpha,beta,wtm,depth-2*INCREMENT_PLY,ply,DO_NULL);
if (abort_search) return(0);
if (value <= alpha) {
value=Search(-MATE,beta,wtm,depth-2*INCREMENT_PLY,ply,DO_NULL);
if (abort_search) return(0);
}
else if (value < beta) {
if ((int) pv[ply-1].path_length >= ply) hash_move[ply]=pv[ply-1].path[ply];
}
else hash_move[ply]=current_move[ply];
last[ply]=last[ply-1];
next_status[ply].phase=FIRST_PHASE;
}
/*
----------------------------------------------------------
| |
| 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 ((current_phase[ply]=(in_check[ply]) ? NextEvasion(ply,wtm) :
NextMove(depth,ply,wtm))) {
extended_reason[ply]&=check_extension;
#if !defined(FAST)
if (ply <= trace_level) SearchTrace(ply,depth,wtm,alpha,beta,"Search",current_phase[ply]);
#endif
/*
----------------------------------------------------------
| |
| 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. |
| |
----------------------------------------------------------
*/
extensions=-INCREMENT_PLY;
if (Captured(current_move[ply]) && Captured(current_move[ply-1]) &&
To(current_move[ply-1]) == To(current_move[ply]) &&
(p_values[Captured(current_move[ply-1])+7] ==
p_values[Captured(current_move[ply])+7] ||
Promote(current_move[ply-1])) &&
!(extended_reason[ply-1]&recapture_extension)) {
extended_reason[ply]|=recapture_extension;
recapture_extensions_done++;
extensions+=RECAPTURE;
}
/*
----------------------------------------------------------
| |
| if we push a passed pawn, we need to look deeper to |
| see if it is a legitimate threat. |
| |
----------------------------------------------------------
*/
if (Piece(current_move[ply])==pawn && !FutileAhead(wtm) &&
((wtm && To(current_move[ply])>H5 && TotalBlackPieces<16 &&
!And(mask_pawn_passed_w[To(current_move[ply])],BlackPawns)) ||
(!wtm && To(current_move[ply])<A4 && TotalWhitePieces<16 &&
!And(mask_pawn_passed_b[To(current_move[ply])],WhitePawns)) ||
push_extensions[To(current_move[ply])]) &&
Swap(From(current_move[ply]),To(current_move[ply]),wtm) >= 0) {
extended_reason[ply]|=passed_pawn_extension;
passed_pawn_extensions_done++;
extensions+=PASSED_PAWN_PUSH;
}
/*
----------------------------------------------------------
| |
| 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(ply,current_move[ply],wtm);
if (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))) {
in_check[ply+1]=1;
extended_reason[ply+1]=check_extension;
check_extensions_done++;
extensions+=IN_CHECK;
}
else {
in_check[ply+1]=0;
extended_reason[ply+1]=no_extension;
}
/*
----------------------------------------------------------
| |
| now we toss in the "razoring" trick, which simply says |
| if we are doing fairly badly, we can reduce the depth |
| an additional ply, if there was nothing at the current |
| ply that caused an extension. |
| |
----------------------------------------------------------
*/
if (depth < 3*INCREMENT_PLY && !in_check[ply] &&
extensions == -INCREMENT_PLY) {
register int val=(wtm) ? Material : -Material;
if (val+1500 < alpha) extensions-=INCREMENT_PLY;
}
/*
----------------------------------------------------------
| |
| if there's only one legal move, extend the search one |
| additional ply since this node is very easy to search. |
| |
----------------------------------------------------------
*/
if (first_move) {
if (last[ply]-last[ply-1] == 1) {
extended_reason[ply]|=one_reply_extension;
one_reply_extensions_done++;
extensions+=ONE_REPLY_TO_CHECK;
}
if (depth+MaxExtensions(extensions) >= INCREMENT_PLY)
value=-Search(-beta,-alpha,ChangeSide(wtm),depth+MaxExtensions(extensions),ply+1,DO_NULL);
else {
value=-Quiesce(-beta,-alpha,ChangeSide(wtm),ply+1);
}
if (abort_search) {
UnMakeMove(ply,current_move[ply],wtm);
return(0);
}
first_move=0;
}
else {
if (depth+MaxExtensions(extensions) >= INCREMENT_PLY)
value=-Search(-alpha-1,-alpha,ChangeSide(wtm),depth+MaxExtensions(extensions),ply+1,DO_NULL);
else {
value=-Quiesce(-alpha-1,-alpha,ChangeSide(wtm),ply+1);
}
if (abort_search) {
UnMakeMove(ply,current_move[ply],wtm);
return(0);
}
if (value>alpha && value<beta) {
if (depth+MaxExtensions(extensions) >= INCREMENT_PLY)
value=-Search(-beta,-alpha,ChangeSide(wtm),depth+MaxExtensions(extensions),ply+1,DO_NULL);
else
value=-Quiesce(-beta,-alpha,ChangeSide(wtm),ply+1);
if (abort_search) {
UnMakeMove(ply,current_move[ply],wtm);
return(0);
}
}
}
if (value > alpha) {
if(value >= beta) {
HistoryRefutation(ply,depth,wtm);
UnMakeMove(ply,current_move[ply],wtm);
StoreRefutation(ply,depth,wtm,beta);
return(beta);
}
alpha=value;
}
}
UnMakeMove(ply,current_move[ply],wtm);
}
/*
----------------------------------------------------------
| |
| 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 (first_move == 1) {
value=(Check(wtm)) ? -(MATE-ply) :
((wtm==root_wtm) ? DrawScore() : -DrawScore());
if(value > beta) value=beta;
else if (value < alpha) value=alpha;
if (value >=alpha && value <beta) {
SavePV(ply,value,0);
#if !defined(FAST)
if (ply <= trace_level) printf("Search() no moves! ply=%d\n",ply);
#endif
}
return(value);
}
else {
if (alpha != initial_alpha) {
memcpy(&pv[ply-1].path[ply],&pv[ply].path[ply],(pv[ply].path_length-ply+1)*4);
memcpy(&pv[ply-1].path_hashed,&pv[ply].path_hashed,3);
pv[ply-1].path[ply-1]=current_move[ply-1];
HistoryBest(ply,depth,wtm);
}
StoreBest(ply,depth,wtm,alpha,initial_alpha);
/*
----------------------------------------------------------
| |
| if the 50-move rule is drawing close, then adjust the |
| score to reflect the impending draw. |
| |
----------------------------------------------------------
*/
if (Rule50Moves(ply) > 99) {
value=(wtm==root_wtm) ? DrawScore() : -DrawScore();
if (value < beta) SavePV(ply,value,0);
#if !defined(FAST)
if(ply <= trace_level) printf("draw by 50-move rule detected, ply=%d.\n",ply);
#endif
return(value);
}
return(alpha);
}
}