Put depth limit on QS

To combat search explosion, QS is limited to 10 ply. This is done by
setting a globalvariable depthLimit when we enter QS. When the limit
is reached, the side to move gets an initiative bonus of 150 on top
of the static eval.

diff --git a/dropper.c b/dropper.c
index 0d97fe6..50106e3 100644 (file)
--- a/dropper.c
+++ b/dropper.c
@@ -789,6 +789,7 @@ Dump (char *s)
 
 #define attacks (rawAttacks + 23)
 static int attackKey, rawAttacks[13*22];
+int depthLimit = MAXPLY;
 
 int
 MoveGen (int stm, MoveStack *m, int castle)
@@ -1102,7 +1103,7 @@ int
 Search (int stm, int alpha, int beta, StackFrame *ff, int depth, int reduction, int maxDepth)
 {
     MoveStack m; StackFrame f; HashEntry *entry;
-    int oldSP = moveSP, *pvStart = pvPtr;
+    int oldSP = moveSP, *pvStart = pvPtr, oldLimit = depthLimit;
     int killer1 = killers[ply][0], killer2 = killers[ply][1], hashMove;
     int bestNr, bestScore, startAlpha, startScore, resultDepth, iterDepth=0, originalReduction = reduction;
     int hit, hashKeyH, ran=0, ipMask=0;
@@ -1192,7 +1193,13 @@ if(hashMove && board[hashMove>>8&255] == 0) {char s[100];sprintf(s,"bad hash mov
            if(curEval >= beta) { ff->depth = 1; moveSP = oldSP; return curEval; } // stand-pat cutoff
            alpha = startScore = curEval; maxDepth = 0; // we will not fail low, so no extra iterations
        }
-       if(maxDepth <= 0 && board[toDecode[hashMove&255]] == 0) hashMove = 0;
+       if(maxDepth <= 0) {
+           if(board[toDecode[hashMove&255]] == 0) hashMove = 0;
+#ifdef IDQS
+           if(ply >= depthLimit) { ff->depth = 1; moveSP = oldSP; return curEval + 150; } // hit depth limit; give hefty stm bonus
+           if(depthLimit == MAXPLY) depthLimit = ply+10; // we just entered pure QS; set up depth limit
+#endif
+       }
     } else if(curEval >= beta && f.checker == CK_NONE) {
        int nullDepth = depth - 3;
        if(nullDepth < 0) nullDepth = 0;
@@ -1229,7 +1236,7 @@ if(hashMove && board[hashMove>>8&255] == 0) {char s[100];sprintf(s,"bad hash mov
        bonus += bonus >> 1;
        int newEval = curEval + bonus;
        if(newEval > alpha) {
-           if(newEval >= beta) { ff->depth = 1; moveSP = oldSP; return newEval; } // stand-pat cutoff
+           if(newEval >= beta) { ff->depth = 1; moveSP = oldSP; depthLimit = oldLimit; return newEval; } // stand-pat cutoff
            alpha = startScore = newEval; maxDepth = 0; // we will not fail low, so no extra iterations
        }
    }
@@ -1324,7 +1331,7 @@ int m=moveStack[curMove];
 printf("%d:%d:%d %2d. %08x %c%d%c%d %6d %6d %6d\n",ply,depth,iterDepth,curMove,m,(m>>8&255)%22+'a',(m>>8&255)/22+1,toDecode[m&255]%22+'a',toDecode[m&255]/22+1,f.pstEval,score,bestScore);
 }
 
-           if(abortFlag) { moveSP = oldSP; return -INF; }
+           if(abortFlag) { moveSP = oldSP; depthLimit = oldLimit; return -INF; }
 
            // minimaxing
            if(f.depth < resultDepth) resultDepth = f.depth;
@@ -1398,7 +1405,7 @@ if(PATH)printf("%d:%d:%d iter end, max=%d, alpha=%d start=%d\n", ply, depth,iter
 if(PATH || f.hashKey == 0x1348708590)printf("%d:%d   Hash store %16llx, d=%d, checker=%x move=%s\n",ply,depth,f.hashKey,resultDepth,entry->checker,MoveToText(entry->move));
 
     // return results
-    moveSP = oldSP; pvPtr = pvStart;
+    moveSP = oldSP; pvPtr = pvStart; depthLimit = oldLimit;
     ff->depth = resultDepth + 1 + originalReduction; // report valid depth as seen from parent
     return bestScore;
 }