From a8806adb87bac158353f851a9ad564941affd153 Mon Sep 17 00:00:00 2001 From: Scott Gasch Date: Thu, 3 Sep 2026 17:10:46 -0700 Subject: Fix draw-score bug in hash-hit path; centralize as g_iDrawScore[2] Search()'s Dieter-Brusser hash-hit-leads-to-draw check only verified that a score of 0 would clear the same alpha/beta bound as the stored iScore -- it didn't establish that iScore itself was accurate. Since playing the hash move actually produces a draw, propagate the draw score upward instead of the stale score computed along a different, non-repeating path. While fixing this, centralized every other place that returned a literal 0 for a draw (search.c's stalemate leaf, searchsup.c's QSearch draw leaf, probe.c's EGTB draw case, which had a dead `// g_iDrawValue[...]` comment suggesting this was intended all along) into a single g_iDrawScore[2] global in draw.c, declared in chess.h. It's indexed by side to move rather than a scalar so a future contempt-factor tweak can bias the draw score per color without touching every call site again; both entries are currently 0, so behavior is unchanged except for the hash-hit bugfix above. Co-Authored-By: Claude Sonnet 5 Claude-Session: https://claude.ai/code/session_012e2SaEaQ27JJq1D3wCqryr --- src/search.c | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-) (limited to 'src/search.c') diff --git a/src/search.c b/src/search.c index 870951b..31220e1 100755 --- a/src/search.c +++ b/src/search.c @@ -275,15 +275,23 @@ Search(IN SEARCHER_THREAD_CONTEXT *ctx, VERIFY(MakeMove(ctx, mvHash)); fIsDraw = IsDraw(ctx); UnmakeMove(ctx, mvHash); - if ((FALSE == fIsDraw) || (iScore == 0) || - ((u == HASH_FLAG_LOWER) && (iScore >= iBeta) && (0 >= iBeta)) || - ((u == HASH_FLAG_UPPER) && (iScore <= iAlpha) && (0 <= iAlpha))) + if ((FALSE == fIsDraw) || (iScore == g_iDrawScore[pos->uToMove]) || + ((u == HASH_FLAG_LOWER) && (iScore >= iBeta) && (g_iDrawScore[pos->uToMove] >= iBeta)) || + ((u == HASH_FLAG_UPPER) && (iScore <= iAlpha) && (g_iDrawScore[pos->uToMove] <= iAlpha))) { - if ((iAlpha < iScore) && (iScore < iBeta)) + // If the hash move leads to a draw, the score actually + // produced by playing it is g_iDrawScore[pos->uToMove] + // (from the mover's point of view), not the stale iScore + // recorded along whatever non-repeating path originally + // stored this entry -- the checks above only established + // that the draw score clears the same bound iScore does, + // not that iScore itself is an accurate value to return. + SCORE iRetScore = fIsDraw ? g_iDrawScore[pos->uToMove] : iScore; + if ((iAlpha < iRetScore) && (iRetScore < iBeta)) { UpdatePV(ctx, HASHMOVE); } - iBestScore = iScore; + iBestScore = iRetScore; goto end; } } @@ -949,7 +957,7 @@ Search(IN SEARCHER_THREAD_CONTEXT *ctx, } else { - iBestScore = 0; + iBestScore = g_iDrawScore[pos->uToMove]; if ((iAlpha < iBestScore) && (iBestScore < iBeta)) { INC(ctx->sCounters.tree.u64LeafCount); -- cgit v1.3