Fix bogus mate scores in some positions

Always before pruning the move, it's important to check that:

bestValue > VALUE_MATED_IN_MAX_PLY

See example position:

8/2p1p3/P1NpP3/3k4/1P1BN3/2P1P3/2Q5/6K1 w - - 0 1
http://support.stockfishchess.org/discussions/problems/268-wrong-declaring-a-forced-mate-in-3-moves

This problem was present in 2.3.1, then it was fixed by my patch.

After 24000 games at 15+0.05
ELO: 2.40 +-4.4 (95%) LOS: 95.7%
Total: 24000 W: 4774 L: 4608 D: 14618

bench: 4465997

diff --git a/src/search.cpp b/src/search.cpp
index 7a06dad93f94035b046c9384938ce6d8d0b983b1..6dcd9f1db794f56f1ab5214e21fcab7587d2b3b0 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -856,7 +856,8 @@ split_point_start: // At split points actual search starts from here
           && !captureOrPromotion
           && !inCheck
           && !dangerous
           && !captureOrPromotion
           && !inCheck
           && !dangerous

-          &&  move != ttMove)
+          &&  move != ttMove
+          &&  bestValue > VALUE_MATED_IN_MAX_PLY)

       {
           // Move count based pruning
           if (   depth < 16 * ONE_PLY
       {
           // Move count based pruning
           if (   depth < 16 * ONE_PLY