Tweak return value in futility pruning

lonfom169 · Disservin · commit 08cdbca56fac · 2023-12-02T11:46:44.000+01:00
In futility pruning, return the average between eval and beta. Passed STC: https://tests.stockfishchess.org/tests/view/65680bb6136acbc5735521d7 LLR: 2.93 (-2.94,2.94) <0.00,2.00> Total: 15200 W: 3926 L: 3642 D: 7632 Ptnml(0-2): 36, 1699, 3867, 1941, 57 Passed LTC: https://tests.stockfishchess.org/tests/view/656817fc136acbc573552304 LLR: 2.94 (-2.94,2.94) <0.50,2.50> Total: 200376 W: 49700 L: 49036 D: 101640 Ptnml(0-2): 110, 22584, 54137, 23246, 111 closes #4897 Bench: 1403703
diff --git a/src/search.cpp b/src/search.cpp
@@ -780,7 +780,7 @@ Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, boo
              >= beta
         && eval >= beta && eval < 29462  // smaller than TB wins
         && (!ttMove || ttCapture))
-        return eval;
+        return (eval + beta) / 2;
 
     // Step 9. Null move search with verification search (~35 Elo)
     if (!PvNode && (ss - 1)->currentMove != MOVE_NULL && (ss - 1)->statScore < 17257 && eval >= beta
