From: Marco Costalba <mcostalba@gmail.com>
Date: Sat, 3 May 2014 09:45:58 +0000 (+0200)
Subject: Add stealmate detection to evaluation
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=9e8bf82350002b826e40581100dbbcd1d65c796e;hp=dc87ec72586d090ceb81fd0600f36f55993e3e1a

Add stealmate detection to evaluation

Currently a stealmate position is misevaluated
in a negative/positive score, this leads qsearch(),
that does not detects stealmates too, to return the
wrong score and this yields to some kind of endgames
to be completely misevaluated.

With this patch is fully fixed follwing position

7k/6p1/6B1/5K1P/8/8/8/8 w - - 0 1

Also in SMP case.

Correct root cause analysys by Ronald de Man.

bench: 8678654
---

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index c2b94b75..29d71ea9 100644
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -787,6 +787,12 @@ namespace {
              sf = ScaleFactor(50 * sf / SCALE_FACTOR_NORMAL);
     }
 
+    // Stealmate detection
+    Color stm = pos.side_to_move();
+    if (   (ei.attackedBy[stm][ALL_PIECES] == ei.attackedBy[stm][KING])
+        && (!(ei.attackedBy[stm][KING] & ~ei.attackedBy[~stm][ALL_PIECES])))
+        sf = SCALE_FACTOR_DRAW;
+
     // Interpolate between a middlegame and a (scaled by 'sf') endgame score
     Value v =  mg_value(score) * int(ei.mi->game_phase())
              + eg_value(score) * int(PHASE_MIDGAME - ei.mi->game_phase()) * sf / SCALE_FACTOR_NORMAL;