From: Chris Caino <chricainogithub@gmail.com>
Date: Mon, 14 Oct 2013 23:09:05 +0000 (+0100)
Subject: Bug fix for KQKRPs endgame
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=2bf18bfc6396ae7292f57fc021b390fc05cd0f95;ds=sidebyside

Bug fix for KQKRPs endgame

This lost position 8/8/3q4/8/5k2/2P1R3/2K2P2/8 w - - 0 1
was previously evaluated as a draw.

The king and rook need to be correctly placed with
respect to the _same_ pawn.

(Note also that the check for the pawn being on RANK_2
in the old version is redundant: it must be on RANK_2 if
it hopes to protect a rook on RANK_3)

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---

diff --git a/src/endgame.cpp b/src/endgame.cpp
index b9ac17a2..5906b4d5 100644
--- a/src/endgame.cpp
+++ b/src/endgame.cpp
@@ -464,10 +464,10 @@ ScaleFactor Endgame<KQKRPs>::operator()(const Position& pos) const {
 
   if (    relative_rank(weakSide, kingSq) <= RANK_2
       &&  relative_rank(weakSide, pos.king_square(strongSide)) >= RANK_4
-      && (pos.pieces(weakSide, ROOK) & rank_bb(relative_rank(weakSide, RANK_3)))
-      && (pos.pieces(weakSide, PAWN) & rank_bb(relative_rank(weakSide, RANK_2)))
-      && (pos.attacks_from<KING>(kingSq) & pos.pieces(weakSide, PAWN))
-      && (pos.attacks_from<PAWN>(rsq, strongSide) & pos.pieces(weakSide, PAWN)))
+      &&  relative_rank(weakSide, rsq) == RANK_3
+      && (  pos.pieces(weakSide, PAWN)
+          & pos.attacks_from<KING>(kingSq)
+          & pos.attacks_from<PAWN>(rsq, strongSide)))
           return SCALE_FACTOR_DRAW;
 
   return SCALE_FACTOR_NONE;