From: Marco Costalba <mcostalba@gmail.com>
Date: Mon, 25 May 2009 14:48:47 +0000 (+0100)
Subject: Passed pawns evaluation tweak
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=738bf66a2d04d27ce8c5481fef52dc3adc3cafe1;ds=sidebyside

Passed pawns evaluation tweak

Do not penalize if in our adavncing pawn's path there are
non-pawns enemy pieces. Especially if they can be attacked
by us.

Patch is mine, but original idea and also fixing of a first, wrong,
version of the patch is from Eelco de Groot.

Tests with Joona framework seems to confirm patch is good

Results for patch 'disabled'   based on 5776 games: Win percentage:
41.309  (+- 0.526)  [+- 1.053]
Results for patch 'enabled'  based on 6400 games: Win percentage:
42.422  (+- 0.500)  [+- 1.000]

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

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 9fd4040e..372313b7 100644
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -933,29 +933,23 @@ namespace {
                         && (squares_behind(us, s) & pos.rooks_and_queens(them)))
                         b3 = b2;
 
-                    if ((b2 & pos.pieces_of_color(them)) == EmptyBoardBB)
-                    {
-                        // There are no enemy pieces in the pawn's path! Are any of the
-                        // squares in the pawn's path attacked by the enemy?
-                        if (b3 == EmptyBoardBB)
-                            // No enemy attacks, huge bonus!
-                            ebonus += Value(tr * (b2 == b4 ? 17 : 15));
-                        else
-                            // OK, there are enemy attacks. Are those squares which are
-                            // attacked by the enemy also attacked by us?  If yes, big bonus
-                            // (but smaller than when there are no enemy attacks), if no,
-                            // somewhat smaller bonus.
-                            ebonus += Value(tr * ((b3 & b4) == b3 ? 13 : 8));
-                    }
+                    // Squares attacked or occupied by enemy pieces
+                    b3 |= (b2 & pos.pieces_of_color(them));
+
+                    // There are no enemy pawns in the pawn's path
+                    assert((b2 & pos.pieces_of_color_and_type(them, PAWN)) == EmptyBoardBB);
+
+                    // Are any of the squares in the pawn's path attacked or occupied by the enemy?
+                    if (b3 == EmptyBoardBB)
+                        // No enemy attacks or pieces, huge bonus!
+                        ebonus += Value(tr * (b2 == b4 ? 17 : 15));
                     else
-                    {
-                        // There are some enemy pieces in the pawn's path. While this is
-                        // sad, we still assign a moderate bonus if all squares in the path
-                        // which are either occupied by or attacked by enemy pieces are
-                        // also attacked by us.
-                        if (((b3 | (b2 & pos.pieces_of_color(them))) & ~b4) == EmptyBoardBB)
-                            ebonus += Value(tr * 6);
-                    }
+                        // OK, there are enemy attacks or pieces (but not pawns). Are those
+                        // squares which are attacked by the enemy also attacked by us?
+                        // If yes, big bonus (but smaller than when there are no enemy attacks),
+                        // if no, somewhat smaller bonus.
+                        ebonus += Value(tr * ((b3 & b4) == b3 ? 13 : 8));
+
                     // At last, add a small bonus when there are no *friendly* pieces
                     // in the pawn's path.
                     if ((b2 & pos.pieces_of_color(us)) == EmptyBoardBB)