From 84408e5cd68a9323292ddababec4d1183abeef2e Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Thu, 20 Nov 2014 12:13:00 +0100
Subject: [PATCH] Fix doubled pawns asymmetry

When evaluating double pawns we use always
lsb() to extract the frontmost square.

This breaks evaluation color symmetry as is
possible to verify with an instrumented evaluate()

  Value evaluate(const Position& pos) {

    Value v = do_evaluate<false>(pos);
    Position p = pos;
    p.flip();
    assert(v == do_evaluate<false>(p));
    return v;
  }

Passed no regression test:

STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 21035 W: 4244 L: 4122 D: 12669

LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 39839 W: 6662 L: 6572 D: 26605

bench: 8255966
---
 src/pawns.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/pawns.cpp b/src/pawns.cpp
index 1099c5a4..fe989e9b 100644
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -165,7 +165,7 @@ namespace {
             value -= UnsupportedPawnPenalty;
 
         if (doubled)
-            value -= Doubled[f] / distance<Rank>(s, Square(lsb(doubled)));
+            value -= Doubled[f] / distance<Rank>(s, frontmost_sq(Us, doubled));
 
         if (backward)
             value -= Backward[opposed][f];
-- 
2.39.2