From: Marco Costalba <mcostalba@gmail.com>
Date: Thu, 20 Nov 2014 11:13:00 +0000 (+0100)
Subject: Fix doubled pawns asymmetry
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=84408e5cd68a9323292ddababec4d1183abeef2e;hp=79232be02a03a5e2225b30f843e9597fd85951dc

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
---

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];