From e6f4b5f46344d638c5e3b27cdbc966899e80e8d6 Mon Sep 17 00:00:00 2001
From: Moez Jellouli <37274752+MJZ1977@users.noreply.github.com>
Date: Fri, 27 Sep 2019 10:18:22 +0200
Subject: [PATCH] More accurate pawn attack span definition

Tweak the pawn attack span for backward pawns and the zone behind
opponent opposing pawns. This is important in positional play and
one of weaknesses of the engine in recent high level games.

STC
LLR: -2.95 (-2.94,2.94) [0.50,4.50]
Total: 66843 W: 14884 L: 14717 D: 37242
http://tests.stockfishchess.org/tests/view/5d8dcb1b0ebc590f3beb2956

LTC
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 77699 W: 12993 L: 12602 D: 52104
http://tests.stockfishchess.org/tests/view/5d8de9bc0ebc590f3beb3d00

See discussion in https://github.com/official-stockfish/Stockfish/pull/2332

Bench: 4012371
---
 src/evaluate.cpp |  2 +-
 src/pawns.cpp    | 25 ++++++++++++++++++-------
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index fadc0fea..ee98da90 100644
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -135,7 +135,7 @@ namespace {
   constexpr Score KnightOnQueen      = S( 16, 12);
   constexpr Score LongDiagonalBishop = S( 45,  0);
   constexpr Score MinorBehindPawn    = S( 18,  3);
-  constexpr Score Outpost            = S( 18,  6);
+  constexpr Score Outpost            = S( 16,  5);
   constexpr Score PassedFile         = S( 11,  8);
   constexpr Score PawnlessFlank      = S( 17, 95);
   constexpr Score RestrictedPiece    = S(  7,  7);
diff --git a/src/pawns.cpp b/src/pawns.cpp
index 50eb3aa3..8022ae51 100644
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -71,10 +71,10 @@ namespace {
     constexpr Color     Them = (Us == WHITE ? BLACK : WHITE);
     constexpr Direction Up   = (Us == WHITE ? NORTH : SOUTH);
 
-    Bitboard neighbours, stoppers, support, phalanx;
+    Bitboard neighbours, stoppers, support, phalanx, opposed;
     Bitboard lever, leverPush;
     Square s;
-    bool opposed, backward, passed, doubled;
+    bool backward, passed, doubled;
     Score score = SCORE_ZERO;
     const Square* pl = pos.squares<PAWN>(Us);
 
@@ -94,8 +94,6 @@ namespace {
 
         Rank r = relative_rank(Us, s);
 
-        e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
-
         // Flag the pawn
         opposed    = theirPawns & forward_file_bb(Us, s);
         stoppers   = theirPawns & passed_pawn_span(Us, s);
@@ -112,6 +110,17 @@ namespace {
         backward =  !(neighbours & forward_ranks_bb(Them, s))
                   && (stoppers & (leverPush | (s + Up)));
 
+        // Span of backward pawns and span behind opposing pawns are not included
+        // in the pawnAttacksSpan bitboard.
+        if (!backward || phalanx)
+        {
+            if (opposed)
+                e->pawnAttacksSpan[Us] |=  pawn_attack_span(Us, s) &
+                                          ~pawn_attack_span(Us, frontmost_sq(Them, opposed));
+            else
+                e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
+        }
+
         // A pawn is passed if one of the three following conditions is true:
         // (a) there is no stoppers except some levers
         // (b) the only stoppers are the leverPush, but we outnumber them
@@ -130,17 +139,19 @@ namespace {
         // Score this pawn
         if (support | phalanx)
         {
-            int v =  Connected[r] * (2 + bool(phalanx) - opposed)
+            int v =  Connected[r] * (2 + bool(phalanx) - bool(opposed))
                    + 21 * popcount(support);
 
             score += make_score(v, v * (r - 2) / 4);
         }
 
         else if (!neighbours)
-            score -= Isolated + WeakUnopposed * !opposed;
+            score -=   Isolated
+                     + WeakUnopposed * !opposed;
 
         else if (backward)
-            score -= Backward + WeakUnopposed * !opposed;
+            score -=   Backward
+                     + WeakUnopposed * !opposed;
 
         if (!support)
             score -=   Doubled * doubled
-- 
2.39.2