Backward simplication

On top of the usual conditions
a) some opponent in front (but no lever)
b) some neighbours (in front) (but no neighbour behind or same rank)
c) < rank_5

to find out if a pawn is backward we look at the squares in front of this pawn to reach the same rank as the next neighbour.

In current master, a pawn is backward if any of those squares is controlled by an enemy pawn on an adjacent file

In this version, a pawn is ALSO backward if any of those squares is occupied by an enemy pawn.

STC:
http://tests.stockfishchess.org/tests/view/56fe7efd0ebc59301a3541f1
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 19051 W: 3557 L: 3433 D: 12061

LTC:
http://tests.stockfishchess.org/tests/view/56febc2d0ebc59301a354209
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 40810 W: 5619 L: 5526 D: 29665

Bench: 7525245

Resolves #614

diff --git a/src/pawns.cpp b/src/pawns.cpp
index 5bf0e82ea98434aee57b171d5604b9ddf3055781..86745924bcfe2f308f04a1f9290a7b22d399f27b 100644 (file)
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -99,9 +99,9 @@ namespace {
     const Square Right = (Us == WHITE ? DELTA_NE : DELTA_SW);
     const Square Left  = (Us == WHITE ? DELTA_NW : DELTA_SE);
 
-    Bitboard b, neighbours, doubled, supported, phalanx;
+    Bitboard b, neighbours, stoppers, doubled, supported, phalanx;
     Square s;
-    bool passed, isolated, opposed, backward, lever, connected;
+    bool opposed, lever, connected, backward;
     Score score = SCORE_ZERO;
     const Square* pl = pos.squares<PAWN>(Us);
     const Bitboard* pawnAttacksBB = StepAttacksBB[make_piece(Us, PAWN)];
@@ -127,55 +127,47 @@ namespace {
         e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
 
         // Flag the pawn
-        neighbours  =   ourPawns   & adjacent_files_bb(f);
-        doubled     =   ourPawns   & forward_bb(Us, s);
-        opposed     =   theirPawns & forward_bb(Us, s);
-        passed      = !(theirPawns & passed_pawn_mask(Us, s));
-        lever       =   theirPawns & pawnAttacksBB[s];
-        phalanx     =   neighbours & rank_bb(s);
-        supported   =   neighbours & rank_bb(s - Up);
-        connected   =   supported | phalanx;
-        isolated    =  !neighbours;
-
-        // Test for backward pawn.
-        // If the pawn is passed, isolated, lever or connected it cannot be
-        // backward. If there are friendly pawns behind on adjacent files
-        // or if it is sufficiently advanced, it cannot be backward either.
-        if (   (passed | isolated | lever | connected)
-            || (ourPawns & pawn_attack_span(Them, s))
-            || (relative_rank(Us, s) >= RANK_5))
+        opposed    = theirPawns & forward_bb(Us, s);
+        stoppers   = theirPawns & passed_pawn_mask(Us, s);
+        lever      = theirPawns & pawnAttacksBB[s];
+        doubled    = ourPawns   & forward_bb(Us, s);
+        neighbours = ourPawns   & adjacent_files_bb(f);
+        phalanx    = neighbours & rank_bb(s);
+        supported  = neighbours & rank_bb(s - Up);
+        connected  = supported | phalanx;
+
+        // A pawn is backward when it is behind all pawns of the same color on the
+        // adjacent files and cannot be safely advanced.
+        if (!neighbours || lever || relative_rank(Us, s) >= RANK_5)
             backward = false;
         else
         {
-            // We now know there are no friendly pawns beside or behind this
-            // pawn on adjacent files. We now check whether the pawn is
-            // backward by looking in the forward direction on the adjacent
-            // files, and picking the closest pawn there.
-            b = pawn_attack_span(Us, s) & (ourPawns | theirPawns);
-            b = pawn_attack_span(Us, s) & rank_bb(backmost_sq(Us, b));
-
-            // If we have an enemy pawn in the same or next rank, the pawn is
-            // backward because it cannot advance without being captured.
-            backward = (b | shift_bb<Up>(b)) & theirPawns;
+            // Find the backmost rank with neighbours or stoppers
+            b = rank_bb(backmost_sq(Us, neighbours | stoppers));
+
+            // The pawn is backward when it cannot safely progress to that rank:
+            // either there is a stopper in the way on this rank, or there is a
+            // stopper on adjacent file which controls the way to that rank.
+            backward = (b | shift_bb<Up>(b & adjacent_files_bb(f))) & stoppers;
+            
+            assert(!backward || !(pawn_attack_span(Them, s + Up) & neighbours));
         }
 
-        assert(opposed | passed | (pawn_attack_span(Us, s) & theirPawns));
-
         // Passed pawns will be properly scored in evaluation because we need
         // full attack info to evaluate them. Only the frontmost passed
         // pawn on each file is considered a true passed pawn.
-        if (passed && !doubled)
+        if (!(stoppers | doubled))
             e->passedPawns[Us] |= s;
 
         // Score this pawn
-        if (isolated)
+        if (!neighbours)
             score -= Isolated[opposed][f];
 
         else if (backward)
             score -= Backward[opposed];
 
         else if (!supported)
-            score -= Unsupported[more_than_one(neighbours & rank_bb(s + Up))];
+            score -= Unsupported[more_than_one(neighbours & pawnAttacksBB[s])];
 
         if (connected)
             score += Connected[opposed][!!phalanx][more_than_one(supported)][relative_rank(Us, s)];