Rewrite backward pawn detection

[stockfish] / src / pawns.cpp

diff --git a/src/pawns.cpp b/src/pawns.cpp
index 5a65422cb009b42360c0ef2c5b83134ad7c75b01..d65b0418f0145f448c3d37dff8ecb7262c86fdaf 100644 (file)
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -92,7 +92,6 @@ namespace {
     Bitboard b;
     Square s;
     File f;
-    Rank r;
     bool passed, isolated, doubled, opposed, chain, backward, candidate;
     Score value = SCORE_ZERO;
     const Square* pl = pos.list<PAWN>(Us);
@@ -113,13 +112,12 @@ namespace {
         assert(pos.piece_on(s) == make_piece(Us, PAWN));
 
         f = file_of(s);
-        r = rank_of(s);
 
         // This file cannot be semi-open
         e->semiopenFiles[Us] &= ~(1 << f);
 
         // Our rank plus previous one. Used for chain detection
-        b = rank_bb(r) | rank_bb(Us == WHITE ? r - Rank(1) : r + Rank(1));
+        b = rank_bb(s) | rank_bb(s - pawn_push(Us));
 
         // Flag the pawn as passed, isolated, doubled or member of a pawn
         // chain (but not the backward one).
@@ -142,16 +140,12 @@ namespace {
             // We now know that 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 seeing whether we meet a friendly or an enemy pawn first.
-            b = pos.attacks_from<PAWN>(s, Us);
+            // 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));
 
-            // Note that we are sure to find something because pawn is not passed
-            // nor isolated, so loop is potentially infinite, but it isn't.
-            while (!(b & (ourPawns | theirPawns)))
-                b = shift_bb<Up>(b);
-
-            // The friendly pawn needs to be at least two ranks closer than the
-            // enemy pawn in order to help the potentially backward pawn advance.
+            // 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;
         }
 
@@ -226,18 +220,16 @@ Value Entry::shelter_storm(const Position& pos, Square ksq) {
   Bitboard ourPawns = b & pos.pieces(Us);
   Bitboard theirPawns = b & pos.pieces(Them);
   Rank rkUs, rkThem;
-  File kf = file_of(ksq);
-
-  kf = (kf == FILE_A) ? FILE_B : (kf == FILE_H) ? FILE_G : kf;
+  File kf = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
 
   for (int f = kf - 1; f <= kf + 1; f++)
   {
       b = ourPawns & FileBB[f];
-      rkUs = b ? relative_rank(Us, Us == WHITE ? lsb(b) : msb(b)) : RANK_1;
+      rkUs = b ? relative_rank(Us, backmost_sq(Us, b)) : RANK_1;
       safety -= ShelterWeakness[rkUs];
 
       b  = theirPawns & FileBB[f];
-      rkThem = b ? relative_rank(Us, Us == WHITE ? lsb(b) : msb(b)) : RANK_1;
+      rkThem = b ? relative_rank(Us, frontmost_sq(Them, b)) : RANK_1;
       safety -= StormDanger[rkUs == RANK_1 ? 0 : rkThem == rkUs + 1 ? 2 : 1][rkThem];
   }