Simplify the backward pawns code

[stockfish] / src / pawns.cpp

diff --git a/src/pawns.cpp b/src/pawns.cpp
index 72556172cb51cef20e3815ed5a4c1271b9b202bb..ca9c302322be35fcc8383cd9f3b925691888fdf9 100644 (file)
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -45,7 +45,7 @@ namespace {
 
   // Strength of pawn shelter for our king by [distance from edge][rank].
   // RANK_1 = 0 is used for files where we have no pawn, or pawn is behind our king.
-  Value ShelterStrength[int(FILE_NB) / 2][RANK_NB] = {
+  constexpr Value ShelterStrength[int(FILE_NB) / 2][RANK_NB] = {
     { V( -9), V(64), V(77), V( 44), V( 4), V( -1), V(-11) },
     { V(-15), V(83), V(51), V(-10), V( 1), V(-10), V(-28) },
     { V(-18), V(84), V(27), V(-12), V(21), V( -7), V(-36) },
@@ -56,10 +56,6 @@ namespace {
   // For the unopposed and unblocked cases, RANK_1 = 0 is used when opponent has
   // no pawn on the given file, or their pawn is behind our king.
   constexpr Value StormDanger[][4][RANK_NB] = {
-    { { V( 0),  V(-290), V(-274), V(57), V(41) },  // BlockedByKing
-      { V( 0),  V(  60), V( 144), V(39), V(13) },
-      { V( 0),  V(  65), V( 141), V(41), V(34) },
-      { V( 0),  V(  53), V( 127), V(56), V(14) } },
     { { V( 4),  V(  73), V( 132), V(46), V(31) },  // Unopposed
       { V( 1),  V(  64), V( 143), V(26), V(13) },
       { V( 1),  V(  47), V( 110), V(44), V(24) },
@@ -120,22 +116,10 @@ namespace {
         phalanx    = neighbours & rank_bb(s);
         supported  = neighbours & rank_bb(s - Up);
 
-        // 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
-        {
-            // 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<Up>(b & adjacent_files_bb(f))) & stoppers;
-
-            assert(!(backward && (forward_ranks_bb(Them, s + Up) & neighbours)));
-        }
+        // A pawn is backward when it is behind all pawns of the same color
+        // on the adjacent files and cannot be safely advanced.
+        backward = !lever && !(ourPawns & pawn_attack_span(Them, s + Up))
+                          &&  (stoppers & (leverPush | (s + Up)));
 
         // Passed pawns will be properly scored in evaluation because we need
         // full attack info to evaluate them. Include also not passed pawns
@@ -183,7 +167,7 @@ namespace Pawns {
 
 void init() {
 
-  static constexpr int Seed[RANK_NB] = { 0, 13, 24, 18, 76, 100, 175, 330 };
+  static constexpr int Seed[RANK_NB] = { 0, 13, 24, 18, 65, 100, 175, 330 };
 
   for (int opposed = 0; opposed <= 1; ++opposed)
       for (int phalanx = 0; phalanx <= 1; ++phalanx)
@@ -228,9 +212,10 @@ Entry* probe(const Position& pos) {
 template<Color Us>
 Value Entry::evaluate_shelter(const Position& pos, Square ksq) {
 
-  enum { BlockedByKing, Unopposed, BlockedByPawn, Unblocked };
+  enum { Unopposed, BlockedByPawn, Unblocked };
   constexpr Color     Them = (Us == WHITE ? BLACK : WHITE);
   constexpr Direction Down = (Us == WHITE ? SOUTH : NORTH);
+  constexpr Bitboard  BlockRanks = (Us == WHITE ? Rank1BB | Rank2BB : Rank8BB | Rank7BB);
 
   Bitboard b = pos.pieces(PAWN) & (forward_ranks_bb(Us, ksq) | rank_bb(ksq));
   Bitboard ourPawns = b & pos.pieces(Us);
@@ -238,6 +223,9 @@ Value Entry::evaluate_shelter(const Position& pos, Square ksq) {
 
   Value safety = (ourPawns & file_bb(ksq)) ? Value(5) : Value(-5);
 
+  if ((shift<Down>(theirPawns) & (FileABB | FileHBB) & BlockRanks) & ksq)
+      safety += 374;
+
   File center = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
   for (File f = File(center - 1); f <= File(center + 1); ++f)
   {
@@ -249,11 +237,9 @@ Value Entry::evaluate_shelter(const Position& pos, Square ksq) {
 
       int d = std::min(f, ~f);
       safety +=  ShelterStrength[d][rkUs]
-               - StormDanger
-                 [(shift<Down>(b) & ksq) ? BlockedByKing :
-                  rkUs   == RANK_1       ? Unopposed     :
-                  rkThem == (rkUs + 1)   ? BlockedByPawn : Unblocked]
-                 [d][rkThem];
+               - StormDanger[rkUs == RANK_1     ? Unopposed     :
+                             rkUs == rkThem - 1 ? BlockedByPawn : Unblocked]
+                            [d][rkThem];
   }
 
   return safety;