Further streamline connected pawn evaluation

Make even more clear what are the terms that
contribute to evaluate connected pawns, and
completely separate them from the weights
that are now fully looked up in a table.

For future tuning makes sense to init the table with
a formula instead of hard-code it. This allows to
reduce problem space cardinality and makes tuning
easier.

And fix a MSVC warning while there:
warning C4804: '>>' : unsafe use of type 'bool' in operation

No functional change.

src/main.cpp

@@ -37,6 +37,7 @@ int main(int argc, char* argv[]) {
   Bitbases::init_kpk();
   Search::init();
   Eval::init();
+  Pawns::init();
   Threads.init();
   TT.resize(Options["Hash"]);
 

src/pawns.cpp

@@ -49,8 +49,8 @@ namespace {
   { S(20, 28), S(29, 31), S(33, 31), S(33, 31),
     S(33, 31), S(33, 31), S(29, 31), S(20, 28) } };
 
-  // Connected bonus by rank
+  // Connected pawn bonus by opposed, phalanx flags and rank
-  const int Connected[RANK_NB] = {0, 6, 15, 10, 57, 75, 135, 258};
+  Score Connected[2][2][RANK_NB];
 
   // Levers bonus by rank
   const Score Lever[RANK_NB] = {
@@ -89,9 +89,9 @@ namespace {
     const Square Right = (Us == WHITE ? DELTA_NE : DELTA_SW);
     const Square Left  = (Us == WHITE ? DELTA_NW : DELTA_SE);
 
-    Bitboard b, p, doubled;
+    Bitboard b, p, doubled, connected;
     Square s;
-    bool passed, isolated, opposed, connected, backward, unsupported, lever;
+    bool passed, isolated, opposed, phalanx, backward, unsupported, lever;
     Score value = SCORE_ZERO;
     const Square* pl = pos.list<PAWN>(Us);
     const Bitboard* pawnAttacksBB = StepAttacksBB[make_piece(Us, PAWN)];
@@ -111,7 +111,6 @@ namespace {
     {
         assert(pos.piece_on(s) == make_piece(Us, PAWN));
 
-        Rank r = rank_of(s), rr = relative_rank(Us, s);
         File f = file_of(s);
 
         // This file cannot be semi-open
@@ -120,12 +119,10 @@ namespace {
         // Previous rank
         p = rank_bb(s - pawn_push(Us));
 
-        // Our rank plus previous one
-        b = rank_bb(s) | p;
-
         // Flag the pawn as passed, isolated, doubled,
         // unsupported or connected (but not the backward one).
-        connected   =   ourPawns   & adjacent_files_bb(f) & b;
+        connected   =   ourPawns   & adjacent_files_bb(f) & (rank_bb(s) | p);
+        phalanx     =   connected  & rank_bb(s);
         unsupported = !(ourPawns   & adjacent_files_bb(f) & p);
         isolated    = !(ourPawns   & adjacent_files_bb(f));
         doubled     =   ourPawns   & forward_bb(Us, s);
@@ -176,15 +173,11 @@ namespace {
         if (backward)
             value -= Backward[opposed][f];
 
-        if (connected) {
+        if (connected)
-            int bonus = Connected[rr];
+            value += Connected[opposed][phalanx][relative_rank(Us, s)];
-            if (ourPawns & adjacent_files_bb(f) & rank_bb(r))
-                bonus += (Connected[rr+1] - Connected[rr]) / 2;
-            value += make_score(bonus / 2, bonus >> opposed);
-        }
 
         if (lever)
-            value += Lever[rr];
+            value += Lever[relative_rank(Us, s)];
     }
 
     b = e->semiopenFiles[Us] ^ 0xFF;
@@ -201,6 +194,19 @@ namespace {
 
 namespace Pawns {
 
+void init()
+{
+  const int c[RANK_NB] = {0, 6, 15, 10, 57, 75, 135, 258};
+
+  for (Rank r = RANK_2; r <= RANK_7; ++r)
+      for (int opposed = false; opposed <= true; ++opposed)
+          for (int phalanx = false; phalanx <= true; ++phalanx)
+          {
+              int bonus = c[r] + (phalanx ? (c[r + 1] - c[r]) / 2 : 0);
+              Connected[opposed][phalanx][r] = make_score(bonus / 2, bonus >> opposed);
+          }
+}
+
 /// probe() takes a position as input, computes a Entry object, and returns a
 /// pointer to it. The result is also stored in a hash table, so we don't have
 /// to recompute everything when the same pawn structure occurs again.

src/pawns.h

@@ -79,6 +79,7 @@ struct Entry {
 
 typedef HashTable<Entry, 16384> Table;
 
+void init();
 Entry* probe(const Position& pos, Table& entries);
 
 } // namespace Pawns