Scale up space weight with number of blocked pawns

This idea is loosely based on stockfish losses in closed positions in different tournaments. Space weight symmetrically increases for both sides the more blocked position is.

passed STC
https://tests.stockfishchess.org/tests/view/5e919eefaf0a0143109dc8ce
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 16994 W: 3389 L: 3172 D: 10433
Ptnml(0-2): 277, 1931, 3918, 2040, 331

passed LTC
https://tests.stockfishchess.org/tests/view/5e91d04faf0a0143109dc8ea
LLR: 2.94 (-2.94,2.94) {0.25,1.75}
Total: 133386 W: 17316 L: 16763 D: 99307
Ptnml(0-2): 945, 12407, 39524, 12784, 1033

closes https://github.com/official-stockfish/Stockfish/pull/2626

Bench: 4966867

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 2698c813dfab118e78a37d1d70d8d1584c4d1e59..cd535d885fc2e617d42c90588f2127f69e6fedfd 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -695,7 +695,7 @@ namespace {
     behind |= shift<Down+Down>(behind);
 
     int bonus = popcount(safe) + popcount(behind & safe & ~attackedBy[Them][ALL_PIECES]);
-    int weight = pos.count<ALL_PIECES>(Us) - 1;
+    int weight = pos.count<ALL_PIECES>(Us) - 2 + pe->blocked_count() / 2;
     Score score = make_score(bonus * weight * weight / 16, 0);
 
     if (T)

diff --git a/src/pawns.cpp b/src/pawns.cpp
index 63bc596fcba4cbb9b280c243af89fccc7fc7bff3..7c4eda0f0b07dc256e48db24327f2be4c634d707 100644 (file)
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -86,6 +86,7 @@ namespace {
     e->passedPawns[Us] = 0;
     e->kingSquares[Us] = SQ_NONE;
     e->pawnAttacks[Us] = e->pawnAttacksSpan[Us] = pawn_attacks_bb<Us>(ourPawns);
+    e->blockedCount[Us] = 0;
 
     // Loop through all pawns of the current color and score each pawn
     while ((s = *pl++) != SQ_NONE)
@@ -105,6 +106,8 @@ namespace {
         phalanx    = neighbours & rank_bb(s);
         support    = neighbours & rank_bb(s - Up);
 
+        e->blockedCount[Us] += bool(blocked);
+
         // A pawn is backward when it is behind all pawns of the same color on
         // the adjacent files and cannot safely advance.
         backward =  !(neighbours & forward_ranks_bb(Them, s + Up))

diff --git a/src/pawns.h b/src/pawns.h
index bd17618f5d8f31fd5009d914b76d78b1a9a7e271..41a88c6f3936935a4475df4e0a28b3a63ffca55d 100644 (file)
--- a/src/pawns.h
+++ b/src/pawns.h
@@ -38,6 +38,7 @@ struct Entry {
   Bitboard passed_pawns(Color c) const { return passedPawns[c]; }
   Bitboard pawn_attacks_span(Color c) const { return pawnAttacksSpan[c]; }
   int passed_count() const { return popcount(passedPawns[WHITE] | passedPawns[BLACK]); }
+  int blocked_count() const { return blockedCount[WHITE] + blockedCount[BLACK]; }
 
   template<Color Us>
   Score king_safety(const Position& pos) {
@@ -59,6 +60,7 @@ struct Entry {
   Square kingSquares[COLOR_NB];
   Score kingSafety[COLOR_NB];
   int castlingRights[COLOR_NB];
+  int blockedCount[COLOR_NB];
 };
 
 typedef HashTable<Entry, 131072> Table;