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 #2626

Bench: 4966867

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)

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))

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;