Calculate min distance between king and his pawns

Just added infrastructure.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/bitboard.cpp

@@ -45,6 +45,7 @@ Bitboard ThisAndAdjacentFilesBB[8];
 Bitboard InFrontBB[2][8];
 Bitboard StepAttacksBB[16][64];
 Bitboard BetweenBB[64][64];
+Bitboard DistanceRingsBB[64][8];
 Bitboard ForwardBB[2][64];
 Bitboard PassedPawnMask[2][64];
 Bitboard AttackSpanMask[2][64];
@@ -195,6 +196,12 @@ void Bitboards::init() {
       for (Square s2 = SQ_A1; s2 <= SQ_H8; s2++)
           SquareDistance[s1][s2] = std::max(file_distance(s1, s2), rank_distance(s1, s2));
 
+  for (Square s1 = SQ_A1; s1 <= SQ_H8; s1++)
+      for (int d = 1; d < 8; d++)
+          for (Square s2 = SQ_A1; s2 <= SQ_H8; s2++)
+              if (SquareDistance[s1][s2] == d)
+                  DistanceRingsBB[s1][d - 1] |= s2;
+
   for (int i = 0; i < 64; i++)
       if (!Is64Bit) // Matt Taylor's folding trick for 32 bit systems
       {

src/bitboard.h

@@ -50,6 +50,7 @@ extern Bitboard ThisAndAdjacentFilesBB[8];
 extern Bitboard InFrontBB[2][8];
 extern Bitboard StepAttacksBB[16][64];
 extern Bitboard BetweenBB[64][64];
+extern Bitboard DistanceRingsBB[64][8];
 extern Bitboard ForwardBB[2][64];
 extern Bitboard PassedPawnMask[2][64];
 extern Bitboard AttackSpanMask[2][64];

src/pawns.cpp

@@ -261,6 +261,11 @@ Score PawnEntry::update_safety(const Position& pos, Square ksq) {
 
   kingSquares[Us] = ksq;
   castleRights[Us] = pos.can_castle(Us);
+  minKPdistance[Us] = 0;
+
+  Bitboard pawns = pos.pieces(Us, PAWN);
+  if (pawns)
+      while (!(DistanceRingsBB[ksq][minKPdistance[Us]++] & pawns)) {}
 
   if (relative_rank(Us, ksq) > RANK_4)
       return kingSafety[Us] = SCORE_ZERO;

src/pawns.h

@@ -59,6 +59,7 @@ private:
   Bitboard passedPawns[2];
   Bitboard pawnAttacks[2];
   Square kingSquares[2];
+  int minKPdistance[2];
   int castleRights[2];
   Score value;
   int halfOpenFiles[2];