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--- chess:programming:pawn_hash_table [2021/10/12 12:39] – created peter
+++ chess:programming:pawn_hash_table [2022/01/06 21:54] (current) – peter
@@ Line 35: / Line 35: @@
 This can be further reduced by mirroring the board, considering symmetric positions or detecting illegal positions.
+----
+===== Potential Pawn Hash Table =====
+mod(49981), which is an appropriate size for a pawn hash table:
+<code cpp>
+unsigned int mod49981 (unsigned __int64 a)
+{
+  // (2**64 + 49981 - 1) / 49981
+  // is not exact divisible without remainder
+  // 369075130023601.0003001140433 ~ 0x14FAC00053EB1
+  unsigned __int64 m = (0x00014FAC00053EB1 * (a >> 32)
+                      + 0x00014FAC * (a & 0xffffffff));
+  int modulus = (int)(a - (__int64)(m>>32)* 49981);
+  if ( modulus < 0 )  // correct negative results
+    modulus += 49981; // due to the remainder of 0.0003
+  return (unsigned int) modulus;
+}
+</code>
+----
+Information to store include:
+  * Color information.
+  * It might be useful to hash the king positions,when much of your pawn evaluation depends on king positions.
+Without king positions in pseudo-C
+<code>
+hash_t RandomIndex[64][2];  // Initialize by Pseudo random numbers.
+  PawnHashIndex = 0;
+  for (pos = A1; pos <= H8; pos++)
+    if (figure[pos] == pawn)
+      PawnHashIndex ^= RandomIndex[pos][color[pos]];
+</code>
+Probably do not do this after every move.
+If you move a pawn , you can update the PawnHashIndex by
+<code>
+// Update the pawn hash for "normal" pawn move.
+PawnHashIndex ^= RandomIndex[from][side]; // Clear the old pawn position.
+PawnHashIndex ^= RandomIndex[to][side];   // Put in the new pawn position.
+</code>
+Similar code is needed for captures of a pawn and pawn promotions.