fix: compute zobrist hash before applying move

1 files changed, 14 insertions, 12 deletions

diff --git a/src/main.c b/src/main.c
index 74d83ce..7985d41 100644
--- a/src/main.c
+++ b/src/main.c
@@ -19,16 +19,19 @@ void generate_zobrist_seed(int* seed) {
 }
 
 int zobrist_hash(int* seed, int* board, int color_to_move) {
-  int result = seed[color_to_move];
+  int result = 1;
+
+  // Last feature means white to move
+  if (color_to_move == WHITE) result ^= seed[MAX_ZOBRIST_SEEDS - 1];
+
   for (int rank = 7; rank >= 0; rank--) {
     for (int file = 0; file < 8; file++) {
       int position = rank * 16 + file;
       int piece = board[position];
-      if (piece != EMPTY) {
-        // Unique index of this piece on this position
-        int zobrist_index = (piece * 128) + position;
-        result ^= seed[zobrist_index];
-      }
+
+      // Unique index of this piece on this position
+      int zobrist_index = piece * 128 + position;
+      result ^= seed[zobrist_index];
     }
   }
   return result;
@@ -38,11 +41,10 @@ int apply_move_zobrist(Move move, int* board, int* zobrist_seed, int hash) {
   int piece = board[move.origin];
   int target_piece = board[move.destination];
 
+  hash ^= zobrist_seed[MAX_ZOBRIST_SEEDS - 1]; // Flip color
   hash ^= zobrist_seed[piece * 128 + move.origin]; // Remove piece from origin
   hash ^= zobrist_seed[piece * 128 + move.destination]; // Add piece to destination
   hash ^= zobrist_seed[target_piece * 128 + move.destination]; // Remove target piece from destination
-  hash ^= zobrist_seed[1 - piece % 2]; // It's now opponent's turn
-  hash ^= zobrist_seed[piece % 2]; // Not a current color's turn
 
   return hash;
 }
@@ -509,12 +511,13 @@ Move find_best_move(int* board, int color, int depth, int alpha, int beta, int*
     Move available_moves[MAX_AVAILABLE_MOVES];
     int available_moves_count = list_available_moves(available_moves, board, color);
     int cache_hits = sort_moves(available_moves, available_moves_count, board, hash, transposition_table, zobrist_seed);
-    if (cache_hits && (double)cache_hits / (double)available_moves_count > 0.05) printf("More than 5 percent cache hits (%i)\n", cache_hits);
+    if (cache_hits && (double)cache_hits / (double)available_moves_count > 0.75) printf("More than 75 percent cache hits (%i)\n", cache_hits);
 
     for (int i = 0; i < available_moves_count; i++) {
       *metrics += 1;
       Move move = available_moves[i];
 
+      int move_hash = apply_move_zobrist(move, board, zobrist_seed, hash);
       int captured_piece = apply_move(move, board);
 
       move.value = depth < MAX_DEPTH
@@ -522,7 +525,6 @@ Move find_best_move(int* board, int color, int depth, int alpha, int beta, int*
         : compute_score(board, available_moves_count, color);
 
       if (depth < MAX_DEPTH) {
-        int move_hash = apply_move_zobrist(move, board, zobrist_seed, hash);
         transposition_table[move_hash % TRANSPOSITION_TABLE_SIZE] = move.value;
       }
 
@@ -559,7 +561,7 @@ int main() {
   print_board(board);
 
   while (1) {
-    printf("Zobrist hash: %i\n", hash);
+    printf("Expected zobrist hash: %i (found %i)\n", hash, zobrist_hash(zobrist_seed, board, color));
 
     if (color == PLAYER) {
       printf("Enter a move for %s:\n", COLORS[color]);
@@ -586,8 +588,8 @@ int main() {
       printf("Checkmate, %s is victorious!\n", COLORS[1 - color]);
       break;
     } else {
-      apply_move(move, board);
       hash = apply_move_zobrist(move, board, zobrist_seed, hash);
+      apply_move(move, board);
 
       print_board(board);