fix: ensure hash always maps to positive index

1 files changed, 16 insertions, 10 deletions

diff --git a/src/main.c b/src/main.c
index 360a4e8..6abea38 100644
--- a/src/main.c
+++ b/src/main.c
@@ -20,6 +20,12 @@ void generate_zobrist_seed(long* seed) {
   }
 }
 
+long crop_hash(long hash) {
+  long modulo = hash % TRANSPOSITION_TABLE_SIZE;
+  if (modulo < 0) modulo += TRANSPOSITION_TABLE_SIZE;
+  return modulo;
+}
+
 int zobrist_hash(long* seed, int* board, int color_to_move) {
   long hash = 1;
 
@@ -36,7 +42,7 @@ int zobrist_hash(long* seed, int* board, int color_to_move) {
       hash ^= seed[zobrist_index];
     }
   }
-  return hash;
+  return crop_hash(hash);
 }
 
 long hash_after_move(Move move, int* board, long* zobrist_seed, long hash) {
@@ -78,7 +84,7 @@ long hash_after_move(Move move, int* board, long* zobrist_seed, long hash) {
     }
   }
 
-  return hash;
+  return crop_hash(hash);
 }
 
 
@@ -563,8 +569,8 @@ int order_moves(Move* moves, int moves_count, int* board, long hash, Transpositi
     int color = board[moves[i].origin] % 2;
     // Lookup move in transpoition table
     long move_hash = hash_after_move(moves[i], board, zobrist_seed, hash);
-    if (transposition_table[move_hash % TRANSPOSITION_TABLE_SIZE].depth > -1) {
-      moves[i].value = transposition_table[move_hash % TRANSPOSITION_TABLE_SIZE].value * (color == WHITE ? 1 : -1);
+    if (transposition_table[move_hash].depth > -1) {
+      moves[i].value = transposition_table[move_hash].value * (color == WHITE ? 1 : -1);
       cache_hits++;
     } else {
       int origin_value = get_piece_raw_value(board[moves[i].origin]);
@@ -603,16 +609,16 @@ Move minimax_search(int* board, int color, int depth, int alpha, int beta, int*
       long move_hash = hash_after_move(move, board, zobrist_seed, hash);
       int captured_piece = make_move(move, board);
 
-      int local_depth = MAX_DEPTH - depth;
+      int relative_depth = MAX_DEPTH - depth;
 
-      move.value = local_depth > 0
+      move.value = relative_depth > 0
         ? minimax_search(board, 1 - color, depth + 1, alpha, beta, metrics, hash, transposition_table, zobrist_seed).value
         : evaluate_position(board, available_moves_count, color);
 
-      if (transposition_table[move_hash % TRANSPOSITION_TABLE_SIZE].depth < local_depth) {
-        if (transposition_table[move_hash % TRANSPOSITION_TABLE_SIZE].depth == -1) TRANSPOSITION_TABLE_CARDINALITY++;
-        transposition_table[move_hash % TRANSPOSITION_TABLE_SIZE].value = move.value;
-        transposition_table[move_hash % TRANSPOSITION_TABLE_SIZE].depth = local_depth;
+      if (transposition_table[move_hash].depth < relative_depth) {
+        if (transposition_table[move_hash].depth == -1) TRANSPOSITION_TABLE_CARDINALITY++;
+        transposition_table[move_hash].value = move.value;
+        transposition_table[move_hash].depth = relative_depth;
       }
 
       unmake_move(move, captured_piece, board);