Add attacker and check logic with tests

engine/src/bitboard.c

@@ -530,6 +530,50 @@ int gen_pseudo_moves(const struct Board *board, struct Move *moves, bool capture
     return count;
 }
 
+uint64_t attackers_to(const struct Board *board, int sq, enum Color by) {
+    uint64_t occ = board->occ[BOTH];
+
+    uint64_t pawns = (by == WHITE) ? board->pieces[P] : board->pieces[p];
+    uint64_t knights = (by == WHITE) ? board->pieces[N] : board->pieces[n];
+    uint64_t bishops = (by == WHITE) ? board->pieces[B] : board->pieces[b];
+    uint64_t rooks = (by == WHITE) ? board->pieces[R] : board->pieces[r];
+    uint64_t queens = (by == WHITE) ? board->pieces[Q] : board->pieces[q];
+    uint64_t kings = (by == WHITE) ? board->pieces[K] : board->pieces[k];
+
+    uint64_t attacks = 0;
+
+    attacks |= PAWN_ATTACKS[by ^ 1][sq] & pawns;
+    attacks |= KNIGHT_ATTACKS[sq] & knights;
+    attacks |= KING_ATTACKS[sq]   & kings;
+
+    // Squares than can see sq given the occupancy of both sides but intersect
+    // with the specified color pieces.
+    attacks |= bishop_attacks(sq, occ) & (bishops | queens);
+    attacks |= rook_attacks(sq, occ) & (rooks | queens);
+
+    return attacks;
+}
+
+bool in_check(const struct Board *board, enum Color side) {
+    uint64_t king_bb = (side == WHITE) ? board->pieces[K] : board->pieces[k];
+    
+    // Do we need this check here?
+    // This would be an illegal position.
+    if (!king_bb) return false;
+
+    int king_sq = pop_lsb_index(&king_bb);
+
+    // Is the king square in the attack map of opposite side?
+    return attackers_to(board, king_sq, side ^ 1) != 0;
+}
+
+bool square_attacked(const struct Board *board, int sq, enum Color by) {
+    // Should we have another way to calculate this and have
+    // earlier exits in the code? For example, we could iterate
+    // each piece type and return boolean sooner and skip the
+    // ray scanning.
+    return attackers_to(board, sq, by) != 0;
+}
 
 void print_board(const struct Board *board) {
     const char PIECE_CH[12] = {