- if(bestMove != 0) {
- MoveStack moves[256];
- int n, j;
- n = generate_legal_moves(pos, moves);
- for(j = 0; j < n; j++)
- if((int(moves[j].move) & 07777) == bestMove)
- return moves[j].move;
- }
+ int bookMove = 0, scoresSum = 0;
+ uint64_t key = book_key(pos);
+ BookEntry entry;
+
+ // Choose a book move among the possible moves for the given position
+ for (int idx = find_key(key); idx < bookSize; idx++)
+ {
+ read_entry(entry, idx);
+ if (entry.key != key)
+ break;
+
+ int score = entry.count;
+
+ assert(score > 0);
+
+ // Choose book move according to its score. If a move has a very
+ // high score it has more probability to be choosen then a one with
+ // lower score. Note that first entry is always chosen.
+ scoresSum += score;
+ if (int(genrand_int32() % scoresSum) < score)
+ bookMove = entry.move;