/**
* Somewhat multi-threaded depth-first search. Performs a DFS of the subtrees from the current
* node in parallel.
*
* @param s The tile sequence
* @param b The board to search
* @param pool The thread pool in which to submit jobs to.
* @return The board with the highest evaluation or null if no board can continue.
*/
private Board solve_pdfs(Board b, ExecutorService pool) {
List<Future<Board>> rets = new ArrayList<>(BOARD_WIDTH);
Board best = null;
int best_score = -1;
for (Direction d : directions) {
Board n = new Board(b);
if (n.move(tileSequence, d)) {
rets.add(pool.submit(new ParallelDFS(n)));
}
}
for (Future<Board> ret : rets) {
try {
Board c = ret.get();
if (c != null) {
int score = evaluate(c);
if (score > best_score) {
best = c;
best_score = score;
}
}
} catch (InterruptedException | ExecutionException e) {
System.err.println("Error: " + e.getMessage());
}
}
return best;
}
private Board solve_dfs(Board b, int depthLimit, int depth) {
if (depth >= depthLimit) { // Cutoff test
return b;
}
Board best = null;
int best_score = -1;
for (int i = 0; i < BOARD_WIDTH; i++) {
Board next = new Board(b);
if (next.move(tileSequence, directions[i])) {
Board candidate = solve_dfs(next, depthLimit, depth + 1);
if (candidate == null && next.finished()) {
candidate = next;
}
if (candidate != null) {
if (candidate.finished()) {
updateBest(candidate);
} else {
int score = evaluate(candidate);
if (score > best_score) {
best_score = score;
best = candidate;
}
}
}
}
}
return best;
}