1

I implemented the iterative deepening a-star search(for the 8-Puzzle problem, but can accept other problems) and ran it on an input. It ran unsuccessfully for 2 hrs. For simpler inputs that are close to goal node it works fine. Others have got it to work for this input. I am not sure whether my implementation is just inefficient or goes into an infinite loop

PuzzleSolver.java$ida

    /** Accepts start node root and string identifying whihc heuristic to use
      * h1 is number of misplaced tiles and h2 is Manhattan distance
      */
    private Node ida(Node root, final String h) {
     PriorityQueue<DNode> frontier = new PriorityQueue<DNode>(10, new Comparator<DNode>(){
        @Override
        public int compare(DNode n1, DNode n2) {
            if(h == "h1") {
                if(n1.depth + h1(n1.node) > n2.depth + h1(n2.node)) return 1;
                if(n1.depth + h1(n1.node) < n2.depth + h1(n2.node)) return -1;
                return 0;
            }
            if(h == "h2") {
                if(n1.depth + h2(n1.node) > n2.depth + h2(n2.node)) return 1;
                if(n1.depth + h2(n1.node) < n2.depth + h2(n2.node)) return -1;
                return 0;
            }
            return 0;
        }});
    ArrayList<Node> explored = new ArrayList<Node>();
    Node soln = null;
    DNode start = new DNode(root, 1);
    frontier.add(start);
    int d = 0;
    int flimit = (h == "h1" ? h1(start.node) : h2(start.node));
    int min = flimit;
    while(true) {
        DNode dn = frontier.poll();
        if(dn == null) {
            frontier.add(start);
            d = 0;
            flimit = min;
            continue;
        }
        d = dn.depth;
        Node n = dn.node;
        //n.print();
        if(goalCheck(n)){
            return n;
        }
        for(int i = 0;i < ops.length;i++) {
            String op = ops[i];
            if(n.applicable(op)) {
                soln = n.applyOp(op);
                int h_cost;
                if(h == "h1") h_cost = h1(soln);
                else h_cost = h2(soln);
                if(!checkDup(explored,soln) && d + 1 + h_cost < flimit) {
                    frontier.add(new DNode(soln, d + 1));
                    DNode least = frontier.peek();
                    min = least.depth + (h == "h1" ? h1(least.node) : h2(least.node));
                }
            }
        }
        explored.add(n);
        max_list_size = Math.max(max_list_size, frontier.size() + explored.size());
    }
}

PuzzleSolver.java$CheckDup

    private boolean checkDup(ArrayList<Node> explored, Node soln) {
    boolean isDuplicate = false;
    for(Node n:explored) {
        boolean equal = true;
        for(int i = 0;i < soln.size; i++) {
            for(int j =0 ;j<soln.size;j++) {
                if(soln.state.get(i).get(j) != n.state.get(i).get(j)) {
                    equal = false; 
                }
            }
        }
        isDuplicate |= equal;
    }
    return isDuplicate;
}

Start state(failed):

1 2 3 
8 - 4
7 6 5

Goal state:

1 3 4 
8 6 2 
7 - 5

(worked for 1 3 4 8 6 0 7 5 2) I have not included Node.java because I am pretty sure it works after running other search algorithms like best-first, dfs. It is difficult to provide an SCCE, so I am just asking for help spotting any obvious bugs in the ida implementation.

EDIT: Solved issue, but still trying to figure out a termination condition when the goal is not reachable. IDA* does not keep a list of explored nodes, so how can I know if I have covered the whole solution space?

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2 回答 2

2

您的checkDup功能非常低效。我建议使用HashSet： http ://docs.oracle.com/javase/7/docs/api/java/util/HashSet.html 您的函数的成本与集合的大小成线性关系，而 的contains方法HashSet有一个常数成本。

java中的字符串与equals： Java String.equals vs ==进行比较

可能还有其他问题，但这是我在快速检查您的代码后发现的两个最明显的问题。

于 2013-11-06T22:46:41.617 回答
1

我计算新 flimit 的方式有误。在其他情况下它不会导致问题，因为后继者的 flimit 不会导致它无限循环。条件也应该 f(current node) <= cutoff。而不是我所接受的'<'。

更新后的版本：

private Node ida(Node root, final String h) {
    PriorityQueue<DNode> frontier = new PriorityQueue<DNode>(10, new Comparator<DNode>(){
        @Override
        public int compare(DNode n1, DNode n2) {
            if(h == "h1") {
                if(n1.depth + h1(n1.node) > n2.depth + h1(n2.node)) return 1;
                if(n1.depth + h1(n1.node) < n2.depth + h1(n2.node)) return -1;
                return 0;
            }
            if(h == "h2") {
                if(n1.depth + h2(n1.node) > n2.depth + h2(n2.node)) return 1;
                if(n1.depth + h2(n1.node) < n2.depth + h2(n2.node)) return -1;
                return 0;
            }
            return 0;
        }});
    ArrayList<Node> explored = new ArrayList<Node>();
    Node soln = null;
    DNode start = new DNode(root, 1);
    frontier.add(start);
    int d = 0;
    int flimit = (h == "h1" ? h1(start.node) : h2(start.node));
    int min = flimit;
    while(true) {
        DNode dn = frontier.poll();
        if(dn == null) {
            explored.clear();
            frontier.add(start);
            d = 0;
            flimit = min;
            continue;
        }
        d = dn.depth;
        Node n = dn.node;
        //n.print();
        if(goalCheck(n)){
            return n;
        }
        min = Integer.MAX_VALUE;
        for(int i = 0;i < ops.length;i++) {
            String op = ops[i];
            if(n.applicable(op)) {
                soln = n.applyOp(op);
                int h_cost;
                if(h == "h1") h_cost = h1(soln);
                else h_cost = h2(soln);
                if(!checkDup(explored,soln))    {
                    if(d + 1 + h_cost <= flimit) {
                        frontier.add(new DNode(soln, d + 1));
                    }
                    else {
                        if(d + 1 + h_cost < min)min = d + 1 + h_cost; 
                    }
                }
            }
        }
        explored.add(n);
        max_list_size = Math.max(max_list_size, frontier.size() + explored.size());
    }
}
于 2013-11-07T05:49:43.767 回答