用Java实现重用搜索结果的迭代的Anytime repair A*算法

Anytime Repair A* (ARA*)算法是一种启发式搜索算法，旨在快速找到最优解，同时允许在搜索过程中随时停止并返回当前最好的解决方案。 ARA*基于A*算法，但使用了两个重要的技术：任意时间截止和重用搜索结果。重用搜索结果是指，在搜索过程中，如果找到一个状态，它已经被搜索过了，那么可以使用之前的搜索结果来加速搜索过程，从而减少搜索时间。

下面是用Java实现重用搜索结果的迭代的Anytime repair A*算法的示例代码：

import java.util.*;

public class ARAStar {
    private static final int MAX_COST = Integer.MAX_VALUE;
    private Map<String, Integer> gScore;
    private Map<String, Integer> hScore;
    private Map<String, Integer> fScore;
    private PriorityQueue<String> openSet;
    private Set<String> closedSet;
    private Map<String, String> cameFrom;

    private int goalX, goalY;
    private int[][] obstacles;

    public ARAStar(int[][] obstacles, int goalX, int goalY) {
        this.gScore = new HashMap<>();
        this.hScore = new HashMap<>();
        this.fScore = new HashMap<>();
        this.openSet = new PriorityQueue<>(Comparator.comparingInt(fScore::get));
        this.closedSet = new HashSet<>();
        this.cameFrom = new HashMap<>();
        this.obstacles = obstacles;
        this.goalX = goalX;
        this.goalY = goalY;
    }

    public List<String> search(String start) {
        gScore.put(start, 0);
        hScore.put(start, heuristic(start));
        fScore.put(start, hScore.get(start));
        openSet.add(start);

        while (!openSet.isEmpty()) {
            String current = openSet.poll();

            if (isGoal(current)) {
                return reconstructPath(current);
            }

            if (fScore.get(current) > getMaxFScore()) {
                continue;
            }

            closedSet.add(current);

            for (String neighbor : getNeighbors(current)) {
                if (closedSet.contains(neighbor)) {
                    continue;
                }

                int tentativeGScore = gScore.get(current) + getCost(current, neighbor);

                if (!gScore.containsKey(neighbor) || tentativeGScore < gScore.get(neighbor)) {
                    cameFrom.put(neighbor, current);
                    gScore.put(neighbor, tentativeGScore);
                    hScore.put(neighbor, heuristic(neighbor));
                    fScore.put(neighbor, tentativeGScore + hScore.get(neighbor));

                    if (!openSet.contains(neighbor)) {
                        openSet.add(neighbor);
                    }
                }
            }
        }

        return null;
    }

    private int getCost(String a, String b) {
        int ax = Integer.parseInt(a.split(",")[0]);
        int ay = Integer.parseInt(a.split(",")[1]);
        int bx = Integer.parseInt(b.split(",")[0]);
        int by = Integer.parseInt(b.split(",")[1]);

        if (obstacles[ax][ay] == MAX_COST || obstacles[bx][by] == MAX_COST) {
            return MAX_COST;
        }

        return Math.abs(ax - bx) + Math.abs(ay - by);
    }

    private List<String> getNeighbors(String node) {
        List<String> neighbors = new ArrayList<>();

        int x = Integer.parseInt(node.split(",")[0]);
        int y = Integer.parseInt(node.split(",")[1]);

        if (x > 0) {
            neighbors.add((x - 1) + "," + y);
        }

        if (y > 0) {
            neighbors.add(x + "," + (y - 1));
        }

        if (x < obstacles.length - 1) {
            neighbors.add((x + 1) + "," + y);
        }

        if (y < obstacles[0].length - 1) {
            neighbors.add(x + "," + (y + 1));
        }

        return neighbors;
    }

    private boolean isGoal(String node) {
        int x = Integer.parseInt(node.split(",")[0]);
        int y = Integer.parseInt(node.split(",")[1]);

        return x == goalX && y == goalY;
    }

    private int heuristic(String node) {
        int x = Integer.parseInt(node.split(",")[0]);
        int y = Integer.parseInt(node.split(",")[1]);

        return Math.abs(x - goalX) + Math.abs(y - goalY);
    }

    private List<String> reconstructPath(String node) {
        List<String> path = new ArrayList<>();
        path.add(node);

        while (cameFrom.containsKey(node)) {
            node = cameFrom.get(node);
            path.add(0, node);
        }

        return path;
    }

    public void setMaxFScore(int maxFScore) {
        for (String node : openSet) {
            if (fScore.get(node) > maxFScore) {
                gScore.put(node, MAX_COST);
                fScore.put(node, MAX_COST);
            }
        }
    }

    private int getMaxFScore() {
        return fScore.get(openSet.peek());
    }
}

在这个示例中，我们使用了一个二维数组来表示地图和障碍物。 `search`方法接受一个起点，并返回一条路径到达目标点。`setMaxFScore`方法用于更新当前的最大f值，以便搜索更接近目标的路径，并且可以重复使用以前的搜索结果。