A*算法 JAVA实现

A*算法的java实现

本文内容参考于 —— [ 理解A*寻路算法具体过程 ]

A*算法是一种启发式最小代价寻路算法，本文是在参考博文的基础上了解A*算法思想之后，使用java实现的，做个记录。
整个过程抽象：

把起始格添加到 "开启列表" 
do 
{ 
       寻找开启列表中F值最低的格子, 我们称它为当前格. 
       把它切换到关闭列表. 
       对当前格相邻的8格中的每一个 
          if (它不可通过 || 已经在 "关闭列表" 中) 
          { 
                什么也不做. 
          } 
          if (它不在开启列表中) 
          { 
                把它添加进 "开启列表", 把当前格作为这一格的父节点, 计算这一格的 FGH 
          }
          if (它已经在开启列表中) 
          { 
                if (用G值为参考检查新的路径是否更好, 更低的G值意味着更好的路径) 
                    { 
                            把这一格的父节点改成当前格, 并且重新计算这一格的 GF 值. 
                    } 
          }
          目标格已经在 "开启列表", 这时候路径被找到跳出循环；
} while(开启列表不为空) 
如果开启列表已经空了,目标格没找到 说明路径不存在.
最后从目标格开始, 沿着每一格的父节点移动直到回到起始格, 这就是路径.

注：这个实现支持斜着走， 如果要实现不支持走沿对角的斜线，可以在此实现的基础上稍作修改即可实现。

---

JAVA实现的代码如下：

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.Stack;

public class AStarAlgorithm {

    private static final int[][] DIREC = {{-1, 0}, {-1, 1}, {0, 1}, {1, 1}, 
        {1, 0}, {1, -1}, {0, -1}, {-1, -1}};

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("please enter (rows cols x1 y1 x2 y2): ");
        final int rows = scanner.nextInt();
        final int cols = scanner.nextInt();
        int x1 = scanner.nextInt();
        int y1 = scanner.nextInt();
        int x2 = scanner.nextInt();
        int y2 = scanner.nextInt();
        scanner.close();

        // generate a two-dimension array filled with 0
        int map[][] = new int[rows][cols];
        for (int i = 0; i < rows; i++) {
            int tmp[] = new int[cols];
            Arrays.fill(tmp, 0);
            map[i] = tmp;
        }
        int midr = rows / 2;
        int midc = cols / 2;
        /*map[midr - 1][midc] = 1;
        map[midr][midc] = 1;
        map[midr + 1][midc] = 1;*/

        for (int i = 1; i < rows - 1; i++) {
            map[i][midc] = 1;
        }
        map[2][6] = 1;
        map[3][6] = 1;
        map[4][6] = 1;
        map[5][6] = 1;


        findPath(map, x1, y1, x2, y2);
    }

    private static void findPath(int[][] map, int x1, int y1, int x2, int y2) {
        List openList = new ArrayList();
        List closeList = new ArrayList();
        boolean findFlag = false;
        Position termPos = null;
        // 起始点
        Position startPos = new Position(x1, y1, calcH(x1, y1, x2, y2));
        openList.add(startPos);
        do {
            // 通过在开启列表中找到F值最小的点作为当前点
            Position currentPos = openList.get(0);
            for (int i = 0; i < openList.size(); i++) {
                if (currentPos.F > openList.get(i).F) {
                    currentPos = openList.get(i);
                }
            }
            // 将找到的当前点放到关闭列表中，并从开启列表中删除
            closeList.add(currentPos);
            openList.remove(currentPos);

            //遍历当前点对应的8个相邻点
            for (int i = 0; i < DIREC.length; i++) {
                int tmpX = currentPos.row + DIREC[i][0];
                int tmpY = currentPos.col + DIREC[i][1];
                if (tmpX < 0 || tmpX >= map.length || tmpY < 0 || tmpY >= map[0].length) {
                    continue;
                }
                //创建对应的点
                Position tmpPos = new Position(tmpX, tmpY, calcH(tmpX, tmpY, x2, y2), currentPos);
                //map中对应的格子中的值为1（障碍）， 或对应的点已经在关闭列表中
                if (map[tmpX][tmpY] == 1 || closeList.contains(tmpPos)) {
                    continue;
                }
                //如果不在开启列表中，则加入到开启列表
                if (!openList.contains(tmpPos)) {
                    openList.add(tmpPos);
                } else {
                    // 如果已经存在开启列表中，则用G值考察新的路径是否更好，如果该路径更好，则把父节点改成当前格并从新计算FGH
                    Position prePos = null;
                    for (Position pos : openList) {
                        if (pos.row == tmpX && pos.col == tmpY) {
                            prePos = pos;
                            break;
                        }
                    }
                    if (tmpPos.G < prePos.G) {
                        prePos.setFaPos(currentPos);
                    }
                }
            }
            // 判断终点是否在开启列表中
            for (Position tpos : openList) {
                if (tpos.row == x2 && tpos.col == y2) {
                    termPos = tpos;
                    findFlag = true;
                    break;
                }
            }

        } while(openList.size() != 0);

        if(!findFlag) {
            System.out.println("no valid path!");
            return;
        }

        Stack resStack = new Stack();
        String pattern = "(%d, %d)";
        if (termPos != null) {
            resStack.push(String.format(pattern, termPos.row, termPos.col));
            while(termPos.fa != null) {
                termPos = termPos.fa;
                resStack.push(String.format(pattern, termPos.row, termPos.col));
            }
        }
        StringBuilder sb = new StringBuilder();
        while (!resStack.empty()) {
            sb.append(resStack.pop());
            if (!resStack.empty()) {
                sb.append(" -> ");
            }
        }
        System.out.println(sb.toString());
    }

    /**
     * 计算某个格子的H值
     * @param x
     * @param y
     * @param tx 终点的x值
     * @param ty 终点的y值
     * @return
     */
    private static int calcH(int x, int y, int tx, int ty) {
        int diff = Math.abs(x - tx) + Math.abs(y - ty);
        return diff * 10;
    }


    static class Position {
        public int F;
        public int G;
        public int H;
        public Position fa;
        public int row;
        public int col;

        public Position() {
        }

        public Position(int row, int col, int H) {
            this(row, col, H, null);
        }

        public Position(int row, int col, int H, Position pos) {
            this.H = H;
            this.row = row;
            this.col = col;
            this.fa = pos;
            this.G = calcG();
            this.F = G + H;
        }

        /** 
         * 计算某个点到起始点的代价G
         * @return
         */
        private int calcG() {
            if (fa == null) return 0;
            if (fa.row != this.row && fa.col !=  this.col) {
                return 14 + fa.G;
            }
            return 10 + fa.G;
        } 

        public void setFaPos(Position pos) {
            this.fa = pos;
            this.G = calcG();
            this.F = G + H;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (!(obj instanceof Position)) {
                return false;
            }
            Position pos = (Position) obj;
            return this.row == pos.row && this.col == pos.col;
        }

        @Override
        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result + row;
            result = prime * result + col;
            return result;
        }

    }

}