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‎DataStructures/Graphs/A_Star.java

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package A_Star;
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import java.util.*;
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public class A_Star {
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private static class Graph {
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//Graph's structure can be changed only applying changes to this class.
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private ArrayList<Edge> [] graph;
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//Initialise ArrayLists in Constructor
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public Graph(int size) {
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this.graph = new ArrayList[size];
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for (int i = 0; i < size; i++) {
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this.graph[i] = new ArrayList<>();
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}
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}
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private ArrayList<Edge> getNeighbours(int from) { return this.graph[from]; }
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//Graph is bidirectional, for just one direction remove second instruction of this method.
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private void addEdge (Edge edge) {
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this.graph[edge.getFrom()].add(new Edge(edge.getFrom(), edge.getTo(), edge.getWeight()));
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this.graph[edge.getTo()].add(new Edge(edge.getTo(), edge.getFrom(), edge.getWeight()));
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}
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private void initializeGraph() {
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this.addEdge(new Edge(0, 19, 75)); this.addEdge(new Edge(0, 15, 140));
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this.addEdge(new Edge(0, 16, 118)); this.addEdge(new Edge(19, 12, 71));
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this.addEdge(new Edge(12, 15, 151));this.addEdge(new Edge(16, 9, 111));
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this.addEdge(new Edge(9, 10, 70)); this.addEdge(new Edge(10, 3, 75));
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this.addEdge(new Edge(3, 2, 120)); this.addEdge(new Edge(2, 14, 146));
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this.addEdge(new Edge(2, 13, 138)); this.addEdge(new Edge(2, 6, 115));
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this.addEdge(new Edge(15, 14, 80)); this.addEdge(new Edge(15, 5, 99));
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this.addEdge(new Edge(14, 13, 97)); this.addEdge(new Edge(5, 1, 211));
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this.addEdge(new Edge(13, 1, 101)); this.addEdge(new Edge(6, 1, 160));
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this.addEdge(new Edge(1, 17, 85)); this.addEdge(new Edge(17, 7, 98));
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this.addEdge(new Edge(7, 4, 86)); this.addEdge(new Edge(17, 18, 142));
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this.addEdge(new Edge(18, 8, 92)); this.addEdge(new Edge(8, 11, 87));
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/*
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.x. node
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(y) cost
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- or | or / bidirectional connection
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( 98)- .7. -(86)- .4.
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|
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( 85)- .17. -(142)- .18. -(92)- .8. -(87)- .11.
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|
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. 1. -------------------- (160)
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| \ |
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(211) \ .6.
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| \ |
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. 5. (101)-.13. -(138) (115)
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| | | /
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( 99) ( 97) | /
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| | | /
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.12. -(151)- .15. -(80)- .14. | /
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| | | | /
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( 71) (140) (146)- .2. -(120)
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| | |
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.19. -( 75)- . 0. .10. -(75)- .3.
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| |
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(118) ( 70)
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| |
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.16. -(111)- .9.
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*/
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}
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}
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private static class Edge {
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private int from;
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private int to;
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private int weight;
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public Edge(int from, int to, int weight) {
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this.from = from;
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this.to = to;
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this.weight = weight;
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}
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public int getFrom() { return from; }
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public int getTo() { return to; }
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public int getWeight() { return weight; }
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}
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//class to iterate during the algorithm execution, and also used to return the solution.
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private static class PathAndDistance {
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private int distance; //distance advanced so far.
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private ArrayList<Integer> path; //list of visited nodes in this path.
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private int estimated; //heuristic value associated to the last node od the path (current node).
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public PathAndDistance(int distance, ArrayList<Integer> path, int estimated) {
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this.distance = distance;
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this.path = path;
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this.estimated = estimated;
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}
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public int getDistance() { return distance; }
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public ArrayList<Integer> getPath() { return path; }
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public int getEstimated() { return estimated; }
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private void printSolution () {
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if (this.path != null)
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System.out.println("Optimal path: " + this.path.toString() +
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", distance: " + this.distance);
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else
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System.out.println("There is no path available to connect the points");
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}
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}
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public static void main(String[] args) {
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//heuristic function optimistic values
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int[] heuristic = {366, 0, 160, 242, 161, 178, 77, 151, 226,
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244, 241, 234, 380, 98, 193, 253, 329, 80, 199, 374};
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Graph graph = new Graph(20);
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graph.initializeGraph();
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PathAndDistance solution = aStar(3, 1, graph, heuristic);
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solution.printSolution();
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}
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public static PathAndDistance aStar(int from, int to, Graph graph, int[] heuristic) {
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//nodes are prioritised by the less value of the current distance of their paths, and the estimated value
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//given by the heuristic function to reach the destination point from the current point.
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PriorityQueue<PathAndDistance> queue = new PriorityQueue<>
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(Comparator.comparingInt(a -> (a.getDistance() + a.getEstimated())));
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//dummy data to start the algorithm from the beginning point
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queue.add(new PathAndDistance(0, new ArrayList<>(Arrays.asList(from)), 0));
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boolean solutionFound = false;
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PathAndDistance currentData = new PathAndDistance(-1, null, -1);
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while (!queue.isEmpty() && !solutionFound) {
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currentData = queue.poll(); //first in the queue, best node so keep exploring.
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int currentPosition = currentData.getPath().get(currentData.getPath().size() - 1); //current node.
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if (currentPosition == to)
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solutionFound = true;
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else
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for (Edge edge : graph.getNeighbours(currentPosition))
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if (!currentData.getPath().contains(edge.getTo())) { //Avoid Cycles
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ArrayList<Integer> updatedPath = new ArrayList<>(currentData.getPath());
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updatedPath.add(edge.getTo()); //Add the new node to the path, update the distance,
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// and the heuristic function value associated to that path.
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queue.add(new PathAndDistance(currentData.getDistance() + edge.getWeight(),
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updatedPath, heuristic[edge.getTo()]));
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}
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}
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return (solutionFound) ? currentData : new PathAndDistance(-1, null, -1);
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//Out of while loop, if there is a solution, the current Data stores the optimal path, and its distance
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}
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}

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