8-Puzzle Solution executes infinitely [migrated]
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Ashwin
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Published on 2012-10-24T16:17:23Z
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I am looking for a solution to 8-puzzle problem using the A* Algorithm. I found this project on the internet. Please see the files - proj1 and EightPuzzle. The proj1 contains the entry point for the program(the main() function) and EightPuzzle describes a particular state of the puzzle. Each state is an object of the 8-puzzle.
I feel that there is nothing wrong in the logic. But it loops forever for these two inputs that I have tried : {8,2,7,5,1,6,3,0,4} and {3,1,6,8,4,5,7,2,0}. Both of them are valid input states. What is wrong with the code?
Note
- For better viewing copy the code in a Notepad++ or some other text editor(which has the capability to recognize java source file) because there are lot of comments in the code.
- Since A* requires a heuristic, they have provided the option of using
manhattan distance and a heuristic that calculates the number of
misplaced tiles. And to ensure that the best heuristic is executed
first, they have implemented a
PriorityQueue. ThecompareTo()function is implemented in theEightPuzzleclass. - The input to the program can be changed by changing the value of
p1din themain()function ofproj1class. - The reason I am telling that there exists solution for the two my above inputs is because the applet here solves them. Please ensure that you select 8-puzzle from teh options in the applet.
EDIT
I gave this input{0,5,7,6,8,1,2,4,3}. It took about10 secondsand gave a result with 26 moves. But the applet gave a result with24 movesin0.0001 secondswithA*.
For quick reference I have pasted the the two classes without the comments :
EightPuzzle
import java.util.*;
public class EightPuzzle implements Comparable <Object> {
int[] puzzle = new int[9];
int h_n= 0;
int hueristic_type = 0;
int g_n = 0;
int f_n = 0;
EightPuzzle parent = null;
public EightPuzzle(int[] p, int h_type, int cost)
{
this.puzzle = p;
this.hueristic_type = h_type;
this.h_n = (h_type == 1) ? h1(p) : h2(p);
this.g_n = cost;
this.f_n = h_n + g_n;
}
public int getF_n()
{
return f_n;
}
public void setParent(EightPuzzle input)
{
this.parent = input;
}
public EightPuzzle getParent()
{
return this.parent;
}
public int inversions()
{
/*
* Definition: For any other configuration besides the goal,
* whenever a tile with a greater number on it precedes a
* tile with a smaller number, the two tiles are said to be inverted
*/
int inversion = 0;
for(int i = 0; i < this.puzzle.length; i++ )
{
for(int j = 0; j < i; j++)
{
if(this.puzzle[i] != 0 && this.puzzle[j] != 0)
{
if(this.puzzle[i] < this.puzzle[j])
inversion++;
}
}
}
return inversion;
}
public int h1(int[] list)
// h1 = the number of misplaced tiles
{
int gn = 0;
for(int i = 0; i < list.length; i++)
{
if(list[i] != i && list[i] != 0)
gn++;
}
return gn;
}
public LinkedList<EightPuzzle> getChildren()
{
LinkedList<EightPuzzle> children = new LinkedList<EightPuzzle>();
int loc = 0;
int temparray[] = new int[this.puzzle.length];
EightPuzzle rightP, upP, downP, leftP;
while(this.puzzle[loc] != 0)
{
loc++;
}
if(loc % 3 == 0){
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc + 1];
temparray[loc + 1] = 0;
rightP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
rightP.setParent(this);
children.add(rightP);
}else if(loc % 3 == 1){
//add one child swaps with right
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc + 1];
temparray[loc + 1] = 0;
rightP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
rightP.setParent(this);
children.add(rightP);
//add one child swaps with left
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc - 1];
temparray[loc - 1] = 0;
leftP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
leftP.setParent(this);
children.add(leftP);
}else if(loc % 3 == 2){
// add one child swaps with left
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc - 1];
temparray[loc - 1] = 0;
leftP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
leftP.setParent(this);
children.add(leftP);
}
if(loc / 3 == 0){
//add one child swaps with lower
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc + 3];
temparray[loc + 3] = 0;
downP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
downP.setParent(this);
children.add(downP);
}else if(loc / 3 == 1 ){
//add one child, swap with upper
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc - 3];
temparray[loc - 3] = 0;
upP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
upP.setParent(this);
children.add(upP);
//add one child, swap with lower
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc + 3];
temparray[loc + 3] = 0;
downP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
downP.setParent(this);
children.add(downP);
}else if (loc / 3 == 2 ){
//add one child, swap with upper
temparray = this.puzzle.clone();
temparray[loc] = temparray[loc - 3];
temparray[loc - 3] = 0;
upP = new EightPuzzle(temparray, this.hueristic_type, this.g_n + 1);
upP.setParent(this);
children.add(upP);
}
return children;
}
public int h2(int[] list)
// h2 = the sum of the distances of the tiles from their goal positions
// for each item find its goal position
// calculate how many positions it needs to move to get into that position
{
int gn = 0;
int row = 0;
int col = 0;
for(int i = 0; i < list.length; i++)
{
if(list[i] != 0)
{
row = list[i] / 3;
col = list[i] % 3;
row = Math.abs(row - (i / 3));
col = Math.abs(col - (i % 3));
gn += row;
gn += col;
}
}
return gn;
}
public String toString()
{
String x = "";
for(int i = 0; i < this.puzzle.length; i++){
x += puzzle[i] + " ";
if((i + 1) % 3 == 0)
x += "\n";
}
return x;
}
public int compareTo(Object input) {
if (this.f_n < ((EightPuzzle) input).getF_n())
return -1;
else if (this.f_n > ((EightPuzzle) input).getF_n())
return 1;
return 0;
}
public boolean equals(EightPuzzle test){
if(this.f_n != test.getF_n())
return false;
for(int i = 0 ; i < this.puzzle.length; i++)
{
if(this.puzzle[i] != test.puzzle[i])
return false;
}
return true;
}
public boolean mapEquals(EightPuzzle test){
for(int i = 0 ; i < this.puzzle.length; i++)
{
if(this.puzzle[i] != test.puzzle[i])
return false;
}
return true;
}
}
proj1
import java.util.*;
public class proj1 {
/**
* @param args
*/
public static void main(String[] args) {
int[] p1d = {1, 4, 2, 3, 0, 5, 6, 7, 8};
int hueristic = 2;
EightPuzzle start = new EightPuzzle(p1d, hueristic, 0);
int[] win = { 0, 1, 2,
3, 4, 5,
6, 7, 8};
EightPuzzle goal = new EightPuzzle(win, hueristic, 0);
astar(start, goal);
}
public static void astar(EightPuzzle start, EightPuzzle goal)
{
if(start.inversions() % 2 == 1)
{
System.out.println("Unsolvable");
return;
}
// function A*(start,goal)
// closedset := the empty set // The set of nodes already evaluated.
LinkedList<EightPuzzle> closedset = new LinkedList<EightPuzzle>();
// openset := set containing the initial node // The set of tentative nodes to be evaluated. priority queue
PriorityQueue<EightPuzzle> openset = new PriorityQueue<EightPuzzle>();
openset.add(start);
while(openset.size() > 0){
// x := the node in openset having the lowest f_score[] value
EightPuzzle x = openset.peek();
// if x = goal
if(x.mapEquals(goal))
{
// return reconstruct_path(came_from, came_from[goal])
Stack<EightPuzzle> toDisplay = reconstruct(x);
System.out.println("Printing solution... ");
System.out.println(start.toString());
print(toDisplay);
return;
}
// remove x from openset
// add x to closedset
closedset.add(openset.poll());
LinkedList <EightPuzzle> neighbor = x.getChildren();
// foreach y in neighbor_nodes(x)
while(neighbor.size() > 0)
{
EightPuzzle y = neighbor.removeFirst();
// if y in closedset
if(closedset.contains(y)){
// continue
continue;
}
// tentative_g_score := g_score[x] + dist_between(x,y)
//
// if y not in openset
if(!closedset.contains(y)){
// add y to openset
openset.add(y);
//
}
//
}
//
}
}
public static void print(Stack<EightPuzzle> x)
{
while(!x.isEmpty())
{
EightPuzzle temp = x.pop();
System.out.println(temp.toString());
}
}
public static Stack<EightPuzzle> reconstruct(EightPuzzle winner)
{
Stack<EightPuzzle> correctOutput = new Stack<EightPuzzle>();
while(winner.getParent() != null)
{
correctOutput.add(winner);
winner = winner.getParent();
}
return correctOutput;
}
}
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