/**
* Sorts an ArrayList of MazeCells based on the value of the Heuristic Function
*
* @param neighbors
* @return
*/
private static ArrayList<MazeCell> sortNeighbors(ArrayList<MazeCell> neighbors) {
for (MazeCell temp : neighbors) {
if (temp.getHeuristicDistance() == Integer.MAX_VALUE) {
neighbors.remove(temp);
}
}
// Lowest index has highest path cost
for (int i = 0; i < neighbors.size(); i++) {
for (int j = 1; j < neighbors.size() - i; j++) {
if (neighbors.get(j - 1).getHeuristicDistance() < neighbors.get(j).getHeuristicDistance()) {
MazeCell temp = neighbors.get(j - 1);
neighbors.set(j - 1, neighbors.get(j));
neighbors.set(j, temp);
}
}
}
return neighbors;
}
/**
* Implements a Greedy Best First Search on a Maze Object
*
* @param myMaze
* @return
*/
public static char[] greedyBestFirstSearch(Maze myMaze) {
// For Greedy Best First Search
Stack<MazeCell> frontier = new Stack<MazeCell>();
// Add the Starting State
frontier.add(myMaze.getCell(myMaze.getStartX(), myMaze.getStartY()));
// Store the current Co-ordinates
int currentX = 0, currentY = 0;
while (!frontier.isEmpty()) {
// Get the Top Node
MazeCell currentNode = frontier.pop();
// Check if this node has been visited
if (currentNode.getVisited() == true) {
continue;
}
// Mark the current node as visited
currentNode.setVisited();
// Obtain the current cordinates
currentX = currentNode.getX();
currentY = currentNode.getY();
// Reached the Goal State
if ((currentX == myMaze.getGoalX()) && (currentY == myMaze.getGoalY())) {
return myMaze.getCell(myMaze.getGoalX(), myMaze.getGoalY()).getPath().toCharArray();
}
// Increase the number of Expanded Nodes
nodesExpanded += 1;
// Check that you're not out of Bounds
if (currentX <= 0
|| currentY <= 0
|| currentX >= myMaze.getHeight() - 1
|| currentY >= myMaze.getWidth() - 1) {
continue;
}
// Initial values of heuristics
int heuristicRight = Integer.MAX_VALUE,
heuristicLeft = Integer.MAX_VALUE,
heuristicUp = Integer.MAX_VALUE,
heuristicDown = Integer.MAX_VALUE;
// Go Rightwards (0)
if ((!myMaze.getCell(currentX, currentY + 1).isWall())
&& (!myMaze.getCell(currentX, currentY + 1).getVisited())) {
// Obtain the heuristic
heuristicRight = manhattan(myMaze.getGoalX(), myMaze.getGoalY(), currentX, currentY + 1);
// Adjust the Path
myMaze
.getCell(currentX, currentY + 1)
.appendPath(myMaze.getCell(currentX, currentY).getPath() + "0");
// Set the Heuristic
myMaze.getCell(currentX, currentY + 1).setHeuristicDistance(heuristicRight);
}
// Go Downwards (1)
if ((!myMaze.getCell(currentX + 1, currentY).isWall())
&& (!myMaze.getCell(currentX + 1, currentY).getVisited())) {
heuristicDown = manhattan(myMaze.getGoalX(), myMaze.getGoalY(), currentX + 1, currentY);
myMaze
.getCell(currentX + 1, currentY)
.appendPath(myMaze.getCell(currentX, currentY).getPath() + "1");
myMaze.getCell(currentX + 1, currentY).setHeuristicDistance(heuristicDown);
}
// Go Leftwards (2)
if ((!myMaze.getCell(currentX, currentY - 1).isWall())
&& (!myMaze.getCell(currentX, currentY - 1).getVisited())) {
heuristicLeft = manhattan(myMaze.getGoalX(), myMaze.getGoalY(), currentX, currentY - 1);
myMaze
.getCell(currentX, currentY - 1)
.appendPath(myMaze.getCell(currentX, currentY).getPath() + "2");
myMaze.getCell(currentX, currentY - 1).setHeuristicDistance(heuristicLeft);
}
// Go Upwards (3)
if ((!myMaze.getCell(currentX - 1, currentY).isWall())
&& (!myMaze.getCell(currentX - 1, currentY).getVisited())) {
heuristicUp = manhattan(myMaze.getGoalX(), myMaze.getGoalY(), currentX - 1, currentY);
myMaze
.getCell(currentX - 1, currentY)
.appendPath(myMaze.getCell(currentX, currentY).getPath() + "3");
myMaze.getCell(currentX - 1, currentY).setHeuristicDistance(heuristicUp);
}
// Used to Sort the Values
ArrayList<MazeCell> myNeighbors = new ArrayList<MazeCell>();
if (heuristicRight != Integer.MAX_VALUE) {
myNeighbors.add(myMaze.getCell(currentX, currentY + 1));
}
if (heuristicDown != Integer.MAX_VALUE) {
myNeighbors.add(myMaze.getCell(currentX + 1, currentY));
}
if (heuristicLeft != Integer.MAX_VALUE) {
myNeighbors.add(myMaze.getCell(currentX, currentY - 1));
}
if (heuristicUp != Integer.MAX_VALUE) {
myNeighbors.add(myMaze.getCell(currentX - 1, currentY));
}
// Sort the Cells based on Heuristic Values
myNeighbors = sortNeighbors(myNeighbors);
// Add the Cells to the Frontier in the Sorted Order
for (MazeCell temp : myNeighbors) {
if (temp.getHeuristicDistance() != Integer.MAX_VALUE) {
frontier.add(temp);
}
}
}
return myMaze.getCell(myMaze.getGoalX(), myMaze.getGoalY()).getPath().toCharArray();
}
