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Find the shortest path in a grid in XNA


I’m currently playing around with a board game engine in XNA, where players can play missions that take place on a tile-based board. I’m now working on using the A* algorithm to find paths between tiles.

About the game

In the game, players have a number of steps to move in each round, and should be able to select which tile they want to move to. The game should then suggest the shortest possible path to that tile, and mark tiles that are too far away in red.

The pathfinding algorithm will also be used by the engine itself, to move enemies around the board. The enemies are controlled by the engine and will have some form of primitive AI, to help them decide things like how and where to move, which player to attack etc.

To improve the illusion of enemy intelligence, the algorithm should also be able to select a random path, if multiple options exist. This will give the enemies a random, unpredictable behavior.

Before we continue with how this is implemented, let’s recap.

Board movement

The players and computer controlled enemies can move sideways, but not diagonally:

Board movement example 1

Factors that limit whether or not a game piece can move from one tile to another (tile A to tile B) are:

  • Tile B doesn’t exist (the piece would move outside of the board boundaries).
  • Tile B is marked as a None or a Nonwalkable tile (see below).
  • Tile B belongs to another room and is separated from tile A by a wall.
  • Tile B is occupied by another piece or furniture (another piece can not stop here).
  • Tile B is occupied by a player or enemy (players and enemies can not walk past eachother).

All these rules are then handled by a bunch of tile-related functions, that take a player or enemy and decide whether or not the character can move to a tile.

Path finding overview

Consider the following map, where a player stands on the green tile and wants to move to the red one:

Board movement example 2

In the example above, multiple “shortest paths” exist. As we will see, the method I use will find a random path every time and has the following steps:

  • Begin at the start tile and set its path length to zero.
  • Recursively handle each sibling.
  • If the sibling hasn’t been handled yet, set its path length to the current length + 1.
  • If the sibling has already been handled, override the its path length if the new length is shorter.
  • When no tile can be improved, start at the end tile and find the shortest path to the start tile.

I call these two processes spreading (a tile spreads its path’s length to its siblings) and tracing (trace the shortest path found while spreading).

Step 1: Spreading

In my game engine, the path-finding operation is started with this Board function:

//Placeholder for the calculated path (not thread safe :)
int[,] pathLengths;

public List<Tile> FindPath(Tile startTile, Tile endTile)
{
	//Abort if start or end tile is null
	if (startTile == null || endTile == null)
	{
		return new List<Tile>();
	}

	//Abort if the end tile is non-stoppable
	if (!endTile.IsStoppable)
	{
		return new List<Tile>();
	}

	//Initialize the path length array
	pathLengths = new int[Tiles.GetLength(0), Tiles.GetLength(1)];
	for (int y = 0; y < pathLengths.GetLength(1); y++)
	{
		for (int x = 0; x < pathLengths.GetLength(0); x++)
	    {
	    	pathLengths[x, y] = int.MaxValue;
	    }
	}
	 

	//Begin at the start tile
	pathLengths[startTile.BoardPosition.X, startTile.BoardPosition.Y] = 0;
	FindPath_Spread(startTile);

	//Once done, backtrack from the end tile
	List<Tile> result = FindPath_Trace(endTile);

	//Only return the path if it contains the start tile
	if (result.Contains(startTile)) {
	 	return result;
	}

	return new List<Tile>();
}

This function corresponds to the first part of the numbered list above. We don’t proceed if any tile is null or if the end tile can not be stopped at.

We then initialize a placeholder array with max length values, then run a spread operation from the start tile. Once the spread is done, we trace the path.

As you can see, this function only returns a path if it contains the start tile. If the trace operation can’t reach the start tile, no path should be returned.

The spread operation is recursive and will eventually handle each tile at least once. It consists of two functions, as can be seen below:

private void FindPath_Spread(Tile tile)
{
	FindPath_Spread(tile, tile.TopSibling);
	FindPath_Spread(tile, tile.LeftSibling);
	FindPath_Spread(tile, tile.RightSibling);
	FindPath_Spread(tile, tile.BottomSibling);
}

private void FindPath_Spread(Tile tile, Tile target)
{
	//Abort if any tile is null
	if (tile == null || target == null) {
		return;
	}

	//Abort if no movement is allowed
	if (!tile.CanMoveTo(target)) {	
		return;
	}

	//Get current path lengths
	int tileLength = FindPath_GetPathLength(tile);
	int targetLength = FindPath_GetPathLength(target);

	//Use length if it improves target
	if (tileLength + 1 < targetLength)
	{
		pathLengths[target.BoardPosition.X, target.BoardPosition.Y] = tileLength + 1;
		FindPath_Spread(target);
	}
}

We initialize the spread operation at the start tile, which has a path length of zero. It then spreads out to the tile’s siblings, but is only handled if a sibling’s path length would be improved. The operation will thus stop once all tiles are as good as they can be.

Step 2: Tracing

Once the spread operation is done, the recursive trace operation will be started. It starts at the end tile and find the shortest way back to the start tile. If the start tile can’t be reached, no path exists between the two tiles. If so, the operation will return an empty list.

The trace operation consists of a single function:

private List<Tile> FindPath_Trace(Tile tile)
{
	//Find the sibling paths
	int tileLength = FindPath_GetPathLength(tile);
	int topLength = FindPath_GetPathLength(tile.TopSibling);
	int leftLength = FindPath_GetPathLength(tile.LeftSibling);
	int rightLength = FindPath_GetPathLength(tile.RightSibling);
	int bottomLength = FindPath_GetPathLength(tile.BottomSibling);

	//Calculate the lowest path length
	int lowestLength =
		Math.Min(tileLength,
		Math.Min(topLength,
		Math.Min(leftLength,
		Math.Min(rightLength, bottomLength))));

	//Add each possible path
	List<Tile> possiblePaths = new List<Tile>();
	if (topLength == lowestLength){
		possiblePaths.Add(tile.TopSibling);
	}
	if (leftLength == lowestLength){
		possiblePaths.Add(tile.LeftSibling);
	}
	if (rightLength == lowestLength) {
		possiblePaths.Add(tile.RightSibling);
	}
	if (bottomLength == lowestLength) {
		possiblePaths.Add(tile.BottomSibling);
	}

	//Continue through a random possible path
	List<Tile> result = new List<Tile>();
	if (possiblePaths.Count() > 0) {
		result = FindPath_Trace(possiblePaths[RandomHelper.GetInt32(0, possiblePaths.Count())]);
	}

	//Add the tile itself, then return
	result.Add(tile);
	return result;
}

FindPath_GetPathLength is a small function that I added to avoid duplicate code:

private int FindPath_GetPathLength(Tile tile)
{
	if (tile == null){
		return int.MaxValue;
	}
	return pathLengths[tile.BoardPosition.X, tile.BoardPosition.Y];
}

Example

Using the map at the beginning of this post, the engine will first parse the map into a game board, as is described in a previous post.

This is how my game (for now) displays the tiles. For now, the walls are missing:

Rendered output

In the image above, all tiles are walkable, to increase the number of “shortest” paths. However, it is not possible to walk to a dark tile from a light one. The green and red tiles are just highlighted display the start and end tile. In the game, they are light grey.

When I run my game-to-be, I auto-generate a path between this green and red tile. As you can see in these images, the game engine suggests different paths each time:

Three different paths

The path finding operation is fast and can handle large board games. However, it wouldn’t be suitable for more complex games, where the world isn’t tile-based.