Killer Game Programming in Java

Storing Tile Details

A TileNode object stores details about a particular tile node, which are used by the A* algorithm when it looks at that node. The most important are values for the getCostFromStart() and costToGoal() functions so the overall score for the node can be worked out.

As explained in the last section, the getCostFromStart() function is the cost of the path that leads to this node from the starting tile. This algorithm's precise definition will vary from game to game, but I use the simplest measure—the length of the path—with the step between adjacent tiles assigned a value of 1. costToGoal() estimates the cost of going from this tile node to the goal.

Tip

This is a little harder to calculate in AlienTiles due to the staggered layout of the tiles as detailed below.

Each TileNode stores a reference to its parent, the tile node that was visited before it. The sequence of nodes from this particular tile node back to the starting tile defines the sprite's path (in reverse).

Calculating the Cost to the Goal

costToGoal() treats the tile coordinates of the current tile and the goal as points on the XY plane and calculates the length of the floor of the straight line between them:

    public void costToGoal(Point goal)
    { double dist = coord.distance(goal.x, goal.y);
      costToGoal = Math.floor(dist);
    }

However, the tiles are positioned in staggered rows, which means the straight line distance can be an inaccurate measure. Therefore, the costToGoal() value may not be less than or equal to ...