Question

What’s the best way to do movement in a 2D square grid system? I have this something that works but it seems wrong/ugly (see below).

x x x x x x x
x x x x x x x
x x x O x x x
x x x U x x x
x x x x x x x
x x x x x x x
x x x x x x x

For example, U is the unit I want to move, and O is an impassable object like another unit or a mountain. If U can move 3 tiles, I want the moveable area (M) to look like this.

x x x x x x x
x x M x M x x
x M M O M M x
M M M U M M M
x x M M M M x
x x M M M x x
x x x M x x x

Here’s my code:

public function possibleMoves(range:uint, cords:Array):void {
var X:uint = cords[0];
var Y:uint = cords[1];

if (range > 0) {
    try {
        theGrid[X + 1][Y].moveable = true;
        if (theGrid[X + 1][Y].getOccupied == false) {
            possibleMoves(range - 1, [X + 1, Y], flag, mtype);
        }
    }   catch (err:Error) { }

    try {
        theGrid[X - 1][Y].moveable = true;
        if (theGrid[X - 1][Y].getOccupied == false) {
            possibleMoves(range - 1, [X - 1, Y], flag, mtype);
        }
    }   catch (err:Error) { }

    try {
        theGrid[X][Y + 1].moveable = true;
        if (theGrid[X][Y + 1].getOccupied == false) {
            possibleMoves(range - 1, [X, Y + 1], flag, mtype);
        }
    }   catch (err:Error) { }

    try {
        theGrid[X][Y - 1].moveable = true;
        if (theGrid[X][Y - 1].getOccupied == false) {
            possibleMoves(range - 1, [X, Y - 1], flag, mtype);
        }
    }   catch (err:Error) { }
}

Answer 1

the data structure of your tileset seems strongly coupled to a “Tile” class that does too many things ; theGrid[X][Y].moveable, theGrid[X][Y].getOccupied… + probably some other methods.

maybe the tileset data structure should only store Boolean values (walkable?true/false) and have a single method to tell wether a tile is walkable or not. in this case, a Vector of Boolean values is enough. testing the 4 ( or 8 with diagonals ) naerby values is pretty fast and spreading the test to the newly found values can be done with a recursive loop.

if you have different types of tiles (walls, objects, characters etc.), you could use a Vector.< int > rather than Booleans ; 0 would be a walkable tile and anything else would be forbidden areas.
this allows a Boolean check : as 0 = false and any other value = true.

I’ve done a sample here http://wonderfl.net/c/bRV8 ; it might be clearer than pasting the code. move the mouse around, you should see a pinky shape the gives you the valid cells.

• l.53 is the “connexity” possible valeus are 4 and 8
• four connected gives

• eight connected

• l.54 is the max recursion depth

as such, the recursion is performed regardless of the starting point. it will spill in a sometimes unexpected way.

if you need to give a specific amount of moves this won’t be enough, you’ll have to set up some kind of pathfinder.

Edit:
It appears that the code provided works, but contains a recursion termination bug that is attempted to be avoided by the following line. This works only in some cases and behaves really weird if you put your character at the edge of the map or give him number of moves other than 5:

        var max:int = ( maxDepth * maxDepth );
        if( maxDepth % 2 == 0 )max--;
        recursiveCheck( valid, tilesetClone, 0, max, connexity );

I checked with different recursion depth, and the bug quickly becomes apparent. Lack of grid and complex map design of this example obscures the bug, but here’s a screenshot below – note that if mouse is positioned in the corner like shown, the field extends 6 squares up and 7 squares left, while it should’ve only been 5.