http://codecombat.com/play/level/gridmancer

gistfile1.js

// Fill the empty space with the minimum number of rectangles.
// The grid size is 1 meter, but the smallest wall/floor tile is 4 meters.
// If you can do better than one rectangle for every tile, let us know!
// We'll help you find a programming job (if you want one).
// Check the guide for more info, and press Contact below to report success.

var ar = this.addRect;
var tileSize = 4;

function draw(x, y, w, h) {
    w = w * tileSize;
    h = h * tileSize;
    ar(x*tileSize+w/2, y*tileSize+h/2, w, h);
}

function occupied(x,y) {
    return grid[y*tileSize][x*tileSize].length > 0;
}

var grid = this.getNavGrid().grid;
var curLines = [];

for(var y = 0; (y+1) * tileSize < grid.length; y += 1) {
    var lastLines = curLines;
    curLines = [];
    for(var x = 0; (x+1) * tileSize < grid[0].length; x += 1) {
        if(!occupied(x,y)) {
            var startx = x;
            while(!occupied(x,y)) {
                x += 1;
            }
            var line = [startx, y, x-startx];
            var i = lastLines.length;
            while(i--){
                var last = lastLines[i];
                if (last[0] === line[0] && last[2] === line[2]) {
                    line = last;
                    lastLines.splice(i, 1);
                    break;
                }
            }
            curLines.push(line);
        }
    }
    var j = lastLines.length;
    while(j--) {
        var l = lastLines[j];
        draw(l[0], l[1], l[2], y-l[1]);
    } 
}