this.move = function(a) {
		if (a == this.empty) return;

		var ay = Math.floor(a / dim);
		var ax = a % dim;

		var by = Math.floor(this.empty / dim);
		var bx = this.empty % dim;

		if ((ax == bx && Math.abs(ay - by) == 1) ||
		    (ay == by && Math.abs(ax - bx) == 1)) {
			this.swap(a, this.empty);
		}
	}