function CellSet(definition) {
	definition = definition || {};
	this.cells = {};
	if(definition.shape == 'rectangle') {
		var xMax = definition.x + definition.width;
		var yMax = definition.y + definition.height;
		for(var i = definition.x; i < xMax; i++) {
			for(var j = definition.y; j < yMax; j++) {
				this.add(i, j);
			}
		}
	} else if(definition.shape == 'points') {
		for(var i = 0; i < definition.points.length; i++) {
			this.cells[id(definition.points[i].i, definition.points[i].j)];
		}
	}
}

CellSet.prototype.add = function(i, j) {
	this.cells[id(i, j)] = true;
}

CellSet.prototype.remove = function(i, j) {
	delete this.cells[id(i, j)];
}

CellSet.prototype.contains = function(i, j) {
	return !!this.cells[id(i, j)];
}

CellSet.prototype.isEmpty = function() {
	return Object.keys(this.cells).length < 1;
}

return {
	make: function(definition) {return new CellSet(definition);},
};

function id(i, j) {
	return i + ':' + j;
}