function CellSet(definition) {
	definition = definition || {};
	this.cells = {};
	if(definition.type == 'rectangle') {
		this.fromRectangle(definition);
	} else if(definition.type == 'isochrome') {
		this.fromIsochrome(definition);
	} else if(definition.type == 'union') {
		this.fromUnion(definition.sets);
	}
}

CellSet.prototype.fromRectangle = function(definition) {
	var iMax = definition.row + definition.height;
	var jMax = definition.column + definition.width;
	for(var i = definition.row; i < iMax; i++) {
		for(var j = definition.column; j < jMax; j++) {
			this.add(i, j);
		}
	}
};

CellSet.prototype.fromIsochrome = function(definition) {
	var originColor = definition.grid[definition.row][definition.column];
	var queue = [{i: definition.row, j: definition.column}];
	while(queue.length > 0) {
		var p = queue[0];
		this.add(p.i, p.j);
		for(var d = -1; d < 2; d += 2) {
			[{i: p.i + d, j: p.j}, {i: p.i, j: p.j + d}].forEach(
				gateKeeper(this, definition.grid, queue, originColor)
			);
		}
		queue.shift();
	}
};

CellSet.prototype.fromUnion = function(sets) {
	for(var i = 0; i < sets.length; i++) {
		for(var key in sets[i].cells) {
			this.cells[key] = true;
		}
	}
}

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;
};

CellSet.prototype.map = function(f) {
	var result = [];
	for(var key in this.cells) {
		result.push(f.apply(null, key.split(':')));
	}
	return result;
}

CellSet.prototype.iter = function(f) {
	this.map(f);
}

CellSet.prototype.difference = function(cellSet) {
	var newCellSet = new CellSet();
	this.iter(function(i, j) {
		if(!cellSet.contains(i, j)) {
			newCellSet.add(i, j);
		}
	});
	return newCellSet;
}

return {
	make: function(definition) {return new CellSet(definition);},
	union: function(sets) {return new CellSet({type: 'union', sets: sets});}
};

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

function gateKeeper(cellSet, grid, queue, originColor) {
	return function(p1) {
		if(p1.i >= 0 && p1.i < grid.length && p1.j >= 0 && p1.j < grid.length
			&& !cellSet.contains(p1.i, p1.j)
			&& grid[p1.i][p1.j] == originColor) {
			queue.push(p1);
		}
	}
}