function Grid(width, height) {

  this.width = width;

  this.height = height;

  this.cells = new Array(width * height);

}

Grid.prototype.valueAt = function(point) {

  return this.cells[point.y * this.width + point.x];

};

Grid.prototype.setValueAt = function(point, value) {

  this.cells[point.y * this.width + point.x] = value;

};

Grid.prototype.isInside = function(point) {

  return point.x >= 0 && point.y >= 0 &&

         point.x < this.width && point.y < this.height;

};

Grid.prototype.moveValue = function(from, to) {

  this.setValueAt(to, this.valueAt(from));

  this.setValueAt(from, undefined);

};