/// <summary>
/// Adds a tile to one of the empty cells on the grid. If no cells are empty,
/// this method throws InvalidOperationException.
/// </summary>
/// <remarks>
/// The tile's position is chosen uniformly at random from the set of empty
/// cells. The tile's value is chosen with a weighted coin flip.
/// </remarks>
/// <returns>the tile that was added</returns>
public Tile AddRandomTile()
{
if (IsFull)
{
throw new InvalidOperationException("There are no empty cells to place a tile in.");
}
var emptyCells = new List <GridCell>(GetEmptyCells());
GridCell cell = RandomProvider.Select(emptyCells);
int number = RandomProvider.FlipCoin(Constants.TILE_PROBABILITY_2) ? 2 : 4;
var tile = new Tile(cell, number);
AddTile(tile);
return(tile);
}