What is the probability distribution of the new tiles in 2048?

Question

This question is about the popular game 2048. I am wondering about the details of the appearance of new tiles at each turn:

• Is it 50% twos and 50% fours?
• Is the probability of appearance evenly distributed on all free slots?

Answer 1

https://github.com/gabrielecirulli/2048/blob/master/js/game_manager.js#L72

// Adds a tile in a random position
GameManager.prototype.addRandomTile = function () {
  if (this.grid.cellsAvailable()) {
    var value = Math.random() < 0.9 ? 2 : 4;
    var tile = new Tile(this.grid.randomAvailableCell(), value);

    this.grid.insertTile(tile);
  }
};

https://github.com/gabrielecirulli/2048/blob/master/js/grid.js#L36

// Find the first available random position
Grid.prototype.randomAvailableCell = function () {
  var cells = this.availableCells();

  if (cells.length) {
    return cells[Math.floor(Math.random() * cells.length)];
  }
};