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function Grid(size, previousState) {
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  this.size = size;
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  this.cells = previousState ? this.fromState(previousState) : this.empty();
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}
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// Build a grid of the specified size
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Grid.prototype.empty = function () {
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  var cells = [];
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  for (var x = 0; x < this.size; x++) {
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    var row = cells[x] = [];
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    for (var y = 0; y < this.size; y++) {
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      row.push(null);
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    }
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  }
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  return cells;
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};
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Grid.prototype.fromState = function (state) {
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  var cells = [];
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  for (var x = 0; x < this.size; x++) {
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    var row = cells[x] = [];
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    for (var y = 0; y < this.size; y++) {
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      var tile = state[x][y];
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      row.push(tile ? new Tile(tile.position, tile.value) : null);
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    }
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  }
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  return cells;
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};
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// Find the first available random position
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Grid.prototype.randomAvailableCell = function () {
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  var cells = this.availableCells();
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  if (cells.length) {
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    return cells[Math.floor(Math.random() * cells.length)];
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  }
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};
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Grid.prototype.availableCells = function () {
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  var cells = [];
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  this.eachCell(function (x, y, tile) {
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    if (!tile) {
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      cells.push({ x: x, y: y });
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    }
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  });
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  return cells;
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};
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// Call callback for every cell
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Grid.prototype.eachCell = function (callback) {
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  for (var x = 0; x < this.size; x++) {
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    for (var y = 0; y < this.size; y++) {
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      callback(x, y, this.cells[x][y]);
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    }
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  }
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};
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// Check if there are any cells available
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Grid.prototype.cellsAvailable = function () {
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  return !!this.availableCells().length;
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};
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// Check if the specified cell is taken
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Grid.prototype.cellAvailable = function (cell) {
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  return !this.cellOccupied(cell);
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};
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Grid.prototype.cellOccupied = function (cell) {
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  return !!this.cellContent(cell);
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};
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Grid.prototype.cellContent = function (cell) {
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  if (this.withinBounds(cell)) {
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    return this.cells[cell.x][cell.y];
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  } else {
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    return null;
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  }
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};
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// Inserts a tile at its position
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Grid.prototype.insertTile = function (tile) {
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  this.cells[tile.x][tile.y] = tile;
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};
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Grid.prototype.removeTile = function (tile) {
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  this.cells[tile.x][tile.y] = null;
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};
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Grid.prototype.withinBounds = function (position) {
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  return position.x >= 0 && position.x < this.size &&
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         position.y >= 0 && position.y < this.size;
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};
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Grid.prototype.serialize = function () {
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  var cellState = [];
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  for (var x = 0; x < this.size; x++) {
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    var row = cellState[x] = [];
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    for (var y = 0; y < this.size; y++) {
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      row.push(this.cells[x][y] ? this.cells[x][y].serialize() : null);
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    }
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  }
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  return {
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    size: this.size,
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    cells: cellState
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  };
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};
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