Workshop › Tetris-style Blocks › Step 28 of 32

A fair bag of pieces

What we are doing

Fully random pieces are fair on average but not in the moment: sometimes the long I does not come for 30 turns. The 7-bag deals each of the seven pieces once, in a shuffled order, then shuffles a new bag.

```js
let bag

  bag = []

// The "7-bag": deal all seven pieces in a random order, then shuffle a new bag.
function takeFromBag() {
  if (bag.length === 0) {
    bag = [0, 1, 2, 3, 4, 5, 6]
    for (let i = bag.length - 1; i > 0; i--) {
      const j = Math.floor(Math.random() * (i + 1))
      ;[bag[i], bag[j]] = [bag[j], bag[i]]
    }
  }
  return SHAPES[bag.pop()].map((row) => [...row])
}

  const shape = takeFromBag()
```

What it means

• An empty bag is refilled with the seven shape numbers and shuffled (Fisher–Yates): from the end to the start, each element swaps with a random one at or before it.
• [a, b] = [b, a] swaps two values. The ; at the start keeps it from being read as part of the line above.
• bag.pop() takes the last number out of the bag; we return a copy of that shape.

Your turn

1. Under let piece write let bag; in newGame, under the board line, write bag = [].
2. Replace randomShape with the comment and takeFromBag.
3. In spawn, call takeFromBag() instead of randomShape().

Skills: Arrays and objects, Game states

← Faster every level The next piece →

All steps

1. The well
2. The board as rows of numbers
3. Draw one cell
4. Draw the whole board
5. The number is the color
6. A piece is a small grid
7. A list of shapes
8. All seven pieces
9. Does it fit inside the well?
10. Blocks are in the way
11. Try, then move
12. Move with the arrows
13. Turn a matrix
14. Turn with Up
15. Wall kicks
16. Falling by itself
17. A random piece at the top
18. Land and lock
19. One function starts a game
20. When the pile reaches the top
21. Game Over on screen
22. Play again
23. Clear full rows
24. Points for lines
25. The side panel
26. Levels
27. Faster every level
28. A fair bag of pieces
29. The next piece
30. Drop it
31. The ghost piece
32. Build it yourself: hold a piece