Workshop › Pac-Man-style Maze Chase › Step 9 of 42

One frame of movement

What we are doing

advance(e, choose) moves anything by one frame. At the center of a tile (progress === 0) it lets choose decide the direction. Then progress grows by one; after frames frames the step is done: the tile changes and progress starts again from 0.

```js
// One frame of movement. Directions are only chosen at the center of a tile, by `choose`.
function advance(e, choose) {
  if (e.progress === 0) choose(e)
  if (same(e.dir, STOP)) return
  e.progress += 1
  if (e.progress < e.frames) return
  e.progress = 0
  e.col = wrap(e.col + e.dir[0])
  e.row += e.dir[1]
}
```

What it means

A function can be passed to another function like any value: choose(e) calls whatever was passed.
A stopped mover does nothing more; return ends the function early.
Only when progress reaches frames does the tile change; the column is wrapped for the tunnel.

Your turn

Under reset, leave an empty line and write the comment and advance.

Skills: Game states, Functions

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