Workshop › Pac-Man-style Maze Chase › Step 13 of 42

Turn back at once

What we are doing

Turning into a side corridor waits for the next tile center, but turning back should not: when a ghost appears ahead you must flee now. The trick: 3 frames into a step from tile 9 to 8 is the same place as 5 frames into a step from tile 8 back to 9. So move to the tile you were heading to, flip dir, and set progress = frames - progress.

```js
const reverse = (dir) => [-dir[0], -dir[1]]

// Turning around in the middle of a tile: step into the next tile and walk back the rest of the way.
function turnAround(e) {
  if (e.progress === 0 || same(e.dir, STOP)) {
    e.dir = reverse(e.dir)
    return
  }
  e.col = wrap(e.col + e.dir[0])
  e.row += e.dir[1]
  e.dir = reverse(e.dir)
  e.progress = e.frames - e.progress
}

function steer(dir) {
  player.want = dir
  if (same(dir, reverse(player.dir)) && !same(dir, STOP)) turnAround(player)
}
```

What it means

reverse flips both numbers of a direction.
At a tile center (or standing still) flipping dir is enough.
Mid-step: jump the tile to the one ahead, flip, and count the remaining frames from the other side. position gives the same point before and after.
steer turns around at once only when the new direction is exactly backwards.

Your turn

Under same, write reverse.
Above steer, write the comment and turnAround.
In steer, under player.want = dir, write the new line.

Skills: Keyboard and mouse input

← Glide between tiles Swipes on a phone →

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