Workshop › Tower Defense › Step 7 of 42

Walking by distance

What we are doing

How do you move something along a road with corners? Steering it gets messy. Much simpler: give each enemy one number, d, the distance walked along the road, and write a function that turns a distance into a point.

```js
// Where on the road an enemy is after walking `d` tiles (null once it is past the end).
function pointAt(d) {
  for (let i = 1; i < PATH.length; i++) {
    const [ax, ay] = PATH[i - 1]
    const [bx, by] = PATH[i]
    const length = Math.abs(bx - ax) + Math.abs(by - ay)
    if (d <= length) return { x: ax + Math.sign(bx - ax) * d, y: ay + Math.sign(by - ay) * d }
    d -= length
  }
  return null
}
```

What it means

A piece from corner a to corner b is |bx - ax| + |by - ay| tiles long (one of the two is 0).
If d fits, the point is d tiles from a towards b; Math.sign gives the direction.
Otherwise the piece is used up: d -= length, and the next piece is tried. Past the last piece: null.

Your turn

Under reset, after an empty line, write the comment and pointAt.

Skills: Functions

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