Workshop › OutRun-style Racer › Step 30 of 44

Distance on a loop

What we are doing

"How far ahead is that car?" is harder on a looping track: a car at 100 may be just ahead of us at 159 900. ahead(a, b) is the distance going forwards from a to b, always between 0 and the track length.

```js
// Distance from a to b going forwards along the looping track.
const ahead = (a, b) => (((b - a) % trackLength) + trackLength) % trackLength
```

What it means

• (b - a) % trackLength can still be negative in JavaScript: -30 % 100 is -30.
• Adding trackLength makes it positive, and the second % brings a positive result back under trackLength.

Your turn

Under the segmentAt line write the comment and ahead.

Skills: Types and values

← Other cars Cars that shrink with distance →

All steps

1. Sky and ground
2. Draw again and again
3. Perspective
4. One piece of road
5. A road made of segments
6. Where we are on the track
7. The road ahead
8. Stripes
9. Paint from far to near
10. Kerbs and a center line
11. Striped grass
12. Moving is one number
13. Which keys are down?
14. The gas pedal
15. Brake, coast and a top speed
16. Your car
17. Speedometer
18. A pedal for phones
19. Move the camera sideways
20. Steering
21. No turning on the spot
22. The grass is slow
23. Steering by touch
24. A track with curves
25. Bend the road
26. No jumps in the curve
27. Which segment am I on?
28. Curves pull you outwards
29. Other cars
30. Distance on a loop
31. Cars that shrink with distance
32. Traffic moves
33. Rear-end crash
34. Counting laps
35. Show the lap
36. A lap timer
37. Show the time
38. Best lap
39. Show the best lap
40. Racing or finished
41. The finish line
42. Finished!
43. Race again
44. Build it yourself: beat the clock