The Ember Ring

Your spell is one function, steer(sensors), called twenty times a second of flight. It answers {"turn": t, "thrust": p} (or [t, p]): turn from -1 (hard left) to 1 (hard right), thrust from -1 (back-beat, to brake) to 1 (full wings). Anything else, or an error, and the dragon glides for that twentieth. It must be quick: a twentieth of a second a call, twelve seconds a lap in all. Variables you keep outside steer last the whole lap.

The sensors

• speed: units a second. forward and slip: the speed along your heading and across it (+ is drifting right). turn_rate: the fastest your heading can swing just now, in radians a second.
• ahead: seven points along the middle of the track, 40, 80, 120, 160, 200, 260 and 320 units on, each {"ahead": a, "right": r, "direction": d}: where it lies from you (a ahead, r to your right) and which way the track runs there, in radians from your heading (+ is to the right).
• rays: how far the track reaches from you at the angles in ray_angles (-90 to 90 degrees from your heading, + to the right), up to 400 units; 0 if you are off it.
• offset: how far you are from the middle of the track (+ is right of it), and half_width: how far the edges are from the middle. track_angle: which way the track runs here, from your heading. on_track: whether you are on it.
• x, y (y grows downward) and heading (radians: 0 is east, and it grows clockwise); progress round the lap from 0 to 1; time in seconds; step.

The rules of flight

• Each twentieth: the heading turns by turn × 2.4 radians a second, but never faster than 90 / speed; wings push along the heading at thrust × 60 units a second a second (a back-beat slows the dragon at -thrust × 90); the sideways part of the speed fades by 1.8 a second (on the track; 1.2 off it); everything fades by drag, 0.5 a second (3.7 off the track).
• So the top speed is 120, and a bend of radius r has a top speed of about the square root of 90 × r: 63 for a hairpin of 45, 95 for a bend of 100. Braking from 120 to 60 at full back-beat takes about 60 units.

Ideas

• Work out from ahead how sharply the track bends, how fast you may take each bend, and when you must start braking for it.
• The shortest way round a bend is not the middle of the track: come in wide, touch the inside, go out wide.
• Keep a little of what you saw in variables outside steer: the lap is one long flight.
• Drift costs speed. A smooth turn early beats a hard one late.