refactor: separate React views, Redux snapshots and Three.js runtime

src/game/systems/vehicle.js

@@ -0,0 +1,35 @@
+// Arcade car physics. Heading h faces world direction (cos h, 0, -sin h), matching Three's rotation.y.
+// maxSpeed 55.6 m/s is 200 km/h.
+export const CAR = {
+  maxSpeed: 55.6,
+  maxReverse: 6,
+  accel: 9.5,
+  brake: 14,
+  drag: 1.6,
+  maxSteer: 0.55,
+  wheelBase: 2.7,
+  radius: 1.6,
+};
+
+export function createCar(x, z, heading = 0) {
+  return { x, z, heading, speed: 0, steer: 0 };
+}
+
+export function stepCar(car, { throttle = 0, steer = 0, handbrake = false }, dt) {
+  car.steer += (steer * CAR.maxSteer - car.steer) * Math.min(1, dt * 6);
+  if (throttle > 0) car.speed += (car.speed < 0 ? CAR.brake : CAR.accel) * throttle * dt;
+  else if (throttle < 0)
+    car.speed += (car.speed > 0 ? -CAR.brake : -CAR.accel * 0.6) * -throttle * dt;
+  else car.speed -= Math.sign(car.speed) * Math.min(Math.abs(car.speed), CAR.drag * dt);
+  if (handbrake)
+    car.speed -= Math.sign(car.speed) * Math.min(Math.abs(car.speed), CAR.brake * 1.4 * dt);
+  car.speed = Math.max(-CAR.maxReverse, Math.min(CAR.maxSpeed, car.speed));
+  // High speed narrows the usable steering angle so the car stays controllable.
+  const steerAngle = car.steer / (1 + Math.abs(car.speed) * 0.045);
+  car.heading -= ((car.speed * Math.tan(steerAngle)) / CAR.wheelBase) * dt;
+  car.x += Math.cos(car.heading) * car.speed * dt;
+  car.z -= Math.sin(car.heading) * car.speed * dt;
+  return car;
+}
+
+export const kmh = (car) => Math.round(Math.abs(car.speed) * 3.6);