/**
 * Car condition and what it does to handling. Pure — no engine imports.
 *
 * Design pillar this serves: "Decline, not reset." Condition only ever falls.
 * Phase 0's whole job is to make that decline *felt* through the wheel, so the
 * handling numbers the physics layer uses are derived here rather than constants.
 */

export interface CarCondition {
  /** 1 = factory fresh, 0 = ruined. */
  engine: number;
  tires: number;
  chassis: number;
}

export interface Handling {
  /** Newtons of drive force available per driven wheel at full throttle. */
  engineForce: number;
  /** Braking impulse per wheel. */
  brakeForce: number;
  /** Max steering angle, radians. */
  maxSteer: number;
  /** Tyre grip. Lower = slides. */
  frictionSlip: number;
  sideFrictionStiffness: number;
  /** Constant tug on the wheel from a bent chassis, radians. Signed. */
  steeringPull: number;
}

export const freshCondition = (): CarCondition => ({ engine: 1, tires: 1, chassis: 1 });

const lerp = (a: number, b: number, t: number) => a + (b - a) * t;
const clamp01 = (v: number) => Math.min(1, Math.max(0, v));

export function deriveHandling(c: CarCondition): Handling {
  return {
    // A tired engine simply cannot push as hard.
    engineForce: lerp(900, 2600, c.engine),
    // Worn pads take longer to haul the car down.
    brakeForce: lerp(4, 14, c.tires),
    maxSteer: lerp(0.35, 0.55, c.chassis),
    // Bald tyres are the most legible failure: the back end starts to leave.
    frictionSlip: lerp(1.6, 5, c.tires),
    sideFrictionStiffness: lerp(0.5, 1, c.tires),
    // A bent chassis pulls to one side. Sign is stable for a given car.
    steeringPull: (1 - c.chassis) * 0.06,
  };
}

export interface WearInput {
  /** Seconds of simulated time. */
  dt: number;
  /** Metres travelled this step. */
  distance: number;
  /** Throttle actually applied, 0..1. */
  throttle: number;
  /** Sum of impact force magnitudes registered this step, newtons. */
  impactForce: number;
}

/**
 * Returns a new condition. Numbers here are deliberately aggressive so a
 * 15-minute session shows visible decline — tune down once the feel is right.
 */
export function applyWear(c: CarCondition, w: WearInput): CarCondition {
  const impact = w.impactForce / 1e5;
  return {
    engine: clamp01(c.engine - w.throttle * w.dt * 6e-4 - impact * 0.01),
    tires: clamp01(c.tires - w.distance * 8e-5 - impact * 0.02),
    chassis: clamp01(c.chassis - impact * 0.05),
  };
}