drive-between-the-lines/src/sim/sim.test.ts

import { describe, expect, it } from 'vitest';
import { generateWorld } from './world';
import { applyWear, deriveHandling, freshCondition, repair, SUBSYSTEMS, type CarCondition } from './car';

describe('world generation', () => {
  it('is reproducible from a seed', () => {
    expect(generateWorld(42)).toEqual(generateWorld(42));
  });

  it('differs between seeds', () => {
    expect(generateWorld(42)).not.toEqual(generateWorld(43));
  });

  it('keeps the spawn point clear', () => {
    const world = generateWorld(7);
    for (const o of world.obstacles) {
      expect(Math.hypot(o.x - world.spawn.x, o.z - world.spawn.z)).toBeGreaterThanOrEqual(12);
    }
  });
});

describe('car condition', () => {
  it('never recovers on its own', () => {
    let c = freshCondition();
    for (let i = 0; i < 600; i++) {
      const next = applyWear(c, { dt: 1 / 60, distance: 0.4, throttle: 1, impactForce: 0 });
      expect(next.level.engine).toBeLessThanOrEqual(c.level.engine);
      expect(next.level.tires).toBeLessThanOrEqual(c.level.tires);
      c = next;
    }
  });

  it('bottoms out rather than going negative', () => {
    let c = freshCondition();
    for (let i = 0; i < 200; i++) {
      c = applyWear(c, { dt: 1 / 60, distance: 5, throttle: 1, impactForce: 5e5 });
    }
    expect(c.level.chassis).toBe(0);
    expect(c.level.tires).toBe(0);
  });

  /**
   * The force magnitude Rapier actually reports for a head-on into a building
   * at 85 km/h, read off the running game. Every number below is anchored to it
   * rather than to a guess about what a collision "should" be worth.
   */
  const HARD_CRASH = 2.47e6;

  it('leaves a car driveable after one hard crash', () => {
    const c = applyWear(freshCondition(), {
      dt: 1 / 60,
      distance: 0.4,
      throttle: 0,
      impactForce: HARD_CRASH,
    });
    // Expensive — the worst single thing that can happen to the car — but the
    // campaign continues. The first pass wrote off the chassis in this one step.
    expect(c.level.chassis).toBeGreaterThan(0.7);
    expect(c.level.chassis).toBeLessThan(0.95);
    expect(c.ceiling.chassis).toBeGreaterThan(0.9);
  });

  it('caps what a single step can take, however violent the contact', () => {
    // A pileup reports contact forces without any natural bound; without a cap
    // one frame of a bad landing is worth more than the crash that caused it.
    const c = applyWear(freshCondition(), {
      dt: 1 / 60,
      distance: 0.4,
      throttle: 1,
      impactForce: 1e9,
    });
    for (const part of SUBSYSTEMS) {
      expect(c.level[part]).toBeGreaterThanOrEqual(0.74);
    }
  });

  it('takes a sustained battering to write the car off', () => {
    let c = freshCondition();
    let crashes = 0;
    while (c.level.chassis > 0 && crashes < 100) {
      c = applyWear(c, { dt: 1 / 60, distance: 0.4, throttle: 0, impactForce: HARD_CRASH });
      crashes++;
    }
    // Enough hard crashes to be a story about the driver, not one bad corner.
    expect(crashes).toBeGreaterThan(5);
  });

  it('makes a worn car measurably worse to drive', () => {
    const fresh = deriveHandling(freshCondition());
    const worn = deriveHandling({
      level: { engine: 0.3, tires: 0.3, chassis: 0.3 },
      ceiling: { engine: 1, tires: 1, chassis: 1 },
    });
    expect(worn.engineForce).toBeLessThan(fresh.engineForce);
    expect(worn.frictionSlip).toBeLessThan(fresh.frictionSlip);
    expect(worn.steeringPull).toBeGreaterThan(fresh.steeringPull);
  });
});

describe('repair', () => {
  const damaged = () => {
    let c = freshCondition();
    for (let i = 0; i < 40; i++) {
      c = applyWear(c, { dt: 1 / 60, distance: 3, throttle: 1, impactForce: 2e5 });
    }
    return c;
  };

  it('lowers the ceiling permanently, so a car never comes back whole', () => {
    const c = damaged();
    expect(c.ceiling.chassis).toBeLessThan(1);
    // No amount of parts can undo it.
    const restored = repair(c, 99).condition;
    expect(restored.level.chassis).toBeLessThan(1);
    expect(restored.level.chassis).toBeCloseTo(c.ceiling.chassis, 6);
  });

  it('never lifts a subsystem above its ceiling', () => {
    const c = damaged();
    const restored = repair(c, 99).condition;
    for (const part of SUBSYSTEMS) {
      expect(restored.level[part]).toBeLessThanOrEqual(restored.ceiling[part] + 1e-9);
    }
  });

  it('hands back parts it could not use', () => {
    const { unused } = repair(freshCondition(), 5);
    expect(unused).toBeCloseTo(5, 6);
  });

  it('spends a scarce part on the worst subsystem', () => {
    const c: CarCondition = {
      level: { engine: 0.9, tires: 0.2, chassis: 0.8 },
      ceiling: { engine: 1, tires: 1, chassis: 1 },
    };
    const restored = repair(c, 0.1).condition;
    expect(restored.level.tires).toBeCloseTo(0.3, 6);
    expect(restored.level.engine).toBe(0.9);
  });

  it('ratchets down over repeated damage-and-repair cycles', () => {
    let c = freshCondition();
    let previousCeiling = 1;
    for (let cycle = 0; cycle < 5; cycle++) {
      for (let i = 0; i < 30; i++) {
        c = applyWear(c, { dt: 1 / 60, distance: 3, throttle: 1, impactForce: 2e4 });
      }
      c = repair(c, 99).condition;
      // Fully repaired every cycle, and still worse off every cycle.
      expect(c.level.chassis).toBeCloseTo(c.ceiling.chassis, 6);
      expect(c.ceiling.chassis).toBeLessThan(previousCeiling);
      previousCeiling = c.ceiling.chassis;
    }
    expect(previousCeiling).toBeGreaterThan(0);
  });
});