tilt/test/skaterSim.mjs

import { SKATE, applyIntent, createSkaterState, speedOf, stepSkater } from '../shared/skaterSim.js';
import { RINK, insideRink } from '../shared/rink.js';
import {
  normalizePlayer, normalizeShotSide, packPlayer, rollShotSide, shotSign, unpackPlayer,
} from '../shared/player.js';
import { done, near, ok, section } from './harness.mjs';

const DT = 1 / 120;

/**
 * Run the sim for `seconds`, optionally editing the state each step.
 *
 * Board clamping is off by default so a test about acceleration is not
 * secretly a test about the end boards. The containment section turns it back
 * on, which is the only place it is the subject.
 */
function run(s, seconds, edit = null, opts = { clampBoards: false }) {
  const steps = Math.round(seconds / DT);
  for (let i = 0; i < steps; i++) {
    if (edit) edit(s, i * DT);
    stepSkater(s, DT, opts);
  }
  return s;
}

/** Down the ice: the rink's long axis is +X, which is yaw = PI/2. */
const START = { x: 0, z: 0, yaw: Math.PI / 2 };
const forward = (s) => {
  s.ix = 1;
  s.iz = 0;
};
const coast = (s) => {
  s.ix = 0;
  s.iz = 0;
};

section('the stride reaches a speed and holds it');
{
  const s = createSkaterState(0, START);
  run(s, 8, forward);
  const cruise = speedOf(s);
  ok(cruise > 4.5, `cruise settles above 4.5 m/s (got ${cruise.toFixed(2)})`);
  ok(cruise <= SKATE.cruiseSpeed, `and never exceeds the cruise ceiling (${cruise.toFixed(2)})`);

  // Another four seconds must not keep adding speed.
  const before = speedOf(s);
  run(s, 4, forward);
  near(speedOf(s), before, 0.05, 'top speed is stable, not creeping');
}

section('acceleration takes time — you cannot jump to top speed');
{
  const s = createSkaterState(0, START);
  run(s, 0.5, forward);
  const half = speedOf(s);
  ok(half > 0.8, `half a second of pushing gets you moving (${half.toFixed(2)} m/s)`);
  ok(half < 4, 'but nowhere near cruise');
}

section('sprinting is meaningfully faster');
{
  const cruiser = createSkaterState(0, START);
  run(cruiser, 8, forward);
  const sprinter = createSkaterState(1, START);
  run(sprinter, 8, (s) => {
    forward(s);
    s.sprint = true;
  });
  ok(
    speedOf(sprinter) > speedOf(cruiser) + 1.5,
    `sprint beats cruise by more than 1.5 m/s (${speedOf(sprinter).toFixed(2)} vs ${speedOf(cruiser).toFixed(2)})`,
  );
  ok(speedOf(sprinter) <= SKATE.sprintSpeed, 'and stays under the sprint ceiling');
}

section('a glide keeps its momentum');
{
  const s = createSkaterState(0, START);
  run(s, 8, forward);
  const entry = speedOf(s);
  run(s, 3, coast);
  const after = speedOf(s);
  ok(after > entry * 0.6, `three seconds of glide keeps most of the speed (${after.toFixed(2)} of ${entry.toFixed(2)})`);
  ok(after < entry, 'but not all of it');
}

section('braking is much faster than gliding');
{
  const glide = createSkaterState(0, START);
  run(glide, 8, forward);
  const brake = createSkaterState(1, START);
  run(brake, 8, forward);
  near(speedOf(glide), speedOf(brake), 0.01, 'both start from the same speed');

  run(glide, 1, coast);
  run(brake, 1, (s) => {
    coast(s);
    s.brake = true;
  });
  ok(speedOf(brake) < 0.6, `a hockey stop is done inside a second (${speedOf(brake).toFixed(2)} m/s left)`);
  ok(speedOf(glide) > speedOf(brake) * 4, 'a glide over the same second is nowhere near stopped');
}

section('the blade kills sideways drift');
{
  const s = createSkaterState(0, START);
  // Thrown across the blade at 4 m/s: body pointing +X, momentum along +Z.
  s.vx = 0;
  s.vz = 4;
  run(s, 1.5, coast);
  const velYaw = Math.atan2(s.vx, s.vz);
  const offBlade = Math.abs(Math.abs(velYaw) - Math.PI / 2);
  ok(offBlade < 0.25, `momentum ends up along the blade, not across it (${offBlade.toFixed(3)} rad off)`);
}

section('a carve redirects momentum instead of destroying it');
{
  const s = createSkaterState(0, START);
  run(s, 6, forward);
  const entry = speedOf(s);
  ok(Math.abs(Math.atan2(s.vx, s.vz) - Math.PI / 2) < 0.05, 'travelling straight down the ice first');

  // Ninety degrees of turn: the stick swings from +X to -Z.
  run(s, 1.2, (st) => {
    st.ix = 0;
    st.iz = -1;
  });

  const velYaw = Math.atan2(s.vx, s.vz);
  ok(velYaw > 2.2, `the velocity vector followed the turn round (${velYaw.toFixed(2)} rad, want ~PI)`);
  ok(speedOf(s) > entry * 0.4, `and kept real speed through it (${speedOf(s).toFixed(2)} of ${entry.toFixed(2)})`);
  ok(speedOf(s) < entry, 'a hard carve is not free');
}

section('momentum resists an instant reversal');
{
  const s = createSkaterState(0, START);
  run(s, 6, forward);
  const entryX = s.vx;
  ok(entryX > 3, 'moving down the ice to begin with');

  // A tenth of a second of "go back the other way" must not flip the velocity.
  run(s, 0.1, (st) => {
    st.ix = -1;
    st.iz = 0;
  });
  ok(s.vx > 0, 'still travelling the original way a tenth of a second later');
  ok(s.vx < entryX, 'but already losing speed to the edges');
}

section('a turn on the spot costs nothing');
{
  const s = createSkaterState(0, { x: 0, z: 0, yaw: 0 });
  run(s, 0.6, (st) => {
    st.ix = 1;
    st.iz = 0;
  });
  ok(Math.abs(s.yaw - Math.PI / 2) < 0.35, `a standing skater can pivot (yaw ${s.yaw.toFixed(2)})`);
}

section('turning is harder at speed than at rest');
{
  const slow = createSkaterState(0, { x: 0, z: 0, yaw: 0 });
  run(slow, 0.3, (st) => {
    st.ix = 1;
    st.iz = 0;
  });
  const fast = createSkaterState(1, { x: 0, z: 0, yaw: 0 });
  run(fast, 6, (st) => {
    st.ix = 0;
    st.iz = 1;
    st.sprint = true;
  });
  const before = fast.yaw;
  run(fast, 0.3, (st) => {
    st.ix = 1;
    st.iz = 0;
    st.sprint = true;
  });
  ok(
    Math.abs(fast.yaw - before) < Math.abs(slow.yaw),
    `a flying skater turns slower than a standing one (${(fast.yaw - before).toFixed(3)} vs ${slow.yaw.toFixed(3)} rad)`,
  );
}

section('nothing leaves the rink');
{
  // Point skaters at the boards from centre ice and hold it for ten seconds.
  for (let i = 0; i < 16; i++) {
    const a = (i / 16) * Math.PI * 2;
    const s = createSkaterState(i, { x: 0, z: 0, yaw: a });
    run(s, 10, (st) => {
      st.ix = Math.sin(a);
      st.iz = Math.cos(a);
      st.sprint = true;
    }, { clampBoards: true });
    ok(insideRink(s.x, s.z, SKATE.radius), `skater driving at heading ${a.toFixed(2)} stayed on the ice`);
    ok(Number.isFinite(s.x) && Number.isFinite(s.z), 'and its position stayed finite');
  }
}

section('the sim is deterministic');
{
  const drive = (st, t) => {
    st.ix = Math.sin(t * 1.3);
    st.iz = Math.cos(t * 0.7);
    st.sprint = t > 3;
  };
  const a = createSkaterState(0, { x: 4, z: -6, yaw: 1 });
  const b = createSkaterState(0, { x: 4, z: -6, yaw: 1 });
  run(a, 12, drive, { clampBoards: true });
  run(b, 12, drive, { clampBoards: true });
  near(a.x, b.x, 0, 'same inputs, same x');
  near(a.z, b.z, 0, 'same inputs, same z');
  near(a.yaw, b.yaw, 0, 'same inputs, same yaw');
}

section('intent from a controller is clamped before the sim sees it');
{
  // This is the seam a gamepad or a network message will come in through, so
  // it has to survive garbage without the sim ever seeing it.
  const s = createSkaterState(0, START);

  applyIntent(s, { ix: 1, iz: 1 });
  near(Math.hypot(s.ix, s.iz), 1, 1e-9, 'a diagonal stick is normalised, not sqrt(2) fast');

  applyIntent(s, { ix: 0.3, iz: -0.4 });
  near(s.ix, 0.3, 1e-9, 'a stick inside the deadzone circle is left alone (x)');
  near(s.iz, -0.4, 1e-9, 'a stick inside the deadzone circle is left alone (z)');

  applyIntent(s, { ix: 40, iz: -40 });
  ok(Math.hypot(s.ix, s.iz) <= 1 + 1e-9, 'an out-of-range stick is clamped');

  applyIntent(s, { ix: NaN, iz: undefined, sprint: 'yes', brake: 0 });
  ok(s.ix === 0 && s.iz === 0, 'NaN and undefined become a centred stick');
  ok(s.sprint === true && s.brake === false, 'and the flags come through as booleans');

  // The clamped state must still step without producing garbage.
  stepSkater(s, DT);

  ok(Number.isFinite(s.x) && Number.isFinite(s.vx), 'and the sim steps cleanly afterwards');
}

section('rink dimensions are the ones we think they are');
{
  near(RINK.halfX * 2, 60.96, 0.01, 'the rink is 200 feet long');
  near(RINK.halfZ * 2, 25.9, 0.02, 'and 85 feet wide');
}

section('player definition carries shot side');
{
  ok(normalizeShotSide('left') === 'left', 'left stays left');
  ok(normalizeShotSide('L') === 'left', 'L is left');
  ok(normalizeShotSide(-1) === 'left', '-1 is left');
  ok(normalizeShotSide('right') === 'right', 'right stays right');
  ok(normalizeShotSide('R') === 'right', 'R is right');
  ok(normalizeShotSide(undefined) === 'right', 'missing defaults to right (authored side)');
  ok(shotSign('left') === -1 && shotSign('right') === 1, 'shotSign is ±1');
  ok(rollShotSide(0.1) === 'left' && rollShotSide(0.9) === 'right', 'roll respects the NHL-ish split');

  const packed = packPlayer({ shotSide: 'left' });
  ok(packed.ss === 'L', 'pack uses a short wire form');
  ok(unpackPlayer(packed).shotSide === 'left', 'unpack restores shot side');
  ok(normalizePlayer({ shotSide: 'left' }).shotSide === 'left', 'normalizePlayer keeps shot side');

  const lefty = createSkaterState(0, START, { shotSide: 'left', name: 'Lefty' });
  const righty = createSkaterState(1, START, { shotSide: 'right' });
  const plain = createSkaterState(2, START);
  ok(lefty.shotSide === 'left', 'state stores left shot');
  ok(righty.shotSide === 'right', 'state stores right shot');
  ok(plain.shotSide === 'right', 'state defaults to right shot');
}

done('skaterSim');