src/scene.js

// The loop, in order:
//   1. Prince enters from the left, running, as a solid black silhouette.
//   2. He stops centre-left.
//   3. A Drained enters from the right, walking. Slow. Unhurried. Never runs.
//   4. Player holds torch -> sword away, light opens ahead of his raised hand.
//   5. Inside the light only, the Drained resolves into line art.
//   6. Release -> light closes, sword returns.
//   7. The Drained reaches him, holds, fades, resets.
//
// SPEC DEPARTURE — combat. Sections 1, 5.4 and 7 of the spec say the Drained
// never accelerates, never reacts to the torch and never attacks, and list
// combat as out of scope. That was overridden deliberately: once a Drained has
// been revealed in the torchlight it rushes, which forces the prince to put the
// torch down and fight. Standing decisions, all easy to reverse:
//   * No health. One touch ends the loop, using the same hold-fade-reset the
//     demo already had. Nothing new on screen.
//   * He cannot kill it. A hit knocks it back and buys time; it gets up and
//     keeps coming. The Drained are people, so the sword does not solve them.
//   * Never raise the torch and nothing changes — it ambles in as before.
//
// Every size, anchor and frame rate comes from the manifest by way of the
// loaded clips. Nothing here hardcodes how big a character is.

import { createPlayer } from './anim.js';
import { createTorch } from './torch.js';
import { DEMO } from './tuning.js';

export const W = 1920;
export const H = 1080;
export const GROUND_Y = 880;

// --- Tunables -------------------------------------------------------------
// The demo's own numbers live in tuning.js under DEMO, separate from the
// level's, so tuning the level cannot silently change the thing that is
// deployed and being played.
//
// NOTE ON LOOP LENGTH: the spec asks for both a "fifteen-second demo" and a
// Drained walk speed of 55px/sec. Those two numbers disagree — at 55px/sec the
// walk across the screen alone is ~21s. Keeping the spec's speed was the call,
// so the loop runs ~24s.
const {
  DRAINED_SPEED, DRAINED_RUSH_SPEED, DRAINED_RUSH_ACCEL,
  KNOCKBACK_SPEED, KNOCKBACK_DECEL, STUN_TIME,
  PRINCE_RUN_SPEED, PRINCE_STOP_X, CONTACT_GAP,
  CONTACT_HOLD, FADE_TIME,
} = DEMO;
// --------------------------------------------------------------------------

const clamp01 = (v) => (v < 0 ? 0 : v > 1 ? 1 : v);


export function createScene(clips, { speedScale = 1 } = {}) {
  // Playtest affordance only. The demo's own numbers are unchanged at scale 1;
  // ?speed=3 just stops a tester waiting out the twenty-second walk-in.
  const baseSpeed = DRAINED_SPEED * speedScale;
  const rushSpeed = DRAINED_RUSH_SPEED * speedScale;
  const runClip = clips.prince_run;
  const idleClip = clips.prince_idle;
  const torchClip = clips.prince_torch;
  const guardClip = clips.prince_guard;
  const attackClip = clips.prince_attack;
  const walkClip = clips.drained_walk;
  const ATTACK_DURATION = attackClip.count / attackClip.fps;

  // The hand the torch is held in, published by the manifest in sprite
  // coordinates. Converted once into an offset from the prince's feet, so
  // moving the pose in the art moves the light with it.
  const hand = torchClip.handPivot ?? [torchClip.pivotX, torchClip.pivotY - 230];
  const HAND_DX = hand[0] - torchClip.pivotX;
  const HAND_DY = hand[1] - torchClip.pivotY;

  // Edges derived from the art, not from numbers typed in here.
  const princeFrontEdge = runClip.width - runClip.pivotX;
  const princeBackEdge = runClip.pivotX;
  const drainedFrontEdge = walkClip.pivotX;          // it faces left
  const drainedBackEdge = walkClip.width - walkClip.pivotX;

  const PRINCE_SPAWN_X = -princeFrontEdge - 10;
  const DRAINED_SPAWN_X = W + drainedFrontEdge;
  const DRAINED_CONTACT_X = PRINCE_STOP_X + princeFrontEdge + drainedFrontEdge + CONTACT_GAP;

  const prince = {
    x: PRINCE_SPAWN_X, prevX: PRINCE_SPAWN_X,
    facing: 1,
    hasSword: true,
    clip: runClip,
    frame: 0,
    run: createPlayer(runClip),
    idle: createPlayer(idleClip),
    torch: createPlayer(torchClip),
    guard: createPlayer(guardClip),
    attacking: false,
    attackTime: 0,
    attackConnected: false,
  };

  const drained = {
    x: DRAINED_SPAWN_X, prevX: DRAINED_SPAWN_X,
    active: false,
    clip: walkClip,
    frame: 0,
    walk: createPlayer(walkClip),
    revealed: false,      // latches the first time it is seen in the light
    speed: baseSpeed,
    stun: 0,
    knockVel: 0,
  };

  const flame = { clip: clips.fire, player: createPlayer(clips.fire), frame: 0 };

  const torch = createTorch();

  const state = {
    prince, drained, torch, flame,
    ground: clips.ground,
    phase: 'enter',
    timer: 0,
    fade: 0, prevFade: 0,
    clock: 0,
    loopElapsed: 0,
    lastLoopLength: 0,
  };

  function reset() {
    torch.reset();
    prince.x = prince.prevX = PRINCE_SPAWN_X;
    prince.hasSword = true;
    prince.run.reset();
    prince.idle.reset();
    drained.x = drained.prevX = DRAINED_SPAWN_X;
    drained.active = false;
    drained.walk.reset();
    drained.revealed = false;
    drained.speed = baseSpeed;
    drained.stun = 0;
    drained.knockVel = 0;
    prince.attacking = false;
    prince.attackTime = 0;
    prince.attackConnected = false;
    state.phase = 'enter';
    state.timer = 0;
    state.fade = state.prevFade = 0;
    state.loopElapsed = 0;
  }

  function update(dt, input) {
    prince.prevX = prince.x;
    drained.prevX = drained.x;
    torch.snapshot();
    state.prevFade = state.fade;
    state.clock += dt;
    state.loopElapsed += dt;

    // --- Torch (spec 5.2, 5.3) -------------------------------------------
    const eased = torch.update(dt, !!input.torch, state.clock,
      prince.x, GROUND_Y, prince.facing, HAND_DX, HAND_DY);

    // The sword rule: torch up means sword away. Not negotiable. There is no
    // sword sprite — the sword lives in the run and idle art and is absent from
    // the torch clip, so swapping the clip IS putting it away.
    prince.hasSword = torch.raise <= 0.001;

    // --- Revealed? ---------------------------------------------------------
    // Latches the first time the Drained is caught inside the lit circle. Once
    // it knows it has been seen, it comes for him, and it does not settle back
    // down.
    if (!drained.revealed && drained.active &&
        torch.covers(drained.x, GROUND_Y - walkClip.height * 0.5)) {
      drained.revealed = true;
    }
    const targetSpeed = drained.revealed ? rushSpeed : baseSpeed;
    if (drained.speed < targetSpeed) {
      drained.speed = Math.min(targetSpeed, drained.speed + DRAINED_RUSH_ACCEL * speedScale * dt);
    }

    // --- The sword ---------------------------------------------------------
    // He can only swing with the sword out, which means the torch must be down,
    // which means he cannot see what he is swinging at. That is the whole trade.
    if (input.attack) {
      input.attack = false;
      // Only while it is still coming. Once it has reached him the loop is
      // over, and a swing that visibly changes nothing reads as a broken input.
      const canSwing = prince.hasSword && !prince.attacking && state.phase === 'approach';
      if (canSwing) {
        prince.attacking = true;
        prince.attackTime = 0;
        prince.attackConnected = false;
      }
    }
    if (prince.attacking) {
      prince.attackTime += dt;
      const frame = Math.min(Math.floor(prince.attackTime * attackClip.fps), attackClip.count - 1);

      // The manifest says which frame connects and how far the blade reaches.
      if (!prince.attackConnected && frame >= (attackClip.hitFrame ?? 0)) {
        prince.attackConnected = true;
        const gap = (drained.x - drainedFrontEdge) - prince.x;
        if (drained.active && gap <= attackClip.reach) {
          // It gets knocked back and it gets up. He cannot kill it — they are
          // people. The sword buys time, it does not solve anything.
          drained.knockVel = KNOCKBACK_SPEED;
          drained.stun = STUN_TIME;
        }
      }
      if (prince.attackTime >= ATTACK_DURATION) prince.attacking = false;
    }

    if (drained.knockVel > 0) {
      drained.x += drained.knockVel * dt;
      drained.knockVel = Math.max(0, drained.knockVel - KNOCKBACK_DECEL * dt);
    }
    if (drained.stun > 0) drained.stun -= dt;

    // --- Prince animation --------------------------------------------------
    if (prince.attacking) {
      prince.clip = attackClip;
      prince.frame = Math.min(Math.floor(prince.attackTime * attackClip.fps), attackClip.count - 1);
    } else if (!prince.hasSword) {
      // Pose is driven by the light, so raising and lowering read as one move.
      // The clip's own fps and frame count still decide how many frames exist.
      prince.torch.time = (eased * (torchClip.count - 1)) / torchClip.fps;
      prince.clip = torchClip;
      prince.frame = prince.torch.index;
      flame.player.advance(dt);
      flame.frame = flame.player.index;
    } else if (state.phase === 'enter') {
      prince.run.advance(dt);
      prince.clip = runClip;
      prince.frame = prince.run.index;
    } else if (drained.revealed) {
      // Once it has been seen, he stands ready whenever he is not swinging.
      prince.guard.advance(dt);
      prince.clip = guardClip;
      prince.frame = prince.guard.index;
    } else {
      prince.idle.advance(dt);
      prince.clip = idleClip;
      prince.frame = prince.idle.index;
    }

    // --- Phases ----------------------------------------------------------
    if (state.phase === 'enter') {
      // He cannot walk while looking. Movement speed drops to zero. (spec 5.2)
      const speed = prince.hasSword ? PRINCE_RUN_SPEED : 0;
      prince.x += speed * dt;
      if (prince.x >= PRINCE_STOP_X) {
        prince.x = PRINCE_STOP_X;
        state.phase = 'approach';
        drained.active = true;
      }
    } else if (state.phase === 'approach') {
      if (drained.stun <= 0 && drained.knockVel <= 0) {
        drained.x -= drained.speed * dt;
      }
      // The walk cycle plays at the same ratio as the movement, so a faster
      // Drained does not moonwalk. Base rate still comes from the manifest.
      drained.walk.advance(dt * (drained.speed / DRAINED_SPEED));
      drained.frame = drained.walk.index;
      if (drained.x <= DRAINED_CONTACT_X) {
        drained.x = DRAINED_CONTACT_X;
        state.phase = 'contact';
        state.timer = CONTACT_HOLD;
      }
    } else if (state.phase === 'contact') {
      state.timer -= dt;
      if (state.timer <= 0) {
        state.phase = 'fade';
        state.timer = FADE_TIME;
      }
    } else if (state.phase === 'fade') {
      state.timer -= dt;
      state.fade = clamp01(1 - state.timer / FADE_TIME);
      if (state.timer <= 0) {
        state.lastLoopLength = state.loopElapsed;
        reset();
      }
    }
  }

  return { state, update, reset };
}