party-player/public/player/motion.js

// Frame motion. Everything here writes to #frameMover's transform and nothing
// else, so additional animation layers can be nested inside without conflict.
//
// An animation is `start(context) => stop()`. `context` carries the element to
// move and a capture handle (possibly null). `stop()` must undo everything the
// animation set up — timers, listeners, DOM — because the next animation takes
// over the same element immediately.

import {
  FRAME_SCALE,
  BOUNCE_SPEED,
  CORNER_INTERVAL_SECONDS,
  CORNER_MARGIN,
  AUTO_ANIMATION,
} from '/config.js';

const ASPECT = 9 / 16;

/**
 * Frame size for a viewport: FRAME_SCALE of the width, but never taller than
 * FRAME_SCALE of the height, so a wide screen does not get a letterbox strip.
 */
export function frameSize(viewportWidth, viewportHeight) {
  let width = viewportWidth * FRAME_SCALE;
  let height = width * ASPECT;

  const maxHeight = viewportHeight * FRAME_SCALE;
  if (height > maxHeight) {
    height = maxHeight;
    width = height / ASPECT;
  }

  return { width, height };
}

/** Size the frame to the current viewport and report the size back. */
function applyFrameSize(element) {
  const size = frameSize(window.innerWidth, window.innerHeight);
  element.style.width = `${size.width}px`;
  element.style.height = `${size.height}px`;
  return size;
}

function place(element, x, y) {
  element.style.transform = `translate3d(${x.toFixed(1)}px, ${y.toFixed(1)}px, 0)`;
}

// --- picker -------------------------------------------------------------

/**
 * Shuffle bag over 0..count-1: every animation comes up exactly once per pass,
 * so selection stays even, and a swap at the bag boundary keeps the same one
 * from landing twice in a row. Pure apart from the injected RNG.
 */
export function createPicker(count, random = Math.random) {
  let bag = [];
  let last = -1;

  function refill() {
    bag = Array.from({ length: count }, (_, i) => i);

    // Fisher–Yates.
    for (let i = bag.length - 1; i > 0; i--) {
      const j = Math.floor(random() * (i + 1));
      [bag[i], bag[j]] = [bag[j], bag[i]];
    }

    // next() pops off the end, so bag[length-1] is served first. A repeat can
    // only happen right here, at the seam between two bags.
    if (bag.length > 1 && bag[bag.length - 1] === last) {
      const j = Math.floor(random() * (bag.length - 1));
      [bag[bag.length - 1], bag[j]] = [bag[j], bag[bag.length - 1]];
    }
  }

  return {
    next() {
      if (count <= 1) return 0;
      if (bag.length === 0) refill();
      last = bag.pop();
      return last;
    },
  };
}

// --- bounce -------------------------------------------------------------

/**
 * Diagonal drift with reflection off the viewport edges. Pure state + step, so
 * the physics can be exercised without a browser and without waiting on rAF.
 */
export function createBouncer({ x = 0, y = 0 } = {}) {
  const bouncer = {
    x,
    y,
    // A shallow diagonal reads better than 45°, and never retraces its path.
    vx: BOUNCE_SPEED,
    vy: BOUNCE_SPEED * 0.62,

    step(dt, bounds) {
      const maxX = Math.max(0, bounds.width);
      const maxY = Math.max(0, bounds.height);

      bouncer.x += bouncer.vx * dt;
      bouncer.y += bouncer.vy * dt;

      // Reflect, mirroring the overshoot back inside the viewport.
      if (bouncer.x <= 0) {
        bouncer.x = Math.min(-bouncer.x, maxX);
        bouncer.vx = Math.abs(bouncer.vx);
      } else if (bouncer.x >= maxX) {
        bouncer.x = Math.max(maxX - (bouncer.x - maxX), 0);
        bouncer.vx = -Math.abs(bouncer.vx);
      }

      if (bouncer.y <= 0) {
        bouncer.y = Math.min(-bouncer.y, maxY);
        bouncer.vy = Math.abs(bouncer.vy);
      } else if (bouncer.y >= maxY) {
        bouncer.y = Math.max(maxY - (bouncer.y - maxY), 0);
        bouncer.vy = -Math.abs(bouncer.vy);
      }

      return bouncer;
    },
  };

  return bouncer;
}

export function startBounce({ element }) {
  let size = applyFrameSize(element);
  const bouncer = createBouncer({
    x: (window.innerWidth - size.width) / 2,
    y: (window.innerHeight - size.height) / 2,
  });

  function bounds() {
    return {
      width: window.innerWidth - size.width,
      height: window.innerHeight - size.height,
    };
  }

  function measure() {
    size = applyFrameSize(element);

    const limit = bounds();
    bouncer.x = Math.min(bouncer.x, Math.max(0, limit.width));
    bouncer.y = Math.min(bouncer.y, Math.max(0, limit.height));
  }

  window.addEventListener('resize', measure);

  let last = performance.now();
  let handle = 0;
  let stopped = false;

  function frame(now) {
    if (stopped) return;

    // Clamped so a stalled tab does not teleport the frame on resume.
    const dt = Math.min((now - last) / 1000, 0.05);
    last = now;

    bouncer.step(dt, bounds());
    place(element, bouncer.x, bouncer.y);
    handle = requestAnimationFrame(frame);
  }

  handle = requestAnimationFrame(frame);

  return function stop() {
    // The flag matters as much as the cancel: a frame already queued would
    // otherwise fire once more and fight whatever animation comes next.
    stopped = true;
    cancelAnimationFrame(handle);
    window.removeEventListener('resize', measure);
  };
}

// --- corners ------------------------------------------------------------

// Clockwise from the top left.
const CORNERS = 4;

/** Top-left offset for a corner, inset by `margin` from the viewport edge. */
export function cornerPosition(index, size, viewport, margin) {
  const right = index === 1 || index === 2;
  const bottom = index === 2 || index === 3;

  return {
    x: right ? Math.max(margin, viewport.width - size.width - margin) : margin,
    y: bottom ? Math.max(margin, viewport.height - size.height - margin) : margin,
  };
}

/**
 * The video parks in one random corner and stays there; every few seconds a
 * screenshot of it lands in the next of the three *other* corners, cycling
 * clockwise. Because the video's own corner is never a slot, a still can never
 * cover the video — and since the crop only ever covers the video's rect, a
 * still never contains another still either.
 */
export function startCorners(context) {
  const { element } = context;
  const layer = document.getElementById('stills');
  const home = Math.floor(Math.random() * CORNERS);

  // The three non-home corners, in clockwise order starting after the video.
  const slots = [1, 2, 3].map((step) => (home + step) % CORNERS);
  const images = slots.map(() => {
    const img = document.createElement('img');
    layer.appendChild(img);
    return img;
  });

  let slot = 0;
  let size;

  function layout() {
    size = applyFrameSize(element);
    const viewport = { width: window.innerWidth, height: window.innerHeight };
    const margin = Math.min(viewport.width, viewport.height) * CORNER_MARGIN;

    const spot = cornerPosition(home, size, viewport, margin);
    place(element, spot.x, spot.y);

    images.forEach((img, i) => {
      const at = cornerPosition(slots[i], size, viewport, margin);
      img.style.width = `${size.width}px`;
      img.style.height = `${size.height}px`;
      place(img, at.x, at.y);
    });
  }

  function shoot() {
    // Read late: the capture handle may still be waiting on the operator to
    // answer the screen-share prompt when this animation starts.
    if (!context.capture) return;

    const rect = element.getBoundingClientRect();
    const shot = context.capture.grab(rect);
    if (!shot) return;

    const img = images[slot];
    img.src = shot;
    img.classList.add('visible');
    slot = (slot + 1) % images.length;
  }

  layout();
  window.addEventListener('resize', layout);
  const timer = setInterval(shoot, CORNER_INTERVAL_SECONDS * 1000);

  return function stop() {
    clearInterval(timer);
    window.removeEventListener('resize', layout);
    for (const img of images) img.remove();
  };
}

// --- registry -----------------------------------------------------------

// Keyed by the names in config.js, which is what travels over the socket.
export const ANIMATIONS = {
  bounce: startBounce,
  corners: startCorners,
};

const NAMES = Object.keys(ANIMATIONS);

/**
 * Run one animation at a time. In auto mode next() swaps to a freshly picked
 * one on every track change; pinned to a name (from the queue page, for
 * testing) it stays put until the mode changes again.
 */
export function startAnimationCycle(context, mode = AUTO_ANIMATION) {
  const picker = createPicker(NAMES.length);
  let stop = null;
  let current = null;

  function run(name) {
    stop?.();
    current = name;
    stop = ANIMATIONS[name](context);
    return current;
  }

  function next() {
    // Pinned: the track changed, but the operator asked for this one.
    if (mode !== AUTO_ANIMATION) return current;
    return run(NAMES[picker.next()]);
  }

  function setMode(value) {
    mode = ANIMATIONS[value] ? value : AUTO_ANIMATION;
    // Always restarts, so switching back to auto takes effect immediately
    // rather than waiting out the current track.
    return mode === AUTO_ANIMATION ? run(NAMES[picker.next()]) : run(mode);
  }

  setMode(mode);

  return {
    next,
    setMode,
    get mode() {
      return mode;
    },
    get name() {
      return current;
    },
  };
}