const WORKER_CDN = 'https://cdnjs.cloudflare.com/ajax/libs/gif.js/0.2.0/gif.worker.js';
let workerBlobUrl = null;

async function getWorkerUrl() {
  if (workerBlobUrl) return workerBlobUrl;
  const res = await fetch(WORKER_CDN);
  const blob = await res.blob();
  workerBlobUrl = URL.createObjectURL(blob);
  return workerBlobUrl;
}

function loadImage(src) {
  return new Promise((resolve, reject) => {
    const img = new Image();
    img.onload = () => resolve(img);
    img.onerror = () => reject(new Error('Failed to load frame image'));
    img.src = src;
  });
}

export async function assembleGif(frames, fps) {
  if (!frames?.length) throw new Error('No frames to assemble.');
  if (!window.GIF) throw new Error('GIF.js not loaded.');

  const workerScript = await getWorkerUrl();
  const images = await Promise.all(frames.map(loadImage));

  const w = images[0].naturalWidth;
  const h = images[0].naturalHeight;
  const delay = Math.round(1000 / fps);

  const canvas = document.createElement('canvas');
  canvas.width = w;
  canvas.height = h;
  const ctx = canvas.getContext('2d');
  if (!ctx) throw new Error('Could not create 2D canvas context.');

  return new Promise((resolve, reject) => {
    const gif = new window.GIF({
      workers: 2,
      quality: 10,
      width: w,
      height: h,
      workerScript,
      repeat: 0
    });

    for (const image of images) {
      ctx.clearRect(0, 0, w, h);
      ctx.fillStyle = '#ffffff';
      ctx.fillRect(0, 0, w, h);
      ctx.drawImage(image, 0, 0, w, h);
      gif.addFrame(ctx, { copy: true, delay });
    }

    gif.on('finished', (blob) => resolve(blob));
    gif.on('error', (err) => reject(err));
    gif.render();
  });
}