// Keep decode, resize and WebP encoding off the scrolling thread. A blob
// worker also works in the standalone wallet without a separately hosted file.
// This function is serialized: all its dependencies must live inside it.
function thumbnailWorker() {
  const encode = async (canvas, quality) => {
    let blob = await canvas.convertToBlob({ type: 'image/webp', quality });
    if (!/webp/.test(blob.type)) blob = await canvas.convertToBlob({ type: 'image/jpeg', quality });
    return blob;
  };
  self.onmessage = async ({ data: { blob, width, tileSize } }) => {
    let bitmap;
    try {
      bitmap = await createImageBitmap(blob);
      const result = { width: bitmap.width, height: bitmap.height, media: null, tile: null };
      // GIFs must keep animating; a canvas copy would freeze their first frame.
      if (!/gif/i.test(blob.type)) {
        if (tileSize) {
          const side = Math.min(bitmap.width, bitmap.height), px = Math.min(tileSize, side);
          let square;
          try {
            square = await createImageBitmap(bitmap, (bitmap.width - side) / 2, (bitmap.height - side) / 2, side, side,
              { resizeWidth: px, resizeHeight: px, resizeQuality: 'high' });
            const canvas = new OffscreenCanvas(px, px);
            canvas.getContext('2d').drawImage(square, 0, 0);
            result.tile = await encode(canvas, 0.8);
          } catch {} finally { square?.close(); }
        }
        if (width && bitmap.width > width * 1.25) {
          try {
            const height = Math.max(1, Math.round(bitmap.height * width / bitmap.width));
            const canvas = new OffscreenCanvas(width, height), ctx = canvas.getContext('2d');
            ctx.imageSmoothingQuality = 'high';
            ctx.drawImage(bitmap, 0, 0, width, height);
            result.media = await encode(canvas, 0.82);
          } catch {}
        }
      }
      self.postMessage(result);
    } catch { self.postMessage(null); } finally { bitmap?.close(); }
  };
}

// One image at a time bounds concurrent decoding/encoding and its CPU/memory
// pressure. If workers are unavailable, callers keep displaying originals.
const JOB_MS = 15000, MAX_QUEUE = 24, MAX_STRIKES = 3;
let worker = null, disabled = false, current = null, timer = null, strikes = 0;
const queue = [];
function finish(job, data) { clearTimeout(timer); timer = null; job?.resolve(data); }
// Workers can't be made here (constructor refused, script won't load):
// thumbnails are off for this session.
function failWorker() {
  disabled = true;
  worker?.terminate(); worker = null;
  const job = current; current = null;
  finish(job, null);
  for (const j of queue.splice(0)) j.resolve(null);
}
// A picture that hung or crashed the worker is given up and the worker
// starts fresh for the next one — one bad file mustn't cost every later
// thumbnail. Three failures in a row and it's treated as broken.
function resetWorker() {
  if (++strikes >= MAX_STRIKES) return failWorker();
  worker?.terminate(); worker = null;
  const job = current; current = null;
  finish(job, null);
  drain();
}
function drain() {
  if (current || !queue.length || disabled) return;
  if (!worker) {
    let url;
    try {
      url = URL.createObjectURL(new Blob(['(' + thumbnailWorker.toString() + ')()'], { type: 'text/javascript' }));
      worker = new Worker(url, { name: 'image-thumbnails' });
      worker.onerror = resetWorker;
      worker.onmessageerror = resetWorker;
      worker.onmessage = ({ data }) => {
        strikes = 0;
        const job = current; current = null;
        finish(job, data);
        drain();
      };
    } catch { failWorker(); return; } finally { if (url) URL.revokeObjectURL(url); }
  }
  current = queue.shift();
  timer = setTimeout(resetWorker, JOB_MS);
  try { worker.postMessage(current.input); } catch { resetWorker(); }
}
export function makeImageThumbnails(blob, { width = 0, tileSize = 0 } = {}) {
  if (disabled || typeof Worker === 'undefined' || typeof OffscreenCanvas === 'undefined'
    || typeof createImageBitmap === 'undefined' || queue.length >= MAX_QUEUE) return Promise.resolve(null);
  return new Promise((resolve) => { queue.push({ input: { blob, width, tileSize }, resolve }); drain(); });
}