1 files changed, 93 insertions, 0 deletions
diff --git a/src/image-thumbnails.js b/src/image-thumbnails.js
new file mode 100644
index 0000000..ffac4b9
--- /dev/null
+++ b/src/image-thumbnails.js
@@ -0,0 +1,93 @@
+// 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(); });
+}