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ts/util/imageToBlurHash.ts

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+import loadImage from 'blueimp-load-image';
+import { encode } from 'blurhash';
+
+type Input = Parameters<typeof loadImage>[0];
+
+const loadImageData = async (input: Input): Promise<ImageData> => {
+  return new Promise((resolve, reject) => {
+    loadImage(
+      input,
+      canvasOrError => {
+        if (canvasOrError instanceof Event && canvasOrError.type === 'error') {
+          const processError = new Error(
+            'imageToBlurHash: Failed to process image'
+          );
+          processError.cause = canvasOrError;
+          reject(processError);
+          return;
+        }
+        if (canvasOrError instanceof HTMLCanvasElement) {
+          const context = canvasOrError.getContext('2d');
+          resolve(
+            context?.getImageData(
+              0,
+              0,
+              canvasOrError.width,
+              canvasOrError.height
+            )
+          );
+        }
+        const error = new Error(
+          'imageToBlurHash: Failed to place image on canvas'
+        );
+        reject(error);
+        return;
+      },
+      // Calculating the blurhash on large images is a long-running and
+      // synchronous operation, so here we ensure the images are a reasonable
+      // size before calculating the blurhash. iOS uses a max size of 200x200
+      // and Android uses a max size of 1/16 the original size. 200x200 is
+      // easier for us.
+      { canvas: true, orientation: true, maxWidth: 200, maxHeight: 200 }
+    );
+  });
+};
+
+export const imageToBlurHash = async (input: Input) => {
+  const { data, width, height } = await loadImageData(input);
+  // 4 horizontal components and 3 vertical components
+  return encode(data, width, height, 4, 3);
+};