export interface DecodedLocalImage {
  readonly source: CanvasImageSource;
  readonly width: number;
  readonly height: number;
  dispose: () => void;
}

const abortError = (): DOMException =>
  new DOMException("Image decoding was cancelled.", "AbortError");

const decodeWithImageElement = (
  file: File,
  signal?: AbortSignal,
): Promise<DecodedLocalImage> =>
  new Promise((resolve, reject) => {
    if (signal?.aborted) {
      reject(abortError());
      return;
    }

    const objectUrl = URL.createObjectURL(file);
    const image = new Image();
    image.decoding = "async";
    let settled = false;
    const cleanup = (): void => {
      image.onload = null;
      image.onerror = null;
      signal?.removeEventListener("abort", onAbort);
      URL.revokeObjectURL(objectUrl);
    };
    const onAbort = (): void => {
      if (settled) return;
      settled = true;
      image.src = "";
      cleanup();
      reject(abortError());
    };
    image.onload = () => {
      if (settled) return;
      settled = true;
      cleanup();
      resolve({
        source: image,
        width: image.naturalWidth,
        height: image.naturalHeight,
        dispose: () => {
          image.src = "";
        },
      });
    };
    image.onerror = () => {
      if (settled) return;
      settled = true;
      cleanup();
      reject(new Error("The browser could not decode this image."));
    };
    signal?.addEventListener("abort", onAbort, { once: true });
    image.src = objectUrl;
  });

export const decodeLocalImage = async (
  file: File,
  signal?: AbortSignal,
): Promise<DecodedLocalImage> => {
  if (signal?.aborted) throw abortError();
  if (typeof createImageBitmap === "function") {
    try {
      const bitmap = await createImageBitmap(file, {
        imageOrientation: "from-image",
      });
      if (signal?.aborted) {
        bitmap.close();
        throw abortError();
      }
      return {
        source: bitmap,
        width: bitmap.width,
        height: bitmap.height,
        dispose: () => bitmap.close(),
      };
    } catch (error) {
      if (
        signal?.aborted ||
        (error instanceof DOMException && error.name === "AbortError")
      )
        throw error;
      // Older browsers sometimes expose createImageBitmap without supporting every format.
    }
  }
  return decodeWithImageElement(file, signal);
};