export type SeedreamLayerCompositionItem = {
  url: string;
  visible: boolean;
  box: [number, number, number, number];
};

export async function downloadSeedreamLayerComposition(options: {
  items: SeedreamLayerCompositionItem[];
  width?: number;
  height?: number;
  fileName: string;
}) {
  if (!options.items.some((item) => item.visible)) throw new Error("请至少显示一个图层后再导出");

  const decoded: Array<{ item: SeedreamLayerCompositionItem; bitmap: ImageBitmap }> = [];
  try {
    for (const item of options.items) decoded.push({ item, bitmap: await fetchBitmap(item.url) });
    const reference = decoded[0].bitmap;
    const width = positiveDimension(options.width) || reference.width;
    const height = positiveDimension(options.height) || reference.height;
    const canvas = document.createElement("canvas");
    canvas.width = width;
    canvas.height = height;
    const context = canvas.getContext("2d");
    if (!context) throw new Error("当前浏览器无法创建合成画布");
    context.clearRect(0, 0, width, height);

    for (const { item, bitmap } of decoded) {
      if (!item.visible) continue;
      const [left, top, right, bottom] = item.box;
      context.drawImage(
        bitmap,
        left / 1000 * width,
        top / 1000 * height,
        Math.max(1, (right - left) / 1000 * width),
        Math.max(1, (bottom - top) / 1000 * height)
      );
    }

    const blob = await canvasBlob(canvas, "image/png");
    downloadBlob(blob, options.fileName);
  } finally {
    for (const { bitmap } of decoded) bitmap.close();
  }
}

async function fetchBitmap(url: string) {
  const response = await fetch(sameOriginBinaryURL(url), { credentials: "same-origin" });
  if (!response.ok) throw new Error(`读取图层失败（HTTP ${response.status}）`);
  return createImageBitmap(await response.blob());
}

function sameOriginBinaryURL(url: string) {
  if (!url.startsWith("/api/assets/") || !url.includes("/download")) return url;
  const [path, rawQuery = ""] = url.split("?", 2);
  const query = new URLSearchParams(rawQuery);
  query.delete("inline");
  const suffix = query.toString();
  return suffix ? `${path}?${suffix}` : path;
}

function canvasBlob(canvas: HTMLCanvasElement, type: string) {
  return new Promise<Blob>((resolve, reject) => {
    canvas.toBlob((blob) => {
      if (blob) resolve(blob);
      else reject(new Error("生成合成图片失败"));
    }, type);
  });
}

function downloadBlob(blob: Blob, fileName: string) {
  const url = URL.createObjectURL(blob);
  const anchor = document.createElement("a");
  anchor.href = url;
  anchor.download = fileName;
  document.body.appendChild(anchor);
  anchor.click();
  anchor.remove();
  window.setTimeout(() => URL.revokeObjectURL(url), 0);
}

function positiveDimension(value?: number) {
  return value && Number.isFinite(value) && value > 1 ? Math.round(value) : undefined;
}