type SaveFilePicker = (options?: {
  suggestedName?: string;
  types?: Array<{
    description?: string;
    accept: Record<string, string[]>;
  }>;
}) => Promise<{
  createWritable: () => Promise<{
    write: (data: Blob) => Promise<void>;
    close: () => Promise<void>;
  }>;
}>;

type NavigatorWithFileShare = Navigator & {
  canShare?: (data: { files?: File[] }) => boolean;
  share?: (data: { files?: File[]; title?: string }) => Promise<void>;
};

type WindowWithSavePicker = Window & {
  showSaveFilePicker?: SaveFilePicker;
};

export async function downloadFile(url: string, filename: string) {
  const res = await fetch(url, { credentials: "include" });
  if (!res.ok) throw new Error(await res.text());

  const blob = await res.blob();
  const safeName = filename || "download";

  if (window.isSecureContext) {
    const picker = (window as WindowWithSavePicker).showSaveFilePicker;
    if (picker) {
      const handle = await picker({
        suggestedName: safeName,
        types: blob.type
          ? [
              {
                description: "File",
                accept: { [blob.type]: [extensionFromName(safeName)] },
              },
            ]
          : undefined,
      });
      const writable = await handle.createWritable();
      await writable.write(blob);
      await writable.close();
      return;
    }
  }

  const file = new File([blob], safeName, {
    type: blob.type || "application/octet-stream",
  });
  const nav = navigator as NavigatorWithFileShare;
  if (nav.canShare?.({ files: [file] }) && nav.share) {
    await nav.share({ files: [file], title: safeName });
    return;
  }

  const objectUrl = URL.createObjectURL(blob);
  triggerDownload(objectUrl, safeName);
  window.setTimeout(() => URL.revokeObjectURL(objectUrl), 30_000);
}

function triggerDownload(url: string, filename: string) {
  const a = document.createElement("a");
  a.href = url;
  a.download = filename;
  a.style.display = "none";
  document.body.append(a);
  a.click();
  a.remove();
}

function extensionFromName(filename: string) {
  const match = /\.[^.]+$/.exec(filename);
  return match?.[0] || ".bin";
}