export type CanvasZipTaskView = {
  id: string; status: "pending" | "running" | "completed" | "failed";
  stage: string; progress: number; totalCount: number; successCount: number; failedCount: number;
  error?: string; downloadUrl?: string;
};
const endpoint = "/api/canvas/download-zip-task";
const wait = (ms: number) => new Promise(resolve => setTimeout(resolve, ms));
async function taskRequest(url: string, init?: RequestInit) {
  for (let attempt = 0; ; attempt++) {
    let response: Response;
    try { response = await fetch(url, { ...init, cache: "no-store", signal: AbortSignal.timeout(20_000) }); }
    catch { if (attempt < 3) { await wait(1000 * (attempt + 1)); continue; } throw new Error("打包连接暂时中断，再次点击可继续查询原任务"); }
    const payload = await response.json().catch(() => null);
    if ((response.status === 429 || response.status >= 500) && attempt < 3) {
      await wait(Math.min(30_000, Math.max(1000, Number(response.headers.get("retry-after") || 2) * 1000))); continue;
    }
    if (!response.ok || !payload?.task) throw new Error(payload?.error || "无法查询打包任务，请稍后重试");
    return payload.task as CanvasZipTaskView;
  }
}
export async function downloadCanvasSelectionZip(params: {
  folderName: string; files: Array<{ url: string; name: string }>; selectionKey: string;
  onProgress: (task: CanvasZipTaskView) => void;
}) {
  const digest = await crypto.subtle.digest("SHA-256", new TextEncoder().encode(JSON.stringify([params.selectionKey, params.files])));
  const key = "canvas-zip:" + Array.from(new Uint8Array(digest), byte => byte.toString(16).padStart(2, "0")).join("");
  let taskId = "";
  try { taskId = sessionStorage.getItem(key) || ""; } catch { /* Storage is optional. */ }
  if (!/^[0-9a-f-]{36}$/i.test(taskId)) taskId = crypto.randomUUID();
  try { sessionStorage.setItem(key, taskId); } catch { /* Downloads work without browser persistence. */ }
  let task = await taskRequest(endpoint, { method: "POST", headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ taskId, folderName: params.folderName, files: params.files, retry: true }) });
  taskId = task.id;
  try { sessionStorage.setItem(key, taskId); } catch {}
  const deadline = Date.now() + 30 * 60_000;
  for (;;) {
    params.onProgress(task);
    if (task.status === "failed") throw new Error(task.error || "打包失败，再次点击可重试");
    if (task.status === "completed") break;
    if (Date.now() > deadline) throw new Error("后台仍在准备图片，再次点击可继续查询进度");
    await wait(2500);
    task = await taskRequest(`${endpoint}?taskId=${encodeURIComponent(taskId)}`);
  }
  if (!task.successCount || !task.downloadUrl) throw new Error("没有成功准备的图片，请稍后重试");
  // A GET for a complete immutable file supports browser retries and byte ranges.
  const frame = document.createElement("iframe");
  frame.hidden = true; frame.src = task.downloadUrl; document.body.appendChild(frame);
  window.setTimeout(() => frame.remove(), 60 * 60_000);
  return task;
}