import { uuid } from "./util";
import { getVisitorId, getSessionId } from "./identity";
import type { CtkConfig } from "./config";

export type TransportConfig = Required<CtkConfig>;

// 批量传输：满 batchSize 或 flushMs 到期即发；4xx（除 429）不重试；
// 指数退避 + 抖动重试；重试耗尽按本批事件数计 dropped；queueMax 溢出丢最旧。
// 红线：任何异常不得冒泡到宿主页（定时器/事件回调内的异常无人捕获）。
// 审查留档（Task 14 结论）：index.ts 的 track 分支已引入宿主可控对象——
// stringify 抛错（如循环引用）时已换出的批次按事件数计 dropped。
export class Transport {
  private q: object[] = [];
  private timer: ReturnType<typeof setTimeout> | null = null;
  private retry = 0;
  dropped = 0;
  constructor(public cfg: TransportConfig) {}

  push(ev: object) {
    if (this.q.length >= this.cfg.queueMax) {
      this.q.shift();
      this.dropped++;
    }
    this.q.push(ev);
    if (this.q.length >= this.cfg.batchSize) this.flush();
    else if (!this.timer)
      this.timer = setTimeout(() => this.flush(), this.cfg.flushMs);
  }

  flush(beacon = false) {
    if (this.timer) {
      clearTimeout(this.timer);
      this.timer = null;
    }
    if (!this.q.length) return;
    let n = 0;
    try {
      const events = this.q;
      this.q = [];
      n = events.length; // 换出后先记数：stringify 抛错时本批已丢，须计 dropped
      const body = JSON.stringify({
        batch_id: uuid(),
        site_id: this.cfg.siteId,
        visitor_id: getVisitorId(),
        session_id: getSessionId(),
        sent_at: new Date().toISOString(),
        // 由 index.ts（Task 15）注入；未注入时 undefined——JSON 序列化自然缺键，照发
        context: (globalThis as { __ctk_ctx?: unknown }).__ctk_ctx,
        events,
      });
      if (beacon && navigator.sendBeacon) {
        // 返回 false＝浏览器拒收（队列满/载荷超 64KB 等）→ 整批丢，须计 dropped
        const ok = navigator.sendBeacon(
          this.cfg.endpoint,
          new Blob([body], { type: "text/plain" })
        );
        if (!ok) this.dropped += n;
        return;
      }
      this.send(body, events.length);
    } catch {
      // 定时器/事件回调内的任何异常不得冒泡到宿主页；
      // stringify 抛错时批次已换出（宿主可控对象），按事件数计丢弃
      this.dropped += n;
    }
  }

  private async send(body: string, count: number) {
    try {
      const res = await fetch(this.cfg.endpoint, {
        method: "POST",
        keepalive: true,
        headers: { "Content-Type": "text/plain" },
        body,
      });
      if (res.status >= 400 && res.status < 500 && res.status !== 429) {
        this.retry = 0;
        return; // 4xx 参数/权限错不重试（403 重试只会掩盖配置错误）
      }
      if (!res.ok) throw new Error(String(res.status));
      this.retry = 0;
    } catch {
      if (this.retry++ < this.cfg.maxRetry) {
        const delay = 500 * 2 ** this.retry * (0.8 + Math.random() * 0.4);
        setTimeout(() => {
          this.send(body, count).catch(() => {});
        }, delay); // 重放同一 body 字符串——同 event_id 重放，服务端幂等去重
      } else {
        this.retry = 0;
        this.dropped += count; // 按事件数计丢弃
      }
    }
  }
}