/**
 * 会话内事件总线（A1.3）。
 *
 * 职责：给事件分配会话内单调递增的 seq、向订阅者广播、支持从某个 seq 之后回放（断点续传）。
 * A3.7 会话落库后，回放改为读存储，这里只保留实时广播；内存缓冲仅作为 A1 的过渡实现。
 */
import { PROTOCOL_VERSION, type PlatformEvent } from '@aps/contracts';

type DistributiveOmit<T, K extends PropertyKey> = T extends unknown ? Omit<T, K> : never;

/** 发布事件的入参：v 与 seq 由总线分配，ts 可由调用方指定 */
export type EventInput = DistributiveOmit<PlatformEvent, 'v' | 'seq' | 'ts'> & { ts?: number };

const REPLAY_BUFFER = 500;

export class EventBus {
  private seq = 0;
  private readonly buffer: PlatformEvent[] = [];
  private readonly listeners = new Set<(event: PlatformEvent) => void>();

  publish(input: EventInput): PlatformEvent {
    this.seq += 1;
    const event = { ...input, v: PROTOCOL_VERSION, seq: this.seq, ts: input.ts ?? Date.now() } as PlatformEvent;
    this.buffer.push(event);
    if (this.buffer.length > REPLAY_BUFFER) this.buffer.shift();
    for (const listener of this.listeners) listener(event);
    return event;
  }

  /** 从 since（不含）之后回放 */
  replay(since = 0): PlatformEvent[] {
    return this.buffer.filter((event) => event.seq > since);
  }

  /** 先回放再订阅：避免回放与订阅之间漏事件 */
  subscribe(since: number, listener: (event: PlatformEvent) => void): () => void {
    for (const event of this.replay(since)) listener(event);
    this.listeners.add(listener);
    return () => {
      this.listeners.delete(listener);
    };
  }

  get lastSeq(): number {
    return this.seq;
  }
}