《互联网产品的客户端架构·内容、社交与实时互动》1.2 social-lab 的第一个决定：端上先画什么

// file: src/core/social-client.ts
// tier: L2
// verify: node --test test/core/social-client.test.ts
import type { ContentItem } from "./content-model.ts";

export type ConnectionState = "idle" | "connecting" | "open" | "closed";

export interface FeedPage {
  items: ContentItem[];
  nextCursor: string | null;
}

export interface AppendResult {
  added: number;
  duplicates: number;
  dropped: number;
}

export class SocialClient {
  #feeds = new Map<string, ContentItem[]>();
  #cursors = new Map<string, string | null>();
  #seen = new Map<string, Set<string>>();
  #connection: ConnectionState = "idle";
  #limit: number;

  constructor(limit = 200) {
    this.#limit = limit;
  }

  get connection(): ConnectionState {
    return this.#connection;
  }

  setConnection(state: ConnectionState): ConnectionState {
    this.#connection = state;
    return state;
  }

  items(feedKey: string): ContentItem[] {
    return [...(this.#feeds.get(feedKey) ?? [])];
  }

  cursor(feedKey: string): string | null {
    return this.#cursors.get(feedKey) ?? null;
  }

  appendPage(feedKey: string, page: FeedPage): AppendResult {
    const current = this.#feeds.get(feedKey) ?? [];
    const seen = this.#seen.get(feedKey) ?? new Set<string>();
    const fresh: ContentItem[] = [];
    let duplicates = 0;
    for (const item of page.items) {
      if (seen.has(item.id)) {
        duplicates += 1;
        continue;
      }
      seen.add(item.id);
      fresh.push(item);
    }
    const merged = [...current, ...fresh];
    const dropped = Math.max(0, merged.length - this.#limit);
    const kept = dropped > 0 ? merged.slice(dropped) : merged;
    if (dropped > 0) {
      // 被淘汰的条目允许在后续分页中重新出现，因此从 seen 中移除。
      for (const item of merged.slice(0, dropped)) seen.delete(item.id);
    }
    this.#feeds.set(feedKey, kept);
    this.#seen.set(feedKey, seen);
    this.#cursors.set(feedKey, page.nextCursor);
    return { added: fresh.length, duplicates, dropped };
  }

  replacePage(feedKey: string, page: FeedPage): number {
    const current = this.#feeds.get(feedKey) ?? [];
    const inserted = page.items.filter((item) => !current.some((old) => old.id === item.id));
    this.#feeds.set(feedKey, [...inserted, ...current].slice(0, this.#limit));
    this.#seen.set(feedKey, new Set([...inserted, ...current].map((i) => i.id)));
    this.#cursors.set(feedKey, page.nextCursor);
    return inserted.length;
  }

  updateInteraction(feedKey: string, id: string, delta: { like?: number; comment?: number }): boolean {
    const list = this.#feeds.get(feedKey);
    if (!list) return false;
    const item = list.find((i) => i.id === id);
    if (!item) return false;
    item.likeCount = Math.max(0, item.likeCount + (delta.like ?? 0));
    item.commentCount = Math.max(0, item.commentCount + (delta.comment ?? 0));
    return true;
  }

  snapshot(feedKey: string): { count: number; cursor: string | null; connection: ConnectionState } {
    return { count: this.items(feedKey).length, cursor: this.cursor(feedKey), connection: this.#connection };
  }
}

$ node --test test/core/social-client.test.ts
✔ 第一页写入后游标与条目都可见 (0.982542ms)
✔ 重复条目只计一次，不改变顺序 (0.103083ms)
✔ 超过上限时从头部淘汰并允许重新出现 (0.107333ms)
✔ 刷新插入新条目到顶部并保持已读位置 (0.536208ms)
✔ 互动计数按增量更新且不为负 (0.119916ms)
✔ 更新不存在的条目返回 false (0.067833ms)
✔ 不同 feed 的去重集合互不干扰 (0.058417ms)
✔ 连接状态可切换并出现在快照中 (0.084ms)
✔ 空页只更新游标 (0.080708ms)
ℹ tests 9
ℹ suites 0
ℹ pass 9
ℹ fail 0
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 261.665792