import type { FeedCursor, FeedItem } from "./types.ts";

export function emptyFeedCursor(): FeedCursor {
  return { lastGuidSet: [], digests: {}, lastFetchedAt: null };
}

export interface ApplyFeedResult {
  /** 窗口内首次出现的 GUID */
  newItems: FeedItem[];
  /** 同 GUID 摘要变化(标题修正、促销、体积、置顶变化等),视为更新 */
  updatedItems: FeedItem[];
  unchangedCount: number;
  /**
   * 疑似缺口:本次条目与上次窗口没有任何重叠,且上次窗口非空。
   * 说明两次抓取之间可能有条目被挤出 Feed,需要调度有预算的列表/搜索补漏(REQ §4.3)。
   */
  possibleGap: boolean;
  cursor: FeedCursor;
}

export interface ApplyFeedOptions {
  /** 游标保留的 GUID 数上限,默认 max(500, 本次条目数 * 3) */
  windowSize?: number;
}

/**
 * 重叠窗口去重:不依赖条目顺序或递增 ID,乱序/置顶/重复出现都按 GUID+摘要判定。
 * 游标只是快速过滤;持久去重仍以数据库 (feed, guid) 唯一约束为准。
 */
export function applyFeedItems(cursor: FeedCursor, items: FeedItem[], opts: ApplyFeedOptions = {}): ApplyFeedResult {
  const newItems: FeedItem[] = [];
  const updatedItems: FeedItem[] = [];
  let unchangedCount = 0;
  let overlap = false;
  const digests: Record<string, string> = { ...cursor.digests };
  const seenThisRound = new Set<string>();
  for (const item of items) {
    if (seenThisRound.has(item.guid)) continue; // 同一次抓取内重复 GUID 只取首条
    seenThisRound.add(item.guid);
    const prev = digests[item.guid];
    if (prev === undefined) {
      newItems.push(item);
    } else {
      overlap = true;
      if (prev !== item.rawDigest) updatedItems.push(item);
      else unchangedCount++;
    }
    digests[item.guid] = item.rawDigest;
  }
  const windowSize = opts.windowSize ?? Math.max(500, items.length * 3);
  const order = [...seenThisRound, ...cursor.lastGuidSet.filter((g) => !seenThisRound.has(g))].slice(0, windowSize);
  const kept: Record<string, string> = {};
  for (const g of order) {
    const d = digests[g];
    if (d !== undefined) kept[g] = d;
  }
  const lastFetchedAt = items.reduce<string | null>(
    (acc, it) => (acc === null || it.fetchedAt > acc ? it.fetchedAt : acc),
    cursor.lastFetchedAt,
  );
  return {
    newItems,
    updatedItems,
    unchangedCount,
    possibleGap: cursor.lastGuidSet.length > 0 && items.length > 0 && !overlap,
    cursor: { lastGuidSet: order, digests: kept, lastFetchedAt },
  };
}