backend/internal/service/hot_rank_worker.go

package service

import (
	"cmp"
	"context"
	"errors"
	"math"
	"slices"
	"sync"
	"time"

	"carrot_bbs/internal/cache"
	"carrot_bbs/internal/config"
	"carrot_bbs/internal/repository"

	"github.com/redis/go-redis/v9"
	"go.uber.org/zap"
)

// HotRankWorker 定时在候选池上重算热门分，写入 Redis ZSET（仅 TopN）及 top_ids（Redis+本地缓存）
type HotRankWorker struct {
	cfg      *config.Config
	postRepo repository.PostRepository
	c        cache.Cache
	mu       sync.Mutex
	stopOnce sync.Once
	stopCh   chan struct{}
	doneCh   chan struct{}
}

// NewHotRankWorker 创建热门榜重算 worker（cache 需为 Redis 实现才有意义）
func NewHotRankWorker(cfg *config.Config, postRepo repository.PostRepository, c cache.Cache) *HotRankWorker {
	return &HotRankWorker{
		cfg:      cfg,
		postRepo: postRepo,
		c:        c,
		stopCh:   make(chan struct{}),
		doneCh:   make(chan struct{}),
	}
}

// Start 启动定时任务（启动时立即执行一次）
func (w *HotRankWorker) Start(ctx context.Context) {
	if w == nil || w.cfg == nil || !w.cfg.HotRank.Enabled || w.c == nil {
		close(w.doneCh)
		return
	}
	interval := time.Duration(w.cfg.HotRank.RefreshIntervalSeconds) * time.Second
	if interval < 30*time.Second {
		interval = 30 * time.Second
	}

	go func() {
		defer close(w.doneCh)

		if err := w.Refresh(ctx); err != nil {
			if !errors.Is(err, cache.ErrKeyNotFound) {
				zap.L().Warn("hot rank initial refresh failed", zap.Error(err))
			}
		}

		ticker := time.NewTicker(interval)
		defer ticker.Stop()

		for {
			select {
			case <-w.stopCh:
				return
			case <-ticker.C:
				if err := w.Refresh(ctx); err != nil {
					if !errors.Is(err, cache.ErrKeyNotFound) {
						zap.L().Warn("hot rank refresh failed", zap.Error(err))
					}
				}
			}
		}
	}()
}

// Stop 停止 worker
func (w *HotRankWorker) Stop() {
	if w == nil {
		return
	}
	w.stopOnce.Do(func() {
		close(w.stopCh)
	})
	<-w.doneCh
}

// Refresh 立即重算热门分（线程安全）
// 算法：候选 ID = 上一期 TopN（Redis）∪ 时间窗内新发帖子 ID，截断至 CandidateCap；
// 仅对候选拉取统计字段并打分，min-max 归一化；已发布置顶帖固定排在最前，其后接热门至 TopN；
// 写入 ZSET（置顶分数高于热门）及 top_ids。
func (w *HotRankWorker) Refresh(ctx context.Context) error {
	if w == nil || w.c == nil {
		return cache.ErrKeyNotFound
	}

	w.mu.Lock()
	defer w.mu.Unlock()

	topN := w.cfg.HotRank.TopN
	if topN <= 0 {
		topN = 100
	}
	topN = min(topN, 500)
	recentHours := w.cfg.HotRank.RecentWindowHours
	if recentHours <= 0 {
		recentHours = 168
	}
	recentLimit := w.cfg.HotRank.RecentFetchLimit
	if recentLimit <= 0 {
		recentLimit = 500
	}
	candidateCap := w.cfg.HotRank.CandidateCap
	if candidateCap <= 0 {
		candidateCap = 800
	}

	zsetKey := cache.HotRankZSetKey()
	topIDsKey := cache.HotRankTopIDsKey()

	pinned, err := w.postRepo.ListPinnedPublishedPostIDs()
	if err != nil {
		return err
	}
	pinned = dedupeIDsPreserveOrder(pinned)

	incumbent, _ := w.c.ZRevRange(ctx, zsetKey, 0, int64(topN-1))

	since := time.Now().Add(-time.Duration(recentHours) * time.Hour)
	recentIDs, err := w.postRepo.ListPublishedPostIDsSince(since, recentLimit)
	if err != nil {
		return err
	}

	candidateIDs := mergeHotRankCandidates(incumbent, recentIDs, candidateCap)
	if len(candidateIDs) == 0 && len(pinned) == 0 {
		if err := w.c.ZReplaceSortedSet(ctx, zsetKey, nil); err != nil {
			return err
		}
		w.c.Delete(topIDsKey)
		return nil
	}

	type scored struct {
		idx  int
		norm float64
	}

	var snapshots []repository.PostHotSnapshot
	var ranked []scored
	if len(candidateIDs) > 0 {
		snapshots, err = w.postRepo.GetPostHotSnapshotsByIDs(candidateIDs)
		if err != nil {
			return err
		}
	}

	if len(snapshots) > 0 {
		now := time.Now()
		raw := make([]float64, len(snapshots))
		for i := range snapshots {
			p := &snapshots[i]
			base := hotRankBaseScore(p.LikesCount, p.CommentsCount, p.FavoritesCount, p.SharesCount, p.ViewsCount)
			tHours := now.Sub(p.CreatedAt).Hours()
			if tHours < 0 {
				tHours = 0
			}
			denom := math.Pow(tHours+2.0, 0.5)
			if denom < 1e-9 {
				denom = 1e-9
			}
			raw[i] = base / denom
		}

		minR := slices.Min(raw)
		maxR := slices.Max(raw)

		ranked = make([]scored, len(snapshots))
		for i := range snapshots {
			var norm float64
			if maxR > minR {
				norm = (raw[i] - minR) / (maxR - minR)
			} else {
				norm = 1
			}
			ranked[i] = scored{idx: i, norm: norm}
		}
		slices.SortFunc(ranked, func(a, b scored) int {
			if a.norm != b.norm {
				return cmp.Compare(b.norm, a.norm) // 降序
			}
			if snapshots[a.idx].CreatedAt.After(snapshots[b.idx].CreatedAt) {
				return -1
			} else if snapshots[a.idx].CreatedAt.Before(snapshots[b.idx].CreatedAt) {
				return 1
			}
			return 0
		})
	}

	pinnedSet := make(map[string]struct{}, len(pinned))
	for _, id := range pinned {
		pinnedSet[id] = struct{}{}
	}

	hotSlots := topN - len(pinned)
	if hotSlots < 0 {
		pinned = pinned[:topN]
		pinnedSet = make(map[string]struct{}, len(pinned))
		for _, id := range pinned {
			pinnedSet[id] = struct{}{}
		}
		hotSlots = 0
	}

	type hotPick struct {
		id   string
		norm float64
	}
	hotPicks := make([]hotPick, 0, hotSlots)
	for _, r := range ranked {
		id := snapshots[r.idx].ID
		if _, isPin := pinnedSet[id]; isPin {
			continue
		}
		hotPicks = append(hotPicks, hotPick{id: id, norm: r.norm})
		if len(hotPicks) >= hotSlots {
			break
		}
	}

	orderedIDs := make([]string, 0, len(pinned)+len(hotPicks))
	orderedIDs = append(orderedIDs, pinned...)
	for _, h := range hotPicks {
		orderedIDs = append(orderedIDs, h.id)
	}

	const pinnedScoreBase = 1000.0
	zMembers := make([]redis.Z, 0, len(orderedIDs))
	for i, id := range pinned {
		zMembers = append(zMembers, redis.Z{
			Score:  pinnedScoreBase - float64(i)*1e-6,
			Member: id,
		})
	}
	for _, h := range hotPicks {
		zMembers = append(zMembers, redis.Z{Score: h.norm * 0.99, Member: h.id})
	}

	if err := w.c.ZReplaceSortedSet(ctx, zsetKey, zMembers); err != nil {
		return err
	}

	ttl := time.Duration(w.cfg.HotRank.RefreshIntervalSeconds) * time.Second * 4
	if ttl < 10*time.Minute {
		ttl = 10 * time.Minute
	}
	w.c.Set(topIDsKey, orderedIDs, ttl)

	zap.L().Info("hot rank refreshed",
		zap.Int("pinned", len(pinned)),
		zap.Int("candidates", len(candidateIDs)),
		zap.Int("scored", len(snapshots)),
		zap.Int("top_n", len(zMembers)),
	)
	return nil
}

func dedupeIDsPreserveOrder(ids []string) []string {
	if len(ids) <= 1 {
		return ids
	}
	seen := make(map[string]struct{}, len(ids))
	out := make([]string, 0, len(ids))
	for _, id := range ids {
		if id == "" {
			continue
		}
		if _, ok := seen[id]; ok {
			continue
		}
		seen[id] = struct{}{}
		out = append(out, id)
	}
	return out
}

func mergeHotRankCandidates(incumbent, recent []string, cap int) []string {
	if cap <= 0 {
		return nil
	}
	seen := make(map[string]struct{}, cap)
	out := make([]string, 0, cap)
	for _, id := range incumbent {
		if id == "" {
			continue
		}
		if _, ok := seen[id]; ok {
			continue
		}
		seen[id] = struct{}{}
		out = append(out, id)
		if len(out) >= cap {
			return out
		}
	}
	for _, id := range recent {
		if id == "" {
			continue
		}
		if _, ok := seen[id]; ok {
			continue
		}
		seen[id] = struct{}{}
		out = append(out, id)
		if len(out) >= cap {
			break
		}
	}
	return out
}

func hotRankBaseScore(likes, comments, favorites, shares, views int) float64 {
	return float64(likes)*3 +
		float64(comments)*5 +
		float64(favorites)*4 +
		float64(shares)*6 +
		float64(views)*0.15
}