package repository

import (
	"context"
	"time"

	"go.uber.org/zap"
)

// feedSeenTTL bounds a home-feed "session": after this idle window the
// per-user seen-set expires and the feed starts fresh from the top.
const feedSeenTTL = 30 * time.Minute

// FeedRepo tracks, per user, which post ids have already been served in the
// current home-feed session (a Redis SET) so paginated requests never repeat
// a post — and so the viewer-state-dependent feed ordering can't cause an
// OFFSET to drift over already-served rows.
//
// Nil-safe: with no Redis configured every method degrades to a no-op /
// empty, and the feed falls back to plain offset paging.
type FeedRepo struct {
	redis  *Redis
	logger *zap.Logger
}

func NewFeedRepo(redis *Redis, logger *zap.Logger) *FeedRepo {
	return &FeedRepo{redis: redis, logger: logger}
}

// Enabled reports whether the Redis-backed seen-set is available.
func (r *FeedRepo) Enabled() bool { return r != nil && r.redis != nil }

func (r *FeedRepo) key(userID string) string { return "feed:seen:" + userID }

// Seen returns the post ids already served to the user this session (empty
// when Redis is unavailable or the session is fresh).
func (r *FeedRepo) Seen(ctx context.Context, userID string) []string {
	if !r.Enabled() {
		return nil
	}
	ids, err := r.redis.Client.SMembers(ctx, r.key(userID)).Result()
	if err != nil {
		// Degrade rather than fail the feed — worst case is a possible repeat.
		r.logger.Warn("feed seen: read failed", zap.String("user", userID), zap.Error(err))
		return nil
	}
	return ids
}

// MarkSeen records the ids just served and refreshes the session TTL.
func (r *FeedRepo) MarkSeen(ctx context.Context, userID string, ids []string) {
	if !r.Enabled() || len(ids) == 0 {
		return
	}
	members := make([]any, len(ids))
	for i, id := range ids {
		members[i] = id
	}
	pipe := r.redis.Client.Pipeline()
	pipe.SAdd(ctx, r.key(userID), members...)
	pipe.Expire(ctx, r.key(userID), feedSeenTTL)
	if _, err := pipe.Exec(ctx); err != nil {
		r.logger.Warn("feed seen: write failed", zap.String("user", userID), zap.Error(err))
	}
}

// Reset clears the user's seen-set — called when a fresh feed session begins
// (the first page request) so reopening the feed serves from the top again.
func (r *FeedRepo) Reset(ctx context.Context, userID string) {
	if !r.Enabled() {
		return
	}
	if err := r.redis.Client.Del(ctx, r.key(userID)).Err(); err != nil {
		r.logger.Warn("feed seen: reset failed", zap.String("user", userID), zap.Error(err))
	}
}