ANTOOZA-BACKEND/Application/Features/Api/Channel/List/Handler.cs

using Application.Abstractions.Cache;
using Application.Abstractions.Data;
using Application.Abstractions.Messaging;
using Application.Abstractions.YouTube;
using Microsoft.EntityFrameworkCore;
using SharedKernel.Results;

namespace Application.Features.Api.Channel.List;

public sealed class Handler(
    IAppDbContext db,
    IYouTubeLiveStateStore liveStateStore,
    IYouTubeChannelCache channelCache,
    ICacheService cache
) : IQueryHandler<Query, Result<Response>>
{
    private const string CacheKey = "channel:list:main";
    private static readonly TimeSpan CacheTtl = TimeSpan.FromSeconds(30);

    public async Task<Result<Response>> Handle(Query request, CancellationToken ct)
    {
        var cached = await cache.GetAsync<List<ChannelItem>>(CacheKey, ct);
        if (cached is not null)
        {
            return new Response(cached);
        }

        var items = await BuildAsync(ct);
        await cache.SetAsync(CacheKey, items, CacheTtl, ct);

        return new Response(items);
    }

    private async Task<List<ChannelItem>> BuildAsync(CancellationToken ct)
    {
        // 1. 활성 채널 목록 조회
        var channels = await db.Channel.AsNoTracking().Where(c => c.IsActive).Select(c => new { c.SID, c.Name, c.Handle }).ToListAsync(ct);

        if (channels.Count == 0)
        {
            return [];
        }

        // 2. YouTube 채널 정보 캐시 일괄 조회 (Redis — API 호출 0)
        var channelIds = channels.Select(c => c.SID).ToList();
        var ytCache = await channelCache.GetManyAsync(channelIds);

        // 3. 현재 라이브 중인 채널 상태 조회 (Redis — 0 unit)
        var allLive = await liveStateStore.GetAllLiveAsync();
        var liveMap = allLive.ToDictionary(l => l.ChannelId);

        // 4. 채널별 정보 조합 — ViewerCount 는 poller 가 30초 주기로 갱신한 Redis 값 사용
        var items = new List<ChannelItem>(channels.Count);
        foreach (var ch in channels)
        {
            var isLive = liveMap.TryGetValue(ch.SID, out var liveInfo);
            var hasYtInfo = ytCache.TryGetValue(ch.SID, out var ytInfo);

            items.Add(new ChannelItem(
                ch.SID,
                ch.Name,
                ch.Handle,
                hasYtInfo ? ytInfo!.ThumbnailUrl : null,
                hasYtInfo ? ytInfo!.SubscriberCount : 0,
                isLive,
                isLive ? liveInfo!.ViewerCount : 0,
                isLive ? liveInfo!.VideoId : null
            ));
        }

        // 5. 정렬: 라이브(시청자 desc) → 비라이브(Fisher-Yates random shuffle)
        var live = items.Where(i => i.IsLive).OrderByDescending(i => i.ViewerCount).ToList();
        var offline = items.Where(i => !i.IsLive).ToList();
        Shuffle(offline);

        return [.. live, .. offline];
    }

    private static void Shuffle<T>(List<T> list)
    {
        var rnd = Random.Shared;
        for (int i = list.Count - 1; i > 0; i--)
        {
            int j = rnd.Next(i + 1);
            (list[i], list[j]) = (list[j], list[i]);
        }
    }
}