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

using Application.Abstractions.Cache;
using Application.Abstractions.Channels;
using Application.Abstractions.Data;
using Application.Abstractions.Messaging;
using Application.Helpers;
using Microsoft.EntityFrameworkCore;

namespace Application.Features.Api.Channel.GetLiveLeaderboard;

internal sealed class Handler(IAppDbContext db, ICacheService cache) : IQueryHandler<Query, Response>
{
    public async Task<Response> Handle(Query request, CancellationToken ct)
    {
        var cfg = await ChatExpConfigLoader.GetAsync(cache, db, ct);
        var maxRank = request.MaxRank ?? cfg.LeaderboardSize;

        var resolved = await ChannelIdentifierResolver.ResolveAsync(db, request.Identifier, ct);

        if (resolved.ChannelSID is null)
        {
            return new Response(null, [], 0, null);
        }

        var channelID = await db.Channel.AsNoTracking()
            .Where(c => c.SID == resolved.ChannelSID)
            .Select(c => (int?)c.ID)
            .FirstOrDefaultAsync(ct);

        if (channelID is null)
        {
            return new Response(null, [], 0, null);
        }

        // 현재 활성 방송 세션 (IsFinalized=false, CreatedAt desc)
        var session = await db.BroadcastSession.AsNoTracking()
            .Where(s => s.ChannelID == channelID.Value && !s.IsFinalized)
            .OrderByDescending(s => s.CreatedAt)
            .Select(s => new { s.ID })
            .FirstOrDefaultAsync(ct);

        if (session is null)
        {
            return new Response(null, [], 0, null);
        }

        // 방송 세션 단위 XP 집계
        var grouped = await db.MemberExpLog.AsNoTracking()
            .Where(e => e.BroadcastSessionID == session.ID)
            .GroupBy(e => e.MemberID)
            .Select(g => new { MemberID = g.Key, Xp = g.Sum(x => x.Amount) })
            .Where(x => x.Xp > 0)
            .OrderByDescending(x => x.Xp)
            .Take(maxRank)
            .ToListAsync(ct);

        var total = grouped.Count;

        if (total == 0)
        {
            return new Response(session.ID, [], 0, MyRank(null, 0));
        }

        var memberIDs = grouped.Select(g => g.MemberID).ToArray();

        // 멤버 정보 + 뱃지 조회 (칭호는 채널 문맥)
        var memberRows = await db.Member.AsNoTracking()
            .Where(m => memberIDs.Contains(m.ID))
            .Select(m => new
            {
                m.ID,
                m.SID,
                m.Name,
                m.Icon,
                GradeImage = m.MemberGrade != null ? m.MemberGrade.Image : null
            })
            .ToDictionaryAsync(x => x.ID, ct);

        var list = new List<LeaderboardItem>(grouped.Count);
        var rank = 0;
        foreach (var g in grouped)
        {
            rank++;
            if (!memberRows.TryGetValue(g.MemberID, out var m))
            {
                continue;
            }

            list.Add(new LeaderboardItem(
                Rank: rank,
                MemberID: m.ID,
                MemberSID: m.SID,
                MemberName: m.Name ?? "익명",
                Xp: g.Xp,
                TitleIconUrl: null,
                GradeImageUrl: m.GradeImage,
                MemberIcon: m.Icon
            ));
        }

        MyRankItem? myRank = null;

        if (request.ViewerMemberID.HasValue)
        {
            var viewerID = request.ViewerMemberID.Value;
            var inTop = list.FirstOrDefault(x => x.MemberID == viewerID);
            if (inTop is not null)
            {
                myRank = new MyRankItem(inTop.Rank, inTop.Xp);
            }
            else
            {
                var myXp = await db.MemberExpLog.AsNoTracking()
                    .Where(e => e.BroadcastSessionID == session.ID && e.MemberID == viewerID)
                    .SumAsync(e => (int?)e.Amount, ct) ?? 0;

                if (myXp > 0)
                {
                    var higher = await db.MemberExpLog.AsNoTracking()
                        .Where(e => e.BroadcastSessionID == session.ID)
                        .GroupBy(e => e.MemberID)
                        .Select(g => g.Sum(x => x.Amount))
                        .CountAsync(xp => xp > myXp, ct);

                    myRank = new MyRankItem(higher + 1, myXp);
                }
                else
                {
                    myRank = new MyRankItem(0, 0);
                }
            }
        }

        return new Response(session.ID, list, total, myRank);
    }

    private static MyRankItem? MyRank(MyRankItem? val, int fallback) => val ?? new MyRankItem(0, fallback);
}