using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using MediaBrowser.Controller.Entities;
using MediaBrowser.Controller.Library;
using MediaBrowser.Model.Entities;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace Jellyfin.Plugin.Floatplane.Channel;

/// <summary>
/// A post is listed twice — under "All videos" (<c>post:…</c>) and under its sub-channel
/// (<c>cpost:…</c>) — because Jellyfin gives a channel item exactly one parent. Jellyfin
/// therefore keeps two sets of user data; this service copies played / resume state from
/// one copy to the other whenever it changes (pattern borrowed from chaosflix-jellyfin).
/// </summary>
public sealed class UserDataMirror : IHostedService
{
    private readonly IUserDataManager _userDataManager;
    private readonly ILibraryManager _libraryManager;
    private readonly IUserManager _userManager;
    private readonly ILogger<UserDataMirror> _logger;

    // UserDataSaved is raised from inside SaveUserData, so our own mirrored save comes
    // back as an event; the copies being written are tracked here while it runs.
    private readonly ConcurrentDictionary<(Guid User, Guid Item), byte> _mirroring = new();

    public UserDataMirror(IUserDataManager userDataManager, ILibraryManager libraryManager, IUserManager userManager, ILogger<UserDataMirror> logger)
    {
        _userDataManager = userDataManager;
        _libraryManager = libraryManager;
        _userManager = userManager;
        _logger = logger;
    }

    /// <inheritdoc />
    public Task StartAsync(CancellationToken cancellationToken)
    {
        _userDataManager.UserDataSaved += OnUserDataSaved;
        _logger.LogDebug("Floatplane user-data mirror active");
        return Task.CompletedTask;
    }

    /// <inheritdoc />
    public Task StopAsync(CancellationToken cancellationToken)
    {
        _userDataManager.UserDataSaved -= OnUserDataSaved;
        return Task.CompletedTask;
    }

    private void OnUserDataSaved(object? sender, UserDataSaveEventArgs e)
    {
        try
        {
            Mirror(e);
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex, "Could not mirror watch state of {Item}", e.Item?.Name);
        }
    }

    private void Mirror(UserDataSaveEventArgs e)
    {
        var item = e.Item;
        var source = e.UserData;
        if (item is null || source is null || !IsWatchState(e.SaveReason))
        {
            return;
        }

        var postId = FloatplaneChannel.PostIdOf(item.ExternalId);
        if (postId is null || _mirroring.ContainsKey((e.UserId, item.Id)))
        {
            return;
        }

        var user = _userManager.GetUserById(e.UserId);
        if (user is null)
        {
            return;
        }

        foreach (var sibling in Siblings(item, postId))
        {
            var target = _userDataManager.GetUserData(user, sibling);
            if (target is null || !Differs(source, target))
            {
                continue;
            }

            target.Played = source.Played;
            target.PlaybackPositionTicks = source.PlaybackPositionTicks;
            target.PlayCount = source.PlayCount;
            target.LastPlayedDate = source.LastPlayedDate;

            if (!_mirroring.TryAdd((e.UserId, sibling.Id), 0))
            {
                continue;
            }

            try
            {
                _userDataManager.SaveUserData(user, sibling, target, UserDataSaveReason.UpdateUserData, CancellationToken.None);
                _logger.LogDebug("Mirrored watch state of {Post} to its other copy", postId);
            }
            finally
            {
                _mirroring.TryRemove((e.UserId, sibling.Id), out _);
            }
        }
    }

    private static bool IsWatchState(UserDataSaveReason reason) => reason is
        UserDataSaveReason.PlaybackStart or
        UserDataSaveReason.PlaybackProgress or
        UserDataSaveReason.PlaybackFinished or
        UserDataSaveReason.TogglePlayed or
        UserDataSaveReason.UpdateUserData;

    private static bool Differs(UserItemData source, UserItemData target) =>
        source.Played != target.Played ||
        source.PlaybackPositionTicks != target.PlaybackPositionTicks ||
        source.PlayCount != target.PlayCount ||
        source.LastPlayedDate != target.LastPlayedDate;

    private IEnumerable<BaseItem> Siblings(BaseItem item, string postId)
    {
        var query = new InternalItemsQuery
        {
            Name = item.Name,
            ChannelIds = item.ChannelId == Guid.Empty ? Array.Empty<Guid>() : new[] { item.ChannelId }
        };

        IReadOnlyList<BaseItem> candidates;
        try
        {
            candidates = _libraryManager.GetItemList(query);
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex, "Could not look up the other copy of {Item}", item.Name);
            return Array.Empty<BaseItem>();
        }

        return candidates
            .Where(c => !c.Id.Equals(item.Id) && string.Equals(FloatplaneChannel.PostIdOf(c.ExternalId), postId, StringComparison.Ordinal))
            // Through the library cache so the instance the server serves gets the new data too.
            .Select(c => _libraryManager.GetItemById(c.Id) ?? c)
            .ToList();
    }
}