using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using MediaBrowser.Controller.Library;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace Jellyfin.Plugin.WatchedTogether.Services;

/// <summary>
/// Keeps group membership consistent with the set of users that actually exist.
/// </summary>
/// <remarks>
/// Jellyfin raises no user-deleted event that carries the removed id, so instead of subscribing to
/// deletions this reconciles configuration against live users at startup. Stale entries are also
/// skipped at read time by <see cref="GroupService.GetEligibleMembers"/>, so this is about keeping
/// stored configuration tidy and disabling groups that have fallen below two members.
/// </remarks>
public sealed class UserLifecycleService : IHostedService
{
    private readonly IUserManager _userManager;
    private readonly IGroupService _groupService;
    private readonly ILibraryAccessService _libraryAccessService;
    private readonly ILogger<UserLifecycleService> _logger;

    /// <summary>
    /// Initializes a new instance of the <see cref="UserLifecycleService"/> class.
    /// </summary>
    /// <param name="userManager">The user manager.</param>
    /// <param name="groupService">The group service.</param>
    /// <param name="libraryAccessService">The library access service.</param>
    /// <param name="logger">The logger.</param>
    public UserLifecycleService(
        IUserManager userManager,
        IGroupService groupService,
        ILibraryAccessService libraryAccessService,
        ILogger<UserLifecycleService> logger)
    {
        _userManager = userManager;
        _groupService = groupService;
        _libraryAccessService = libraryAccessService;
        _logger = logger;
    }

    /// <inheritdoc />
    public async Task StartAsync(CancellationToken cancellationToken)
    {
        try
        {
            Reconcile();
            await ReapplyLibraryAccessAsync().ConfigureAwait(false);
        }
#pragma warning disable CA1031 // Reconciliation must never prevent the server from starting.
        catch (Exception ex)
#pragma warning restore CA1031
        {
            _logger.LogError(ex, "Failed to reconcile Watched Together groups at startup");
        }
    }

    /// <summary>
    /// Recomputes every group's library access.
    /// </summary>
    /// <remarks>
    /// A member's own library access can be narrowed at any time through the normal user editor,
    /// which would leave a group's stored intersection too wide. Recomputing at startup brings
    /// shared accounts back in line without needing to hook every policy change.
    /// </remarks>
    /// <returns>A task representing the update.</returns>
    private async Task ReapplyLibraryAccessAsync()
    {
        var config = Plugin.Instance?.Configuration;
        if (config is null)
        {
            return;
        }

        foreach (var group in config.Groups.ToList())
        {
            await _libraryAccessService
                .ApplyIntersectionAsync(group.SharedUserId, group.MemberUserIds)
                .ConfigureAwait(false);
        }
    }

    /// <inheritdoc />
    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;

    /// <summary>
    /// Removes references to users that no longer exist.
    /// </summary>
    private void Reconcile()
    {
        var config = Plugin.Instance?.Configuration;
        if (config is null || config.Groups.Count == 0)
        {
            return;
        }

        var liveIds = _userManager.UsersIds.ToHashSet();

        var referenced = config.Groups
            .SelectMany(g => g.MemberUserIds.Append(g.SharedUserId))
            .Distinct()
            .ToList();

        foreach (var id in referenced.Where(id => !liveIds.Contains(id)))
        {
            _logger.LogInformation("Pruning deleted user {UserId} from Watched Together groups", id);
            _groupService.PruneDeletedUser(id);
        }
    }
}