using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Jellyfin.Data;
using Jellyfin.Database.Implementations.Entities;
using Jellyfin.Database.Implementations.Enums;
using MediaBrowser.Controller.Library;
using Microsoft.Extensions.Logging;

namespace Jellyfin.Plugin.WatchedTogether.Services;

/// <summary>
/// Grants a shared account only the libraries that <em>every</em> member can already reach.
/// </summary>
/// <remarks>
/// Sharing an account must never be a way to gain access. If Alice is blocked from a library, a
/// group she belongs to cannot see it either, even when every other member can. The result is the
/// intersection of the members' access, so the shared account is always a subset of what each
/// member could reach alone.
/// </remarks>
public class LibraryAccessService : ILibraryAccessService
{
    private readonly IUserManager _userManager;
    private readonly ILibraryManager _libraryManager;
    private readonly ILogger<LibraryAccessService> _logger;

    /// <summary>
    /// Initializes a new instance of the <see cref="LibraryAccessService"/> class.
    /// </summary>
    /// <param name="userManager">The user manager.</param>
    /// <param name="libraryManager">The library manager.</param>
    /// <param name="logger">The logger.</param>
    public LibraryAccessService(
        IUserManager userManager,
        ILibraryManager libraryManager,
        ILogger<LibraryAccessService> logger)
    {
        _userManager = userManager;
        _libraryManager = libraryManager;
        _logger = logger;
    }

    /// <inheritdoc />
    public IReadOnlyList<Guid> ComputeIntersection(IReadOnlyList<Guid> memberIds)
    {
        ArgumentNullException.ThrowIfNull(memberIds);

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

        HashSet<Guid>? intersection = null;

        foreach (var memberId in memberIds)
        {
            var member = _userManager.GetUserById(memberId);
            if (member is null)
            {
                // A member we cannot resolve contributes no access. Treating it as "everything"
                // would silently widen the group.
                _logger.LogWarning(
                    "Member {MemberId} could not be resolved; treating its library access as empty",
                    memberId);
                return [];
            }

            var accessible = GetAccessibleLibraries(member);

            if (intersection is null)
            {
                intersection = accessible;
            }
            else
            {
                intersection.IntersectWith(accessible);
            }

            // Nothing left in common; no later member can widen it again.
            if (intersection.Count == 0)
            {
                break;
            }
        }

        return intersection?.ToList() ?? (IReadOnlyList<Guid>)[];
    }

    /// <inheritdoc />
    public async Task<IReadOnlyList<Guid>> ApplyIntersectionAsync(
        Guid sharedUserId,
        IReadOnlyList<Guid> memberIds)
    {
        var granted = ComputeIntersection(memberIds);

        var sharedUser = _userManager.GetUserById(sharedUserId);
        if (sharedUser is null)
        {
            return granted;
        }

        // Always an explicit list, never EnableAllFolders: "all" would keep growing as libraries
        // are added, silently granting the group access nobody agreed to.
        var policy = _userManager.GetUserDto(sharedUser).Policy;
        if (policy is null)
        {
            _logger.LogWarning(
                "Could not read the policy for shared account {SharedUserId}; library access unchanged",
                sharedUserId);
            return granted;
        }

        policy.EnableAllFolders = false;
        policy.EnabledFolders = granted.ToArray();

        await _userManager.UpdatePolicyAsync(sharedUserId, policy).ConfigureAwait(false);

        if (granted.Count == 0)
        {
            _logger.LogWarning(
                "Shared account {SharedUserId} has no libraries: its members have none in common",
                sharedUserId);
        }
        else
        {
            _logger.LogInformation(
                "Shared account {SharedUserId} granted {Count} libraries common to all members",
                sharedUserId,
                granted.Count);
        }

        return granted;
    }

    /// <summary>
    /// Resolves the concrete set of libraries a single user can reach.
    /// </summary>
    /// <param name="user">The user to inspect.</param>
    /// <returns>The library identifiers this user can see.</returns>
    private HashSet<Guid> GetAccessibleLibraries(User user)
    {
        // "Enable all folders" has to be expanded into concrete ids before it can be intersected
        // with another member's explicit list.
        var accessible = user.HasPermission(PermissionKind.EnableAllFolders)
            ? GetAllLibraryIds()
            : user.GetPreferenceValues<Guid>(PreferenceKind.EnabledFolders).ToHashSet();

        // An explicitly blocked folder is removed even when the user otherwise has access to all.
        accessible.ExceptWith(user.GetPreferenceValues<Guid>(PreferenceKind.BlockedMediaFolders));

        return accessible;
    }

    /// <summary>
    /// Gets the identifiers of every top-level library on the server.
    /// </summary>
    /// <returns>All library identifiers.</returns>
    private HashSet<Guid> GetAllLibraryIds()
    {
        try
        {
            return _libraryManager.GetUserRootFolder()
                .Children
                .Select(c => c.Id)
                .ToHashSet();
        }
#pragma warning disable CA1031 // Failing closed is the safe direction for an access decision.
        catch (Exception ex)
#pragma warning restore CA1031
        {
            _logger.LogError(ex, "Could not enumerate libraries; treating access as empty");
            return [];
        }
    }
}