using System.Collections.Concurrent;
using System.DirectoryServices.AccountManagement;
using System.Runtime.Versioning;
using System.Security.Principal;
using SQLInsight.Core.Data;

namespace SQLInsight.Agent.Security;

/// <summary>
/// Unfolds a Windows group (DOMAIN\Group) to the people in it, recursively, by asking Active Directory as the agent's
/// account (the computer account for the default virtual account, which may read the directory in a domain).
/// Answers are cached for an hour (failures for ten minutes) and each lookup is cut off after 20 seconds, so a slow or
/// unreachable domain controller never holds up the monitoring.
/// </summary>
[SupportedOSPlatform("windows")]
public sealed partial class ActiveDirectoryGroupResolver(TimeProvider time, ILogger<ActiveDirectoryGroupResolver> logger) : IGroupMemberResolver
{
    public static readonly TimeSpan CacheFor = TimeSpan.FromHours(1);
    public static readonly TimeSpan CacheFailuresFor = TimeSpan.FromMinutes(10);
    public static readonly TimeSpan Timeout = TimeSpan.FromSeconds(20);

    private readonly ConcurrentDictionary<string, (DateTimeOffset At, GroupExpansion Result)> _cache = new(StringComparer.OrdinalIgnoreCase);

    public async Task<GroupExpansion> ExpandAsync(string group, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(group);
        var now = time.GetUtcNow();
        if (_cache.TryGetValue(group, out var cached) && now - cached.At < (cached.Result.Error is null ? CacheFor : CacheFailuresFor))
        {
            return cached.Result;
        }

        GroupExpansion result;
        try
        {
            result = await Task.Run(() => Expand(group), cancellationToken).WaitAsync(Timeout, cancellationToken).ConfigureAwait(false);
        }
        catch (TimeoutException)
        {
            result = GroupExpansion.Failed($"Active Directory svarede ikke inden for {Timeout.TotalSeconds:0} sekunder.");
        }
        catch (Exception ex) when (ex is PrincipalException or System.DirectoryServices.DirectoryServicesCOMException or System.Runtime.InteropServices.COMException
                                       or InvalidOperationException or UnauthorizedAccessException or ArgumentException)
        {
            result = GroupExpansion.Failed($"Active Directory kunne ikke spørges: {ex.Message}");
        }

        if (result.Error is not null)
        {
            LogExpansionFailed(logger, group, result.Error);
        }

        _cache[group] = (now, result);
        return result;
    }

    private static GroupExpansion Expand(string group)
    {
        var separator = group.IndexOf('\\', StringComparison.Ordinal);
        if (separator <= 0)
        {
            return GroupExpansion.Failed("Gruppen har intet domæne (forventet DOMÆNE\\gruppe).");
        }

        var domain = group[..separator];
        var name = group[(separator + 1)..];
        if (domain.Equals("BUILTIN", StringComparison.OrdinalIgnoreCase) || domain.Equals("NT AUTHORITY", StringComparison.OrdinalIgnoreCase))
        {
            return GroupExpansion.Failed("En lokal gruppe på SQL Server-maskinen; medlemmerne kan ikke læses fra Active Directory.");
        }

        var local = domain.Equals(Environment.MachineName, StringComparison.OrdinalIgnoreCase);
        using var context = local ? new PrincipalContext(ContextType.Machine) : new PrincipalContext(ContextType.Domain, domain);
        using var principal = GroupPrincipal.FindByIdentity(context, IdentityType.SamAccountName, name);
        if (principal is null)
        {
            return GroupExpansion.Failed(local ? "Gruppen findes ikke på maskinen." : $"Gruppen findes ikke i domænet {domain}.");
        }

        var members = new List<GroupMember>();
        using var found = principal.GetMembers(recursive: true);
        foreach (var member in found)
        {
            using (member)
            {
                var account = Account(member, domain);
                var enabled = member is AuthenticablePrincipal authenticable ? authenticable.Enabled ?? true : true;
                members.Add(new GroupMember(account, member.DisplayName, enabled));
            }
        }

        return new GroupExpansion(members.DistinctBy(m => m.Account, StringComparer.OrdinalIgnoreCase).OrderBy(m => m.Account, StringComparer.OrdinalIgnoreCase).ToList());
    }

    /// <summary>DOMAIN\sam, from the SID so members of other domains get their own domain.</summary>
    private static string Account(Principal member, string fallbackDomain)
    {
        try
        {
            if (member.Sid?.Translate(typeof(NTAccount)) is NTAccount account)
            {
                return account.Value;
            }
        }
        catch (IdentityNotMappedException)
        {
        }

        return $@"{fallbackDomain}\{member.SamAccountName}";
    }

    [LoggerMessage(Level = LogLevel.Warning, Message = "Windows group {Group} could not be unfolded: {Error}")]
    private static partial void LogExpansionFailed(ILogger logger, string group, string error);
}