/*************************************************************************
 * ModernUO                                                              *
 * Copyright 2019-2025 - ModernUO Development Team                       *
 * Email: hi@modernuo.com                                                *
 * File: LoginKeys.cs                                                    *
 *                                                                       *
 * This program is free software: you can redistribute it and/or modify  *
 * it under the terms of the GNU General Public License as published by  *
 * the Free Software Foundation, either version 3 of the License, or     *
 * (at your option) any later version.                                   *
 *                                                                       *
 * You should have received a copy of the GNU General Public License     *
 * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
 *************************************************************************/

using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace Server.Network;

/// <summary>
/// Represents encryption keys derived from a client version.
/// Used for login packet encryption/decryption.
/// </summary>
public readonly struct LoginKeys
{
    public static readonly LoginKeys Empty = new(0, 0);

    private static readonly Dictionary<ClientVersion, LoginKeys> _cache = [];

    public uint Key1 { get; }
    public uint Key2 { get; }

    private LoginKeys(uint key1, uint key2)
    {
        Key1 = key1;
        Key2 = key2;
    }

    /// <summary>
    /// Gets or computes encryption keys for the specified client version.
    /// Results are cached for performance.
    /// </summary>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static LoginKeys GetKeys(ClientVersion version)
    {
        if (version == null)
        {
            return Empty;
        }

        if (_cache.TryGetValue(version, out var keys))
        {
            return keys;
        }

        keys = ComputeKeys(version);
        _cache[version] = keys;
        return keys;
    }

    /// <summary>
    /// Computes encryption keys from client version using the UO key derivation algorithm.
    /// </summary>
    private static LoginKeys ComputeKeys(ClientVersion version)
    {
        uint major = (uint)version.Major;
        uint minor = (uint)version.Minor;
        uint revision = (uint)version.Revision;

        // Key1 derivation
        uint key1 = (major << 23) | (minor << 14) | (revision << 4);
        key1 ^= (revision * revision) << 9;
        key1 ^= minor * minor;
        key1 ^= (minor * 11) << 24;
        key1 ^= (revision * 7) << 19;
        key1 ^= 0x2C13A5FD;

        // Key2 derivation
        uint key2 = (major << 22) | (revision << 13) | (minor << 3);
        key2 ^= (revision * revision * 3) << 10;
        key2 ^= minor * minor;
        key2 ^= (minor * 13) << 23;
        key2 ^= (revision * 7) << 18;
        key2 ^= 0xA31D527F;

        return new LoginKeys(key1, key2);
    }
}