using System;

namespace ADOFAIRenderer.Util.OS;

/// <summary>
/// Read-only access to <c>HKEY_LOCAL_MACHINE</c> (64-bit registry view) through reflection.
/// <para>
/// The mod targets netstandard2.1, which does not expose <c>Microsoft.Win32.RegistryKey</c>; the game runtime (Mono)
/// provides it in its core library. The types and methods are looked up once in the static constructor and invoked through
/// reflection.
/// </para>
/// <para>
/// Intended for Windows. On other platforms the result depends on the runtime's registry support; when the types or methods
/// are not found, or a call throws, every method returns null.
/// All failures (missing key, access denied, reflection errors) are caught and reported as null; nothing is logged.
/// Stateless and safe to call from any thread; each call opens and closes its own keys.
/// </para>
/// </summary>
internal static class RegistryUtil
{
    // Reflected Microsoft.Win32 types; null when the runtime does not provide them.
    private static readonly Type? RegistryKeyType;
    private static readonly Type? RegistryHiveType;
    private static readonly Type? RegistryViewType;
    // RegistryKey.OpenBaseKey(RegistryHive, RegistryView) (static), OpenSubKey(string) (read-only open),
    // GetSubKeyNames() and GetValue(string).
    private static readonly System.Reflection.MethodInfo? OpenBaseKeyMethod;
    private static readonly System.Reflection.MethodInfo? OpenSubKeyMethod;
    private static readonly System.Reflection.MethodInfo? GetSubKeyNamesMethod;
    private static readonly System.Reflection.MethodInfo? GetValueMethod;

    /// <summary>
    /// Resolves the registry types by name (<see cref="Type.GetType(string)"/>, then the core library that defines
    /// <see cref="object"/>) and their methods. Leaves the fields null on any failure.
    /// </summary>
    static RegistryUtil()
    {
        try
        {
            RegistryKeyType = Type.GetType("Microsoft.Win32.RegistryKey");
            RegistryHiveType = Type.GetType("Microsoft.Win32.RegistryHive");
            RegistryViewType = Type.GetType("Microsoft.Win32.RegistryView");

            if (RegistryKeyType == null || RegistryHiveType == null || RegistryViewType == null)
            {
                var mscorlib = typeof(object).Assembly;
                RegistryKeyType = mscorlib.GetType("Microsoft.Win32.RegistryKey");
                RegistryHiveType = mscorlib.GetType("Microsoft.Win32.RegistryHive");
                RegistryViewType = mscorlib.GetType("Microsoft.Win32.RegistryView");
            }

            if (RegistryKeyType != null)
            {
                OpenBaseKeyMethod = RegistryKeyType.GetMethod("OpenBaseKey", new[] { RegistryHiveType!, RegistryViewType! });
                OpenSubKeyMethod = RegistryKeyType.GetMethod("OpenSubKey", new[] { typeof(string) });
                GetSubKeyNamesMethod = RegistryKeyType.GetMethod("GetSubKeyNames");
                GetValueMethod = RegistryKeyType.GetMethod("GetValue", new[] { typeof(string) });
            }
        }
        catch
        {
            // ignored
        }
    }

    /// <summary>Closes a reflected RegistryKey (Dispose, or Close when it is not IDisposable). Ignores null and errors.</summary>
    /// <param name="key">RegistryKey instance returned by a reflected call, or null.</param>
    private static void CloseKey(object? key)
    {
        if (key == null || RegistryKeyType == null) return;
        try
        {
            if (key is IDisposable disposable)
            {
                disposable.Dispose();
            }
            else
            {
                var closeMethod = RegistryKeyType.GetMethod("Close");
                closeMethod?.Invoke(key, null);
            }
        }
        catch
        {
            // ignored
        }
    }

    /// <summary>
    /// Returns the names of the direct subkeys of <c>HKEY_LOCAL_MACHINE\&lt;parentKeyPath&gt;</c> in the 64-bit registry view.
    /// </summary>
    /// <param name="parentKeyPath">Key path relative to HKEY_LOCAL_MACHINE, with backslash separators.</param>
    /// <returns>Subkey names, or null when the key does not exist, cannot be opened, or the registry is unavailable.</returns>
    // ReSharper disable once InconsistentNaming
    public static string[]? GetHKLMSubKeyNames(string parentKeyPath)
    {
        if (RegistryKeyType == null || OpenBaseKeyMethod == null || OpenSubKeyMethod == null || GetSubKeyNamesMethod == null)
            return null;

        try
        {
            // RegistryHive.LocalMachine = 0x80000002 (-2147483646), RegistryView.Registry64 = 256.
            // On a 32-bit OS a Registry64 request returns the 32-bit view.
            // Reference: https://learn.microsoft.com/en-us/dotnet/api/microsoft.win32.registryhive
            // Reference: https://learn.microsoft.com/en-us/dotnet/api/microsoft.win32.registryview
            object hiveLocalMachine = Enum.ToObject(RegistryHiveType!, 0x80000002);
            object viewRegistry64 = Enum.ToObject(RegistryViewType!, 256);

            object? baseKey = OpenBaseKeyMethod.Invoke(null, new[] { hiveLocalMachine, viewRegistry64 });
            if (baseKey == null) return null;

            object? key = OpenSubKeyMethod.Invoke(baseKey, new object[] { parentKeyPath });
            if (key == null)
            {
                CloseKey(baseKey);
                return null;
            }

            string[]? subkeyNames = GetSubKeyNamesMethod.Invoke(key, null) as string[];
            CloseKey(key);
            CloseKey(baseKey);
            return subkeyNames;
        }
        catch
        {
            return null;
        }
    }

    /// <summary>
    /// Reads a string value of <c>HKEY_LOCAL_MACHINE\&lt;keyPath&gt;</c> in the 64-bit registry view.
    /// </summary>
    /// <param name="keyPath">Key path relative to HKEY_LOCAL_MACHINE, with backslash separators.</param>
    /// <param name="valueName">Value name; an empty string reads the key's default value.</param>
    /// <returns>
    /// The value when it exists and is a string (REG_SZ / REG_EXPAND_SZ); null when the key or value does not exist, the value
    /// has another type, or the registry is unavailable.
    /// </returns>
    // ReSharper disable once InconsistentNaming
    public static string? GetHKLMValue(string keyPath, string valueName)
    {
        if (RegistryKeyType == null || OpenBaseKeyMethod == null || OpenSubKeyMethod == null || GetValueMethod == null)
            return null;

        try
        {
            // Same hive and view constants as GetHKLMSubKeyNames.
            object hiveLocalMachine = Enum.ToObject(RegistryHiveType!, 0x80000002);
            object viewRegistry64 = Enum.ToObject(RegistryViewType!, 256);

            object? baseKey = OpenBaseKeyMethod.Invoke(null, new[] { hiveLocalMachine, viewRegistry64 });
            if (baseKey == null) return null;

            object? key = OpenSubKeyMethod.Invoke(baseKey, new object[] { keyPath });
            if (key == null)
            {
                CloseKey(baseKey);
                return null;
            }

            string? value = GetValueMethod.Invoke(key, new object[] { valueName }) as string;
            CloseKey(key);
            CloseKey(baseKey);
            return value;
        }
        catch
        {
            return null;
        }
    }
}