using System.Runtime.InteropServices;

namespace CursorLang.Core.Interop;

/// <summary>
/// Win32 API for reading the layout of the active application.
/// </summary>
internal static class KeyboardLayoutNative
{
    private const uint WmInputLangChangeRequest = 0x0050;

    /// <summary>Take the next layout from the system list.</summary>
    private static readonly IntPtr InputLangChangeForward = new(0x0002);

    /// <summary>HKL_NEXT — the same request in the language of older Windows versions.</summary>
    private static readonly IntPtr HklNext = new(1);

    [DllImport("user32.dll")]
    internal static extern IntPtr GetForegroundWindow();

    [DllImport("user32.dll")]
    private static extern uint GetWindowThreadProcessId(IntPtr hWnd, IntPtr lpdwProcessId);

    [DllImport("user32.dll")]
    private static extern IntPtr GetKeyboardLayout(uint idThread);

    [DllImport("user32.dll")]
    private static extern bool PostMessage(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam);

    /// <summary>
    /// The layout the user is currently typing with.
    /// </summary>
    internal static int GetActiveLocaleId() => GetLocaleIdOf(GetInputWindow());

    /// <summary>
    /// Asks the active application to switch to the next layout from the system list.
    /// </summary>
    /// <remarks>
    /// Synthesizing a system shortcut such as Alt+Shift will not do: the user can
    /// reassign it in the Windows settings or turn it off entirely. A request sent as
    /// a message does not depend on those settings and works in another process.
    /// </remarks>
    internal static void RequestNextLayout()
    {
        IntPtr target = GetInputWindow();
        if (target == IntPtr.Zero)
        {
            return;
        }

        // Both parameters mean the same thing: different Windows versions and different
        // UI frameworks look either at the flag or at lParam
        PostMessage(target, WmInputLangChangeRequest, InputLangChangeForward, HklNext);
    }

    /// <summary>
    /// The window that owns keyboard input.
    /// </summary>
    /// <remarks>
    /// We ask the window with keyboard focus rather than the foreground window: in
    /// Windows 11 Notepad, the Start menu and other WinUI applications the input field
    /// lives in a separate thread, and the layout changes only for that thread. For the
    /// main window's thread it stays the same, and the switch goes unnoticed.
    /// </remarks>
    private static IntPtr GetInputWindow()
    {
        if (ForegroundInputNative.TryGetInfo(out ForegroundInputNative.GuiThreadInfo info) &&
            info.hwndFocus != IntPtr.Zero)
        {
            return info.hwndFocus;
        }

        return GetForegroundWindow();
    }

    /// <summary>
    /// The locale identifier for a window. In Windows the layout is bound to a thread,
    /// so this reveals it for any application, not only for our own.
    /// </summary>
    internal static int GetLocaleIdOf(IntPtr hWnd)
    {
        uint threadId = GetWindowThreadProcessId(hWnd, IntPtr.Zero);
        return (int)GetKeyboardLayout(threadId).ToInt64() & 0xFFFF;
    }
}