using System.Runtime.InteropServices;

namespace CursorLang.Core.Threading;

/// <summary>
/// The Win32 message loop — what the application has instead of a dispatcher.
/// </summary>
/// <remarks>
/// It lives in Core rather than in the agent because the things that need a pumping
/// thread do: <see cref="MessageTimer"/> is used by the layout polling and by saving
/// the settings, and both are Core's. The settings window has a loop of its own, run
/// by WPF, and everything here works inside it just the same.
/// </remarks>
public static class MessageLoop
{
    /// <summary>
    /// Pumps messages until <c>WM_QUIT</c> and returns its exit code.
    /// </summary>
    /// <remarks>
    /// A -1 from GetMessage means the window handle has already gone; going round
    /// again would spin forever, so the loop gives up instead.
    /// </remarks>
    public static int Run()
    {
        while (true)
        {
            int result = GetMessage(out Message message, IntPtr.Zero, 0, 0);
            if (result is 0 or -1)
            {
                return result == 0 ? (int)message.wParam : 1;
            }

            TranslateMessage(ref message);
            DispatchMessage(ref message);
        }
    }

    /// <summary>Asks the loop on this thread to finish.</summary>
    public static void Quit(int exitCode = 0) => PostQuitMessage(exitCode);


    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "GetMessageW")]
    private static extern int GetMessage(out Message lpMsg, IntPtr hWnd, uint filterMin, uint filterMax);

    [DllImport("user32.dll")]
    private static extern bool TranslateMessage(ref Message lpMsg);

    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "DispatchMessageW")]
    private static extern IntPtr DispatchMessage(ref Message lpMsg);

    [DllImport("user32.dll")]
    private static extern void PostQuitMessage(int exitCode);

    [StructLayout(LayoutKind.Sequential)]
    private struct Message
    {
        public IntPtr hwnd;
        public uint message;
        public IntPtr wParam;
        public IntPtr lParam;
        public uint time;
        public int x;
        public int y;
        public uint lPrivate;
    }
}