using System.Collections.Concurrent;
using CursorLang.Agent.Interop;

namespace CursorLang.Agent.Windows;

/// <summary>
/// The window the agent lives around: never shown, but it owns the tray icon and it
/// is the way back onto the message loop from a callback.
/// </summary>
/// <remarks>
/// A window with no <c>WS_VISIBLE</c> shows nowhere, yet is a window in every other
/// way. A message-only window would do as well were it not for the news of Explorer
/// restarting: that one is broadcast, and broadcasts pass such windows by.
/// </remarks>
internal sealed class AgentWindow : NativeWindow
{
    /// <summary>
    /// A message hook. Returning <c>true</c> means the message has been dealt with.
    /// </summary>
    internal delegate bool MessageFilter(uint message, IntPtr wParam, IntPtr lParam);

    private const string ClassName = "CursorLang.Agent.Window";

    /// <summary>Drain the queue of posted work. WM_APP is free for the application.</summary>
    private const uint WM_INVOKE = WindowNative.WM_APP + 100;

    private readonly List<MessageFilter> _filters = [];
    private readonly ConcurrentQueue<Action> _posted = new();

    internal AgentWindow()
        : base(ClassName, "CursorLang agent", WindowNative.WS_POPUP, WindowNative.WS_EX_TOOLWINDOW)
    {
    }

    internal void AddFilter(MessageFilter filter) => _filters.Add(filter);

    /// <summary>
    /// Runs the action on the message loop, after the current message is done with.
    /// </summary>
    /// <remarks>
    /// This is what the agent has instead of <c>Dispatcher.BeginInvoke</c>. The caller
    /// that matters is the keyboard hook: Windows removes a hook whose procedure takes
    /// too long, so the procedure only records what happened and the answer — showing
    /// the popup, switching the layout — waits for the message after this one.
    /// </remarks>
    internal void Post(Action action)
    {
        _posted.Enqueue(action);
        WindowNative.PostMessage(Handle, WM_INVOKE, IntPtr.Zero, IntPtr.Zero);
    }

    /// <summary>Asks the message loop to finish.</summary>
    internal void Quit() => WindowNative.PostQuitMessage(0);

    protected override bool OnMessage(uint message, IntPtr wParam, IntPtr lParam, out IntPtr result)
    {
        result = IntPtr.Zero;

        if (message == WM_INVOKE)
        {
            while (_posted.TryDequeue(out Action? action))
            {
                action();
            }

            return true;
        }

        if (message is WindowNative.WM_CLOSE or WindowNative.WM_ENDSESSION)
        {
            Quit();
            return true;
        }

        foreach (MessageFilter filter in _filters)
        {
            if (filter(message, wParam, lParam))
            {
                return true;
            }
        }

        return false;
    }
}