using CursorLang.Core.Interop;
using CursorLang.Core.Models;
using CursorLang.Core.Threading;

namespace CursorLang.Core.Services;

/// <summary>
/// The settings of layout tracking.
/// </summary>
public sealed class KeyboardLayoutOptions
{
    /// <summary>How often to check the layout of the active window.</summary>
    public TimeSpan PollInterval { get; init; } = TimeSpan.FromMilliseconds(150);
}

/// <summary>
/// Polls the active window on a timer.
/// </summary>
/// <remarks>
/// Polling was chosen not for simplicity: there is no event-based way to learn about
/// a layout change in another process from managed code. HSHELL_LANGUAGE from
/// RegisterShellHookWindow does not arrive on Windows 10/11, and TSF notifications
/// (ITfLanguageProfileNotifySink) report only language changes inside our own process
/// — both options were tried and did not work. One tick is three Win32 calls reading
/// data from kernel memory.
///
/// The timer ticks on the message loop of whatever thread starts it, the same as a
/// dispatcher timer did: the agent has a plain Win32 loop and no dispatcher to offer.
/// </remarks>
public sealed class KeyboardLayoutService : IKeyboardLayoutService, IDisposable
{
    private readonly MessageTimer _pollTimer;
    private readonly Func<IntPtr> _getForegroundWindow;
    private readonly Func<int> _getActiveLocaleId;
    private readonly Action _requestNextLayout;

    private int _lastLocaleId = -1;
    private IntPtr _lastForegroundWindow;

    public KeyboardLayoutService(KeyboardLayoutOptions options)
        : this(
            options,
            KeyboardLayoutNative.GetForegroundWindow,
            KeyboardLayoutNative.GetActiveLocaleId,
            KeyboardLayoutNative.RequestNextLayout)
    {
    }

    /// <summary>
    /// Takes the sources of system information explicitly: in tests the layout and the
    /// active window are not provided by Windows.
    /// </summary>
    internal KeyboardLayoutService(
        KeyboardLayoutOptions options,
        Func<IntPtr> getForegroundWindow,
        Func<int> getActiveLocaleId,
        Action requestNextLayout)
    {
        _getForegroundWindow = getForegroundWindow;
        _getActiveLocaleId = getActiveLocaleId;
        _requestNextLayout = requestNextLayout;

        _pollTimer = new MessageTimer { Interval = options.PollInterval };
        _pollTimer.Tick += OnTick;
    }

    public event EventHandler<LayoutChangedEventArgs>? LayoutChanged;

    public KeyboardLayout Current => KeyboardLayout.FromLocaleId(_getActiveLocaleId());

    public void Start()
    {
        _lastForegroundWindow = _getForegroundWindow();
        _lastLocaleId = _getActiveLocaleId();
        _pollTimer.Start();
    }

    public void Stop() => _pollTimer.Stop();

    public void SwitchToNext() => _requestNextLayout();

    public void Dispose()
    {
        _pollTimer.Tick -= OnTick;
        _pollTimer.Dispose();
    }

    private void OnTick(object? sender, EventArgs e) => Poll();

    // A single poll step. Called by the timer, and in tests — directly:
    // there is no point waiting for a tick to check how the reason for a layout
    // change is decided
    internal void Poll()
    {
        IntPtr foreground = _getForegroundWindow();
        if (foreground == IntPtr.Zero)
        {
            return;
        }

        bool appSwitched = foreground != _lastForegroundWindow;
        _lastForegroundWindow = foreground;

        int localeId = _getActiveLocaleId();
        if (localeId == _lastLocaleId)
        {
            return;
        }

        _lastLocaleId = localeId;

        // Moving to another application with a layout of its own is not the same as
        // the user switching the layout, and the subscribers are free to react to
        // these cases differently
        LayoutChangeReason reason = appSwitched
            ? LayoutChangeReason.ApplicationSwitched
            : LayoutChangeReason.UserSwitched;

        LayoutChanged?.Invoke(this, new LayoutChangedEventArgs(KeyboardLayout.FromLocaleId(localeId), reason));
    }
}