using System.Runtime.InteropServices;

namespace CursorLang.Core.Threading;

/// <summary>
/// A timer that ticks on the message loop — the agent's stand-in for
/// <c>DispatcherTimer</c>.
/// </summary>
/// <remarks>
/// <c>SetTimer</c> with a null window binds the timer to the thread rather than to a
/// window, and <c>DispatchMessage</c> calls the callback straight from the loop. The
/// upshot is the same as with a dispatcher timer: the tick arrives on the thread that
/// owns the hook and the popup, so nothing needs marshalling and nothing races.
///
/// The callback lives in a field for the reason a hook procedure does: the only
/// reference to it is held by Win32, and a collected delegate takes the process down
/// with it at the first tick.
/// </remarks>
internal sealed class MessageTimer : IDisposable
{
    /// <summary>Windows will not go below this, and pretending otherwise misleads.</summary>
    private const uint MinimumIntervalMilliseconds = 10;

    private readonly TimerProc _callback;

    private nuint _id;

    internal MessageTimer() => _callback = OnTimer;

    internal event EventHandler? Tick;

    internal TimeSpan Interval { get; set; } = TimeSpan.FromMilliseconds(100);

    internal bool IsRunning => _id != 0;

    /// <summary>Starts the timer, or restarts it from zero when it is already running.</summary>
    internal void Start()
    {
        Stop();

        var milliseconds = (uint)Math.Clamp(
            Math.Round(Interval.TotalMilliseconds), MinimumIntervalMilliseconds, int.MaxValue);

        _id = SetTimer(IntPtr.Zero, 0, milliseconds, _callback);
    }

    internal void Stop()
    {
        if (_id == 0)
        {
            return;
        }

        KillTimer(IntPtr.Zero, _id);
        _id = 0;
    }

    public void Dispose() => Stop();

    private void OnTimer(IntPtr window, uint message, nuint id, uint time) =>
        Tick?.Invoke(this, EventArgs.Empty);

    private delegate void TimerProc(IntPtr hWnd, uint message, nuint idEvent, uint time);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern nuint SetTimer(IntPtr hWnd, nuint nIDEvent, uint uElapse, TimerProc lpTimerFunc);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern bool KillTimer(IntPtr hWnd, nuint uIDEvent);
}