using System.Runtime.InteropServices;

namespace CursorLang.Core.Services;

/// <summary>
/// Tells the agent that settings.json has changed.
/// </summary>
/// <remarks>
/// The message carries nothing but the fact. The temptation to send the changed values
/// along has to be resisted: the file would stop being the only source of truth, and
/// the two would part company the first time somebody edits it by hand. Nothing else
/// tells the agent — it does not watch the file — so a message that goes missing means
/// settings it does not pick up until it is restarted.
///
/// Order is what makes it safe. The settings window writes the file whole, moves it
/// into place in one step and only then signals, so by the time the agent reads there
/// is nothing half-written to read.
///
/// A registered message rather than <c>WM_APP + n</c>: the identifier is unique across
/// the system, so it cannot be confused with anything else that finds its way to that
/// window.
/// </remarks>
internal static class SettingsSignal
{
    /// <summary>The window class the agent registers for its hidden window.</summary>
    internal const string AgentWindowClass = "CursorLang.Agent.Window";

    /// <summary>The message both sides agree on.</summary>
    internal static uint Message { get; } = RegisterWindowMessage("CursorLang.SettingsChanged");

    /// <summary>
    /// Wakes the agent, if one is running in this session. Silence is a normal
    /// answer: the settings window is perfectly usable with no agent behind it.
    /// </summary>
    internal static void NotifyAgent()
    {
        IntPtr agent = FindWindow(AgentWindowClass, null);
        if (agent != IntPtr.Zero)
        {
            PostMessage(agent, Message, IntPtr.Zero, IntPtr.Zero);
        }
    }

    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "RegisterWindowMessageW")]
    private static extern uint RegisterWindowMessage(string lpString);

    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "FindWindowW")]
    private static extern IntPtr FindWindow(string lpClassName, string? lpWindowName);

    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "PostMessageW")]
    private static extern bool PostMessage(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);
}