using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using NetVips.Internal;

using GSignalMatchType = NetVips.Internal.Enums.GSignalMatchType;

namespace NetVips;

/// <summary>
/// Manage <see cref="Internal.GObject"/> lifetime.
/// </summary>
public class GObject : SafeHandle
{
    /// <summary>
    /// We have to record all of the <see cref="SignalConnect{T}"/> delegates to
    /// prevent them from being re-located or disposed of by the garbage collector.
    /// </summary>
    /// <remarks>
    /// All recorded delegates are freed in <see cref="ReleaseDelegates"/>.
    /// </remarks>
    private readonly ICollection<GCHandle> _handles = new List<GCHandle>();

    /// <summary>
    /// Hint of how much native memory is actually occupied by the object.
    /// </summary>
    internal long MemoryPressure;

    // Handy for debugging
    // public static int NObjects;

    /// <summary>
    /// Initializes a new instance of the <see cref="GObject"/> class
    /// with the specified pointer to wrap around.
    /// </summary>
    /// <remarks>
    /// Wraps a GObject instance around an underlying GValue. When the
    /// instance is garbage-collected, the underlying object is unreferenced.
    /// </remarks>
    /// <param name="pointer">The pointer to wrap around.</param>
    internal GObject(nint pointer) : base(IntPtr.Zero, true)
    {
        // record the pointer we were given to manage
        SetHandle(pointer);

        // NObjects++;
    }

    /// <summary>
    /// Connects a callback function (<paramref name="callback"/>) to a signal on this object.
    /// </summary>
    /// <remarks>
    /// The callback will be triggered every time this signal is issued on this instance.
    /// </remarks>
    /// <typeparam name="T">The type of the callback to connect.</typeparam>
    /// <param name="detailedSignal">A string of the form "signal-name::detail".</param>
    /// <param name="callback">The callback to connect.</param>
    /// <param name="data">Data to pass to handler calls.</param>
    /// <returns>The handler id.</returns>
    /// <exception cref="T:System.ArgumentException">If it failed to connect the signal.</exception>
    public ulong SignalConnect<T>(string detailedSignal, T callback, nint data = default)
        where T : notnull
    {
        // add a weak reference callback to ensure all handles are released on finalization
        if (_handles.Count == 0)
        {
            GWeakNotify notify = ReleaseDelegates;
            var notifyHandle = GCHandle.Alloc(notify);

            Internal.GObject.WeakRef(this, notify, GCHandle.ToIntPtr(notifyHandle));
        }

        // prevent the delegate from being re-located or disposed of by the garbage collector
        var delegateHandle = GCHandle.Alloc(callback);
        _handles.Add(delegateHandle);

        var cHandler = Marshal.GetFunctionPointerForDelegate(callback);
        var ret = GSignal.ConnectData(this, detailedSignal, cHandler, data, null, default);
        if (ret == 0)
        {
            throw new ArgumentException("Failed to connect signal " + detailedSignal);
        }

        return ret;
    }

    /// <summary>
    /// Disconnects a handler from this object.
    /// </summary>
    /// <remarks>
    /// If the <paramref name="handlerId"/> is 0 then this function does nothing.
    /// </remarks>
    /// <param name="handlerId">Handler id of the handler to be disconnected.</param>
    public void SignalHandlerDisconnect(ulong handlerId)
    {
        if (handlerId != 0)
        {
            GSignal.HandlerDisconnect(this, handlerId);
        }
    }

    /// <summary>
    /// Disconnects all handlers from this object that match <paramref name="func"/> and
    /// <paramref name="data"/>.
    /// </summary>
    /// <typeparam name="T">The type of the func.</typeparam>
    /// <param name="func">The func of the handlers.</param>
    /// <param name="data">The data of the handlers.</param>
    /// <returns>The number of handlers that matched.</returns>
    public uint SignalHandlersDisconnectByFunc<T>(T func, nint data = default)
        where T : notnull
    {
        var funcPtr = Marshal.GetFunctionPointerForDelegate(func);
        return GSignal.HandlersDisconnectMatched(this,
            GSignalMatchType.G_SIGNAL_MATCH_FUNC | GSignalMatchType.G_SIGNAL_MATCH_DATA,
            0, 0, IntPtr.Zero, funcPtr, data);
    }

    /// <summary>
    /// Disconnects all handlers from this object that match <paramref name="data"/>.
    /// </summary>
    /// <param name="data">The data of the handlers.</param>
    /// <returns>The number of handlers that matched.</returns>
    public uint SignalHandlersDisconnectByData(nint data)
    {
        return GSignal.HandlersDisconnectMatched(this,
            GSignalMatchType.G_SIGNAL_MATCH_DATA,
            0, 0, IntPtr.Zero, IntPtr.Zero, data);
    }

    /// <summary>
    /// Decreases the reference count of object.
    /// When its reference count drops to 0, the object is finalized (i.e. its memory is freed).
    /// </summary>
    /// <returns><see langword="true"/> if the handle is released successfully; otherwise,
    /// in the event of a catastrophic failure, <see langword="false"/>.</returns>
    protected override bool ReleaseHandle()
    {
        if (!IsInvalid)
        {
            Internal.GObject.Unref(handle);
        }
        // NObjects--;

        return true;
    }

    /// <summary>
    /// Release all the <see cref="SignalConnect{T}"/> delegates by this object on finalization.
    /// </summary>
    /// <remarks>
    /// This function is only called when <see cref="SignalConnect{T}"/> was used on this object.
    /// </remarks>
    /// <param name="data">Data that was provided when the weak reference was established.</param>
    /// <param name="objectPointer">The object being disposed.</param>
    internal void ReleaseDelegates(nint data, nint objectPointer)
    {
        foreach (var gcHandle in _handles)
        {
            if (gcHandle.IsAllocated)
            {
                gcHandle.Free();
            }
        }

        // All GCHandles are free'd. Clear the list to prevent inadvertent use.
        _handles.Clear();

        // Free the GCHandle used by this GWeakNotify
        var notifyHandle = GCHandle.FromIntPtr(data);
        if (notifyHandle.IsAllocated)
        {
            notifyHandle.Free();
        }
    }

    /// <summary>
    /// Increases the reference count of object.
    /// </summary>
    internal nint ObjectRef()
    {
        return Internal.GObject.Ref(handle);
    }

    /// <summary>
    /// Gets a value indicating whether the handle is invalid.
    /// </summary>
    /// <returns><see langword="true"/> if the handle is not valid; otherwise, <see langword="false"/>.</returns>
    public override bool IsInvalid => handle == IntPtr.Zero;

    /// <summary>
    /// Get the reference count of object. Handy for debugging.
    /// </summary>
    internal uint RefCount => Marshal.PtrToStructure<Internal.GObject.Struct>(handle).RefCount;

    // Do not provide a finalizer - SafeHandle's critical finalizer will
    // call ReleaseHandle for us.
}