using System.Buffers;
using System.IO;
using System.Runtime.InteropServices;

namespace NetVips;

/// <summary>
/// An target you can connect delegates to implement behaviour.
/// </summary>
public class TargetCustom : Target
{
    /// <summary>
    /// A write delegate.
    /// </summary>
    /// <remarks>
    /// The interface is the same as <see cref="Stream.Write(byte[], int, int)"/>.
    /// The handler is given a bytes-like object to write. However, the handler MUST
    /// return the number of bytes written.
    /// </remarks>
    /// <param name="buffer">An array of bytes.</param>
    /// <param name="length">The number of bytes to be written to the current target.</param>
    /// <returns>The total number of bytes written to the target.</returns>
    public delegate long WriteDelegate(byte[] buffer, int length);

    /// <summary>
    /// A read delegate.
    /// </summary>
    /// <remarks>
    /// libtiff needs to be able to read on targets, unfortunately.
    /// </remarks>
    /// <param name="buffer">An array of bytes.</param>
    /// <param name="length">The maximum number of bytes to be read.</param>
    /// <returns>The total number of bytes read into the buffer.</returns>
    public delegate int ReadDelegate(byte[] buffer, int length);

    /// <summary>
    /// A seek delegate.
    /// </summary>
    /// <remarks>
    /// libtiff needs to be able to seek on targets, unfortunately.
    /// </remarks>
    /// <param name="offset">A byte offset relative to the <paramref name="origin"/>
    /// parameter.</param>
    /// <param name="origin">A value of type <see cref="SeekOrigin"/> indicating the
    /// reference point used to obtain the new position.</param>
    /// <returns>The new position within the current target.</returns>
    public delegate long SeekDelegate(long offset, SeekOrigin origin);

    /// <summary>
    /// A end delegate.
    /// </summary>
    /// <remarks>
    /// This optional handler is called at the end of write. It should do any
    /// cleaning up, if necessary.
    /// </remarks>
    /// <returns>0 on success, -1 on error.</returns>
    public delegate int EndDelegate();

    /// <summary>
    /// Attach a write delegate.
    /// </summary>
    public event WriteDelegate OnWrite;

    /// <summary>
    /// Attach a read delegate.
    /// </summary>
    /// <remarks>
    /// This is not called prior libvips 8.13.
    /// </remarks>
    public event ReadDelegate OnRead;

    /// <summary>
    /// Attach a seek delegate.
    /// </summary>
    /// <remarks>
    /// This is not called prior libvips 8.13.
    /// </remarks>
    public event SeekDelegate OnSeek;

    /// <summary>
    /// Attach a end delegate.
    /// </summary>
    public event EndDelegate OnEnd;

    /// <inheritdoc cref="Target"/>
    public TargetCustom() : base(Internal.VipsTargetCustom.New())
    {
        var vips813 = NetVips.AtLeastLibvips(8, 13);

        SignalConnect("write", (Internal.VipsTargetCustom.WriteSignal)WriteHandler);
        if (vips813)
        {
            SignalConnect("read", (Internal.VipsTargetCustom.ReadSignal)ReadHandler);
            SignalConnect("seek", (Internal.VipsTargetCustom.SeekSignal)SeekHandler);
        }
        SignalConnect(vips813 ? "end" : "finish", (Internal.VipsTargetCustom.EndSignal)EndHandler);
    }

    /// <summary>
    /// The internal write handler.
    /// </summary>
    /// <param name="targetPtr">The underlying pointer to the target.</param>
    /// <param name="buffer">An array of bytes.</param>
    /// <param name="length">The number of bytes to be written to the current target.</param>
    /// <param name="userDataPtr">User data associated with the target.</param>
    /// <returns>The total number of bytes written to the target.</returns>
    internal long WriteHandler(nint targetPtr, byte[] buffer, int length, nint userDataPtr)
    {
        var bytesWritten = OnWrite?.Invoke(buffer, length);
        return bytesWritten ?? -1;
    }

    /// <summary>
    /// The internal read handler.
    /// </summary>
    /// <param name="targetPtr">The underlying pointer to the target.</param>
    /// <param name="buffer">A pointer to an array of bytes.</param>
    /// <param name="length">The maximum number of bytes to be read.</param>
    /// <param name="userDataPtr">User data associated with the target.</param>
    /// <returns>The total number of bytes read into the buffer.</returns>
    internal long ReadHandler(nint targetPtr, nint buffer, long length, nint userDataPtr)
    {
        if (length <= 0)
        {
            return 0;
        }

        var tempArray = ArrayPool<byte>.Shared.Rent((int)length);
        try
        {
            var readLength = OnRead?.Invoke(tempArray, (int)length);
            if (!readLength.HasValue)
            {
                return -1;
            }

            if (readLength.Value > 0)
            {
                Marshal.Copy(tempArray, 0, buffer, readLength.Value);
            }

            return readLength.Value;
        }
        catch
        {
            return -1;
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(tempArray);
        }
    }

    /// <summary>
    /// The internal seek handler.
    /// </summary>
    /// <param name="targetPtr">The underlying pointer to the target.</param>
    /// <param name="offset">A byte offset relative to the <paramref name="whence"/>
    /// parameter.</param>
    /// <param name="whence">A value of type <see cref="SeekOrigin"/> indicating the
    /// reference point used to obtain the new position.</param>
    /// <param name="userDataPtr">User data associated with the target.</param>
    /// <returns>The new position within the current target.</returns>
    internal long SeekHandler(nint targetPtr, long offset, int whence, nint userDataPtr)
    {
        var newPosition = OnSeek?.Invoke(offset, (SeekOrigin)whence);
        return newPosition ?? -1;
    }

    /// <summary>
    /// The internal end handler.
    /// </summary>
    /// <param name="targetPtr">The underlying pointer to the target.</param>
    /// <param name="userDataPtr">User data associated with the target.</param>
    /// <returns>0 on success, -1 on error.</returns>
    internal int EndHandler(nint targetPtr, nint userDataPtr)
    {
        return OnEnd?.Invoke() ?? 0;
    }
}