// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Buffers;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using Microsoft.Extensions.Logging;

namespace Libuv.Internal
{
  /// <summary>
  /// Summary description for UvWriteRequest
  /// </summary>
  public class UvWriteReq : UvRequest
  {
    private static readonly LibuvFunctions.uv_write_cb _uv_write_cb = (ptr, status) => UvWriteCb(ptr, status);

    private IntPtr _bufs;

    private Action<UvWriteReq, int, UvException, object> _callback;
    private object _state;
    private const int BUFFER_COUNT = 4;

    private LibuvAwaitable<UvWriteReq> _awaitable = new LibuvAwaitable<UvWriteReq>();
    private List<GCHandle> _pins = new List<GCHandle>(BUFFER_COUNT + 1);
    private List<MemoryHandle> _handles = new List<MemoryHandle>(BUFFER_COUNT + 1);

    public UvWriteReq(ILibuvTrace logger) : base(logger)
    {
    }

    public override void Init(LibuvThread thread)
    {
      DangerousInit(thread.Loop);

      base.Init(thread);
    }

    public void DangerousInit(UvLoopHandle loop)
    {
      var requestSize = loop.Libuv.req_size(LibuvFunctions.RequestType.WRITE);
      var bufferSize = Marshal.SizeOf<LibuvFunctions.uv_buf_t>() * BUFFER_COUNT;
      CreateMemory(
        loop.Libuv,
        loop.ThreadId,
        requestSize + bufferSize);
      _bufs = handle + requestSize;
    }

    public LibuvAwaitable<UvWriteReq> WriteAsync(UvStreamHandle handle, in ReadOnlySequence<byte> buffer)
    {
      Write(handle, buffer, LibuvAwaitable<UvWriteReq>.Callback, _awaitable);
      return _awaitable;
    }

    public LibuvAwaitable<UvWriteReq> WriteAsync(UvStreamHandle handle, ArraySegment<ArraySegment<byte>> bufs)
    {
      Write(handle, bufs, LibuvAwaitable<UvWriteReq>.Callback, _awaitable);
      return _awaitable;
    }

    private unsafe void Write(
      UvStreamHandle handle,
      in ReadOnlySequence<byte> buffer,
      Action<UvWriteReq, int, UvException, object> callback,
      object state)
    {
      try
      {
        var nBuffers = 0;
        if (buffer.IsSingleSegment)
          nBuffers = 1;
        else
          foreach (var _ in buffer)
            nBuffers++;

        var pBuffers = (LibuvFunctions.uv_buf_t*)_bufs;
        if (pBuffers == null)
          throw new NullReferenceException();

        if (nBuffers > BUFFER_COUNT)
        {
          // create and pin buffer array when it's larger than the pre-allocated one
          var bufArray = new LibuvFunctions.uv_buf_t[nBuffers];
          var gcHandle = GCHandle.Alloc(bufArray, GCHandleType.Pinned);
          _pins.Add(gcHandle);
          pBuffers = (LibuvFunctions.uv_buf_t*)gcHandle.AddrOfPinnedObject();
          if (pBuffers == null)
            throw new NullReferenceException();
        }

        if (nBuffers == 1)
        {
          var memory = buffer.First;
          var memoryHandle = memory.Pin();
          _handles.Add(memoryHandle);

          // Fast path for single buffer
          pBuffers[0] = Libuv.buf_init(
            (IntPtr)memoryHandle.Pointer,
            memory.Length);
        }
        else
        {
          var index = 0;
          foreach (var memory in buffer)
          {
            // This won't actually pin the buffer since we're already using pinned memory
            var memoryHandle = memory.Pin();
            _handles.Add(memoryHandle);

            // create and pin each segment being written
            pBuffers[index] = Libuv.buf_init(
              (IntPtr)memoryHandle.Pointer,
              memory.Length);
            index++;
          }
        }

        _callback = callback;
        _state = state;
        _uv.write(this, handle, pBuffers, nBuffers, _uv_write_cb);
      }
      catch
      {
        _callback = null;
        _state = null;
        UnpinGcHandles();
        throw;
      }
    }

    private void Write(
      UvStreamHandle handle,
      ArraySegment<ArraySegment<byte>> bufs,
      Action<UvWriteReq, int, UvException, object> callback,
      object state)
    {
      WriteArraySegmentInternal(handle, bufs, null, callback, state);
    }

    public void Write2(
      UvStreamHandle handle,
      ArraySegment<ArraySegment<byte>> bufs,
      UvStreamHandle sendHandle,
      Action<UvWriteReq, int, UvException, object> callback,
      object state)
    {
      WriteArraySegmentInternal(handle, bufs, sendHandle, callback, state);
    }

    private unsafe void WriteArraySegmentInternal(
      UvStreamHandle handle,
      ArraySegment<ArraySegment<byte>> bufs,
      UvStreamHandle sendHandle,
      Action<UvWriteReq, int, UvException, object> callback, object state
    )
    {
      try
      {
        var pBuffers = (LibuvFunctions.uv_buf_t*)_bufs;
        if (pBuffers == null)
          throw new NullReferenceException();

        var nBuffers = bufs.Count;
        if (nBuffers > BUFFER_COUNT)
        {
          // create and pin buffer array when it's larger than the pre-allocated one
          var bufArray = new LibuvFunctions.uv_buf_t[nBuffers];
          var gcHandle = GCHandle.Alloc(bufArray, GCHandleType.Pinned);
          _pins.Add(gcHandle);
          pBuffers = (LibuvFunctions.uv_buf_t*)gcHandle.AddrOfPinnedObject();
          if (pBuffers == null)
            throw new NullReferenceException();
        }

        for (var index = 0; index < nBuffers; index++)
        {
          // create and pin each segment being written
          var buf = bufs.Array?[bufs.Offset + index] ?? throw new Exception("buffs.Array is null");

          var gcHandle = GCHandle.Alloc(buf.Array, GCHandleType.Pinned);
          _pins.Add(gcHandle);
          pBuffers[index] = Libuv.buf_init(
            gcHandle.AddrOfPinnedObject() + buf.Offset,
            buf.Count);
        }

        _callback = callback;
        _state = state;

        if (sendHandle == null)
          _uv.write(this, handle, pBuffers, nBuffers, _uv_write_cb);
        else
          _uv.write2(this, handle, pBuffers, nBuffers, sendHandle, _uv_write_cb);
      }
      catch
      {
        _callback = null;
        _state = null;
        UnpinGcHandles();
        throw;
      }
    }

    // Safe handle has instance method called Unpin
    // so using UnpinGcHandles to avoid conflict
    private void UnpinGcHandles()
    {
      var pinList = _pins;
      var count = pinList.Count;
      for (var i = 0; i < count; i++) pinList[i].Free();
      pinList.Clear();

      var handleList = _handles;
      count = handleList.Count;
      for (var i = 0; i < count; i++) handleList[i].Dispose();
      handleList.Clear();
    }

    private static void UvWriteCb(IntPtr ptr, int status)
    {
      var req = FromIntPtr<UvWriteReq>(ptr);
      req.UnpinGcHandles();

      var callback = req._callback;
      req._callback = null;

      var state = req._state;
      req._state = null;

      UvException error = null;
      if (status < 0) req.Libuv.Check(status, out error);

      try
      {
        callback(req, status, error, state);
      }
      catch (Exception ex)
      {
        req._log.LogError(0, ex, "UvWriteCb");
        throw;
      }
    }
  }
}