// 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.IO;
using System.Threading;
using System.Threading.Tasks;

namespace System.IO.Pipelines
{
    // Write only stream implementation for efficiently writing bytes from the request body
    internal class PipeWriterStream : Stream
    {
        private long _length;
        private readonly PipeWriter _pipeWriter;

        public PipeWriterStream(PipeWriter pipeWriter)
        {
            _pipeWriter = pipeWriter;
        }

        public override bool CanRead => false;

        public override bool CanSeek => false;

        public override bool CanWrite => true;

        public override long Length => _length;

        public override long Position { get => throw new NotSupportedException(); set => throw new NotSupportedException(); }

        public override void Flush()
        {
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            throw new NotSupportedException();
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            throw new NotSupportedException();
        }

        public override void SetLength(long value)
        {
            throw new NotSupportedException();
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            _pipeWriter.Write(new ReadOnlySpan<byte>(buffer, offset, count));
            _length += count;
        }

        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        {
            return WriteCoreAsync(buffer.AsMemory(offset, count), cancellationToken).AsTask();
        }

#if NETCOREAPP2_1
        public override ValueTask WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default)
        {
            return WriteCoreAsync(source, cancellationToken);
        }
#endif

        private ValueTask WriteCoreAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default)
        {
            if (cancellationToken.IsCancellationRequested)
            {
                return new ValueTask(Task.FromCanceled(cancellationToken));
            }

            _length += source.Length;
            var task = _pipeWriter.WriteAsync(source);
            if (task.IsCompletedSuccessfully)
            {
                // Cancellation can be triggered by PipeWriter.CancelPendingFlush
                if (task.Result.IsCanceled)
                {
                    throw new OperationCanceledException();
                }
            }
            else if (!task.IsCompletedSuccessfully)
            {
                return WriteSlowAsync(task);
            }

            return default;

            async ValueTask WriteSlowAsync(ValueTask<FlushResult> flushTask)
            {
                var flushResult = await flushTask;

                // Cancellation can be triggered by PipeWriter.CancelPendingFlush
                if (flushResult.IsCanceled)
                {
                    throw new OperationCanceledException();
                }
            }
        }

        public void Reset()
        {
            _length = 0;
        }
    }
}