csharp/src/Google.Protobuf/WriteBufferHelper.cs

#region Copyright notice and license
// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#endregion

using System;
using System.Buffers;
using System.Runtime.CompilerServices;
using System.Security;

namespace Google.Protobuf
{
    /// <summary>
    /// Abstraction for writing to a steam / IBufferWriter
    /// </summary>
    [SecuritySafeCritical]
    internal struct WriteBufferHelper
    {
        private IBufferWriter<byte> bufferWriter;
        private CodedOutputStream codedOutputStream;

        public CodedOutputStream CodedOutputStream => codedOutputStream;

        /// <summary>
        /// Initialize an instance with a coded output stream.
        /// This approach is faster than using a constructor because the instance to initialize is passed by reference
        /// and we can write directly into it without copying.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void Initialize(CodedOutputStream codedOutputStream, out WriteBufferHelper instance)
        {
            instance.bufferWriter = null;
            instance.codedOutputStream = codedOutputStream;
        }

        /// <summary>
        /// Initialize an instance with a buffer writer.
        /// This approach is faster than using a constructor because the instance to initialize is passed by reference
        /// and we can write directly into it without copying.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void Initialize(IBufferWriter<byte> bufferWriter, out WriteBufferHelper instance, out Span<byte> buffer)
        {
            instance.bufferWriter = bufferWriter;
            instance.codedOutputStream = null;
            buffer = default;  // TODO: initialize the initial buffer so that the first write is not via slowpath.
        }

        /// <summary>
        /// Initialize an instance with a buffer represented by a single span (i.e. buffer cannot be refreshed)
        /// This approach is faster than using a constructor because the instance to initialize is passed by reference
        /// and we can write directly into it without copying.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void InitializeNonRefreshable(out WriteBufferHelper instance)
        {
            instance.bufferWriter = null;
            instance.codedOutputStream = null;
        }

        /// <summary>
        /// Verifies that SpaceLeft returns zero.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void CheckNoSpaceLeft(ref WriterInternalState state)
        {
            if (GetSpaceLeft(ref state) != 0)
            {
                throw new InvalidOperationException("Did not write as much data as expected.");
            }
        }

        /// <summary>
        /// If writing to a flat array, returns the space left in the array. Otherwise,
        /// throws an InvalidOperationException.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int GetSpaceLeft(ref WriterInternalState state)
        {
            if (state.writeBufferHelper.codedOutputStream?.InternalOutputStream == null && state.writeBufferHelper.bufferWriter == null)
            {
                return state.limit - state.position;
            }
            else
            {
                throw new InvalidOperationException(
                    "SpaceLeft can only be called on CodedOutputStreams that are " +
                        "writing to a flat array or when writing to a single span.");
            }
        }

        [MethodImpl(MethodImplOptions.NoInlining)]
        public static void RefreshBuffer(ref Span<byte> buffer, ref WriterInternalState state)
        {
            if (state.writeBufferHelper.codedOutputStream?.InternalOutputStream != null)
            {
                // because we're using coded output stream, we know that "buffer" and codedOutputStream.InternalBuffer are identical.
                state.writeBufferHelper.codedOutputStream.InternalOutputStream.Write(state.writeBufferHelper.codedOutputStream.InternalBuffer, 0, state.position);
                // reset position, limit stays the same because we are reusing the codedOutputStream's internal buffer.
                state.position = 0;
            }
            else if (state.writeBufferHelper.bufferWriter != null)
            {
                // commit the bytes and get a new buffer to write to.
                state.writeBufferHelper.bufferWriter.Advance(state.position);
                state.position = 0;
                buffer = state.writeBufferHelper.bufferWriter.GetSpan();
                state.limit = buffer.Length;
            }
            else
            {
                // We're writing to a single buffer.
                throw new CodedOutputStream.OutOfSpaceException();
            }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void Flush(ref Span<byte> buffer, ref WriterInternalState state)
        {
            if (state.writeBufferHelper.codedOutputStream?.InternalOutputStream != null)
            {
                // because we're using coded output stream, we know that "buffer" and codedOutputStream.InternalBuffer are identical.
                state.writeBufferHelper.codedOutputStream.InternalOutputStream.Write(state.writeBufferHelper.codedOutputStream.InternalBuffer, 0, state.position);
                state.position = 0;
            }
            else if (state.writeBufferHelper.bufferWriter != null)
            {
                // calling Advance invalidates the current buffer and we must not continue writing to it,
                // so we set the current buffer to point to an empty Span. If any subsequent writes happen,
                // the first subsequent write will trigger refresing of the buffer.
                state.writeBufferHelper.bufferWriter.Advance(state.position);
                state.position = 0;
                state.limit = 0;
                buffer = default;  // invalidate the current buffer
            }
        }
    }
}