// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Collections.Immutable;
using System.Diagnostics;
using System.IO;
using System.Reflection.Metadata;
using Microsoft.CodeAnalysis.PooledObjects;
using Microsoft.CodeAnalysis;

namespace Roslyn.Utilities
{
    /// <summary>
    /// A write-only memory stream backed by a <see cref="BlobBuilder"/>.
    /// </summary>
    internal sealed class BlobBuildingStream : Stream
    {
        private static readonly ObjectPool<BlobBuildingStream> s_pool = new ObjectPool<BlobBuildingStream>(() => new BlobBuildingStream());
        private readonly BlobBuilder _builder;

        /// <summary>
        /// The chunk size to be used by the underlying BlobBuilder.
        /// </summary>
        /// <remarks>
        /// The current single use case for this type is embedded sources in PDBs.
        ///
        /// 32 KB is:
        ///
        /// * Large enough to handle 99.6% all VB and C# files in Roslyn and CoreFX 
        ///   without allocating additional chunks.
        ///
        /// * Small enough to avoid the large object heap.
        ///
        /// * Large enough to handle the files in the 0.4% case without allocating tons
        ///   of small chunks. Very large source files are often generated in build
        ///   (e.g. Syntax.xml.Generated.vb is 390KB compressed!) and those are actually
        ///   attractive candidates for embedding, so we don't want to discount the large
        ///   case too heavily.)
        ///
        /// * We pool the outer BlobBuildingStream but only retain the first allocated chunk.
        /// </remarks>
        public const int ChunkSize = 32 * 1024;

        public override bool CanWrite => true;
        public override bool CanRead => false;
        public override bool CanSeek => false;
        public override long Length => _builder.Count;

        public static BlobBuildingStream GetInstance()
        {
            return s_pool.Allocate();
        }

        private BlobBuildingStream()
        {
            // NOTE: We pool the wrapping BlobBuildingStream, but not individual chunks.
            // The first chunk will be reused, but any further chunks will be freed when we're done building blob.
            _builder = new BlobBuilder(ChunkSize);
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            _builder.WriteBytes(buffer, offset, count);
        }

        public override void WriteByte(byte value)
        {
            _builder.WriteByte(value);
        }

        public void WriteInt32(int value)
        {
            _builder.WriteInt32(value);
        }

        public Blob ReserveBytes(int byteCount)
        {
            return _builder.ReserveBytes(byteCount);
        }

        public ImmutableArray<byte> ToImmutableArray()
        {
            return _builder.ToImmutableArray();
        }

        public void Free()
        {
            _builder.Clear();  // frees all but first chunk
            s_pool.Free(this); // return first chunk to pool
        }

        public override void Flush()
        {
        }

        protected override void Dispose(bool disposing)
        {
            Debug.Assert(disposing);
            Free();
        }

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

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

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

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

    }
}