private IValueCodec WriteMetadataCore <T>(Stream stream, Schema schema, int col, string kind, ColumnType type, out CompressionKind compressionKind)
{
_host.Assert(typeof(T) == type.RawType);
IValueCodec generalCodec;
if (!_factory.TryGetCodec(type, out generalCodec))
{
compressionKind = default(CompressionKind);
return(null);
}
IValueCodec <T> codec = (IValueCodec <T>)generalCodec;
T value = default(T);
schema[col].Metadata.GetValue(kind, ref value);
// Metadatas will often be pretty small, so that compression makes no sense.
// We try both a compressed and uncompressed version of metadata and
// opportunistically pick whichever is smallest.
MemoryStream uncompressedMem = _memPool.Get();
using (IValueWriter <T> writer = codec.OpenWriter(uncompressedMem))
{
writer.Write(in value);
writer.Commit();
}
MemoryStream compressedMem = _memPool.Get();
ArraySegment <byte> buffer;
bool tmp = uncompressedMem.TryGetBuffer(out buffer);
_host.Assert(tmp);
using (Stream compressStream = _compression.CompressStream(compressedMem))
compressStream.Write(buffer.Array, buffer.Offset, buffer.Count);
if (uncompressedMem.Length <= compressedMem.Length)
{
// Write uncompressed.
compressionKind = CompressionKind.None;
}
else
{
// Write compressed.
compressionKind = _compression;
tmp = compressedMem.TryGetBuffer(out buffer);
_host.Assert(tmp);
}
stream.Write(buffer.Array, buffer.Offset, buffer.Count);
_memPool.Return(ref uncompressedMem);
_memPool.Return(ref compressedMem);
return(codec);
}
