/// <summary>
/// A helper method to query and write metadata to the stream.
/// </summary>
/// <param name="writer">A binary writer, which if metadata exists for the
/// indicated column the base stream will be positioned just past the end of
/// the written metadata table of contents, and if metadata does not exist
/// remains unchanged</param>
/// <param name="schema">The schema to query for metadat</param>
/// <param name="col">The column we are attempting to get metadata for</param>
/// <param name="ch">The channel to which we write any diagnostic information</param>
/// <returns>The offset of the metadata table of contents, or 0 if there was
/// no metadata</returns>
private long WriteMetadata(BinaryWriter writer, Schema schema, int col, IChannel ch)
{
_host.AssertValue(writer);
_host.AssertValue(schema);
_host.Assert(0 <= col && col < schema.Count);
int count = 0;
WriteMetadataCoreDelegate del = WriteMetadataCore <int>;
MethodInfo methInfo = del.GetMethodInfo().GetGenericMethodDefinition();
object[] args = new object[] { writer.BaseStream, schema, col, null, null, null };
List <long> offsets = new List <long>();
offsets.Add(writer.BaseStream.Position);
var metadataInfos = new List <Tuple <string, IValueCodec, CompressionKind> >();
var kinds = new HashSet <string>();
// Write all metadata blocks for this column to the file, one after the other, keeping
// track of the location and size of each for when we write the metadata table of contents.
// (To be clear, this specific layout is not required by the format.)
foreach (var metaColumn in schema[col].Metadata.Schema)
{
_host.Check(!string.IsNullOrEmpty(metaColumn.Name), "Metadata with null or empty kind detected, disallowed");
_host.Check(metaColumn.Type != null, "Metadata with null type detected, disallowed");
if (!kinds.Add(metaColumn.Name))
{
throw _host.Except("Metadata with duplicate kind '{0}' encountered, disallowed", metaColumn.Name, schema[col].Name);
}
args[3] = metaColumn.Name;
args[4] = metaColumn.Type;
IValueCodec codec = (IValueCodec)methInfo.MakeGenericMethod(metaColumn.Type.RawType).Invoke(this, args);
if (codec == null)
{
// Nothing was written.
ch.Warning("Could not get codec for type {0}, dropping column '{1}' index {2} metadata kind '{3}'",
metaColumn.Type, schema[col].Name, col, metaColumn.Name);
continue;
}
offsets.Add(writer.BaseStream.Position);
_host.CheckIO(offsets[offsets.Count - 1] > offsets[offsets.Count - 2], "Bad offsets detected during write");
metadataInfos.Add(Tuple.Create(metaColumn.Name, codec, (CompressionKind)args[5]));
count++;
}
if (metadataInfos.Count == 0)
{
_host.CheckIO(writer.BaseStream.Position == offsets[0], "unexpected offset after no writing of metadata");
return(0);
}
// Write the metadata table of contents just past the end of the last metadata block.
// *** Metadata TOC format ***
// LEB128 int: Number of metadata TOC entries
// Metadata TOC entries: As many of these as indicated by the count above
long expectedPosition = offsets[metadataInfos.Count];
writer.WriteLeb128Int((ulong)metadataInfos.Count);
expectedPosition += Utils.Leb128IntLength((ulong)metadataInfos.Count);
for (int i = 0; i < metadataInfos.Count; ++i)
{
// *** Metadata TOC entry format ***
// string: metadata kind
// codec definition: metadata codec
// CompressionKind(byte): block compression strategy
// long: Offset into the stream of the start of the metadata block
// LEB128 int: Byte size of the metadata block in the file
writer.Write(metadataInfos[i].Item1);
int stringLen = Encoding.UTF8.GetByteCount(metadataInfos[i].Item1);
expectedPosition += Utils.Leb128IntLength((ulong)stringLen) + stringLen;
_host.CheckIO(writer.BaseStream.Position == expectedPosition, "unexpected offsets after metadata table of contents kind");
expectedPosition += _factory.WriteCodec(writer.BaseStream, metadataInfos[i].Item2);
_host.CheckIO(writer.BaseStream.Position == expectedPosition, "unexpected offsets after metadata table of contents type description");
writer.Write((byte)metadataInfos[i].Item3);
expectedPosition++;
writer.Write(offsets[i]);
expectedPosition += sizeof(long);
long blockSize = offsets[i + 1] - offsets[i];
writer.WriteLeb128Int((ulong)blockSize);
expectedPosition += Utils.Leb128IntLength((ulong)blockSize);
_host.CheckIO(writer.BaseStream.Position == expectedPosition, "unexpected offsets after metadata table of contents location");
}
_host.Assert(metadataInfos.Count == offsets.Count - 1);
return(offsets[metadataInfos.Count]);
}