public override void FinalizeHeader()
{
var blockWriter = new SpanWriter(Buffer);
blockWriter.Position = LastPosition;
// Write up the block info - write what we can write - the entry count
blockWriter.Position = 0;
blockWriter.WriteUInt16((ushort)Type);
blockWriter.WriteUInt16((ushort)((EntryCount * 2) + 1));
// Write end offset terminator - skip to last of block toc and write it behind it
blockWriter.Position = BlockSize - (EntryCount * TocEntrySize);
blockWriter.Position -= 2;
blockWriter.WriteInt16((short)LastPosition);
}