Return a |_Chunk| subclass instance appropriate to *chunk_type* parsed
    from *stream_rdr* at *offset*.

def _ChunkFactory(chunk_type, stream_rdr, offset):
    """
    Return a |_Chunk| subclass instance appropriate to *chunk_type* parsed
    from *stream_rdr* at *offset*.
    """
    chunk_cls_map = {
        PNG_CHUNK_TYPE.IHDR: _IHDRChunk,
        PNG_CHUNK_TYPE.pHYs: _pHYsChunk,
    }
    chunk_cls = chunk_cls_map.get(chunk_type, _Chunk)
    return chunk_cls.from_offset(chunk_type, stream_rdr, offset)