def retimestamp_tags_inplace(f, fu):
flv = FLV(f)
offset = None
for tag in flv.iter_tags():
if offset is None and is_nonheader_media(tag):
offset = tag.timestamp
log.debug("Determined the offset to be %d", offset)
# optimise for offset == 0, which in case of inplace updating is a noop
if offset is not None and offset != 0:
fu.seek(tag.offset + 4, os.SEEK_SET)
fu.write(make_si32_extended(tag.timestamp - offset))
