def calculate_etl(self, downloaded, total, start=None):
"""Calculates the estimated time left, based on how long it took us
to reach "downloaded" and how many messages we have left.
If no start time is given, the time will simply by estimated by how
many chunks are left, which will NOT work if what is being downloaded is media"""
left = total - downloaded
if not start:
# We add chunk size - 1 because division will truncate the decimal places,
# so for example, if we had a chunk size of 8:
# 7 messages + 7 = 14 -> 14 // 8 = 1 chunk download required
# 8 messages + 7 = 15 -> 15 // 8 = 1 chunk download required
# 9 messages + 7 = 16 -> 16 // 8 = 2 chunks download required
#
# Clearly, both 7 and 8 fit in one chunk, but 9 doesn't.
chunks_left = (left + self.download_chunk_size - 1) // self.download_chunk_size
etl = chunks_left * self.download_delay
else:
if downloaded:
delta_time = (datetime.now() - start).total_seconds() / downloaded
etl = left * delta_time
else:
etl = 0
return timedelta(seconds=round(etl, 1))
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