def perform_carryforward():
"""Perform as many carryforwards as needed.
Walk back through weeks until the week is non-persistent or carryforward is true. Then perform
carry-forward on the oldest week without carryforward set. Repeat until we're carrying forward
last week until this week.
"""
offset = 1
nonpersistentweeks = 0
while True:
weekname = naming.relative_name(offset)
week = reader.DiaryReader().read_file(weekname)
if not week.persistent:
nonpersistentweeks += 1
if week.has_carryforward() or nonpersistentweeks == MAXIMUM_HOLIDAY_WEEKS:
break
offset += 1
while offset > 1:
offset -= 1
fromname = naming.relative_name(offset)
toname = naming.relative_name(offset - 1)
fromweek = reader.DiaryReader().read_file(fromname)
toweek = reader.DiaryReader().read_file(toname)
_carryforward(fromweek, toweek)
writer.DiaryWriter().write_file(fromname, fromweek)
writer.DiaryWriter().write_file(toname, toweek)
return
