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class AbstractShrinkList(object):
"""A mixin base class. You should subclass it and add a method
must_keep(). Behaves like a list with the method append(), and
you can read *for reading* the list of items by calling items().
The twist is that occasionally append() will throw away the
items for which must_keep() returns False. (It does so without
changing the order.)
"""
_mixin_ = True
def __init__(self):
self._list = []
self._next_shrink = 16
def append(self, x):
self._do_shrink()
self._list.append(x)
def items(self):
return self._list
def _do_shrink(self):
if len(self._list) >= self._next_shrink:
rest = 0
for x in self._list:
if self.must_keep(x):
self._list[rest] = x
rest += 1
del self._list[rest:]
self._next_shrink = 16 + 2 * rest
def must_keep(self, x):
raise NotImplementedError
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