61 lines
2.3 KiB
Python
61 lines
2.3 KiB
Python
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import bisect, time
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class CacheDict(dict):
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"""This class works like a basic dictionary except that you can put
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constraints on its size. Once that size is reached, the key that was
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inserted or accessed the least recently is removed every time a new key
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is added."""
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def __init__(self, max_size=1000000):
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'''CacheDict(max_size=1000000): build a cache'''
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# once max_size is reached, the oldest cache entry will be
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# pushed out to make room for each new one
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self.max_size = max_size
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dict.__init__(self)
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# _times_dict will map keys to timestamps
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self._times_dict = {}
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# _times_list will store (timestamp, key) pairs in sorted
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# order (oldest first)
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self._times_list = []
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def timestamp(self, key):
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'''find the timestamp for key'''
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assert key in self
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# construct a (timestamp, key) item
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item = (self._times_dict[key], key)
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# look for the item in the (sorted) list
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i = bisect.bisect_left(self._times_list, item)
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# make sure the index we are returning really is valid
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if item != self._times_list[i]:
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raise LookupError
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return i
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def __getitem__(self, key):
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# find the value in the dict
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value = dict.__getitem__(self, key)
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# do this to update the timestamp on this key
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self[key] = value
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return value
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def __setitem__(self, key, value):
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# delete any old instance of the key to make way for the new
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if key in self:
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del self._times_list[self.timestamp(key)]
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# remove old keys until we have enough space to add this one
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while len(self._times_list) >= self.max_size:
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key = self._times_list[0][1]
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del self[key]
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# add this key, create a timestamp, and update our other data
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# structures accordingly
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t = time.time()
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dict.__setitem__(self, key, value)
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self._times_dict[key] = t
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# make sure we keep the list sorted
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bisect.insort_left(self._times_list, (t, key))
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def __delitem__(self, key):
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# we need to make sure we delete this key out of all three of
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# our data structures
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del self._times_list[self.timestamp(key)]
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del self._times_dict[key]
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dict.__delitem__(self, key)
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