pmacs3/cache.py

import bisect, time

class CacheDict(dict):
    """This class works like a basic dictionary except that you can put
    constraints on its size. Once that size is reached, the key that was
    inserted or accessed the least recently is removed every time a new key
    is added."""
    def __init__(self, max_size=10000):
        '''build a cache'''
        # once max_size is reached, the oldest cache entry will be pushed out to
        # make room for each new one.
        self.max_size = max_size
        dict.__init__(self)
        # _times_dict will map keys to timestamps.
        self._times_dict = {}
        # _times_list will store (timestamp, key) pairs, sorted oldest first.
        self._times_list = []

    def _timestamp_index(self, key):
        '''find the index in the list of timestamps for the given key'''
        assert key in self
        item = (self._times_dict[key], key)
        # look for the item in the (sorted) list
        i = bisect.bisect_left(self._times_list, item)
        # make sure the index we are returning really is valid
        if item != self._times_list[i]:
            raise LookupError("key %r was not found" % key)
        return i

    def __getitem__(self, key):
        '''implements d[key]'''
        # find the value in the dict
        value = dict.__getitem__(self, key)
        # do this to update the timestamp on this key
        self[key] = value
        return value

    def __setitem__(self, key, value):
        '''implements d[key] = value'''
        # delete any old instance of the key to make way for the new
        if key in self:
            del self._times_list[self._timestamp_index(key)]
        # remove old keys until we have enough space to add this one
        while len(self._times_list) >= self.max_size:
            key = self._times_list[0][1]
            del self[key]
        # add this key, create a timestamp, and update our other data
        # structures accordingly
        t = time.time()
        dict.__setitem__(self, key, value)
        self._times_dict[key] = t
        # make sure we keep the list sorted
        bisect.insort_left(self._times_list, (t, key))

    def __delitem__(self, key):
        '''implements del d[key]'''
        # we need to make sure we delete this key out of all three of our data
        # structures
        del self._times_list[self._timestamp_index(key)]
        del self._times_dict[key]
        dict.__delitem__(self, key)