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Python 最近最少使用算法

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Python 最近最少使用算法


# lrucache.py -- a simple LRU (Least-Recently-Used) cache class 

# Copyright 2004 Evan Prodromou <evan@bad.dynu.ca> 
# Licensed under the Academic Free License 2.1 

# Licensed for ftputil under the revised BSD license 
# with permission by the author, Evan Prodromou. Many 
# thanks, Evan! :-) 
# 
# The original file is available at 
# http://pypi.python.org/pypi/lrucache/0.2 . 

# arch-tag: LRU cache main module 

"""a simple LRU (Least-Recently-Used) cache module 

This module provides very simple LRU (Least-Recently-Used) cache 
functionality. 

An *in-memory cache* is useful for storing the results of an 
'expe\nsive' process (one that takes a lot of time or resources) for 
later re-use. Typical examples are accessing data from the filesystem, 
a database, or a network location. If you know you'll need to re-read 
the data again, it can help to keep it in a cache. 

You *can* use a Python dictionary as a cache for some purposes. 
However, if the results you're caching are large, or you have a lot of 
possible results, this can be impractical memory-wise. 

An *LRU cache*, on the other hand, only keeps _some_ of the results in 
memory, which keeps you from overusing resources. The cache is bounded 
by a maximum size; if you try to add more values to the cache, it will 
automatically discard the values that you haven't read or written to 
in the longest time. In other words, the least-recently-used items are 
discarded. [1]_ 

.. [1]: 'Discarded' here means 'removed from the cache'. 

""" 

from __future__ import generators 
import time 
from heapq import heappush, heappop, heapify 

# the suffix after the hyphen denotes modifications by the 
#  ftputil project with respect to the original version 
__version__ = "0.2-1" 
__all__ = ['CacheKeyError', 'LRUCache', 'DEFAULT_SIZE'] 
__docformat__ = 'reStructuredText en' 

DEFAULT_SIZE = 16 
"""Default size of a new LRUCache object, if no 'size' argument is given.""" 

class CacheKeyError(KeyError): 
    """Error raised when cache requests fail 

    When a cache record is accessed which no longer exists (or never did), 
    this error is raised. To avoid it, you may want to check for the existence 
    of a cache record before reading or deleting it.""" 
    pass 

class LRUCache(object): 
    """Least-Recently-Used (LRU) cache. 

    Instances of this class provide a least-recently-used (LRU) cache. They 
    emulate a Python mapping type. You can use an LRU cache more or less like 
    a Python dictionary, with the exception that objects you put into the 
    cache may be discarded before you take them out. 

    Some example usage:: 

    cache = LRUCache(32) # new cache 
    cache['foo'] = get_file_contents('foo') # or whatever 

    if 'foo' in cache: # if it's still in cache... 
        # use cached version 
        contents = cache['foo'] 
    else: 
        # recalculate 
        contents = get_file_contents('foo') 
        # store in cache for next time 
        cache['foo'] = contents 

    print cache.size # Maximum size 

    print len(cache) # 0 <= len(cache) <= cache.size 

    cache.size = 10 # Auto-shrink on size assignment 

    for i in range(50): # note: larger than cache size 
        cache[i] = i 

    if 0 not in cache: print 'Zero was discarded.' 

    if 42 in cache: 
        del cache[42] # Manual deletion 

    for j in cache:   # iterate (in LRU order) 
        print j, cache[j] # iterator produces keys, not values 
    """ 

    class __Node(object): 
        """Record of a cached value. Not for public consumption.""" 

        def __init__(self, key, obj, timestamp, sort_key): 
            object.__init__(self) 
            self.key = key 
            self.obj = obj 
            self.atime = timestamp 
            self.mtime = self.atime 
            self._sort_key = sort_key 

        def __cmp__(self, other): 
            return cmp(self._sort_key, other._sort_key) 

        def __repr__(self): 
            return "<%s %s => %s (%s)>" % \ 
                   (self.__class__, self.key, self.obj, \ 
                    time.asctime(time.localtime(self.atime))) 

    def __init__(self, size=DEFAULT_SIZE): 
        # Check arguments 
        if size <= 0: 
            raise ValueError, size 
        elif type(size) is not type(0): 
            raise TypeError, size 
        object.__init__(self) 
        self.__heap = [] 
        self.__dict = {} 
        """Maximum size of the cache. 
        If more than 'size' elements are added to the cache, 
        the least-recently-used ones will be discarded.""" 
        self.size = size 
        self.__counter = 0 

    def _sort_key(self): 
        """Return a new integer value upon every call. 
        
        Cache nodes need a monotonically increasing time indicator. 
        time.time() and time.clock() don't guarantee this in a 
        platform-independent way. 
        """ 
        self.__counter += 1 
        return self.__counter 

    def __len__(self): 
        return len(self.__heap) 

    def __contains__(self, key): 
        return self.__dict.has_key(key) 

    def __setitem__(self, key, obj): 
        if self.__dict.has_key(key): 
            node = self.__dict[key] 
            # update node object in-place 
            node.obj = obj 
            node.atime = time.time() 
            node.mtime = node.atime 
            node._sort_key = self._sort_key() 
            heapify(self.__heap) 
        else: 
            # size may have been reset, so we loop 
            while len(self.__heap) >= self.size: 
                lru = heappop(self.__heap) 
                del self.__dict[lru.key] 
            node = self.__Node(key, obj, time.time(), self._sort_key()) 
            self.__dict[key] = node 
            heappush(self.__heap, node) 

    def __getitem__(self, key): 
        if not self.__dict.has_key(key): 
            raise CacheKeyError(key) 
        else: 
            node = self.__dict[key] 
            # update node object in-place 
            node.atime = time.time() 
            node._sort_key = self._sort_key() 
            heapify(self.__heap) 
            return node.obj 

    def __delitem__(self, key): 
        if not self.__dict.has_key(key): 
            raise CacheKeyError(key) 
        else: 
            node = self.__dict[key] 
            del self.__dict[key] 
            self.__heap.remove(node) 
            heapify(self.__heap) 
            return node.obj 

    def __iter__(self): 
        copy = self.__heap[:] 
        while len(copy) > 0: 
            node = heappop(copy) 
            yield node.key 
        raise StopIteration 

    def __setattr__(self, name, value): 
        object.__setattr__(self, name, value) 
        # automagically shrink heap on resize 
        if name == 'size': 
            while len(self.__heap) > value: 
                lru = heappop(self.__heap) 
                del self.__dict[lru.key] 

    def __repr__(self): 
        return "<%s (%d elements)>" % (str(self.__class__), len(self.__heap)) 

    def mtime(self, key): 
        """Return the last modification time for the cache record with key. 
        May be useful for cache instances where the stored values can get 
        'stale', such as caching file or network resource contents.""" 
        if not self.__dict.has_key(key): 
            raise CacheKeyError(key) 
        else: 
            node = self.__dict[key] 
            return node.mtime 

if __name__ == "__main__": 
    cache = LRUCache(25) 
    print cache 
    for i in range(50): 
        cache[i] = str(i) 
    print cache 
    if 46 in cache: 
        print "46 in cache" 
        del cache[46] 
    print cache 
    cache.size = 10 
    print cache 
    cache[46] = '46' 
    print cache 
    print len(cache) 
    for c in cache: 
        print c 
    print cache 
    print cache.mtime(46) 
    for c in cache: 
        print c 


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