mirror of
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105 lines
4.2 KiB
Python
105 lines
4.2 KiB
Python
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"""
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LRU cache implementation for Python 2.7
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Ported from http://code.activestate.com/recipes/578078/ and simplified for our
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use (only support maxsize > 0 and positional arguments).
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"""
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from collections import namedtuple
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from functools import update_wrapper
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from threading import RLock
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_CacheInfo = namedtuple("CacheInfo", ["hits", "misses", "maxsize", "currsize"])
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def lru_cache(maxsize=100):
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"""Least-recently-used cache decorator.
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Arguments to the cached function must be hashable.
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See: http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used
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"""
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def decorating_function(user_function):
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cache = dict()
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stats = [0, 0] # make statistics updateable non-locally
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HITS, MISSES = 0, 1 # names for the stats fields
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cache_get = cache.get # bound method to lookup key or return None
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_len = len # localize the global len() function
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lock = RLock() # linkedlist updates aren't threadsafe
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root = [] # root of the circular doubly linked list
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root[:] = [root, root, None, None] # initialize by pointing to self
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nonlocal_root = [root] # make updateable non-locally
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PREV, NEXT, KEY, RESULT = 0, 1, 2, 3 # names for the link fields
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assert maxsize and maxsize > 0, "maxsize %s not supported" % maxsize
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def wrapper(*args):
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# size limited caching that tracks accesses by recency
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key = args
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with lock:
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link = cache_get(key)
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if link is not None:
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# record recent use of the key by moving it to the
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# front of the list
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root, = nonlocal_root
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link_prev, link_next, key, result = link
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link_prev[NEXT] = link_next
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link_next[PREV] = link_prev
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last = root[PREV]
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last[NEXT] = root[PREV] = link
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link[PREV] = last
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link[NEXT] = root
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stats[HITS] += 1
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return result
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result = user_function(*args)
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with lock:
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root, = nonlocal_root
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if key in cache:
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# getting here means that this same key was added to the
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# cache while the lock was released. since the link
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# update is already done, we need only return the
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# computed result and update the count of misses.
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pass
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elif _len(cache) >= maxsize:
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# use the old root to store the new key and result
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oldroot = root
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oldroot[KEY] = key
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oldroot[RESULT] = result
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# empty the oldest link and make it the new root
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root = nonlocal_root[0] = oldroot[NEXT]
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oldkey = root[KEY]
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# oldvalue = root[RESULT]
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root[KEY] = root[RESULT] = None
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# now update the cache dictionary for the new links
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del cache[oldkey]
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cache[key] = oldroot
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else:
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# put result in a new link at the front of the list
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last = root[PREV]
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link = [last, root, key, result]
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last[NEXT] = root[PREV] = cache[key] = link
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stats[MISSES] += 1
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return result
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def cache_info():
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"""Report cache statistics"""
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with lock:
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return _CacheInfo(stats[HITS], stats[MISSES], maxsize, len(cache))
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def cache_clear():
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"""Clear the cache and cache statistics"""
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with lock:
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cache.clear()
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root = nonlocal_root[0]
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root[:] = [root, root, None, None]
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stats[:] = [0, 0]
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wrapper.__wrapped__ = user_function
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wrapper.cache_info = cache_info
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wrapper.cache_clear = cache_clear
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return update_wrapper(wrapper, user_function)
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return decorating_function
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