Created
October 11, 2016 23:53
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LRU cache from hettinger for p27
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""" | |
LRU Cache | |
Recipe created by Raymond Hettinger | |
http://code.activestate.com/recipes/578078-py26-and-py30-backport-of-python-33s-lru-cache/ | |
TODO: refactor out xpt.adt.cachedict | |
""" | |
from collections import namedtuple | |
from functools import update_wrapper | |
from threading import RLock | |
_CacheInfo = namedtuple("CacheInfo", ["hits", "misses", "maxsize", "currsize"]) | |
class _HashedSeq(list): | |
__slots__ = 'hashvalue' | |
def __init__(self, tup, hash=hash): | |
self[:] = tup | |
self.hashvalue = hash(tup) | |
def __hash__(self): | |
return self.hashvalue | |
def _make_key(args, kwds, typed, | |
kwd_mark=(object(),), | |
fasttypes={int, str, frozenset, type(None)}, | |
sorted=sorted, tuple=tuple, type=type, len=len): | |
'Make a cache key from optionally typed positional and keyword arguments' | |
key = args | |
if kwds: | |
sorted_items = sorted(kwds.items()) | |
key += kwd_mark | |
for item in sorted_items: | |
key += item | |
if typed: | |
key += tuple(type(v) for v in args) | |
if kwds: | |
key += tuple(type(v) for k, v in sorted_items) | |
elif len(key) == 1 and type(key[0]) in fasttypes: | |
return key[0] | |
return _HashedSeq(key) | |
def lru_cache(maxsize=100, typed=False): | |
"""Least-recently-used cache decorator. | |
If *maxsize* is set to None, the LRU features are disabled and the cache | |
can grow without bound. | |
If *typed* is True, arguments of different types will be cached separately. | |
For example, f(3.0) and f(3) will be treated as distinct calls with | |
distinct results. | |
Arguments to the cached function must be hashable. | |
View the cache statistics named tuple (hits, misses, maxsize, currsize) with | |
f.cache_info(). Clear the cache and statistics with f.cache_clear(). | |
Access the underlying function with f.__wrapped__. | |
See: http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used | |
""" | |
# Users should only access the lru_cache through its public API: | |
# cache_info, cache_clear, and f.__wrapped__ | |
# The internals of the lru_cache are encapsulated for thread safety and | |
# to allow the implementation to change (including a possible C version). | |
def decorating_function(user_function): | |
cache = dict() | |
stats = [0, 0] # make statistics updateable non-locally | |
HITS, MISSES = 0, 1 # names for the stats fields | |
make_key = _make_key | |
cache_get = cache.get # bound method to lookup key or return None | |
_len = len # localize the global len() function | |
lock = RLock() # because linkedlist updates aren't threadsafe | |
root = [] # root of the circular doubly linked list | |
root[:] = [root, root, None, None] # initialize by pointing to self | |
nonlocal_root = [root] # make updateable non-locally | |
PREV, NEXT, KEY, RESULT = 0, 1, 2, 3 # names for the link fields | |
if maxsize == 0: | |
def wrapper(*args, **kwds): | |
# no caching, just do a statistics update after a successful call | |
result = user_function(*args, **kwds) | |
stats[MISSES] += 1 | |
return result | |
elif maxsize is None: | |
def wrapper(*args, **kwds): | |
# simple caching without ordering or size limit | |
key = make_key(args, kwds, typed) | |
result = cache_get(key, root) # root used here as a unique not-found sentinel | |
if result is not root: | |
stats[HITS] += 1 | |
return result | |
result = user_function(*args, **kwds) | |
cache[key] = result | |
stats[MISSES] += 1 | |
return result | |
else: | |
def wrapper(*args, **kwds): | |
# size limited caching that tracks accesses by recency | |
key = make_key(args, kwds, typed) if kwds or typed else args | |
with lock: | |
link = cache_get(key) | |
if link is not None: | |
# record recent use of the key by moving it to the front of the list | |
root, = nonlocal_root | |
link_prev, link_next, key, result = link | |
link_prev[NEXT] = link_next | |
link_next[PREV] = link_prev | |
last = root[PREV] | |
last[NEXT] = root[PREV] = link | |
link[PREV] = last | |
link[NEXT] = root | |
stats[HITS] += 1 | |
return result | |
result = user_function(*args, **kwds) | |
with lock: | |
root, = nonlocal_root | |
if key in cache: | |
# getting here means that this same key was added to the | |
# cache while the lock was released. since the link | |
# update is already done, we need only return the | |
# computed result and update the count of misses. | |
pass | |
elif _len(cache) >= maxsize: | |
# use the old root to store the new key and result | |
oldroot = root | |
oldroot[KEY] = key | |
oldroot[RESULT] = result | |
# empty the oldest link and make it the new root | |
root = nonlocal_root[0] = oldroot[NEXT] | |
oldkey = root[KEY] | |
oldvalue = root[RESULT] | |
root[KEY] = root[RESULT] = None | |
# now update the cache dictionary for the new links | |
del cache[oldkey] | |
cache[key] = oldroot | |
else: | |
# put result in a new link at the front of the list | |
last = root[PREV] | |
link = [last, root, key, result] | |
last[NEXT] = root[PREV] = cache[key] = link | |
stats[MISSES] += 1 | |
return result | |
def cache_info(): | |
"""Report cache statistics""" | |
with lock: | |
return _CacheInfo(stats[HITS], stats[MISSES], maxsize, len(cache)) | |
def cache_clear(): | |
"""Clear the cache and cache statistics""" | |
with lock: | |
cache.clear() | |
root = nonlocal_root[0] | |
root[:] = [root, root, None, None] | |
stats[:] = [0, 0] | |
wrapper.__wrapped__ = user_function | |
wrapper.cache_info = cache_info | |
wrapper.cache_clear = cache_clear | |
return update_wrapper(wrapper, user_function) | |
return decorating_function |
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