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156 lines (114 loc) · 5.26 KB
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"""
A fork of Python 3.6's stdlib lru_cache (found in Python's 'cpython/Lib/functools.py')
adapted into a data structure for single threaded uses.
https://github.com/python/cpython/blob/v3.6.12/Lib/functools.py
Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Python Software Foundation;
All Rights Reserved
PYTHON SOFTWARE FOUNDATION LICENSE VERSION 2
--------------------------------------------
1. This LICENSE AGREEMENT is between the Python Software Foundation
("PSF"), and the Individual or Organization ("Licensee") accessing and
otherwise using this software ("Python") in source or binary form and
its associated documentation.
2. Subject to the terms and conditions of this License Agreement, PSF hereby
grants Licensee a nonexclusive, royalty-free, world-wide license to reproduce,
analyze, test, perform and/or display publicly, prepare derivative works,
distribute, and otherwise use Python alone or in any derivative version,
provided, however, that PSF's License Agreement and PSF's notice of copyright,
i.e., "Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Python Software Foundation;
All Rights Reserved" are retained in Python alone or in any derivative version
prepared by Licensee.
3. In the event Licensee prepares a derivative work that is based on
or incorporates Python or any part thereof, and wants to make
the derivative work available to others as provided herein, then
Licensee hereby agrees to include in any such work a brief summary of
the changes made to Python.
4. PSF is making Python available to Licensee on an "AS IS"
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IMPLIED. BY WAY OF EXAMPLE, BUT NOT LIMITATION, PSF MAKES NO AND
DISCLAIMS ANY REPRESENTATION OR WARRANTY OF MERCHANTABILITY OR FITNESS
FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF PYTHON WILL NOT
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5. PSF SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF PYTHON
FOR ANY INCIDENTAL, SPECIAL, OR CONSEQUENTIAL DAMAGES OR LOSS AS
A RESULT OF MODIFYING, DISTRIBUTING, OR OTHERWISE USING PYTHON,
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agrees to be bound by the terms and conditions of this License
Agreement.
"""
SENTINEL = object()
# aliases to the entries in a node
PREV = 0
NEXT = 1
KEY = 2
VALUE = 3
class LRUCache(object):
def __init__(self, max_size):
assert max_size > 0
self.max_size = max_size
self.full = False
self.cache = {}
# root of the circularly linked list to keep track of
# the least recently used key
self.root = [] # type: ignore
# the node looks like [PREV, NEXT, KEY, VALUE]
self.root[:] = [self.root, self.root, None, None]
self.hits = self.misses = 0
def set(self, key, value):
link = self.cache.get(key, SENTINEL)
if link is not SENTINEL:
# have to move the node to the front of the linked list
link_prev, link_next, _key, _value = link
# first remove the node from the lsnked list
link_prev[NEXT] = link_next
link_next[PREV] = link_prev
# insert the node between the root and the last
last = self.root[PREV]
last[NEXT] = self.root[PREV] = link
link[PREV] = last
link[NEXT] = self.root
# update the value
link[VALUE] = value
elif self.full:
# reuse the root node, so update its key/value
old_root = self.root
old_root[KEY] = key
old_root[VALUE] = value
self.root = old_root[NEXT]
old_key = self.root[KEY]
self.root[KEY] = self.root[VALUE] = None
del self.cache[old_key]
self.cache[key] = old_root
else:
# insert new node after last
last = self.root[PREV]
link = [last, self.root, key, value]
last[NEXT] = self.root[PREV] = self.cache[key] = link
self.full = len(self.cache) >= self.max_size
def get(self, key, default=None):
link = self.cache.get(key, SENTINEL)
if link is SENTINEL:
self.misses += 1
return default
# have to move the node to the front of the linked list
link_prev, link_next, _key, _value = link
# first remove the node from the lsnked list
link_prev[NEXT] = link_next
link_next[PREV] = link_prev
# insert the node between the root and the last
last = self.root[PREV]
last[NEXT] = self.root[PREV] = link
link[PREV] = last
link[NEXT] = self.root
self.hits += 1
return link[VALUE]