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# Cache implementaion with a Least Recently Used (LRU) replacement policy and
# a basic dictionary interface.
# Copyright (C) 2006, 2009, 2010, 2011 Jay Hutchinson
# This program is free software; you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the Free
# Software Foundation; either version 2 of the License, or (at your option)
# any later version.
# This program is distributed in the hope that it will be useful, but WITHOUT
# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
# FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
# more details.
# You should have received a copy of the GNU General Public License along
# with this program; if not, write to the Free Software Foundation, Inc., 51
# Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
# The cache is implemented using a combination of a python dictionary (hash
# table) and a circular doubly linked list. Items in the cache are stored in
# nodes. These nodes make up the linked list. The list is used to efficiently
# maintain the order that the items have been used in. The front or head of
# the list contains the most recently used item, the tail of the list
# contains the least recently used item. When an item is used it can easily
# (in a constant amount of time) be moved to the front of the list, thus
# updating its position in the ordering. These nodes are also placed in the
# hash table under their associated key. The hash table allows efficient
# lookup of values by key.
# Class for the node objects.
class _dlnode(object):
def __init__(self):
self.empty = True
class lrucache(object):
def __init__(self, size, callback=None):
self.callback = callback
# Create an empty hash table.
self.table = {}
# Initialize the doubly linked list with one empty node. This is an
# invariant. The cache size must always be greater than zero. Each
# node has a 'prev' and 'next' variable to hold the node that comes
# before it and after it respectively. Initially the two variables
# each point to the head node itself, creating a circular doubly
# linked list of size one. Then the size() method is used to adjust
# the list to the desired size.
self.head = _dlnode()
self.head.next = self.head
self.head.prev = self.head
self.listSize = 1
# Adjust the size
self.size(size)
def __len__(self):
return len(self.table)
def clear(self):
for node in self.dli():
node.empty = True
node.key = None
node.value = None
self.table.clear()
def __contains__(self, key):
return key in self.table
# Looks up a value in the cache without affecting cache order.
def peek(self, key):
# Look up the node
node = self.table[key]
return node.value
def __getitem__(self, key):
# Look up the node
node = self.table[key]
# Update the list ordering. Move this node so that is directly
# proceeds the head node. Then set the 'head' variable to it. This
# makes it the new head of the list.
self.mtf(node)
self.head = node
# Return the value.
return node.value
def get(self, key, default=None):
"""Get an item - return default (None) if not present"""
try:
return self[key]
except KeyError:
return default
def __setitem__(self, key, value):
# First, see if any value is stored under 'key' in the cache already.
# If so we are going to replace that value with the new one.
if key in self.table:
# Lookup the node
node = self.table[key]
# Replace the value.
node.value = value
# Update the list ordering.
self.mtf(node)
self.head = node
return
# Ok, no value is currently stored under 'key' in the cache. We need
# to choose a node to place the new item in. There are two cases. If
# the cache is full some item will have to be pushed out of the
# cache. We want to choose the node with the least recently used
# item. This is the node at the tail of the list. If the cache is not
# full we want to choose a node that is empty. Because of the way the
# list is managed, the empty nodes are always together at the tail
# end of the list. Thus, in either case, by chooseing the node at the
# tail of the list our conditions are satisfied.
# Since the list is circular, the tail node directly preceeds the
# 'head' node.
node = self.head.prev
# If the node already contains something we need to remove the old
# key from the dictionary.
if not node.empty:
if self.callback is not None:
self.callback(node.key, node.value)
del self.table[node.key]
# Place the new key and value in the node
node.empty = False
node.key = key
node.value = value
# Add the node to the dictionary under the new key.
self.table[key] = node
# We need to move the node to the head of the list. The node is the
# tail node, so it directly preceeds the head node due to the list
# being circular. Therefore, the ordering is already correct, we just
# need to adjust the 'head' variable.
self.head = node
def __delitem__(self, key):
# Lookup the node, then remove it from the hash table.
node = self.table[key]
del self.table[key]
node.empty = True
# Not strictly necessary.
node.key = None
node.value = None
# Because this node is now empty we want to reuse it before any
# non-empty node. To do that we want to move it to the tail of the
# list. We move it so that it directly preceeds the 'head' node. This
# makes it the tail node. The 'head' is then adjusted. This
# adjustment ensures correctness even for the case where the 'node'
# is the 'head' node.
self.mtf(node)
self.head = node.next
def __iter__(self):
# Return an iterator that returns the keys in the cache in order from
# the most recently to least recently used. Does not modify the cache
# order.
for node in self.dli():
yield node.key
def items(self):
# Return an iterator that returns the (key, value) pairs in the cache
# in order from the most recently to least recently used. Does not
# modify the cache order.
for node in self.dli():
yield (node.key, node.value)
def keys(self):
# Return an iterator that returns the keys in the cache in order from
# the most recently to least recently used. Does not modify the cache
# order.
for node in self.dli():
yield node.key
def values(self):
# Return an iterator that returns the values in the cache in order
# from the most recently to least recently used. Does not modify the
# cache order.
for node in self.dli():
yield node.value
def size(self, size=None):
if size is not None:
assert size > 0
if size > self.listSize:
self.addTailNode(size - self.listSize)
elif size < self.listSize:
self.removeTailNode(self.listSize - size)
return self.listSize
# Increases the size of the cache by inserting n empty nodes at the tail
# of the list.
def addTailNode(self, n):
for i in range(n):
node = _dlnode()
node.next = self.head
node.prev = self.head.prev
self.head.prev.next = node
self.head.prev = node
self.listSize += n
# Decreases the size of the list by removing n nodes from the tail of the
# list.
def removeTailNode(self, n):
assert self.listSize > n
for i in range(n):
node = self.head.prev
if not node.empty:
if self.callback is not None:
self.callback(node.key, node.value)
del self.table[node.key]
# Splice the tail node out of the list
self.head.prev = node.prev
node.prev.next = self.head
# The next four lines are not strictly necessary.
node.prev = None
node.next = None
node.key = None
node.value = None
self.listSize -= n
# This method adjusts the ordering of the doubly linked list so that
# 'node' directly precedes the 'head' node. Because of the order of
# operations, if 'node' already directly precedes the 'head' node or if
# 'node' is the 'head' node the order of the list will be unchanged.
def mtf(self, node):
node.prev.next = node.next
node.next.prev = node.prev
node.prev = self.head.prev
node.next = self.head.prev.next
node.next.prev = node
node.prev.next = node
# This method returns an iterator that iterates over the non-empty nodes
# in the doubly linked list in order from the most recently to the least
# recently used.
def dli(self):
node = self.head
for i in range(len(self.table)):
yield node
node = node.next
class WriteThroughCacheManager(object):
def __init__(self, store, size):
self.store = store
self.cache = lrucache(size)
def __len__(self):
return len(self.store)
# Returns/sets the size of the managed cache.
def size(self, size=None):
return self.cache.size(size)
def clear(self):
self.cache.clear()
self.store.clear()
def __contains__(self, key):
# Check the cache first. If it is there we can return quickly.
if key in self.cache:
return True
# Not in the cache. Might be in the underlying store.
if key in self.store:
return True
return False
def __getitem__(self, key):
# First we try the cache. If successful we just return the value. If
# not we catch KeyError and ignore it since that just means the key
# was not in the cache.
try:
return self.cache[key]
except KeyError:
pass
# It wasn't in the cache. Look it up in the store, add the entry to
# the cache, and return the value.
value = self.store[key]
self.cache[key] = value
return value
def get(self, key, default=None):
"""Get an item - return default (None) if not present"""
try:
return self[key]
except KeyError:
return default
def __setitem__(self, key, value):
# Add the key/value pair to the cache and store.
self.cache[key] = value
self.store[key] = value
def __delitem__(self, key):
# Write-through behavior cache and store should be consistent. Delete
# it from the store.
del self.store[key]
try:
# Ok, delete from the store was successful. It might also be in
# the cache, try and delete it. If not we catch the KeyError and
# ignore it.
del self.cache[key]
except KeyError:
pass
def __iter__(self):
return self.keys()
def keys(self):
return self.store.keys()
def values(self):
return self.store.values()
def items(self):
return self.store.items()
class WriteBackCacheManager(object):
def __init__(self, store, size):
self.store = store
# Create a set to hold the dirty keys.
self.dirty = set()
# Define a callback function to be called by the cache when a
# key/value pair is about to be ejected. This callback will check to
# see if the key is in the dirty set. If so, then it will update the
# store object and remove the key from the dirty set.
def callback(key, value):
if key in self.dirty:
self.store[key] = value
self.dirty.remove(key)
# Create a cache and give it the callback function.
self.cache = lrucache(size, callback)
# Returns/sets the size of the managed cache.
def size(self, size=None):
return self.cache.size(size)
def clear(self):
self.cache.clear()
self.dirty.clear()
self.store.clear()
def __contains__(self, key):
# Check the cache first, since if it is there we can return quickly.
if key in self.cache:
return True
# Not in the cache. Might be in the underlying store.
if key in self.store:
return True
return False
def __getitem__(self, key):
# First we try the cache. If successful we just return the value. If
# not we catch KeyError and ignore it since that just means the key
# was not in the cache.
try:
return self.cache[key]
except KeyError:
pass
# It wasn't in the cache. Look it up in the store, add the entry to
# the cache, and return the value.
value = self.store[key]
self.cache[key] = value
return value
def get(self, key, default=None):
"""Get an item - return default (None) if not present"""
try:
return self[key]
except KeyError:
return default
def __setitem__(self, key, value):
# Add the key/value pair to the cache.
self.cache[key] = value
self.dirty.add(key)
def __delitem__(self, key):
found = False
try:
del self.cache[key]
found = True
self.dirty.remove(key)
except KeyError:
pass
try:
del self.store[key]
found = True
except KeyError:
pass
if not found: # If not found in cache or store, raise error.
raise KeyError
def __iter__(self):
return self.keys()
def keys(self):
for key in self.store.keys():
if key not in self.dirty:
yield key
for key in self.dirty:
yield key
def values(self):
for key, value in self.items():
yield value
def items(self):
for key, value in self.store.items():
if key not in self.dirty:
yield (key, value)
for key in self.dirty:
value = self.cache.peek(key)
yield (key, value)
def sync(self):
# For each dirty key, peek at its value in the cache and update the
# store. Doesn't change the cache's order.
for key in self.dirty:
self.store[key] = self.cache.peek(key)
# There are no dirty keys now.
self.dirty.clear()
def flush(self):
self.sync()
self.cache.clear()
def __enter__(self):
return self
def __exit__(self, exc_type, exc_val, exc_tb):
self.sync()
return False
class FunctionCacheManager(object):
def __init__(self, func, size):
self.func = func
self.cache = lrucache(size)
def size(self, size=None):
return self.cache.size(size)
def clear(self):
self.cache.clear()
def __call__(self, *args, **kwargs):
kwtuple = tuple((key, kwargs[key]) for key in sorted(kwargs.keys()))
key = (args, kwtuple)
try:
return self.cache[key]
except KeyError:
pass
value = self.func(*args, **kwargs)
self.cache[key] = value
return value
def lruwrap(store, size, writeback=False):
if writeback:
return WriteBackCacheManager(store, size)
else:
return WriteThroughCacheManager(store, size)
import functools
class lrudecorator(object):
def __init__(self, size):
self.cache = lrucache(size)
def __call__(self, func):
def wrapper(*args, **kwargs):
kwtuple = tuple((key, kwargs[key]) for key in sorted(kwargs.keys()))
key = (args, kwtuple)
try:
return self.cache[key]
except KeyError:
pass
value = func(*args, **kwargs)
self.cache[key] = value
return value
wrapper.cache = self.cache
wrapper.size = self.cache.size
wrapper.clear = self.cache.clear
return functools.update_wrapper(wrapper, func)