diff --git a/synapse/util/lrucache.py b/synapse/util/lrucache.py
new file mode 100644
index 0000000000..a45c673d32
--- /dev/null
+++ b/synapse/util/lrucache.py
@@ -0,0 +1,110 @@
+# -*- coding: utf-8 -*-
+# Copyright 2015 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+class LruCache(object):
+ """Least-recently-used cache."""
+ # TODO(mjark) Add hit/miss counters
+ # TODO(mjark) Add mutex for linked list for thread safety.
+ def __init__(self, max_size):
+ cache = {}
+ list_root = []
+ list_root[:] = [list_root, list_root, None, None]
+
+ PREV, NEXT, KEY, VALUE = 0, 1, 2, 3
+
+ def add_node(key, value):
+ prev_node = list_root
+ next_node = prev_node[NEXT]
+ node = [prev_node, next_node, key, value]
+ prev_node[NEXT] = node
+ next_node[PREV] = node
+ cache[key] = node
+
+ def move_node_to_front(node):
+ prev_node = node[PREV]
+ next_node = node[NEXT]
+ prev_node[NEXT] = next_node
+ next_node[PREV] = prev_node
+ prev_node = list_root
+ next_node = prev_node[NEXT]
+ node[PREV] = prev_node
+ node[NEXT] = next_node
+ prev_node[NEXT] = node
+ next_node[PREV] = node
+
+ def delete_node(node):
+ prev_node = node[PREV]
+ next_node = node[NEXT]
+ prev_node[NEXT] = next_node
+ next_node[PREV] = prev_node
+ cache.pop(node[KEY], None)
+
+ def cache_get(key, default=None):
+ node = cache.get(key, None)
+ if node is not None:
+ move_node_to_front(node)
+ return node[VALUE]
+ else:
+ return default
+
+ def cache_set(key, value):
+ node = cache.get(key, None)
+ if node is not None:
+ move_node_to_front(node)
+ node[VALUE] = value
+ else:
+ add_node(key, value)
+ if len(cache) > max_size:
+ delete_node(list_root[PREV])
+
+ def cache_set_default(key, value):
+ node = cache.get(key, None)
+ if node is not None:
+ return node[VALUE]
+ else:
+ add_node(key, value)
+ if len(cache) > max_size:
+ delete_node(list_root[PREV])
+ return value
+
+ def cache_pop(key, default=None):
+ node = cache.get(key, None)
+ if node:
+ delete_node(node)
+ return node[VALUE]
+ else:
+ return default
+
+ self.sentinel = object()
+ self.get = cache_get
+ self.set = cache_set
+ self.setdefault = cache_set_default
+ self.pop = cache_pop
+
+ def __getitem__(self, key):
+ result = self.get(key, self.sentinel)
+ if result is self.sentinel:
+ raise KeyError()
+ else:
+ return result
+
+ def __setitem__(self, key, value):
+ self.set(key, value)
+
+ def __delitem__(self, key, value):
+ result = self.pop(key, self.sentinel)
+ if result is self.sentinel:
+ raise KeyError()
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