util: add an LRU cache dict

In certain cases we would like to have a cache of the last N results of a
given computation, where N is small. This will be used in an upcoming patch to
increase the size of the manifest cache from 1 to 3.

tests/test-lrucachedict.py

@@ -0,0 +1,35 b''
+from mercurial import util
+
+def printifpresent(d, xs):
+    for x in xs:
+        present = x in d
+        print "'%s' in d: %s" % (x, present)
+        if present:
+            print "d['%s']: %s" % (x, d[x])
+
+def test_lrucachedict():
+    d = util.lrucachedict(4)
+    d['a'] = 'va'
+    d['b'] = 'vb'
+    d['c'] = 'vc'
+    d['d'] = 'vd'
+
+    # all of these should be present
+    printifpresent(d, ['a', 'b', 'c', 'd'])
+
+    # 'a' should be dropped because it was least recently used
+    d['e'] = 've'
+    printifpresent(d, ['a', 'b', 'c', 'd', 'e'])
+
+    # touch entries in some order (get or set).
+    d['e']
+    d['c'] = 'vc2'
+    d['d']
+    d['b'] = 'vb2'
+
+    # 'e' should be dropped now
+    d['f'] = 'vf'
+    printifpresent(d, ['b', 'c', 'd', 'e', 'f'])
+
+if __name__ == '__main__':
+    test_lrucachedict()

tests/test-lrucachedict.py.out

@@ -0,0 +1,26 b''
+'a' in d: True
+d['a']: va
+'b' in d: True
+d['b']: vb
+'c' in d: True
+d['c']: vc
+'d' in d: True
+d['d']: vd
+'a' in d: False
+'b' in d: True
+d['b']: vb
+'c' in d: True
+d['c']: vc
+'d' in d: True
+d['d']: vd
+'e' in d: True
+d['e']: ve
+'b' in d: True
+d['b']: vb2
+'c' in d: True
+d['c']: vc2
+'d' in d: True
+d['d']: vd
+'e' in d: False
+'f' in d: True
+d['f']: vf

mercurial/util.py

@@ -211,6 +211,31 b' except AttributeError:'
                     del self[i]
                     break
 
+class lrucachedict(object):
+    '''cache most recent gets from or sets to this dictionary'''
+    def __init__(self, maxsize):
+        self._cache = {}
+        self._maxsize = maxsize
+        self._order = deque()
+
+    def __getitem__(self, key):
+        value = self._cache[key]
+        self._order.remove(key)
+        self._order.append(key)
+        return value
+
+    def __setitem__(self, key, value):
+        if key not in self._cache:
+            if len(self._cache) >= self._maxsize:
+                del self._cache[self._order.popleft()]
+        else:
+            self._order.remove(key)
+        self._cache[key] = value
+        self._order.append(key)
+
+    def __contains__(self, key):
+        return key in self._cache
+
 def lrucachefunc(func):
     '''cache most recent results of function calls'''
     cache = {}