summaryrefslogtreecommitdiffstats
path: root/python/pylru/pylru.py
blob: e69cadb76c54cbde19a778abb0d809bde48a879e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556

# 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)