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; Copyright (c) Rich Hickey. All rights reserved.
; The use and distribution terms for this software are covered by the
; Eclipse Public License 1.0 (
; which can be found in the file epl-v10.html at the root of this distribution.
; By using this software in any fashion, you are agreeing to be bound by
; the terms of this license.
; You must not remove this notice, or any other, from this software.
(ns clojure.core.memoize
"core.memoize is a memoization library offering functionality above Clojure's core `memoize`
function in the following ways:
**Pluggable memoization**
core.memoize allows for different back-end cache implmentations to be used as appropriate without
changing the memoization modus operandi.
**Manipulable memoization**
Because core.memoize allows you to access a function's memoization store, you do interesting things like
clear it, modify it, and save it for later.
{:author "fogus"}
(:require [clojure.core.cache :as cache])
(:import [clojure.core.cache CacheProtocol]))
;; Plugging Interface
(deftype PluggableMemoization [f cache]
(has? [_ item]
(clojure.core.cache/has? cache item))
(hit [_ item]
(PluggableMemoization. f (clojure.core.cache/hit cache item)))
(miss [_ item result]
(PluggableMemoization. f (clojure.core.cache/miss cache item result)))
(evict [_ key]
(PluggableMemoization. f (clojure.core.cache/evict cache key)))
(lookup [_ item]
(clojure.core.cache/lookup cache item))
(seed [_ base]
(PluggableMemoization. f (clojure.core.cache/seed cache base)))
(toString [_] (str cache)))
;; # Auxilliary functions
(defn through* [cache f item]
"The basic hit/miss logic for the cache system based on `core.cache/through`.
Clojure delays are used to hold the cache value."
#(delay (%1 %2))
#(clojure.core/apply f %)
(def ^{:private true
:doc "Returns a function's cache identity."}
cache-id #(:unk (meta %)))
;; # Public Utilities API
(defn snapshot
"Returns a snapshot of an unk-placed memoization cache. By snapshot
you can infer that what you get is only the cache contents at a
moment in time."
(when-let [cache (:unk (meta memoized-fn))]
(into {}
(for [[k v] (.cache @cache)]
[(vec k) @v]))))
(defn memoized?
"Returns true if a function has an unk-placed cache, false otherwise."
(boolean (:unk (meta f))))
(defn memo-clear!
"Reaches into an unk-memoized function and clears the cache. This is a
destructive operation and should be used with care.
Keep in mind that depending on what other threads or doing, an
immediate call to `snapshot` may not yield an empty cache. That's
cool though, we've learned to deal with that stuff in Clojure by
(when-let [cache (cache-id f)]
(swap! cache (constantly (clojure.core.cache/seed @cache {})))))
(defn memo-swap!
"Takes an unk-populated function and a map and replaces the memoization cache
with the supplied map. This is potentially some serious voodoo,
since you can effectively change the semantics of a function on the fly.
(def id (memo identity))
(memo-swap! id '{[13] :omg})
(id 13)
;=> :omg
With great power comes ... yadda yadda yadda."
[f base]
(when-let [cache (cache-id f)]
(swap! cache
(constantly (clojure.core.cache/seed @cache
(into {}
(for [[k v] base]
[k (reify
(deref [this] v))])))))))
(defn memo-unwrap
(:unk-orig (meta f)))
;; # Public memoization API
(defn build-memoizer
"Builds a function that given a function, returns a pluggable memoized
version of it. `build-memoizer` Takes a cache factory function, a function
to memoize, and the arguments to the factory. At least one of those
functions should be the function to be memoized."
([cache-factory f & args]
(let [cache (atom (apply cache-factory f args))]
(fn [& args]
(let [cs (swap! cache through* f args)]
@(clojure.core.cache/lookup cs args)))
{:unk cache
:unk-orig f}))))
(defn memo
"Used as a more flexible alternative to Clojure's core `memoization`
function. Memoized functions built using `memo` will respond to
the core unk manipulable memoization utilities. As a nice bonus,
you can use `memo` in place of `memoize` without any additional
The default way to use this function is to simply apply a function
that will be memoized. Additionally, you may also supply a map
of the form `'{[42] 42, [108] 108}` where keys are a vector
mapping expected argument values to arity positions. The map values
are the return values of the memoized function.
You can access the memoization cache directly via the `:unk` key
on the memoized function's metadata. However, it is advised to
use the unk primitives instead as implementation details may
change over time."
([f] (memo f {}))
([f seed]
#(PluggableMemoization. %1 (cache/basic-cache-factory %2))
(defn memo-fifo
"Works the same as the basic memoization function (i.e. `memo` and `core.memoize` except
when a given threshold is breached. Observe the following:
(def id (memo-fifo identity 2))
(id 42)
(id 43)
(snapshot id)
;=> {[42] 42, [43] 43}
As you see, the limit of `2` has not been breached yet, but if you call again with another
value, then it will:
(id 44)
(snapshot id)
;=> {[44] 44, [43] 43}
That is, the oldest entry `42` is pushed out of the memoization cache. This is the standard
**F**irst **I**n **F**irst **O**ut behavior."
([f] (memo-fifo f 32 {}))
([f limit] (memo-fifo f limit {}))
([f limit base]
#(PluggableMemoization. %1 (cache/fifo-cache-factory %3 :threshold %2))
(defn memo-lru
"Works the same as the basic memoization function (i.e. `memo` and `core.memoize` except
when a given threshold is breached. Observe the following:
(def id (memo-lru identity 2))
(id 42)
(id 43)
(snapshot id)
;=> {[42] 42, [43] 43}
At this point the cache has not yet crossed the set threshold of `2`, but if you execute
yet another call the story will change:
(id 44)
(snapshot id)
;=> {[44] 44, [43] 43}
At this point the operation of the LRU cache looks exactly the same at the FIFO cache.
However, the difference becomes apparent on further use:
(id 43)
(id 0)
(snapshot id)
;=> {[0] 0, [43] 43}
As you see, once again calling `id` with the argument `43` will expose the LRU nature
of the underlying cache. That is, when the threshold is passed, the cache will expel
the **L**east **R**ecently **U**sed element in favor of the new."
([f] (memo-lru f 32))
([f limit] (memo-lru f limit {}))
([f limit base]
#(PluggableMemoization. %1 (cache/lru-cache-factory %3 :threshold %2))
(defn memo-ttl
"Unlike many of the other unk memoization functions, `memo-ttl`'s cache policy is time-based
rather than algortihmic or explicit. When memoizing a function using `memo-ttl` you should
should provide a **T**ime **T**o **L**ive parameter in milliseconds.
(def id (memo-ttl identity 5000))
(id 42)
(snapshot id)
;=> {[42] 42}
... wait 5 seconds ...
(id 43)
(snapshot id)
;=> {[43] 43}
The expired cache entries will be removed on each cache miss."
([f] (memo-ttl f 3000 {}))
([f limit] (memo-ttl f limit {}))
([f limit base]
#(PluggableMemoization. %1 (cache/ttl-cache-factory %3 :ttl %2))
(defn memo-lu
"Similar to the implementation of memo-lru, except that this function removes all cache
values whose usage value is smallest.
(def id (memo-lu identity 3))
(id 42)
(id 42)
(id 43)
(id 44)
(snapshot id)
;=> {[44] 44, [42] 42}
The **L**east **U**sed values are cleared on cache misses."
([f] (memo-lu f 32))
([f limit] (memo-lu f limit {}))
([f limit base]
#(PluggableMemoization. %1 (cache/lu-cache-factory %3 :threshold %2))
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