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Fogus August 13, 2013
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; Copyright (c) Rich Hickey and Michael Fogus. All rights reserved.
; The use and distribution terms for this software are covered by the
; Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
; 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]))

;; Plugging Interface

(deftype PluggableMemoization [f cache]
  cache/CacheProtocol
  (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)))
  Object
  (toString [_] (str cache)))


(defn ^:private d-lay [fun]
  (let [memory (atom {})]
    (reify
      clojure.lang.IDeref
      (deref [this]
        (if-let [e (find @memory fun)]
          (val e)
          (let [ret (fun)]
            (swap! memory assoc fun ret)
            ret))))))


;; # 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."
  (clojure.core.cache/through
    (fn [f a] (d-lay #(f a)))
    #(clojure.core/apply f %)
    cache
    item))

(def ^{:private true
       :doc "Returns a function's cache identity."}
  cache-id #(::cache (meta %)))


;; # Public Utilities API

(defn snapshot
  "Returns a snapshot of a core.memo-placed memoization cache. By snapshot
you can infer that what you get is only the cache contents at a
moment in time."
  [memoized-fn]
  (when-let [cache (::cache (meta memoized-fn))]
    (into {}
          (for [[k v] (.cache @cache)]
            [(vec k) @v]))))

(defn memoized?
  "Returns true if a function has an core.memo-placed cache, false otherwise."
  [f]
  (boolean (::cache (meta f))))

(defn memo-clear!
  "Reaches into an core.memo-memoized function and clears the cache. This is a
destructive operation and should be used with care.

When the second argument is a vector of input arguments, clears cache only
for argument vector.

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
now."
  ([f]
     (when-let [cache (cache-id f)]
       (swap! cache (constantly (clojure.core.cache/seed @cache {})))))
  ([f args]
     (when-let [cache (cache-id f)]
       (swap! cache (constantly (clojure.core.cache/evict @cache args))))))

(defn memo-swap!
  "Takes a core.memo-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
                                          clojure.lang.IDeref
                                          (deref [this] v))])))))))

(defn memo-unwrap
  [f]
  (::original (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))]
       (with-meta
        (fn [& args]
          (let [cs (swap! cache through* f args)
                val (clojure.core.cache/lookup cs args)]
            ;; The assumption here is that if what we got
            ;; from the cache was non-nil, then we can dereference
            ;; it. core.memo currently wraps all of its values in
            ;; a `delay`.
            (and val @val)))
        {::cache cache
         ::original 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.memo manipulable memoization utilities. As a nice bonus,
you can use `memo` in place of `memoize` without any additional
changes.

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 `:clojure.core.memoize/cache` key
on the memoized function's metadata. However, it is advised to
use the core.memo primitives instead as implementation details may
change over time."
  ([f] (memo f {}))
  ([f seed]
     (build-memoizer
       #(PluggableMemoization. %1 (cache/basic-cache-factory %2))
       f
       seed)))

;; ## Utilities

(defn ^{:private true} !! [c]
  (println "WARNING - Deprecated construction method for"
           c
           "cache; prefered way is:"
           (str "(clojure.core.memoize/" c " function <base> <:" c "/threshold num>)")))

(defmacro ^{:private true} def-deprecated [nom ds & arities]
  `(defn ~(symbol (str "memo-" (name nom))) ~ds
      ~@(for [[args body] arities]
          (list args `(!! (quote ~nom)) body))))

(defmacro ^{:private true} massert [condition msg]
  `(when-not ~condition
      (throw (new AssertionError (str "clojure.core.memoize/" ~msg "\n" (pr-str '~condition))))))

(defmacro ^{:private true} check-args [nom f base key threshold]
  (when *assert*
    (let [good-key (keyword nom "threshold")
          key-error `(str "Incorrect threshold key " ~key)
          fun-error `(str ~nom " expects a function as its first argument; given " ~f)
          thresh-error `(str ~nom " expects an integer for its " ~good-key " argument; given " ~threshold)]
      `(do (massert (= ~key ~good-key) ~key-error)
           (massert (some #{clojure.lang.IFn
                            clojure.lang.AFn
                            java.lang.Runnable
                            java.util.concurrent.Callable}
                          (ancestors (class ~f)))
                    ~fun-error)
           (massert (number? ~threshold) ~thresh-error)))))

;; ## Main API functions

;; ### FIFO

(def-deprecated fifo
  "DEPRECATED: Please use clojure.core.memoize/fifo instead."
  ([f] (memo-fifo f 32 {}))
  ([f limit] (memo-fifo f limit {}))
  ([f limit base]
     (build-memoizer
       #(PluggableMemoization. %1 (cache/fifo-cache-factory %3 :threshold %2))
       f
       limit
       base)))

(defn 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:

(require '[clojure.core.memoize :as memo])

(def id (memo/fifo identity :fifo/threshold 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 is:

(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] (fifo f {} :fifo/threshold 32))
  ([f base] (fifo f base :fifo/threshold 32))
  ([f tkey threshold] (fifo f {} tkey threshold))
  ([f base key threshold]
    (check-args "fifo" f base key threshold)

    (build-memoizer
       #(PluggableMemoization. %1 (cache/fifo-cache-factory %3 :threshold %2))
       f
       threshold
       base)))

;; ### LRU

(def-deprecated lru
  "DEPRECATED: Please use clojure.core.memoize/lru instead."
  ([f] (memo-lru f 32))
  ([f limit] (memo-lru f limit {}))
  ([f limit base]
     (build-memoizer
       #(PluggableMemoization. %1 (cache/lru-cache-factory %3 :threshold %2))
       f
       limit
       base)))

(defn 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:

(require '[clojure.core.memoize :as memo])

(def id (memo/lru identity :lru/threshold 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] (lru f {} :lru/threshold 32))
  ([f base] (lru f base :lru/threshold 32))
  ([f tkey threshold] (lru f {} tkey threshold))
  ([f base key threshold]
    (check-args "lru" f base key threshold)

    (build-memoizer
       #(PluggableMemoization. %1 (cache/lru-cache-factory %3 :threshold %2))
       f
       threshold
       base)))

;; ### TTL

(def-deprecated ttl
  "DEPRECATED: Please use clojure.core.memoize/ttl instead."
  ([f] (memo-ttl f 3000 {}))
  ([f limit] (memo-ttl f limit {}))
  ([f limit base]
     (build-memoizer
       #(PluggableMemoization. %1 (cache/ttl-cache-factory %3 :ttl %2))
       f
       limit
       {})))

(defn ttl
  "Unlike many of the other core.memo memoization functions,
`memo-ttl`'s cache policy is time-based rather than algortihmic
or explicit. When memoizing a function using `memo-ttl` you
should provide a **T**ime **T**o **L**ive parameter in
milliseconds.

(require '[clojure.core.memoize :as memo])

(def id (memo/ttl identity :ttl/threshold 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] (ttl f {} :ttl/threshold 32))
  ([f base] (ttl f base :ttl/threshold 32))
  ([f tkey threshold] (ttl f {} tkey threshold))
  ([f base key threshold]
    (check-args "ttl" f base key threshold)

    (build-memoizer
       #(PluggableMemoization. %1 (cache/ttl-cache-factory %3 :ttl %2))
       f
       threshold
       base)))

;; ### LU

(def-deprecated lu
  "DEPRECATED: Please use clojure.core.memoize/lu instead."
  ([f] (memo-lu f 32))
  ([f limit] (memo-lu f limit {}))
  ([f limit base]
     (build-memoizer
       #(PluggableMemoization. %1 (cache/lu-cache-factory %3 :threshold %2))
       f
       limit
       base)))

(defn lu
  "Similar to the implementation of memo-lru, except that this
function removes all cache values whose usage value is
smallest:

(require '[clojure.core.memoize :as memo])

(def id (memo/lu identity :lu/threshold 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] (lu f {} :lu/threshold 32))
  ([f base] (lu f base :lu/threshold 32))
  ([f tkey threshold] (lu f {} tkey threshold))
  ([f base key threshold]
    (check-args "lu" f base key threshold)

    (build-memoizer
       #(PluggableMemoization. %1 (cache/lu-cache-factory %3 :threshold %2))
       f
       threshold
       base)))
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