cache.clj

;   Copyright (c) Rich Hickey. All rights reserved.
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;   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
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(ns ^{:doc "A caching library for Clojure."
      :author "Fogus"}
  clojure.core.cache)

;; # Protocols and Types

(defprotocol CacheProtocol
  "This is the protocol describing the basic cache capability."
  (lookup [cache e]
          [cache e not-found]
   "Retrieve the value associated with `e` if it exists, else `nil` in
   the 2-arg case.  Retrieve the value associated with `e` if it exists,
   else `not-found` in the 3-arg case.")
  (has?    [cache e]
   "Checks if the cache contains a value associtaed with `e`")
  (hit     [cache e]
   "Is meant to be called if the cache is determined to contain a value
   associated with `e`")
  (miss    [cache e ret]
   "Is meant to be called if the cache is determined to **not** contain a
   value associated with `e`")
  (evict  [cache e]
   "Removes an entry from the cache")
  (seed    [cache base]
   "Is used to signal that the cache should be created with a seed.
   The contract is that said cache should return an instance of its
   own type."))


(defmacro defcache
  [type-name fields & specifics]
  (let [[base-field & _] fields]
    `(deftype ~type-name [~@fields]
       ~@specifics
     
       clojure.lang.ILookup
       (valAt [this# key#]
              (lookup this# key#))
       (valAt [this# key# not-found#]
              (if (has? this# key#)
                (lookup this# key#)
                not-found#))

       clojure.lang.IPersistentMap
       (assoc [this# k# v#]
         (miss this# k# v#))
       (without [this# k#]
         (evict this# k#))

       clojure.lang.Associative
       (containsKey [this# k#]
         (has? this# k#))
       (entryAt [this# k#]
         (when (has? this# k#)
           (clojure.lang.MapEntry. k# (lookup this# k#))))

       clojure.lang.Counted
       (count [this#]
         (clojure.core/count ~base-field))

       clojure.lang.IPersistentCollection
       (cons [_# elem#]
         (clojure.core/cons ~base-field elem#))
       (empty [this#]
         (seed this# (empty ~base-field)))
       (equiv [_# other#]
         (.equiv ~base-field other#))

       clojure.lang.Seqable
       (seq [_#]
         (seq ~base-field))

       ;; Java interfaces
       java.lang.Iterable
       (iterator [this#] (.iterator ~base-field)))))

(defcache BasicCache [cache]
  CacheProtocol
  (lookup [_ item]
    (get cache item))
  (lookup [_ item not-found]
    (get cache item not-found))
  (has? [_ item]
    (contains? cache item))
  (hit [this item] this)
  (miss [_ item result]
    (BasicCache. (assoc cache item result)))
  (evict [_ key]
    (BasicCache. (dissoc cache key)))
  (seed [_ base]
    (BasicCache. base))
  Object
  (toString [_] (str cache)))

;; FnCache

(defcache FnCache [cache f]
  CacheProtocol
  (lookup [_ item]
    (f (get cache item)))
  (lookup [_ item not-found]
    (let [ret (get cache item not-found)]
      (if (= ret not-found)
        not-found
        (f ret))))
  (has? [_ item]
    (contains? cache item))
  (hit [this item] this)
  (miss [_ item result]
    (BasicCache. (assoc cache item result)))
  (evict [_ key]
    (BasicCache. (dissoc cache key)))
  (seed [_ base]
    (BasicCache. base))
  Object
  (toString [_] (str cache)))

;; # FIFO

(defn- describe-layout [mappy limit]
  (let [q clojure.lang.PersistentQueue/EMPTY 
        ks (keys mappy)
        [dropping keeping] (split-at (- (count ks) limit) ks)]
    {:dropping dropping
     :keeping  keeping
     :queue
     (into q (concat (repeat (- limit (count keeping)) ::free)
                     (take limit keeping)))}))

(defn- dissoc-keys [m ks]
  (if ks
    (recur (dissoc m (first ks)) (next ks))
    m))

(defn- prune-queue [q ks]
  (into clojure.lang.PersistentQueue/EMPTY
        (filter (complement (set ks)) q)))

(defcache FIFOCache [cache q limit]
  CacheProtocol
  (lookup [_ item]
    (get cache item))
  (lookup [_ item not-found]
    (get cache item not-found))
  (has? [_ item]
    (contains? cache item))
  (hit [this item]
    this)
  (miss [_ item result]
    (let [k (peek q)]
      (FIFOCache. (-> cache (dissoc k) (assoc item result))
                  (-> q pop (conj item))
                  limit)))
  (evict [this key]
    (let [v (get cache key ::miss)]
      (if (= v ::miss)
        this
        (FIFOCache. (dissoc cache key)
                    (prune-queue q [key])
                    limit))))
  (seed [_ base]
    (let [{dropping :dropping
           q :queue} (describe-layout base limit)]
      (FIFOCache. (dissoc-keys base dropping)
                  q
                  limit)))
  Object
  (toString [_]
    (str cache \, \space (pr-str q))))


(defcache LRUCache [cache lru tick limit]
  CacheProtocol
  (lookup [_ item]
    (get cache item))
  (lookup [_ item not-found]
    (get cache item not-found))
  (has? [_ item]
    (contains? cache item))
  (hit [_ item]
    (let [tick+ (inc tick)]
      (LRUCache. cache
                 (assoc lru item tick+)
                 tick+
                 limit)))
  (miss [_ item result]
    (let [tick+ (inc tick)]
      (if-let [ks (keys lru)]
        (let [k (apply min-key lru ks)]
          (LRUCache. (-> cache (dissoc k) (assoc item result))  ;; expulsion case
                     (-> lru (dissoc k) (assoc item tick+))
                     tick+
                     limit))
        (LRUCache. (assoc cache item result)  ;; no change case
                   (assoc lru item tick+)
                   tick+
                   limit))))
  (evict [this key]
    (let [v (get cache key ::miss)]
      (if (= v ::miss)
        this
        (LRUCache. (dissoc cache key)
                   (dissoc lru key)
                   tick
                   limit))))
  (seed [_ base]
    (LRUCache. base
               (into {} (for [x (range (- limit) 0)] [x x]))
               0
               limit))
  Object
  (toString [_]
    (str cache \, \space lru \, \space tick \, \space limit)))

(declare key-killer)

(defcache TTLCache [cache ttl limit]
  CacheProtocol
  (lookup [_ item]
    (get cache item))
  (lookup [_ item not-found]
    (get cache item not-found))
  (has? [_ item]
    (when-let [t (get ttl item)]
      (< (- (System/currentTimeMillis)
            t)
         limit)))
  (hit [this item] this)
  (miss [this item result]
    (let [now  (System/currentTimeMillis)
          kill-old (key-killer ttl limit now)]
      (TTLCache. (assoc (kill-old cache) item result)
                 (assoc (kill-old ttl) item now)
                 limit)))
  (seed [_ base]
    (let [now (System/currentTimeMillis)]
      (TTLCache. base
                 (into {} (for [x base] [(key x) now]))
                 limit)))
  (evict [_ key]
    (TTLCache. (dissoc cache key)
               (dissoc ttl key)
               limit))
  Object
  (toString [_]
    (str cache \, \space ttl \, \space limit)))


(defn- key-killer
  [ttl limit now]
  (let [ks (map key (filter #(> (- now (val %)) limit)
                            ttl))]
    #(apply dissoc % ks)))


(defcache LUCache [cache lu limit]
  CacheProtocol
  (lookup [_ item]
    (get cache item))
  (lookup [_ item not-found]
    (get cache item not-found))
  (has? [_ item]
    (contains? cache item))
  (hit [_ item]
    (LUCache. cache (update-in lu [item] inc) limit))
  (miss [_ item result]
    (if-let [ks (keys lu)]
      (let [k (apply min-key lu ks)]
        (LUCache. (-> cache (dissoc k) (assoc item result))  ;; expulsion case
                  (-> lu (dissoc k) (assoc item 0))
                  limit))
      (LUCache. (assoc cache item result)  ;; no change case
                (assoc lu item 0)
                limit)))
  (evict [this key]
    (let [v (get cache key ::miss)]
      (if (= v ::miss)
        this
        (LUCache. (dissoc cache key)
                   (dissoc lu key)
                   limit))))
  (seed [_ base]
    (LUCache. base
              (into {} (for [x (range (- limit) 0)] [x x]))
              limit))
  Object
  (toString [_]
    (str cache \, \space lu \, \space limit)))


;; # LIRS
;; *initial Clojure implementation by Jan Oberhagemann*

;;  A
;;  [LIRS](http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.116.2184)
;;  cache consists of two LRU lists, `S` and `Q`, and keeps more history
;;  than a LRU cache. Every cached item is either a LIR, HIR or
;;  non-resident HIR block. `Q` contains only HIR blocks, `S` contains
;;  LIR, HIR, non-resident HIR blocks. The total cache size is
;;  |`S`|+|`Q`|, |`S`| is typically 99% of the cache size.

;;  * LIR block:
;;    Low Inter-Reference block, a cached item with a short interval
;;    between accesses. A block `x`, `x` ∈ `S` ∧ `x` ∉ `Q` ∧ `x` ∈
;;    `cache`, is a LIR block.

;;  * HIR block:
;;    High Inter-Reference block, a cached item with rare accesses and
;;    long interval. A block `x`, `x` ∈ `Q` ∧ `x` ∈ `cache`, is a HIR block.

;;  * non-resident HIR block:
;;    only the key of the HIR block is cached, without the corresponding
;;    value a test (has?) for the corresponding key is always a
;;    miss. Used for additional history information. A block `x`, `x` ∈
;;    `S` ∧ `x` ∉ `Q` ∧ `x` ∉ `cache`, is a non-resident HIR block.

;; ## Outline of the implemented algorithm

;; `cache` is used to store the key value pairs.
;; `S` and `Q` maintain the relative order of accesses of the keys, like
;; a LRU list.

;; Definition of `prune stack`:
;;
;;         repeatedly remove oldest item from S until an item k, k ∉ Q ∧
;;         k ∈ cache (a LIR block), is found


;; In case of a miss for key `k` and value `v` (`k` ∉ cache) and
;;
;;  * (1.1) `S` is not filled, |`S`| < `limitS`
;;         add k to S
;;         add k to the cache

;;  * (1.2) `k` ∉ `S`, never seen or not seen for a long, long time
;;             remove oldest item x from Q
;;             remove x from cache
;;             add k to S
;;             add k to Q
;;             add k to the cache

;;  * (1.3) `k` ∈ `S`, this is a non-resident HIR block
;;         remove oldest item x from Q
;;         remove x from cache
;;         add k to S
;;         remove oldest item y from S
;;         add y to Q
;;         prune stack


;; In case of a hit for key `k` (`k` ∈ cache) and

;;  * (2.1) `k` ∈ `S` ∧ `k` ∉ `Q`, a LIR block
;;         add k to S / refresh
;;         prune stack if k was the oldest item in S

;;  * (2.2) `k` ∈ `S` ∧ `k` ∈ `Q`, a HIR block
;;         add k to S / refresh
;;         remove k from Q
;;         remove oldest item x from S
;;         add x to Q
;;         prune stack

;;  * (2.3) `k` ∉ `S` ∧ `k` ∈ `Q`, a HIR block, only older than the oldest item in S
;;         add k to S
;;         add k to Q / refresh

(defn- prune-stack [lruS lruQ cache]
  (loop [s lruS q lruQ c cache]
    (let [k (apply min-key s (keys s))]
      (if (or (contains? q k)               ; HIR item
              (not (contains? c k)))        ; non-resident HIR item
        (recur (dissoc s k) q c)
        s))))

(defcache LIRSCache [cache lruS lruQ tick limitS limitQ]
  CacheProtocol
  (lookup [_ item]
    (get cache item))
  (lookup [_ item not-found]
    (get cache item not-found))
  (has? [_ item]
    (contains? cache item))
  (hit [_ item]
    (let [tick+ (inc tick)]
      (if (not (contains? lruS item))
                                        ; (2.3) item ∉ S ∧ item ∈ Q
        (LIRSCache. cache (assoc lruS item tick+) (assoc lruQ item tick+) tick+ limitS limitQ)
        (let [k (apply min-key lruS (keys lruS))]
          (if (contains? lruQ item)
                                        ; (2.2) item ∈ S ∧ item ∈ Q
            (let [new-lruQ (-> lruQ (dissoc item) (assoc k tick+))]
              (LIRSCache. cache
                          (-> lruS (dissoc k) (assoc item tick+) (prune-stack new-lruQ cache))
                          new-lruQ
                          tick+
                          limitS
                          limitQ))
                                        ; (2.1) item ∈ S ∧ item ∉ Q
            (LIRSCache. cache
                        (-> lruS (assoc item tick+) (prune-stack lruQ cache))
                        lruQ
                        tick+
                        limitS
                        limitQ))))))

  (miss [_ item result]
    (let [tick+ (inc tick)]
      (if (< (count cache) limitS)
                                        ; (1.1)
        (let [k (apply min-key lruS (keys lruS))]
          (LIRSCache. (assoc cache item result)
                      (-> lruS (dissoc k) (assoc item tick+))
                      lruQ
                      tick+
                      limitS
                      limitQ))
        (let [k (apply min-key lruQ (keys lruQ))
              new-lruQ (dissoc lruQ k)
              new-cache (-> cache  (dissoc k) (assoc item result))]
          (if (contains? lruS item)
                                        ; (1.3)
            (let [lastS (apply min-key lruS (keys lruS))]
              (LIRSCache. new-cache
                          (-> lruS (dissoc lastS) (assoc item tick+) (prune-stack new-lruQ new-cache))
                          (assoc new-lruQ lastS tick+)
                          tick+
                          limitS
                          limitQ))
                                        ; (1.2)
            (LIRSCache. new-cache
                        (assoc lruS item tick+)
                        (assoc new-lruQ item tick+)
                        tick+
                        limitS
                        limitQ))))))
  (seed [_ base]
    (LIRSCache. base
                (into {} (for [x (range (- limitS) 0)] [x x]))
                (into {} (for [x (range (- limitQ) 0)] [x x]))
                0
                limitS
                limitQ))
  Object
  (toString [_]
    (str cache \, \space lruS \, \space lruQ \, \space tick \, \space limitS \, \space limitQ)))

;; Factories

(defn basic-cache-factory
  "Returns a pluggable basic cache initialied to `base`"
  [base]
  {:pre [(map? base)]}
  (BasicCache. base))

(defn fifo-cache-factory
  "Returns a FIFO cache with the cache and FIFO queue initialized to `base` --
   the queue is filled as the values are pulled out of `base`.  If the associative
   structure can guarantee ordering, then the said ordering will define the
   eventual eviction order.  Otherwise, there are no guarantees for the eventual
   eviction ordering.

   If the number of elements in `base` is greater than the limit then some items
   in `base` will be dropped from the resulting cache.  If the associative
   structure used as `base` can guarantee sorting, then the last `limit` elements
   will be used as the cache seed values.  Otherwise, there are no guarantees about
   the elements in the resulting cache."
  [limit base]
  {:pre [(number? limit) (< 0 limit)
         (map? base)]
   :post [(== limit (count (.q %)))]}
  (clojure.core.cache/seed (FIFOCache. {} clojure.lang.PersistentQueue/EMPTY limit) base))

(defn lru-cache-factory
  "Returns an LRU cache with the cache and usage-table initialied to `base` --
   each entry is initialized with the same usage value. (maybe this should be
   randomized?)"
  [limit base]
  {:pre [(number? limit) (< 0 limit)
         (map? base)]}
  (clojure.core.cache/seed (LRUCache. {} {} 0 limit) base))

(defn ttl-cache-factory
  "Returns a TTL cache with the cache and expiration-table initialied to `base` --
   each with the same time-to-live."
  [ttl base]
  {:pre [(number? ttl) (<= 0 ttl)
         (map? base)]}
  (clojure.core.cache/seed (TTLCache. {} {} ttl) base))

(defn lu-cache-factory
  "Returns an LU cache with the cache and usage-table initialied to `base`."
  [limit base]
  {:pre [(number? limit) (< 0 limit)
         (map? base)]}
  (clojure.core.cache/seed (LUCache. {} {} limit) base))

(defn lirs-cache-factory
  "Returns an LIRS cache with the S & R LRU lists set to the indicated
   limts."
  [s-history-limit q-history-limit base]
  {:pre [(number? s-history-limit) (< 0 s-history-limit)
         (number? q-history-limit) (< 0 q-history-limit)
         (map? base)]}
  (seed (LIRSCache. {} {} {} 0 s-history-limit q-history-limit) base))