Added lru-use function

private/lru-cache.rkt

@@ -11,6 +11,7 @@
          lru-expire
          set-lru-expire!
          lru-has?
+         lru-use
          lru-add!
          lru-count
          lru-empty?
@@ -47,19 +48,19 @@
 (define (expire? e)
   (or (eq? e #f) (max-count? e)))
 
-(define (find-and-bubble count max-count e cache cmp expire-s current-s el)
-  (if (or (null? cache)
-          (>= count max-count))
+(define (find-and-bubble count max-count e cache cmp expire-s current-s el el-found-cb)
+  (if (or (null? cache) (>= count max-count))
       '()
       (let ((front (car cache)))
-        (if (and (not (eq? expire-s #f))
-                 (< (cadr front) expire-s))
-            (find-and-bubble count max-count e (cdr cache) cmp expire-s current-s el)
+        (if (and (not (eq? expire-s #f)) (< (cadr front) expire-s))
+            (find-and-bubble count max-count e (cdr cache) cmp expire-s current-s el el-found-cb)
             (if (cmp e (car front))
-                (find-and-bubble count max-count e (cdr cache) cmp expire-s current-s (car front))
+                (begin
+                  (el-found-cb (car front))
+                  (find-and-bubble count max-count e (cdr cache) cmp expire-s current-s (car front) el-found-cb))
                 (cons front
                       (find-and-bubble (+ count 1) max-count
-                                       e (cdr cache) cmp expire-s current-s el))
+                                       e (cdr cache) cmp expire-s current-s el el-found-cb))
                 )
             )
         )
@@ -91,18 +92,38 @@
         'cleaned)))
 
 (define (find-and/or-add lru el)
-  (let ((ncache (find-and-bubble
-                 1 (lru*-max-count lru)
-                 el (unbox (lru*-cache lru))
-                 (lru*-compare lru)
-                 (if (eq? (lru*-expire-in-seconds lru) #f)
-                     #f
-                     (- (current-seconds) (lru*-expire-in-seconds lru)))
-                 (current-seconds)
-                 #f)))
+  (let ((ncache (find-and-bubble 1 (lru*-max-count lru) el
+                                 (unbox (lru*-cache lru))
+                                 (lru*-compare lru)
+                                 (if (eq? (lru*-expire-in-seconds lru) #f)
+                                     #f
+                                     (- (current-seconds) (lru*-expire-in-seconds lru)))
+                                 (current-seconds)
+                                 #f
+                                 (λ (el) #t)
+                                 )))
     (set-box! (lru*-cache lru) (cons (list el (current-seconds)) ncache)))
   lru)
 
+(define (find-and-get lru key)
+  (let* ((el #f)
+         (found #f))
+    (let ((ncache (find-and-bubble 1 (lru*-max-count lru) key
+                                   (unbox (lru*-cache lru))
+                                   (lru*-compare lru)
+                                   (if (eq? (lru*-expire-in-seconds lru) #f)
+                                       #f
+                                       (- (current-seconds) (lru*-expire-in-seconds lru)))
+                                   (current-seconds)
+                                   #f
+                                   (λ (e)
+                                     (set! found #t)
+                                     (set! el e)))))
+      (set-box! (lru*-cache lru) (if found
+                                     (cons (list el (current-seconds)) ncache)
+                                     ncache)))
+    (values found el)))
+
 (define-syntax with-lock
   (syntax-rules ()
     ((_ lru code ...)
@@ -132,6 +153,15 @@
     )
   )
 
+(define/contract (lru-use lru key def-el)
+  (-> lru*? any/c any/c any/c)
+  (with-lock lru
+    (let-values (((found el) (find-and-get lru key)))
+      (if found
+          el
+          def-el))))
+    
+    
 (define/contract (lru-add! lru el)
   (-> lru*? any/c lru*?)
   (with-lock lru

scribblings/lru-cache.scrbl

@@ -74,6 +74,15 @@ If capacity is reached, the least recently used item is removed.
 Returns @racket[l].
 }
 
+@defproc[(lry-use [l lru?] [key any/c] [default any/c]) any/c]{
+Finds the given @racket[item] by @racket[key] (this must be accomodated
+in the compare function (see @tt{#:cmp}). When the item is found
+using the key, it will not only return the item, but also move the
+item to the front of the cache and update the expiry parameter.
+
+When the item is not found, it will return @racket{default}. 
+}
+
 @defproc[(lru-has? [l lru?] [item any/c]) boolean?]{
 Returns @racket[#t] if @racket[item] is currently present (and not expired),
 otherwise @racket[#f]. This call also performs lazy cleanup of expired items.