one/eleven.scm: add alt. version.

(make-cache, alt-recursive-fn): new functions.

1 files changed, 30 insertions, 1 deletions

diff --git a/one/eleven.scm b/one/eleven.scm
index d47fced..a95b7ab 100644
--- a/one/eleven.scm
+++ b/one/eleven.scm
@@ -3,10 +3,39 @@
 ;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
 
 (define-module (one eleven)
-  #:export (recursive-fn))
+  #:export (recursive-fn
+            make-cache
+            alt-recursive-fn))
 
 (define (recursive-fn n)
   (cond ((<= n 3) n)
         (else (+ (* 1 (recursive-fn (- n 1)))
                  (* 2 (recursive-fn (- n 2)))
                  (* 3 (recursive-fn (- n 3)))))))
+
+;;;; alternative version. faster but ugly.
+
+(define (make-cache)
+  (let ((cache (make-hash-table)))
+    (define (get key)
+      (hash-get-handle cache key))
+    (define (set key value)
+      (hash-create-handle! cache key value)
+      ;; return value as a side effect.
+      value)
+    (lambda args
+      (apply (case (car args)
+               ((get) get)
+               ((set) set)
+               (else (error "Invalid method")))
+             (cdr args)))))
+
+(define (alt-recursive-fn cache n)
+  (let ((v (cache 'get n)))
+    (cond (v (cdr v))
+          ((<= n 3)
+           (cache 'set n n))
+          (else (cache 'set n
+                       (+ (* 1 (alt-recursive-fn cache (- n 1)))
+                          (* 2 (alt-recursive-fn cache (- n 2)))
+                          (* 3 (alt-recursive-fn cache (- n 3)))))))))