1 files changed, 41 insertions, 0 deletions
diff --git a/net/ricketyspace/sicp/one/eleven.scm b/net/ricketyspace/sicp/one/eleven.scm
new file mode 100644
index 0000000..baa129b
--- /dev/null
+++ b/net/ricketyspace/sicp/one/eleven.scm
@@ -0,0 +1,41 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one eleven)
+  #: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)))))))))