net: Add `ifast-*` function.

* net/ricketyspace/sicp/one/eighteen.scm (ifast-iter-*)
(ifast-*): New functions.
* net/ricketyspace/sicp/one/seventeen.scm (double, halve): Add
functions to export.
(even?): Add function to replace.

2 files changed, 22 insertions, 1 deletions

diff --git a/net/ricketyspace/sicp/one/eighteen.scm b/net/ricketyspace/sicp/one/eighteen.scm
new file mode 100644
index 0000000..0c75311
--- /dev/null
+++ b/net/ricketyspace/sicp/one/eighteen.scm
@@ -0,0 +1,18 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one eighteen)
+  #:use-module (net ricketyspace sicp one seventeen)
+  #:export (ifast-*))
+
+(define (ifast-iter-* a b n)
+  (cond ((= b 0) n)
+        ((even? b) (ifast-iter-* (double a) (halve b) n))
+        (else (ifast-iter-* a (- b 1) (+ a n)))))
+
+(define (ifast-* a b)
+  "Compute A * B.
+
+Iterative version of fast-*."
+  (ifast-iter-* a b 0))
diff --git a/net/ricketyspace/sicp/one/seventeen.scm b/net/ricketyspace/sicp/one/seventeen.scm
index 48717e9..1ef69cc 100644
--- a/net/ricketyspace/sicp/one/seventeen.scm
+++ b/net/ricketyspace/sicp/one/seventeen.scm
@@ -20,7 +20,10 @@
 ;;;
 
 (define-module (net ricketyspace sicp one seventeen)
-  #:export (fast-*))
+  #:export (double
+            halve
+            fast-*)
+  #:replace (even?))
 
 (define (double a)
   "Return A + A."