net: Add (net ricketyspace sicp one twentythree).

* net/ricketyspace/sicp/one/twentythree.scm
(net ricketyspace sicp one twentythree): Add module.

1 files changed, 25 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/one/twentythree.scm b/net/ricketyspace/sicp/one/twentythree.scm
new file mode 100644
index 0000000..f88b05d
--- /dev/null
+++ b/net/ricketyspace/sicp/one/twentythree.scm
@@ -0,0 +1,25 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one twentythree)
+  #:use-module (net ricketyspace sicp utils)
+  #:export (smallest-divisor))
+
+(define (square x)
+  (expt x 2))
+
+(define (divides? a b)
+  (= (remainder b a) 0))
+
+(define (next n)
+  (cond ((= n 2) 3)
+        (else (+ n 2))))
+
+(define (find-divisor n test-divisor)
+  (cond ((> (square test-divisor) n) n)
+        ((divides? test-divisor n) test-divisor)
+        (else (find-divisor n (next test-divisor)))))
+
+(define (smallest-divisor n)
+  (find-divisor n 2))