Add net/ricketyspace/sicp/one/seventeen.scm

1 files changed, 46 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/one/seventeen.scm b/net/ricketyspace/sicp/one/seventeen.scm
new file mode 100644
index 0000000..1a5ec15
--- /dev/null
+++ b/net/ricketyspace/sicp/one/seventeen.scm
@@ -0,0 +1,46 @@
+
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+;;;
+;;;  __________________
+;;; < +, - everything. >
+;;;  ------------------
+;;;   \            .    .     .
+;;;    \      .  . .     `  ,
+;;;     \    .; .  : .' :  :  : .
+;;;      \   i..`: i` i.i.,i  i .
+;;;       \   `,--.|i |i|ii|ii|i:
+;;;            U**U\.'@@@@@@`.||'
+;;;            \__/(@@@@@@@@@@)'
+;;;                 (@@@@@@@@)
+;;;                 `YY~~~~YY'
+;;;                  ||    ||
+;;;
+
+(define-module (net ricketyspace sicp one seventeen)
+  #:export (fast-*))
+
+(define (double a)
+  "Return A + A."
+  (+ a a))
+
+(define (halve b)
+  "Return B/2.
+
+B must be an even number."
+  (cond ((= b 0) 0)
+        (else (+ 1 (halve (- b 2))))))
+
+(define (even? b)
+  "Return #t if B is divisible by 2; #f otherwise."
+  (cond ((= b 1) #f)
+        ((= b 0) #t)
+        (else (even? (- b 2)))))
+
+(define (fast-* a b)
+  "Return A * B."
+  (cond ((= b 1) a)
+        ((even? b) (double (fast-* a (halve b))))
+        (else (+ a (fast-* a (- b 1))))))