one -> net/ricketyspace/sicp

1 files changed, 31 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/one/fifteen.scm b/net/ricketyspace/sicp/one/fifteen.scm
new file mode 100644
index 0000000..a61fa8f
--- /dev/null
+++ b/net/ricketyspace/sicp/one/fifteen.scm
@@ -0,0 +1,31 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one fifteen)
+  #:use-module (ice-9 regex)
+  #:export (sine))
+
+(define (sine angle)
+  (let ((out ""))
+    (define (log angle)
+      (set! out (string-append out " (p (sine "
+                               (number->string (/ angle 3.0)))))
+    (define (close-parens s)
+      (string-trim (string-append
+                    s (fold-matches "\\([p,s]" s ""
+                                    (lambda (match prev)
+                                      (string-append prev ")"))))))
+    (define (cube x) (* x x x))
+    (define (p x) (- (* 3 x) (* 4 (cube x))))
+    (define (s angle)
+      (if (not (> (abs angle) 0.1))
+          angle
+          (begin
+            (log angle)
+            (p (s (/ angle 3.0))))))
+    (s angle)
+    (close-parens out)))
+;;
+;; (sine 12.15)
+;; $25 = "(p (sine 4.05 (p (sine 1.3499999999999999 (p (sine 0.44999999999999996 (p (sine 0.15 (p (sine 0.049999999999999996))))))))))"