add one/twelve.scm

made some progress on ex. 1.12.

1 files changed, 30 insertions, 0 deletions

diff --git a/one/twelve.scm b/one/twelve.scm
new file mode 100644
index 0000000..39c3d8b
--- /dev/null
+++ b/one/twelve.scm
@@ -0,0 +1,30 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+;;; pascal's fucking triangle.
+
+(define-module (one twelve)
+  #:export (compute-triangle
+            compute-line
+            display-line
+            value-at))
+
+(define (compute-triangle n)
+  (cond ((= n 1) (list (compute-line n 1)))
+        (else (cons (compute-line n 1)
+                    (compute-triangle (- n 1))))))
+
+(define (compute-line n pos)
+  (cond ((> pos n) '())
+        (else
+         (cons (value-at n pos)
+               (compute-line n (+ pos 1))))))
+
+(define (value-at line pos)
+  (cond ((= line 1) 1)
+        ((= pos 1) 1)
+        ((= line pos) 1)
+        (else (+
+               (value-at (- line 1) (- pos 1))
+               (value-at (- line 1) pos)))))