Initial commit

2017/17/day17.scm

@@ -0,0 +1,25 @@
+;; $ csc day17.scm -o day17 -O5
+;; $ ./day17 -:hm8g
+
+(use srfi-1)
+(declare (fixnum))
+
+(define (prepend! x list)
+  (set-cdr! list (cons (car list) (cdr list)))
+  (set-car! list x))
+
+(define (spin steps n after)
+  (let ((buffer (circular-list 0)))
+    (do ((i 1 (+ i 1)))
+	((> i n) (cadr (member after buffer)))
+      (let ((new (drop buffer (+ steps 1))))
+	(prepend! i new)
+	(set! buffer new)))))
+
+(write (spin 3 2017 2017))
+(newline)
+(write (spin 312 2017 2017))
+(newline)
+(write (spin 312 50000000 0))
+(newline)
+