A-M baby

Task/Hailstone-sequence/Racket/hailstone-sequence.rkt

@@ -0,0 +1,19 @@
+#lang racket
+(define memo (make-hash))
+(hash-set! memo 1 '((1) 1))
+
+(define (hailstone n)
+  (hash-ref memo n
+   (λ ()
+     (define h (hailstone (if (even? n) (/ n 2) (+ (* 3 n) 1))))
+     (hash-set! memo n (list (cons n (first h)) (+ (second h) 1)))
+     (hash-ref memo n))))
+
+(define h27 (first (hailstone 27)))
+
+(printf "first 4 elements of h(27): ~v\n" (take h27 4))
+(printf "last  4 elements of h(27): ~v\n" (take-right h27 4))
+
+(printf "x < 10000 such that h(x) gives the longest sequence: ")
+(for/fold ([m 0]) ([n (in-range 1 100000)])
+  (max m (second (hailstone n))))