#lang racket

(provide hailstone)
(define hailstone
  (let ([t (make-hasheq)])
    (hash-set! t 1 '(1))
    (λ(n) (hash-ref! t n
            (λ() (cons n (hailstone (if (even? n) (/ n 2) (+ (* 3 n) 1)))))))))

(module+ main
  (define h27 (hailstone 27))
  (printf "h(27) = ~s, ~s items\n"
          `(,@(take h27 4) ... ,@(take-right h27 4))
          (length h27))
  (define N 100000)
  (define longest
    (for/fold ([m #f]) ([i (in-range 1 (add1 N))])
      (define h (hailstone i))
      (if (and m (> (cdr m) (length h))) m (cons i (length h)))))
  (printf "for x<=~s, ~s has the longest sequence with ~s items\n"
          N (car longest) (cdr longest)))