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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: Euler 14.
Date: Mon, 5 Aug 2024 02:24:03 -0000 (UTC)
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B. Pym wrote:

> The following iterative sequence is defined for the set of positive
> integers:
> 
> 	  n -> n/2 (n is even)
> 	  n -> 3n + 1 (n is odd)
> 
> Using the rule above and starting with 13, we generate the following
> sequence:
> 
> 	  13 -> 40 -> 20 -> 10 -> 5 -> 16 -> 8 -> 4 -> 2 -> 1
> 
> It can be seen that this sequence (starting at 13 and
> finishing at 1) contains 	10 terms. Although it has not been
> proved yet (Collatz Problem), it is thought 	that all starting
> numbers finish at 1.
> 
> Which starting number, under one million, produces the longest chain?
> 
> NOTE: Once the chain starts the terms are allowed to go above one
> million.
> 
> 
> Gauche Scheme
> 
> (use gauche.collection) ;; find-max
> 
> 
> (define (cltz n) (if (odd? n) (+ 1 (* n 3)) (/ n 2)))
> 
> (define (d c n)
>   (if (= n 1) c (d (+ 1 c) (cltz n))))
> 
> (find-max (lrange 1 1000000) :key (pa$ d 1))
> 
>   ===>
> 837799

newLISP

(define (cltz n) (if (odd? n) (+ 1 (* n 3)) (/ n 2)))

(define (d c n) (if (= n 1) c (d (+ 1 c) (cltz n))))

(local (best-n  best-cnt  cnt)
  (for (n 1 999999)
    (setq cnt (d 1 n))
    (if (> cnt best-cnt) (setq best-n n  best-cnt cnt)))
  (list best-n best-cnt))

(837799 525)