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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: Non-determinism
Date: Mon, 5 Aug 2024 21:35:07 -0000 (UTC)
Organization: A noiseless patient Spider
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B. Pym wrote:

> Problem 4.42 in SICP
> 
> Five school girls took an exam. As they think thattheir
> parents are too much interested in their score, they promise
> that they write one correct and one wrong informations to
> their parents. Followings are parts of their letters
> concerning their result:
> 
> Betty: 	Kitty was the second and I third.
> Ethel: 	I won the top and Joan the second.
> Joan: 	I was the third and poor Ethel the last.
> Kitty: 	I was the second and Mary the fourth.
> Mary: 	I was the fourth. Betty won the top.
> 
> Guess the real order of the five school girls. 

newLISP

;; Iterate over all permutations of a list, and
;; call a function on each.
(define (permute permute.seq permute.func (permute.built '()))
  (if (null? permute.seq)
    (permute.func permute.built)
    (let (seq (copy permute.seq))
      (dotimes (i (length seq))
        (unless (zero? i) (rotate seq -1))
        (permute
          (rest seq)
          permute.func
          (cons (first seq) permute.built))))))

(define (xor a b) (if a (not b) b))

(define (find* x xs) (+ 1 (find x xs)))

(define (either a m  b n  lst)
  (xor (= m (find* a lst))
       (= n (find* b lst))))

(define (check answer)
  (if
    (and
      (either 'kitty 2  'betty 3  answer)
      (either 'kitty 2  'mary 4  answer)
      (either 'mary 4  'betty 1  answer)
      (either 'ethel 1  'joan 2  answer))
    (println answer)))

(permute '(kitty betty ethel joan mary) check)
  ===>
(kitty joan betty mary ethel)