euler-55.scm

#!/usr/local/bin/gosh

;;; Last Update: 2010-10-09 00:01+09:00.

;;; Project Euler - Problem-55:
;;; http://projecteuler.net/index.php?section=problems&id=55

;;; Kim, Yi-Chul < kimarx@gmail.com >

(use srfi-1)
(use srfi-13)


(print (lychrel-counter 0 (iota 10000 1)))

(define (reverse-num n)
  (string->number
   (string-reverse
    (number->string n))))

;;; Add a given number to its reversed number.
(define (add-rev n)
  (+ n (reverse-num n)))

(define (palindromic-num? n)
  (eqv? n (reverse-num n)))

(define (rec-palindromic? init n)
  (let ((num init))
    (cond [(> num 50) #f]
          [(palindromic-num? n) #t]
          [else (rec-palindromic? (+ num 1) (add-rev n))])))

(define (lychrel? n)
  (not (rec-palindromic? 1 (add-rev n))))

(define (lychrel-counter init lis)
  (let ((count init))
    (if (null? lis)
        count
        (if (lychrel? (car lis))
            (lychrel-counter (+ count 1) (cdr lis))
            (lychrel-counter count (cdr lis))))))