Project Euler

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# [[Problem 4]]

```scheme

#!r6rs

#!chezscheme

(import (chezscheme))

(define (RotationNumber? num)

  (let* ((num-string (number->string num))

         (str-length (string-length num-string))

         (count (div str-length 2)))

    (let loop ((index 0))

      (cond

        [(<= count index) #t]

        [(equal? (string-ref num-string index) (string-ref num-string (- str-length 1 index)))

         (loop (add1 index))]

        [else #f]))))

(define (answer-4)

  (let loop1 ((num1 100)

              (result 0))

    (if (< 999 num1)

      result

      (let loop2 ((num2 100)

                  (result result))

        (if (< 999 num2)

          (loop1 (add1 num1) result)

          (let ((num (* num1 num2)))

            (if (RotationNumber? num)

              (loop2 (add1 num2) (max result num))

              (loop2 (add1 num2) result))))))))

(printf "4: ~D~%" answer-4)

```