;;  -----------------------------------------------------------------------
;; |                                                                       |
;; |   FILE           :   positions-of.rkt                                 |
;; |   AUTHOR         :   Eugene Wallingford                               |
;; |   CREATION DATE  :   2020/02/18                                       |
;; |                                                                       |
;; |   DESCRIPTION    :   Solution to Problem 5 on Homework 4.  Notice     |
;; |                      that the helper function positions-of-at         |
;; |                      is tail recursive!                               |
;; |                                                                       |
;; |   REQUIRES       :   Uses Rackunit for testing.                       |
;; |                                                                       |
;;  -----------------------------------------------------------------------

#lang racket
(require rackunit)

; -------------------------------------------------------------------------
; positions-of                                        (interface procedure)
; -------------------------------------------------------------------------

(define positions-of
  (lambda (s los)
    (positions-of-at s los 0)))

(define positions-of-at
  (lambda (s los position)
    (if (null? los)
        '()
        (if (eq? s (first los))
            (cons position
                  (positions-of-at s (rest los) (add1 position)))
            (positions-of-at s (rest los) (add1 position))))))

(check-equal? (positions-of 'a '(a b a c d a e f g a h i j k))
              '(0 2 5 9))
(check-equal? (positions-of 'a '(b c d e f g h i j k))
              '())

; -------------------------------------------------------------------------