#lang racket

(provide clear-bookmarks bookmarks-empty? bookmarks-push bookmarks-pop)

;; ----------------------------------------------------------------------------
;; Bookmarks
;; Bookmarks are a list (stack) of positions
(define bookmarks null)

(define (clear-bookmarks) (set! bookmarks null))

(define (bookmarks-empty?) (null? bookmarks))

;; bookmarks-push : position -> void
(define (bookmarks-push bm)
  (if (null? bookmarks)
      (set! bookmarks (cons bm bookmarks))
      (when (not (= bm (car bookmarks))) ; only push if top bookmark != bm
        (set! bookmarks (cons bm bookmarks)))))

;; bookmarks-pop : position -> position or #f
;; pops bookmarks until one that is not = current-pot is found and returns it
;; returns #f if no such bookmark exists
(define (bookmarks-pop current-pos)
  (let loop ([bms bookmarks])
    (if (null? bms)
        #f
        (if (and current-pos (= current-pos (car bms)))
            (loop (cdr bms))
            (begin0
              (car bms)
              (set! bookmarks (cdr bms)))))))