#lang racket

;1 zad
; ( 1 2 1 1 2 3 1)
; ((1 . 4) (2 . 2) (3 . 1))
(define (occurence-x x xs)
  (define (helper x xs counter)
    (cond
      ((null? xs) counter)
      ((eq? x (car xs)) (helper x (cdr xs) (+ counter 1)))
      (else (helper x (cdr xs) counter))))
  (helper x xs 0))

(define (remove-element x xs)
  (cond
    ((null? xs) '())
    ((eq? x (car xs)) (remove-element x (cdr xs)))
    (else (cons (car xs) (remove-element x (cdr xs))))))

(define (occurence-each xs)
  (define (helper xs newlist)
    (if (null? xs)
        (reverse newlist)
        (helper (remove-element (car xs) xs) (cons (cons (car xs) (occurence-x (car xs) xs)) newlist))))
  (helper xs '()))


;2 zadacha
; ( 1 1 1 3 2 2 5 6 1 )
; ( (1 . 3) (3 . 1) (2 . 2) (5 . 1) (6 . 1) (1 . 1 ) )

(define (drop n xs)
  (define (drop-helper n xs newlist)
    (cond
      ((null? xs ) newlist)
      ((= n 0) (drop-helper n (cdr xs) (append newlist (list (car xs)))))
      (else (drop-helper (- n 1) (cdr xs) newlist))))
  (drop-helper n xs '()))

(define (prefix xs)
  (define (prefix-helper xs lastElement counter)
    (if (or (null? xs)(not (= lastElement (car xs))))
          counter
      (prefix-helper (cdr xs) (car xs) (+ counter 1))))
  (prefix-helper (cdr xs) (car xs) 1))

(define (occurence-each-prefix xs)
  (define (helper xs newlist)
    (if (null? xs)
        (reverse newlist)
        (helper (drop (prefix xs) xs) (cons (cons (car xs) (prefix xs)) newlist))))
  (helper xs '()))

;3 zadacha

(define (list-number-n-times number times)
  (if (= times 0)
      '()
      (cons number (list-number-n-times number (- times 1)))))

(define (restore xs)
  (define (restore-helper xs newlist)
    (if (null? xs)
        newlist
        (restore-helper (drop 1 xs) (append newlist (list-number-n-times (caar xs) (cdar xs))))))
  (restore-helper xs '()))