#lang racket

;; --------------------------------------------------
;; Cursul al II-lea:
;;
;; Reducerea listelor. Funcționalele foldl/foldr.

;; Trei probleme:
;; 1. suma unei liste
;; 2. maximul unei liste
;; 3. multiplii lui trei dintr-o listă

(define sum-list
  (λ (lst)
    (if (null? lst) 0 (+ (car lst) (sum-list (cdr lst))))))

(define (max-list l)
    (cond ((null? (cdr l)) (car l))
          ((> (car l) (max-list (cdr l))) (car l))
          (else (max-list (cdr l)))))

(define (unique-helper l acc)
  (cond ((null? l) (car acc))
        ((member (car l) (cdr acc)) (unique-helper (cdr l) acc))
        (else (unique-helper (cdr l) `(,(add1 (car acc)) . (,(car l) . ,(cdr acc)))))))

(define (unique l) (unique-helper l '(0)))

;; fold-right => natural pe stivă

(define (fold-right f default l)
  (if (null? l)
      default
      (f (car l) (fold-right f default (cdr l)))
      ))

(define (sum-list-right l)
  (fold-right (λ (x y) (+ x y))
              0
              l))

(define (unique-list-right l)
  (car (fold-right
        (λ (e set)
          (if (member e (cdr set))
              set
              (cons (add1 (car set)) (cons e (cdr set)))))
        '(0)
        l)))

;; fold-left => natural pe coadă

(define (fold-left f acc l)
  (if (null? l)
      acc
      (fold-left f (f (car l) acc) (cdr l))
      ))

(define (sum-list-left l)
  (fold-right (λ (x y) (+ x y))
              0
              l))

(define (unique-list-left l)
  (car
   (fold-left
    (λ (e set)
      (if (member e (cdr set))
          set
          (cons (add1 (car set)) (cons e (cdr set)))))
    '(0)
    l)))