;; A Grade is: (make-grade Symbol Number)
(define-struct grade (letter num))

;; The Symbol in a Grade represents

;;    'A  >= 90
;;    'B  >= 80
;;    'C  >= 70
;;    'D  >= 60
;;    'F  < 60

;; A [Listof Grades] ...
(define grades
  (list (make-grade 'D 62) (make-grade 'C 79) (make-grade 'A 93) (make-grade 'B 84)
        (make-grade 'F 57) (make-grade 'F 38) (make-grade 'A 90) (make-grade 'A 95)
        (make-grade 'C 76) (make-grade 'A 90) (make-grade 'F 55) (make-grade 'C 74)
        (make-grade 'A 92) (make-grade 'B 86) (make-grade 'F 43) (make-grade 'C 73)))

;; A1.1
(check-expect (log->lon grades)
              (list 62 79 93 84 57 38 90 95 76 90 55 74 92 86 43 73))
(define (log->lon log)
  (local [(define (grade->number grade)
            (grade-num grade))]
    (map grade->number log)))

;; A1.2
(check-expect (best-grade grades) (make-grade 'A 95))
(define (best-grade log)
  (local [(define (higher-grade grade1 grade2)
            (cond [(> (grade-num grade1)
                      (grade-num grade2)) grade1]
                  [else grade2]))]
    (foldr higher-grade (first log) log)))

;; A1.3
(check-expect (just-As grades) (list
                                (make-grade 'A 93)
                                (make-grade 'A 90)
                                (make-grade 'A 95)
                                (make-grade 'A 90)
                                (make-grade 'A 92)))

(define (just-As log)
  (local [(define (check-a grade)
            (symbol=? (grade-letter grade) 'A))]
    (filter check-a log)))

;; A1.4
(check-expect (all-pass? grades) false)
(define (all-pass? log)
  (local [(define (check-f grade)
            (not (symbol=? (grade-letter grade) 'F)))]
    (andmap check-f log)))

;; A1.5
(check-expect (bonus grades)
              (list
               (make-grade 'D 67)
               (make-grade 'B 84)
               (make-grade 'A 98)
               (make-grade 'B 89)
               (make-grade 'D 62)
               (make-grade 'F 43)
               (make-grade 'A 95)
               (make-grade 'A 100)
               (make-grade 'B 81)
               (make-grade 'A 95)
               (make-grade 'D 60)
               (make-grade 'C 79)
               (make-grade 'A 97)
               (make-grade 'A 91)
               (make-grade 'F 48)
               (make-grade 'C 78)))
(define (bonus log)
  (local [(define (add5 grade)
            (+ 5 (grade-num grade)))
          (define (grade-check num)
            (cond [(> num 89) 'A]
                  [(and (> num 79) (< num 90)) 'B]
                  [(and (> num 69) (< num 80)) 'C]
                  [(and (> num 59) (< num 70)) 'D]
                  [else 'F]))
          (define (update grade)
            (make-grade (grade-check (add5 grade))
                        (add5 grade)))]
    (map update log)))