; Exercise 3:
; Design a function widen-river to make every merge point wider in a river network
; by a given amount.
; You may omit the purpose statement.


(define RIVER-S1 (make-stream 3))
(define RIVER-S2 (make-stream 5))
(define RIVER-S3 (make-stream 1))
(define RIVER-S4 (make-stream 6))
(define RIVER-S5 (make-stream 2))
(define RIVER-M1 (make-merge 50 RIVER-S1 RIVER-S2))
(define RIVER-M2 (make-merge 80 RIVER-S3 RIVER-M1))
(define RIVER-M3 (make-merge 50 RIVER-S4 RIVER-S5))
(define RIVER-M4 (make-merge 100 RIVER-M2 RIVER-M3))

; S1:3  S2:5
;   \    /
;   M1:50  S3:1  S4:6  S5:2
;     \     /      \    /
;       M2:80      M3:50
;           \      /
;            M4:100
;

;; widen-river : River Number -> River
;; updates each merge point in a river network by the given number

(check-expect (widen-river RIVER-S1 5) RIVER-S1)
(check-expect (widen-river RIVER-M1 10) (make-merge 60 RIVER-S1 RIVER-S2))
(check-expect (widen-river RIVER-M4 2) (make-merge 102
                                                   (make-merge 82 RIVER-S3 (make-merge 52 RIVER-S1 RIVER-S2))
                                                   (make-merge 52 RIVER-S4 RIVER-S5)))


(define (widen-river r n)
  (cond [(stream? r) r]
        [(merge? r) (make-merge
                     (+ (merge-width r) n)
                     (widen-river (merge-left r) n)
                     (widen-river (merge-rightrp) n))]))