[racket] missing solution 16.3.3 ex:file-du

A candidate for a solution.

;; model 3
(define-struct file (name size content))
(define-struct dir (name dirs files))

;; files:
(define hang (make-file 'hang 8 empty))
(define draw (make-file 'draw 2 empty))
(define read (make-file 'read! 19 empty))
(define one  (make-file 'part1 99 empty))
(define two  (make-file 'part2 52 empty))
(define thre (make-file 'part3 17 empty))
(define rdme (make-file 'read 10 empty))

;; directories:
(define Code (make-dir 'Code '() (list hang draw)))
(define Docs (make-dir 'Docs '() (list read)))
(define Libs (make-dir 'Libs (list Code Docs) '()))
(define Text (make-dir 'Text '() (list one two thre)))
(define Top  (make-dir 'TS (list Text Libs) (list rdme)))

;; dur-dir :: dir -> number
;; consumes a directory producing a sum of the list of files in each.
;; to the sum, it'll add 1 for each directory it finds. (we're assuming
;; each directory is of size 1).
(define (du-dir d)
  (+ 1 (sum-files (dir-files d)) (du-dir-subdir (dir-dirs d))))

(define (du-dir-subdir ls)
  (cond
   [(empty? ls) 0]
   [(dir? (first ls))
    (+ (du-dir (first ls))
       (du-dir-subdir (rest ls)))]))

;; sum-files :: ls-of-files -> number
;; consumes a list of files producing the sum of their sizes
(define (sum-files ls)
  (cond
   [(empty? ls) 0]
   [else (+ (file-size (first ls))
            (sum-files (rest ls)))]))

;; tests
(check-expect
 (sum-files (list hang draw))
 10)

(check-expect
 (sum-files empty)
 0)

(check-expect
 (du-dir Libs)
 (+ 8 2 19 1 1 1))

Source:
> http://www.htdp.org/2003-09-26/Solutions/file-du.html
> No Solution Written, yetUnfortunately, the solution to this exercise has
> not yet been written. To submit a solution you have written to this
> problem, or to complain that the solution isn't available, please contact Robby
> Findler
> <robby at eecs.northwestern.edu?subject=missing%20solution%2016.3.3%20ex:file-du%20>
> .
>
> To see the list of solutions, visit the table of contents
> <http://www.htdp.org/2003-09-26/Solutions/contents.html>. Each of the
> hyperlinked exercise numbers has a solution.
>
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