Location: contrib, 134 Lines

; Scheme 9 from Empty Space, Function Library
; By Nils M Holm, 2009-2012
; Placed in the Public Domain
; (draw-tree object)  ==>  unspecific
; (dt)                ==>  unspecific
; Print a tree structure resembling a Scheme datum. Each cons
; cell is represented by [o|o] with lines leading to their car
; and cdr parts. Conses with a cdr value of () are represented
; by [o|/].
; DT is an abbrevation for DRAW-TREE.
; (Example): (draw-tree '((a) (b . c) (d e)))  ==>  unspecific
;            Output:  [o|o]---[o|o]---[o|/]
;                      |       |       |
;                     [o|/]    |      [o|o]---[o|/]
;                      |       |       |       |
;                      a       |       d       e
;                              |
;                             [o|o]--- c
;                              |
;                              b

(define (draw-tree n)

  (define *nothing* (cons 'N '()))

  (define *visited* (cons 'V '()))

  (define (empty? x) (eq? x *nothing*))

  (define (visited? x) (eq? (car x) *visited*))

  (define (mark-visited x) (cons *visited* x))

  (define (members-of x) (cdr x))

  (define (done? x)
    (and (pair? x)
         (visited? x)
         (null? (cdr x))))

  (define (draw-fixed-string s)
    (let* ((b (make-string 8 #\space))
           (k (string-length s))
           (s (if (> k 7) (substring s 0 7) s))
           (s (if (< k 3) (string-append " " s) s))
           (k (string-length s)))
      (display (string-append s (substring b 0 (- 8 k))))))

  (define (draw-atom n)
    (cond ((null? n)
            (draw-fixed-string "()"))
          ((symbol? n)
            (draw-fixed-string (symbol->string n)))
          ((number? n)
            (draw-fixed-string (number->string n)))
          ((string? n)
            (draw-fixed-string (string-append "\"" n "\"")))
          ((char? n)
            (draw-fixed-string (string-append "#\\" (string n))))
          ((eq? n #t)
            (draw-fixed-string "#t"))
          ((eq? n #f)
            (draw-fixed-string "#f"))
            (error "draw-atom: unknown type" n))))

  (define (draw-conses n)
    (let draw-conses ((n n)
                      (r '()))
      (cond ((not (pair? n))
              (draw-atom n)
              (reverse! r))
            ((null? (cdr n))
              (display "[o|/]")
              (reverse! (cons (car n) r)))
              (display "[o|o]---")
              (draw-conses (cdr n) (cons (car n) r))))))

  (define (draw-bars n)
    (let draw-bars ((n (members-of n)))
      (cond ((not (pair? n)))
            ((empty? (car n))
              (draw-fixed-string "")
              (draw-bars (cdr n)))
            ((and (pair? (car n))
                  (visited? (car n)))
              (draw-bars (members-of (car n)))
              (draw-bars (cdr n)))
              (draw-fixed-string "|")
              (draw-bars (cdr n))))))

  (define (skip-empty n)
    (if (and (pair? n)
             (or (empty? (car n))
                 (done? (car n))))
        (skip-empty (cdr n))

  (define (remove-trailing-nothing n)
    (reverse (skip-empty (reverse n))))

  (define (all-vertical? n)
    (or (not (pair? n))
        (and (null? (cdr n))
             (all-vertical? (car n)))))

  (define (draw-members n)
    (let draw-members ((n (members-of n))
                       (r '()))
      (cond ((not (pair? n))
                  (reverse r))))
            ((empty? (car n))
              (draw-fixed-string "")
              (draw-members (cdr n)
                            (cons *nothing* r)))
            ((not (pair? (car n)))
              (draw-atom (car n))
              (draw-members (cdr n)
                            (cons *nothing* r)))
            ((null? (cdr n))
              (draw-members (cdr n)
                            (cons (draw-final (car n)) r)))
            ((all-vertical? (car n))
              (draw-fixed-string "[o|/]")
              (draw-members (cdr n)
                            (cons (caar n) r)))
              (draw-fixed-string "|")
              (draw-members (cdr n)
                            (cons (car n) r))))))

  (define (draw-final n)
    (cond ((not (pair? n))
            (draw-atom n)
          ((visited? n)
            (draw-members n))
            (mark-visited (draw-conses n)))))

  (if (not (pair? n))
      (draw-atom n)
      (let draw-tree ((n (mark-visited (draw-conses n))))
        (if (not (done? n))
            (begin (newline)
                   (draw-bars n)
                   (draw-tree (draw-members n))))))

(define dt draw-tree)

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