\version "2.13.36"

\header {
  texidoc = "Use @code{define-event-class}, scheme engraver methods,
and grob creation methods to create a fully functional text spanner
in scheme."
}

#(define-event-class 'scheme-text-span-event
   '(scheme-text-span-event
     span-event
     music-event
     StreamEvent))

#(define (add-grob-definition grob-name grob-entry)
   (let* ((meta-entry   (assoc-get 'meta grob-entry))
          (class        (assoc-get 'class meta-entry))
          (ifaces-entry (assoc-get 'interfaces meta-entry)))
     (set-object-property! grob-name 'translation-type? list?)
     (set-object-property! grob-name 'is-grob? #t)
     (set! ifaces-entry (append (case class
                                  ((Item) '(item-interface))
                                  ((Spanner) '(spanner-interface))
                                  ((Paper_column) '((item-interface
                                                     paper-column-interface)))
                                  ((System) '((system-interface
                                               spanner-interface)))
                                  (else '(unknown-interface)))
                                ifaces-entry))
     (set! ifaces-entry (uniq-list (sort ifaces-entry symbol<?)))
     (set! ifaces-entry (cons 'grob-interface ifaces-entry))
     (set! meta-entry (assoc-set! meta-entry 'name grob-name))
     (set! meta-entry (assoc-set! meta-entry 'interfaces
                                  ifaces-entry))
     (set! grob-entry (assoc-set! grob-entry 'meta meta-entry))
     (set! all-grob-descriptions
           (cons (cons grob-name grob-entry)
                 all-grob-descriptions))))

#(add-grob-definition
  'SchemeTextSpanner
  `(
    (bound-details . ((left . ((Y . 0)
                               (padding . 0.25)
                               (attach-dir . ,LEFT)
                               ))
                      (left-broken . ((end-on-note . #t)))
                      (right . ((Y . 0)
                                (padding . 0.25)
                                ))
                      ))
    (dash-fraction . 0.2)
    (dash-period . 3.0)
    (direction . ,UP)
    (font-shape . italic)
    (left-bound-info . ,ly:line-spanner::calc-left-bound-info)
    (outside-staff-priority . 350)
    (right-bound-info . ,ly:line-spanner::calc-right-bound-info)
    (staff-padding . 0.8)
    (stencil . ,ly:line-spanner::print)
    (style . dashed-line)

    (meta . ((class . Spanner)
             (interfaces . (font-interface
                            line-interface
                            line-spanner-interface
                            side-position-interface))))))

#(define scheme-event-spanner-types
   '(
     (SchemeTextSpanEvent
      . ((description . "Used to signal where scheme text spanner brackets
start and stop.")
         (types . (general-music scheme-text-span-event span-event event))
         ))
     ))

#(set!
  scheme-event-spanner-types
  (map (lambda (x)
         (set-object-property! (car x)
                               'music-description
                               (cdr (assq 'description (cdr x))))
         (let ((lst (cdr x)))
           (set! lst (assoc-set! lst 'name (car x)))
           (set! lst (assq-remove! lst 'description))
           (hashq-set! music-name-to-property-table (car x) lst)
           (cons (car x) lst)))
       scheme-event-spanner-types))

#(set! music-descriptions
       (append scheme-event-spanner-types music-descriptions))

#(set! music-descriptions
       (sort music-descriptions alist<?))

#(define (add-bound-item spanner item)
   (if (null? (ly:spanner-bound spanner LEFT))
       (ly:spanner-set-bound! spanner LEFT item)
       (ly:spanner-set-bound! spanner RIGHT item)))

#(define (axis-offset-symbol axis)
   (if (eq? axis X) 'X-offset 'Y-offset))

#(define (set-axis! grob axis)
  (if (not (number? (ly:grob-property grob 'side-axis)))
      (begin
        (set! (ly:grob-property grob 'side-axis) axis)
        (ly:grob-chain-callback
         grob
         (if (eq? axis X)
             ly:side-position-interface::x-aligned-side
             ly:side-position-interface::y-aligned-side)
         (axis-offset-symbol axis)))))

schemeTextSpannerEngraver =
#(lambda (context)
   (let ((span '())
         (finished '())
         (current-event '())
         (event-drul '(() . ())))
     (list (cons 'listeners
                 (list (cons 'scheme-text-span-event
                             (lambda (engraver event)
                               (if (= START (ly:event-property event 'span-direction))
                                   (set-car! event-drul event)
                                   (set-cdr! event-drul event))))))
           (cons 'acknowledgers
                 (list (cons 'note-column-interface
                             (lambda (engraver grob source-engraver)
                               (if (ly:spanner? span)
                                   (begin
                                     (ly:pointer-group-interface::add-grob span 'note-columns grob)
                                     (add-bound-item span grob)))
                               (if (ly:spanner? finished)
                                   (begin
                                     (ly:pointer-group-interface::add-grob finished 'note-columns grob)
                                     (add-bound-item finished grob)))))))
           (cons 'process-music
                 (lambda (trans)
                   (if (ly:stream-event? (cdr event-drul))
                       (if (null? span)
                           (ly:warning "You're trying to end a scheme text spanner but you haven't started one.")
                           (begin (set! finished span)
                                  (ly:engraver-announce-end-grob trans finished current-event)
                                  (set! span '())
                                  (set! current-event '())
                                  (set-cdr! event-drul '()))))
                   (if (ly:stream-event? (car event-drul))
                       (begin (set! current-event (car event-drul))
                              (set! span (ly:engraver-make-grob trans 'SchemeTextSpanner current-event))
                              (set-axis! span Y)
                              (set-car! event-drul '())))))
           (cons 'stop-translation-timestep
                 (lambda (trans)
                   (if (and (ly:spanner? span)
                            (null? (ly:spanner-bound span LEFT)))
                       (set! (ly:spanner-bound span LEFT)
                             (ly:context-property context 'currentMusicalColumn)))
                   (if (ly:spanner? finished)
                       (begin
                         (if (null? (ly:spanner-bound finished RIGHT))
                             (set! (ly:spanner-bound finished RIGHT)
                                   (ly:context-property context 'currentMusicalColumn)))
                         (set! finished '())
                         (set! event-drul '(() . ()))))))
           (cons 'finalize
                 (lambda (trans)
                   (if (ly:spanner? finished)
                       (begin
                         (if (null? (ly:spanner-bound finished RIGHT))
                             (set! (ly:spanner-bound finished RIGHT)
                                   (ly:context-property context 'currentMusicalColumn)))
                         (set! finished '())))
                   (if (ly:spanner? span)
                       (begin
                         (ly:warning "I think there's a dangling scheme text spanner :-(")
                         (ly:grob-suicide! span)
                         (set! span '()))))))))

schemeTextSpannerStart =
#(make-span-event 'SchemeTextSpanEvent START)

schemeTextSpannerEnd =
#(make-span-event 'SchemeTextSpanEvent STOP)

\layout {
  \context {
    \Global
    \grobdescriptions #all-grob-descriptions
  }
  \context {
    \Voice
    \consists \schemeTextSpannerEngraver
  }
}

\relative c' {
  a4 b\schemeTextSpannerStart c d |
  \repeat unfold 20 { a4 b c d | }
  a4 b c\schemeTextSpannerEnd d |
  \override SchemeTextSpanner #'to-barline = ##t
  a4\schemeTextSpannerStart b d c |
  \repeat unfold 20 { a4 b c d | }
  a1\schemeTextSpannerEnd |
}