\version "2.18.0"

% Merge identical TextScript objects in multiple voices of a staff,
% like they are produced by the partcombiner.

% Based on an engraver by Jay Anderson, from
% "http://www.mail-archive.com/lilypond-user%40gnu.org/msg69608.html"

% Engraver listening for TextScript grobs.
% When two or more TextScripts are present at the same moment
% and their text content is identical, all but the first one
% are omitted through setting their 'stencil property to false.
#(define merge-markups-engraver
   (lambda (context)
     (let ((markups '())
           (processed-markups '()))

       `((start-translation-timestep .
           ,(lambda (trans)
              (set! markups '())))

         (stop-translation-timestep .
           ,(lambda (trans)
              (if (> (length markups) 1)
                  (begin
                   ;; There are more than one markups at the time-step
                   (set! processed-markups '())
                   (for-each
                    (lambda (mkup)
                      ;; Retrieve the text content for the iterated markup grob
                      (let ((text (markup->string (ly:grob-property mkup 'text))))
                        (if (member text processed-markups)
                            ;; If we have already encountered this text omit the grob ...
                            (ly:grob-set-property! mkup 'stencil #f)
                            ;; ... else accept it this time and add it to the list of
                            ;; encountered texts
                            (set! processed-markups
                                  (append processed-markups (list text))))))
                    (reverse markups))))))

         (acknowledgers
          (text-interface .
            ,(lambda (engraver grob source-engraver)
               (if (member
                    (assoc-ref (ly:grob-property grob 'meta) 'name)
                    '(TextScript
                      DynamicText
                      ))
                   (set! markups (cons grob markups))))))))))


%%{
musica = \relative c'' {
  \partcombineApart
  c1^\markup "Test"
  c1^\markup "Test"
  c1^\markup "Test"

  %%%%%%%%%%%%
  % TODO
  % Also handle hairpins (or other spanners).
  % Obviously removing the stencil of the middle \p doesn't do any harm
  % to the behaviour of the hairpins.
  c1 \<
  c1 \p
  c1 \>
  c1 \!
}

musicb = \relative c' {
  c1_\markup "Test"
  c1_\markup "Prost"
  c1 _\markup "Test" ^\markup "Test-alternative"
  c1 \<
  c1 \p
  c1 \>
  c1 \!
}

\score {
  \new Staff \with {
    \consists #merge-markups-engraver
  }
  \partcombine \musica \musicb
  \layout {}
}

%}