\version "2.17.17"

%%%%%%%%%%%%%%%%%%%%%%

#(define (define-grob-property symbol type? description)
  (if (not (equal? (object-property symbol 'backend-doc) #f))
      (ly:error (_ "symbol ~S redefined") symbol))

  (set-object-property! symbol 'backend-type? type?)
  (set-object-property! symbol 'backend-doc description)
  symbol)

#(map
  (lambda (x)
    (apply define-grob-property x))

  `(
    (tagged ,boolean? "is this grob marked?")
    (syllables ,array? "the lyric syllables at a timestep")
  ))

%%%%%%%%%%%%%%%%%%%%%%%%

#(define (Lyric_text_align_engraver ctx)
"If the property 'tagged is set, collect all lyric syllables at that
timestep in the grob-array `syllables'"
  (let ((syl '()))
    (make-engraver
     (acknowledgers
      ((lyric-syllable-interface trans grob source)
       (set! syl (cons grob syl))))
     ((stop-translation-timestep trans)
      (if (any (lambda (x) (eq? #t (ly:grob-property x 'tagged)))
               syl)
          (for-each
            (lambda (x)
               (for-each
                 (lambda (y)
                   (ly:pointer-group-interface::add-grob x 'syllables y))
                 syl))
            syl))
      (set! syl '())))))

#(define (X-offset-callback grob)
  (let* ((target (ly:grob-object grob 'syllables))
         (target
           (if (ly:grob-array? target)
               (ly:grob-array->list target)
               '())))
    (if (pair? target)
        (let ((longest
                (car
                  (sort target
                        (lambda (x y)
                          (> (interval-length
                               (ly:stencil-extent
                                 (grob-interpret-markup
                                   grob (ly:grob-property x 'text))
                                 X))
                             (interval-length
                               (ly:stencil-extent
                                 (grob-interpret-markup
                                   grob (ly:grob-property y 'text))
                                 X))))))))
          (if (eq? grob longest)
              ; if our grob has the longest syllable, return its default
              ; value for 'X-offset
              (ly:self-alignment-interface::aligned-on-x-parent grob)
              (ly:grob-property longest 'X-offset)))
        (ly:self-alignment-interface::aligned-on-x-parent grob))))
         
tagIt = \once \override Lyrics.LyricText #'tagged = ##t 

%%%%%%%%%%%%%%%%  EXAMPLE


\layout {
  ragged-right = ##f
  \context {
    \Lyrics
    \override LyricText.X-offset = #X-offset-callback
  }
  \context {
    \Score
    \consists #Lyric_text_align_engraver
  }
}



\score {
  \new Staff <<
    \new Voice = "hymn" {
      \relative c'' {
        \partial 2 a4 a \bar "|"
      }
    }
    \new Lyrics \lyricsto "hymn" {
      \tagIt with my
    }
    \new Lyrics \lyricsto "hymn" { 
      through the
    }
    \new Lyrics \lyricsto "hymn" {
      praise the
    }
    \new Lyrics \lyricsto "hymn" {
      bless -- ed
    }
  >>
}