\version "2.16.0"

% Taken from improved version from
% http://lilypond.1069038.n5.nabble.com/new-snippet-combine-multimeasure-rests-td144688.html
% Improved by Urs Liska to also merge over empty music expressions


#(define (add-durations dur1 dur2)
   (let* ((len1 (ly:duration-length dur1))
          (len2 (ly:duration-length dur2))
          (mult (ly:moment-div (ly:moment-add len1 len2) len1)))
     (ly:make-duration (ly:duration-log dur1)
       (ly:duration-dot-count dur1)
       (* (ly:duration-scale dur1) (ly:moment-main mult)))))

#(define (combinable-rest? rest)
   (and (ly:music? rest)
        (or (eq? 'MultiMeasureRestMusic (ly:music-property rest 'name))
            (eq? 'SkipEvent (ly:music-property rest 'name)))
        (null? (ly:music-property rest 'articulations))))

#(define (combine-rests rest1 rest2)
   ;; create one rest/skip with the sum of both lengths
   (make-music (ly:music-property rest1 'name)
     'duration (add-durations (ly:music-property rest1 'duration)
                 (ly:music-property rest2 'duration))
     'articulations '()))

#(define (consolidator curr rest)
   ;; determine ir we have consecutive MultimeasureRests or skips
   (if (and (combinable-rest? curr)
            (not (null? rest)))
       ;; -> we have a combinable rest left and 'something' right
       (if (and (combinable-rest? (car rest))
                (eq? (ly:music-property curr 'name) (ly:music-property (car rest) 'name)))
           ;; -> we also have a combinable rest right and both are the same type,
           ;; recurse by first merging rests and then looking for the next item
           (consolidator (combine-rests curr (car rest))
             (cdr rest))
           ;; -> right is either no combinable rest or one of different type.
           (if (or (eq? 'BarCheck (ly:music-property (car rest) 'name))
                   (eq? 'Music (ly:music-property (car rest) 'name)))
               ;; -> right is one of the 'skippable' types,
               ;; so recurse using left and the next one to the right
               (consolidator curr (cdr rest))
               ;; just return left followed by righ
               (cons curr rest)))
       ;; -> no combinable rest to the left
       ; But what happens when rest *is* null?
       (cons curr rest)))

#(define (accumulate-result output input)
   ;; recurse over the elements of 'music', appending consolidated
   ;; items (i.e. the item or a merged rest) to 'output'
   (if (null? input)
       output
       (let ((done output)
             (curr (car input))
             (rest (cdr input)))
         (if (null? rest)
             (append done (list curr))
             (let ((prev (consolidator curr rest)))
               (accumulate-result (append done (list (car prev))) (cdr prev)))))))

#(define (condense music)
   ;; recurse over the music list and condense consecutive rests
   (let* ((output music)
          (elts (ly:music-property output 'elements))
          (elt (ly:music-property output 'element)))
     (if (pair? elts)
         (ly:music-set-property! output 'elements (map condense (accumulate-result '() elts))))
     (if (ly:music? elt)
         (ly:music-set-property! output 'element (condense elt)))
     output))

combineMMRests =
#(define-music-function (parser location music) (ly:music?)
   ;; process the 'music' argument and merge consecutive MultimeasureRests
       (condense music))

\combineMMRests {
  \compressFullBarRests
  c''1 \repeat unfold 3 R1 R1 R1 s1 s1 c''1
}

{
  \displayMusic {
    \repeat unfold 4 { R1 }
  }
}

{
  \displayMusic {
    R1*4
  }
}

{
  \displayMusic {
    R1 R1 R1 R1
  }
}