%%http://lsr.di.unimi.it/LSR/Item?id=880

% original code (for zig-zag lines) by Thomas Morley (Harm)
% -> http://lists.gnu.org/archive/html/lilypond-user/2012-12/msg00715.html
% slightly modified to create dashed lines by Paul Morris

dashedStaffSymbolLines =
#(define-music-function (dash-space bool-list)
 ((number-pair? '(0.5 . 0.5)) list?)
"
Replaces specified lines of a StaffSymbol with dashed lines.

The lines to be changed should be given as a list containing booleans, with
the meaning:
  #f - no dashes, print a normal line
  #t - print a dashed line
The order of the bool-list corresponds with the order of the given list of
'line-positions or if not specified, with the default.
If the length of the bool-list and the 'line-positions doesn't match a warning
is printed.

The width of the dashes and the spacing between them can be altered by adding a pair
as first argument while calling the function:
\\dashedStaffSymbolLines #'(1 . 1) #'(#f #f #t #f #f)
the first number of the pair is the width, the second the spacing
"
#{
 \override Staff.StaffSymbol.after-line-breaking =
   #(lambda (grob)
     (let* ((staff-stencil (ly:grob-property grob 'stencil))
            (staff-line-positions 
              (if (equal? (ly:grob-property grob 'line-positions) '() )
                '(-4 -2 0 2 4)
                (ly:grob-property grob 'line-positions)))
            (staff-width
              (interval-length
                (ly:stencil-extent staff-stencil X)))
            (staff-space (ly:staff-symbol-staff-space grob))
            (staff-line-thickness (ly:staff-symbol-line-thickness grob))
            ;; width of the dash
            (dash-width (car dash-space))
            ;; space between dashes
            (space-width (cdr dash-space))
            ;; Construct the first dash
            (sample-path `((moveto 0 0)
                           (lineto ,dash-width 0)
                           ))
            ;; Make a stencil of the first dash
            (dash-stencil
              (grob-interpret-markup
                grob
                (markup
                  #:path staff-line-thickness sample-path)))
           ;; width of both dash and space
           (dash-space-width (+ dash-width space-width))
           
           ;; another way: get width of dash from the dash stencil
           ;; (stil-width
           ;;   (interval-length
           ;;     (ly:stencil-extent dash-stencil X)))
           ;; (dash-space-width (+ stil-width space-width))
           
            ;; Make a guess how many dashes are needed.
            (count-dashes
              (inexact->exact
                (round
                  (/ staff-width
                     (- dash-space-width
                        staff-line-thickness)))))
            ;; Construct a stencil of dashes with the guessed count
            (dashed-stil
                (ly:stencil-aligned-to
                  (apply ly:stencil-add
                    (map
                      (lambda (x)
                        (ly:stencil-translate-axis
                          dash-stencil
                          (* (- dash-space-width staff-line-thickness) x)
                          X))
                      (iota count-dashes)))
                  Y
                  CENTER))
            ;; Get the the length of that dashed stencil
            (stil-x-length
              (interval-length
                (ly:stencil-extent dashed-stil  X)))
            ;; Construct a line-stencil to replace the staff-lines.
            (line-stil
              (make-line-stencil staff-line-thickness 0 0 staff-width 0))
            ;; Calculate the factor to scale the dashed-stil to fit
            ;; the width of the original staff-symbol-stencil
            (corr-factor
              (/ staff-width (- stil-x-length staff-line-thickness)))
            ;; Construct the new staff-symbol
            (new-stil
              (apply
                ly:stencil-add
                  (map
                    (lambda (x y)
                      (ly:stencil-translate
                          (if (eq? y #f)
                            line-stil
                            (ly:stencil-scale
                              dashed-stil
                              corr-factor 1))
                          (cons (/ staff-line-thickness 2)
                                (* (/ x 2) staff-space))))
                    staff-line-positions bool-list))))
       
      (if (= (length bool-list)(length staff-line-positions))
        (ly:grob-set-property! grob 'stencil new-stil)
        (ly:warning
          "length of dashed line bool-list doesn't match the line-positions - ignoring"))))
#})


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

music =
\relative f' {
  c2 d e f g a b
}

<<
 \new Staff \with {
           \dashedStaffSymbolLines #'(#f #f #t #f #f)
 }
 \music
 
 
 \new Staff \with {
           \dashedStaffSymbolLines #'(0.2 . 0.3) #'(#f #t #f #t #f)
 }
 \music

 \new Staff \with {
           \override StaffSymbol.line-positions = #'(-4  0  4)
           \dashedStaffSymbolLines #'(#f #t #f)
           % make staff smaller:
           fontSize = #-3
           \override StaffSymbol.staff-space = #(magstep -3)
           \override StaffSymbol.thickness = #(magstep -3)
 }
 \music
>>