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Re: Issue #768 - chord repetition shortcut: patch for review

From: Alexander Kobel
Subject: Re: Issue #768 - chord repetition shortcut: patch for review
Date: Mon, 30 Nov 2009 23:42:10 +0100
User-agent: Thunderbird (X11/20090817)

Hi, Nicolas,

I recognized two potential sources of trouble with your new q command. (Excellent! Wonderful! Tremendous! idea, by the way.)
I might have a solution for one of them, but I don't trust myself at all.

Firstly, the one I have no clue about. Is it reasonable to hope for an addition s.t. q ignores preceding rests or skips? If I want to repeat those, I can just write r or s, but it'd be great if they don't overwrite the previous-chord register. E.g. for the following
  { <c c,>8 q r q  q q r q }
instead of
  { <c c,>8 q r <c c,>  q q r <c c,> }
? AFAICS, the previous-chord is set during parsing/lexing (?), so this once more needs coding in the C++ part, and this is well beyond my scope right now.

Secondly, q behaves somewhat unintuitive in relative mode when repeating chords where the octave changes for the "main" note (is there a better name for this one?).
When I enter
  \relative c { <c c,> <c' c,> q }
I expect the outcome to be (in absolute mode)
  { <c c,> <c' c> <c' c> },
but instead it's
  { <c c,> <c' c> <c'' c'> }.
I kind of seem to have a fix for this - which is a very naive "I'm feeling lucky"-modification of your default-repeat-chord repetition function. I attach it, and it seems to work well, but to be honest - I have not the slightest clue why, and I don't trust it at all. I'd probably feel better if I had (ly:make-pitch 0 ...) in line 13 (which was my initial guess), but it might well be false trust also then.

And, of course, what's missing (if at all possible without C++ hacking) is to find out whether the function is called inside a relative or an absolute context, thus choosing the appropriate repetition function. I'm not even sure if that's reasonable, since I don't know when the function actually gets evaluated, and, alas, one can define music in absolute context and put a \relative before it later on. See \parallelMusic, which suffers from the same problem, as I understand.

Okay, looking forward to read your reply,

#(define-public (relative-repeat-chord previous-chord duration articulations)
  "Copy the previous chord, filter out events which are not notes, set the
chord duration, add articulations."
  (let ((new-chord (ly:music-deep-copy previous-chord)))
   (set! (ly:music-property new-chord 'elements)
    (append! articulations
     (let ((notes (filter (lambda (event)
                           (eqv? (ly:music-property event 'name) 'NoteEvent))
                   (ly:music-property new-chord 'elements))))
      (if (not (null? notes))
       (let ((pitch (ly:music-property (car notes) 'pitch)))
        (set! (ly:music-property (car notes) 'pitch)
         (ly:make-pitch -1 (ly:pitch-notename pitch) (ly:pitch-alteration 
   (for-each (lambda (event)
              (if (ly:duration? (ly:music-property event 'duration))
               (set! (ly:music-property event 'duration) duration)))
    (ly:music-property new-chord 'elements))

#(ly:parser-set-repetition-symbol parser 'q)
#(ly:parser-set-repetition-function parser relative-repeat-chord)

\relative c { <f'' f,,> q q q | <a, e a'> q q q }

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