Transposes all chords on an e-chords.com song page.

echords_transpose

function transpose_page(semitone_shift) {
  
  $('u > a').each(function() {
    old_chord = $(this).attr('rel');
    new_chord = transpose_chord(old_chord, semitone_shift);
    $(this).html(new_chord);
  });

  function transpose_chord(chord, semitone_shift) {

    function note_from_chord(chord) {
      matches = chord.match(/^[A-G|a-g][#|b]?/);
      return !!matches ? matches[0] : null
    }

    old_note = note_from_chord(chord);
    new_note = transpose_note(old_note, semitone_shift);
    return chord.replace(old_note, new_note); // TODO this only replaces first occurrence?
  }



  function transpose_note(note, semitone_shift) {
    function chromatic_scale() { return chromatic_scale_sharps(); }
    function chromatic_scale_sharps() { return ['C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#', 'A', 'A#', 'B']; }
    function chromatic_scale_flats()  { return ['C', 'Db', 'D', 'Eb', 'E', 'F', 'Gb', 'G', 'Ab', 'A', 'Bb', 'B']; }

    function note_sharp_to_corresponding_flat(note) { return chromatic_scale_flats()[chromatic_scale_sharps().indexOf(note)] }
    function note_flat_to_corresponding_sharp(note) { return chromatic_scale_sharps()[chromatic_scale_flats().indexOf(note)] }

    function note_is_sharp(note) { return !!note.match(/#$/); }
    function note_is_flat(note)  { return !!note.match(/b$/); }

    function note_to_semitone(note) {
      scale = note_is_sharp(note) ? chromatic_scale_sharps() : chromatic_scale_flats();
      return scale.indexOf(note);
    }

    function semitone_to_note(semitone, inflector) {
      if(inflector == undefined) { inflector = '#' }
      scale = (inflector == '#') ? chromatic_scale_sharps() : chromatic_scale_flats();
      return scale[normalize(semitone)];
    }
    
    function normalize(semitone) {
      if(semitone < 0) {
        correction = Math.ceil(Math.abs(semitone/12)) * 12
        return semitone + correction;
      } else {
        return semitone % 12;
      }
    }

    inflector = note_is_sharp(note) ? '#' : 'b'
    return semitone_to_note(note_to_semitone(note) + semitone_shift, inflector);
  }


}