Automatically display key names above key signatures

This snippet automatically displays the name of the key above the key signature. This is helpful when learning the key signatures, or to easily see whether a key is major, minor, or modal (dorian, phrygian, lydian, etc.). It works by creating a custom key signature engraver.

\version "2.24"

%% http://lsr.di.unimi.it/LSR/Item?id=856

\paper { tagline = ##f }

%% see also lists.gnu.org/archive/html/lilypond-user/2013-12/msg00828.html
% by Paul Morris

% tonic-num: number of the tonic note 0-6, C=0, B=6
% acc-type: the accidental sign type, 1/2=sharp, -1/2=flat
% acc-count: the number and type of accidentals in the key signature
%                  values are: -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
%                  (negative = flats, positive = sharps)
% tonic-acc: #f if the tonic note is not sharp or flat, otherwise a pair
% maj-num: number of the tonic note 0-6, if the key sig were major
% mode-num: number of the mode 0-6
% txt-size: size of key name text
% padd: padding between key signature and key name
% mult: for correct resizing when the staff is resized

#(define Custom_key_engraver
   (make-engraver
    (acknowledgers
     ((key-signature-interface engraver grob source-engraver)
      ;; if key cancellation then do nothing,
      ;; else modify the key signature stencil
      (if (not (eq? 'KeyCancellation
                    (grob::name grob)))
          (let* ((context (ly:translator-context engraver))
                 (tonic-pitch (ly:context-property context 'tonic))
                 (tonic-num (ly:pitch-notename tonic-pitch))
                 (acc-list (ly:grob-property grob 'alteration-alist))

                 (acc-type (if (null? acc-list)
                               0 (cdr (list-ref acc-list 0))))

                 (acc-count (* (length acc-list)
                              (if (< acc-type 0) -1 1)))

                 (maj-num (case acc-count
                            ((0) 0)
                            ((1) 4) ((2) 1) ((3) 5) ((4) 2) ((5) 6) ((6) 3) ((7) 0)
                            ((-1) 3) ((-2) 6) ((-3) 2) ((-4) 5) ((-5) 1) ((-6) 4) ((-7) 0)))

                 (mode-num (modulo (- tonic-num maj-num) 7))
                 (key-letter (case tonic-num
                               ((0) "C" ) ((1) "D" ) ((2) "E" ) ((3) "F" )
                               ((4) "G" ) ((5) "A" ) ((6) "B" )))

                 (mult (magstep (ly:grob-property grob 'font-size 0.0)))

                 (txt-sharp #{ \markup {
                   \translate #(cons (* mult -0.3) (* mult 0.8))
                   \magnify #(* mult 0.9) \sharp
                   } #})

                 (txt-flat #{ \markup {
                   \translate #(cons (* mult -0.2) (* mult 0.4))
                   \magnify #(* mult 0.9) \flat
                   } #})

                 (tonic-acc (if (pair? (assq tonic-num acc-list))
                                (if (= acc-type 0.5) txt-sharp txt-flat)
                                ""))

                 (key-mode (case mode-num
                             ((0) "Major") ((1) "Dorian") ((2) "Phrygian") ((3) "Lydian")
                             ((4) "Mixolydian") ((5) "Minor") ((6) "Locrian")))

                 ;; F Major and D Minor each get extra padding
                 ;; so the name appears above the staff
                 (padd (if (= acc-count -1)
                           (+ 0.5 (* mult 1.2))
                           0.5))

                 (txt-size 0.7)
                 (key-name #{ \markup { #key-letter #tonic-acc #key-mode } #})
                 (key-name-scaled (ly:stencil-scale
                                   (grob-interpret-markup grob key-name)
                                   txt-size txt-size))

                 (key-sig-stencil (ly:key-signature-interface::print grob))
                 (grob-y-ext (ly:grob-property grob 'Y-extent)))

            ;; Names for C Major, A Minor, etc. have to be moved up above the staff
            ;; padd has no effect if there are no sharps or flats in the key signature
            (if (equal? grob-y-ext (cons +inf.0 -inf.0))
                (set! key-name-scaled
                      (ly:stencil-translate-axis key-name-scaled 3 Y)))

            (ly:grob-set-property! grob 'stencil
              (ly:stencil-combine-at-edge key-sig-stencil 1 1
                key-name-scaled padd))))))))

\layout {
  \context {
    \Staff
    \consists \Custom_key_engraver
  }
}

\markup \null % avoid clipped text

\new Staff \relative f'' {
  \key c \major
  c1
  \key a \minor
  a1
  \key a \major
  a1
  \key fis \minor
  fis1
  \key ees \major
  ees1
  \key c \minor
  c'1
  \key c \dorian
  c1
  \key c \phrygian
  c1
  \key c \lydian
  c1
  \key c \mixolydian
  c1
  \key c \locrian
  c1
}