Create a function for transposing musical chords

Question

Your mission is to create a function for transposing music chords.

Copied from SO with permission of the original asker, I just really wanted to see what you guys could do with this one:

Since I don't expect everyone to be a musician here, I'll try to explain how it works in music theory. I hope I don't forget something. If yes, musicians, please, correct me.

1) The simple chords

The simple chords are almost as simple as an alphabet and it goes like this:

C, C#, D, D#, E, F, F#, G, G#, A, A# B

From B it loops all over again to C. Therefore, If the original chord is E and we want to transpose +1, the resulting chord is F. If we transpose +4, the resulting chord is G#.

2) Expanded chords

They work almost like the simple chords, but contain a few more characters, which can safely be ignored when transposing. For example:

Cmi, C#7, Dsus7, Emi, Fsus4, F#mi, G ...

So again, as with the simple chords, if we transpose Dsus7 + 3 = Fsus7

3) Non-root bass tone

A problem arises when the bass plays a different tone than the chord root tone. This is marked by a slash after the chord and also needs to be transposed. Examples:

C/G, Dmi/A, F#sus7/A#

As with examples 1 and 2, everything is the same, but the part after the slash needs transpose too, therefore:

C/G + 5 = F/C

F#sus7/A# + 1 = Gsus7/B

I think this should be all, unless I forgot something.

So basically, imagine you have a javascript variable called chord and the transpose value transpose. What code would transpose the chord?

Example:

var chord = 'F#sus7/C#';
var transpose = 3; // remember this value also may be negative, like "-4"
... code here ...
var result; // expected result = 'Asus7/E';

original question here: https://stackoverflow.com/questions/7936843/how-do-i-transpose-music-chords-using-javascript (note that he's asking for JS, for this challenge I don't care what language its in)

Answer 1

Haskell, 146

c=zip(words"C C# D D# E F F# G G# A A# B")[0..]
x#n=maybe x(\i->fst$c!!mod(i+n)12)$lookup x c
(x:'#':y)%n=(x:"#")#n++y%n
(x:y)%n=[x]#n++y%n
x%_=x

This defines an operator % that transposes a chord by a given amount, so you'd use it like this:

*Main> "E"%1
"F"
*Main> "E"%4
"G#"
*Main> "Dsus7"%3
"Fsus7"
*Main> "C/G"%5
"F/C"
*Main> "F#sus7/A#"%1
"Gsus7/B"

Answer 2

OK, within the limitations of the question as asked and not catering for a lot of things you'd need to allow for if using with real music (flats being the most obvious):

function t(c,a) {
  var s=["C","C#","D","D#","E","F","F#","G","G#","A","A#","B"],i;
  return c.replace(/[A-G]#?/g,function(m){return s[(i=(s.indexOf(m)+a)%s.length)<0?i+s.length:i];});
}

This is my first attempt at code-golf, so I hope I've taken the right approach. Following is the readable version that I originally posted at stackoverflow.com:

function transposeChord(chord, amount) {
  var scale = ["C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"];
  return chord.replace(/[CDEFGAB]#?/g,
                       function(match) {
                         var i = (scale.indexOf(match) + amount) % scale.length;
                         return scale[ i < 0 ? i + scale.length : i ];
                       });
}

Answer 3

As a preliminary effort, I'd like to flesh-out some of what "the full solution" should take into account.

Sharps and Flats

When transposing a musical key, one adds sharps or flats to the key signature according to the distance traversed in the "circle of fourths" (or fifths, depending on the direction you're going).

fifths from C up to the enharmonic
key: C G  D  A  E  B  F# C# G#  D#  A#  E#  B#
add:   F# C# G# D# A# E# B# F## C## G## D## A##

fourths from C down to the enharmonic
key: C F  Bb Eb Ab Db Gb Cb Fb  Bbb Ebb Abb Dbb
add:   Bb Eb Ab Db Gb Cb Fb Bbb Ebb Abb Dbb Gbb

So the key of C can be written with zero sharps or flats, or with twelve sharps, or with twelve flats. The first options is clearly superior.

But we only really need the middle of the table above because all those double-sharps and double-flats mean that some note is being given an overly-complicated name. So extracting the middle and arranging in sequence gives us this list of keys.

C  Db(5 'b's) D(2#) Eb(3b) E(4#) F(1b) F#(6#)[or Gb(6b)]
G(1#) Ab(4b) A(3#) Bb(2b) B(5#) C

And, dropping the counts and picking between F# and Gb (all flats looks prettier than one lonely sharp) gives:

C Db D Eb F Gb G Ab A Bb B C

But for named chords, the situation is less clear because the names carry accidentals with them.