Converts from music notes to number of semitones

gistfile1.fs

type Notes =
  | A      = 0
  | ASharp = 1
  | B      = 2
  | C      = 3
  | CSharp = 4
  | D      = 5
  | DSharp = 6
  | E      = 7
  | F      = 8
  | FSharp = 9
  | G      = 10
  | GSharp = 11;;
  
// Incomplete http://www.seventhstring.com/resources/notefrequencies.html
let notesToFreq = 
  Map[
     (Notes.A, 4),      440.0;
     (Notes.ASharp, 4), 466.2;
     (Notes.B, 4),      493.9;
     (Notes.C, 5),      523.3;
     (Notes.CSharp, 5), 554.4;
     (Notes.D, 5),      587.3;
     (Notes.DSharp, 5), 622.3;
     (Notes.E, 5),      659.3;
     (Notes.F, 5),      698.5;
     (Notes.FSharp, 5), 740.0;
     (Notes.G, 5),      784.0;
     (Notes.GSharp, 5), 830.6];;
     
let semitoneInterval = 1.0595;;

let numberOfSemitones first second = log(Map.find second notesToFreq / Map.find first notesToFreq)/log(semitoneInterval) |> round |> int;;

// Test
numberOfSemitones (Notes.A,4) (Notes.GSharp, 5) = 11;;