ChordNotes

chordNotes.swift

import Foundation

let noteToSemitone: [String: Int] = [
    "C": 0, "C#": 1, "Db": 1,
    "D": 2, "D#": 3, "Eb": 3,
    "E": 4, "Fb": 4, "E#": 5,
    "F": 5, "F#": 6, "Gb": 6,
    "G": 7, "G#": 8, "Ab": 8,
    "A": 9, "A#": 10, "Bb": 10,
    "B": 11, "Cb": 11, "B#": 0
]

let semitoneToNote: [Int: (sharp: String, flat: String)] = [
    0: ("C", "C"),
    1: ("C#", "Db"),
    2: ("D", "D"),
    3: ("D#", "Eb"),
    4: ("E", "Fb"),
    5: ("F", "F"),
    6: ("F#", "Gb"),
    7: ("G", "G"),
    8: ("G#", "Ab"),
    9: ("A", "A"),
    10: ("A#", "Bb"),
    11: ("B", "Cb")
]

func chordNotes(chord: String) -> [String] {
    let root = String(chord.prefix(while: { ["C", "D", "E", "F", "G", "A", "B"].contains($0) || $0 == "#" || $0 == "b" }))
    let suffix = String(chord.dropFirst(root.count))

    guard let rootSemitone = noteToSemitone[root] else {
        return []
    }

    var intervals =
        switch suffix {
        case "m"    : [0, 3, 7]
        case "7"    : [0, 4, 7, 10]
        case "m7"   : [0, 3, 7, 10]
        case "M7"   : [0, 4, 7, 11]
        case "add9" : [0, 4, 7, 14]
        case "madd9": [0, 3, 7, 14]
        case "dim"  : [0, 3, 6]
        case "aug"  : [0, 4, 8]
        case "m7b5" : [0, 3, 6, 10]
        default     : [0, 4, 7]
    }

    let notes = intervals.map {
        let semitone = ($0 + rootSemitone) % 12
        return root.contains("#") 
            ? semitoneToNote[semitone]!.sharp
            : semitoneToNote[semitone]!.flat
    }

    return notes
}

// テスト例

let chord1 = "Cm7"
print("\(chord1): \(chordNotes(chord: chord1))")

let chord2 = "Fadd9"
print("\(chord2): \(chordNotes(chord: chord2))")

let chord3 = "F#m7b5"
print("\(chord3): \(chordNotes(chord: chord3))")

let chord4 = "AbM7"
print("\(chord4): \(chordNotes(chord: chord4))")