ScalesByTones.swift

let notes = ["A", "A♯", "B", "C", "C♯", "D", "D♯", "E", "F", "F♯", "G", "G♯"]

extension Strideable {
  func strides<S : SequenceType where S.Generator.Element == Stride>(ss: S) -> [Self] {
    var i = self
    return ss.map { s in
      defer { i = i.advancedBy(s) }
      return i
    }
  }
}

extension CollectionType where Index : IntegerArithmeticType {
  subscript(mod i: Index) -> Generator.Element {
    return self[i % endIndex]
  }
}
extension CollectionType where Index : IntegerArithmeticType, Index : Strideable {
  func rotateIntervals<S : SequenceType where S.Generator.Element == Index.Stride>(from: Index, by: S) -> [Generator.Element] {
    return from.strides(by).map { i in self[mod: i] }
  }
}

let major = [2, 2, 1, 2, 2, 2, 1]

notes.rotateIntervals(0, by: major) // ["A", "B", "C♯", "D", "E", "F♯", "G♯"]
notes.rotateIntervals(3, by: major) // ["C", "D", "E", "F", "G", "A", "B"]

let minor = [2, 1, 2, 2, 2, 2, 2]

notes.rotateIntervals(0, by: minor) // ["A", "B", "C", "D", "E", "F♯", "G♯"]