Bowling frame

struct Frame {
  enum Change {
    case addRoll(_ roll: Int)
    case setCurrentFrame(_ newValue: Bool)
  }

  static let Strike = 10
  static let FullRackOfPins = 10
  static let Gutter = 0
  static let LastFrame = 10
  static let EmptyBoxScore = "  "

  let frameNumber: Int
  var rolls = [Int]()
  var isCurrentFrame = false

  // computed properties
  var isStrike: Bool { (rolls.first == 10) }
  var isSpare: Bool { rolls.count >= 2 && (rolls[0] + rolls[1] == 10) }
  var showExtraRolls: Bool { frameNumber == Self.LastFrame }
  var isAcceptingRolls: Bool { isCurrentFrame || ((isSpare || isStrike) && (rolls.count < 3)) }
  var isComplete: Bool { ((isSpare || isStrike) && (rolls.count == 3)) || ((!isSpare && !isStrike) && rolls.count == 2) }
  var value: Int { rolls.reduce(0, +) }
  var firstRollRepresentation: String {
    isStrike ? " " : rolls.first == 0 ? "-" : rolls.count == 0 ? " " : "\(rolls.first!)"
  }
  var secondRollRepresentation: String {
    isStrike ? "X" : isSpare ? "/" : rolls.count < 2 ? " " : "\(rolls[1])"
  }
  var boxScore: String { "\(firstRollRepresentation)\(secondRollRepresentation)" }

  func alter(_ c: Change...) -> Self { c.reduce(self) { $0.alter($1) } }
  private func alter(_ change: Change) -> Self {
    var updatedFrame = self
    switch change {
    case .addRoll(let roll):
      if isAcceptingRolls {
        updatedFrame.rolls.append(roll)
      }
    case .setCurrentFrame(let newValue):
      updatedFrame.isCurrentFrame = newValue
    }
    return updatedFrame
  }
}

extension Frame: CustomStringConvertible {
  var description: String {
    let status = isSpare ? "spare" : isStrike ? "strike" :  isComplete ? "open" : "incomplete"
    return
      "Frame \(frameNumber) boxScore: \(boxScore) rolls: \(rolls) status: \(status) value: \(value) completed: \(isComplete)"
  }
}


var frame1 = Frame(frameNumber: 1, isCurrentFrame: true)
var frame2 = Frame(frameNumber: 2, isCurrentFrame: false)
var frame3 = Frame(frameNumber: 3, isCurrentFrame: false)

print(frame1, "\n", frame2, "\n", frame3)
print("--- before any rolls ---")


// bowl first frame
frame1 = frame1.alter(.addRoll(10))
print(frame1, "\n", frame2, "\n", frame3)
print("--- end of frame 1 ---")

// bowl second frame
frame1 = frame1.alter(.setCurrentFrame(false))
frame2 = frame2.alter(.setCurrentFrame(true))
frame2 = frame2.alter(.addRoll(7))
frame1 = frame1.alter(.addRoll(7))
frame2 = frame2.alter(.addRoll(3))
frame1 = frame1.alter(.addRoll(3))
print(frame1, "\n", frame2, "\n", frame3)
print("--- end of frame 2 ---")

// bowl third frame
frame2 = frame2.alter(.setCurrentFrame(false))
frame3 = frame3.alter(.setCurrentFrame(true))
frame3 = frame3.alter(.addRoll(9))
frame2 = frame2.alter(.addRoll(9))
frame3 = frame3.alter(.addRoll(0))
print(frame1, "\n", frame2, "\n", frame3)
print("--- end of frame 3 ---")