extensions.swift

typealias Velocity = Double
extension Velocity {
  var kph: Velocity { return self * 1.60934 }
  var mph: Velocity { return self }
}
protocol Vehicle {
  var topSpeed: Velocity { get }
  var numberOfDoors: Int { get }
  var hasFlatbed: Bool { get }
}
struct Car {
  let make: String
  let model: String
  let year: Int
  let color: String
  let nickname: String
  var gasLevel: Double {
    willSet {
      precondition(newValue <= 1.0 && newValue >= 0.0, "New value must be between 0 and 1")
    }
  }
}
extension Car: Vehicle {
  var topSpeed: Velocity { return 100 }
  var numberOfDoors: Int { return 4 }
  var hasFlatbed: Bool { return false }
}
extension Car {
  init(make: String, model: String, year: Int) {
    self.init(make: make,
          model: model,
          year: year,
          color: "Black",
          nickname: "N/A",
          gasLevel: 1.0)
  }
}

var c = Car(make: "Mercedes", model: "CLA 250", year: 2017)
print(c)

extension Car {
  enum Kind {
    case coupe, sedan
  }
  var kind: Kind {
    if numberOfDoors == 2 {
      return .coupe
    } else {
      return .sedan
    }
  }
}

print(c.kind)

extension Car {
  mutating func emptyGas(by amount: Double) {
    precondition(amount <= 1 && amount > 0, "Amount to remove must be between 0 and 1")
    gasLevel -= amount
  }
  mutating func fillGas() {
    gasLevel = 1.0
  }
}

c.emptyGas(by: 0.3)
print(c.gasLevel)
c.fillGas()
print(c.gasLevel)