protocol CircularEnum: CaseIterable where Self.AllCases.Element: Equatable {
    var currentIndex: Self.AllCases.Index { get }
    func next() -> Self.AllCases.Element
    func previous() -> Self.AllCases.Element
}

extension CircularEnum {
    var currentIndex: Self.AllCases.Index {
        return Self.allCases.firstIndex(of: self)!
    }

    func next() -> Self.AllCases.Element {
        let allCases = Self.allCases
        let nextIndex = allCases.index(after: currentIndex)
        if nextIndex == allCases.endIndex {
            return allCases.first!
        }

        return allCases[nextIndex]
    }

    func previous() -> Self.AllCases.Element {
        let allCases = Self.allCases
        var index = currentIndex
        if index == allCases.startIndex {
            index = allCases.endIndex
        }
        index = allCases.index(index, offsetBy: -1)

        return allCases[index]
    }
}

enum Season: CircularEnum {
    case winter
    case spring
    case summer
    case fall
}

let season = Season.fall
season.previous() // summer
season.next() // winter