lldb_and_xcode_4.swift

struct Atbash: Cipher {
    private let alphabet = Alphabet.letters
    
    func encrypt(_ text: String) -> String {
        var output = [Character]()
        
        for character in text {
            if character != " " {
                guard let index = alphabet.index(of: character) else { continue }
                output.append(alphabet[alphabet.count - index - 1])
            } else {
                output.append(" ")
            }
        }
        
        return String(output)
    }
    
    func decrypt(_ text: String) -> String {
        return encrypt(text)
    }
}