decrypting_gravity_falls6.swift

class NumbersDecryptor: Decryptor {
    
    fileprivate var cipherDictionary = [String : String]()
    
    override func decrypt(_ input: String) -> String? {
        cipherDictionary.removeAll()
        // The most annoying part of this is to properly replace characters in message
        // so when we do .separatedBy we just get strings we can change to numbers.
        // This may break depending on what text you will use and whether you are using characters
        // that are currently supported or not. I even noticed that – is not the same as -.
        let numbers = input.replacingOccurrences(of: " ", with: "-")
            .replacingOccurrences(of: ":", with: "")
            .replacingOccurrences(of: "\"", with: "")
            .replacingOccurrences(of: ",", with: "")
            .replacingOccurrences(of: "`", with: "")
            .replacingOccurrences(of: ".", with: "")
            .replacingOccurrences(of: "'", with: "-")
            .replacingOccurrences(of: "?", with: "")
            .replacingOccurrences(of: "!", with: "")
            .replacingOccurrences(of: "'", with: "-")
            .replacingOccurrences(of: "–", with: "-")
            .replacingOccurrences(of: "“", with: "")
            .replacingOccurrences(of: "”", with: "")
            .components(separatedBy: "-")
        
        for number in numbers {
            guard Int(number) != nil else {
                assertionFailure("Unsupported character found in _\(number)_. Please remove it or add another .replacingOccurrences(of:")
                break }
                
            guard cipherDictionary[number] == nil else { continue }
            cipherDictionary[number] = substitute(number)
        }
        var output = input
        let allNumbers = cipherDictionary.keys.sorted { Int($0)! > Int($1)! }
        
        for number in allNumbers {
            guard let text = cipherDictionary[number] else { continue }
            
            output = output.replacingOccurrences(of: number, with: text)
        }
        
        return output.replacingOccurrences(of: "-", with: "")
            .replacingOccurrences(of: "–", with: "")
    }
    
    override func substitute(_ string: String) -> String {
        guard let index = Int(string),
            index > 0, index < alphabet.count else { return "?" }
        return alphabet[index - 1].isEmpty ? string : alphabet[index - 1]
    }
}