Swift random password generator

main.swift

import Foundation

struct PasswordGenerator {
    
    var length: Int
    
    var characterSet: CharacterSet {
        didSet {
            // re-compute the characters array after the character set changing.
            characters = Array(characterSet.characters)
        }
    }
    
    // cached characters.
    private var characters: [Character]
    
    // default options of just using letters (upper and lowercase)
    init(length: Int, characterSet: CharacterSet = .letters) {
        self.length = length
        self.characterSet = characterSet
        self.characters = Array(characterSet.characters)
    }
    
    /// Generate a new password from the given character set.
    ///
    /// - Precondition: The character set must be non-empty.
    func generate() -> String {
        
        // we cannot generate a password if there's no characters to generate it from.
        precondition(!characters.isEmpty,
                     "Cannot generate password from an empty character set.")
        
        let characterCount = UInt32(characters.count)
        
        let result = (0 ..< length).map { _ in
            characters[Int(arc4random_uniform(characterCount))]
        }
        
        return String(result)
    }
}

extension PasswordGenerator {
    
    struct CharacterSet : OptionSet {
        
        static let lowercase = CharacterSet(rawValue: 1 << 0)
        static let uppercase = CharacterSet(rawValue: 1 << 1)
        static let numbers   = CharacterSet(rawValue: 1 << 2)
        static let symbols   = CharacterSet(rawValue: 1 << 3)
        
        static let letters: CharacterSet = [.lowercase, .uppercase]
        static let alphanumeric: CharacterSet = [.letters, .numbers]
        
        let rawValue: Int

        var characters: Set<Character> {
            
            var characters = Set<Character>()
            
            if contains(.lowercase) { // in Swift 4, remove ".characters"
                characters.formUnion("abcdefghijklmnopqrstuvwxyz".characters)
            }
            
            if contains(.uppercase) {
                characters.formUnion("ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters)
            }
            
            if contains(.numbers) {
                characters.formUnion("1234567890".characters)
            }
            
            if contains(.symbols) {
                characters.formUnion("!@#$%^&*".characters)
            }
            
            return characters
        }
    }
}


var passwordGenerator = PasswordGenerator(length: 16, characterSet: .alphanumeric)
print(passwordGenerator.generate()) // GcWrH81pYU46m4lx

passwordGenerator.length = 20
print(passwordGenerator.generate()) // 9SNiPlgYqRCpvpP96cuT

passwordGenerator.characterSet.subtract(.lowercase)
print(passwordGenerator.generate()) // YX1WELZEC7GN4L2JTAVR