Rot13

// Original Source http://www.dotnetperls.com/rot13-swift
// Edited version


extension String {
    
    func rot13() -> String {
        // Empty character array.
        var result = [Character]()
        // Some ASCII constants.
        // A = 65
        // M = 77
        // Z = 90
        // a = 97
        // m = 109
        // z = 122
        let upperA = 65
        let upperZ = 90
        let lowerA = 97
        let lowerZ = 122
        
        // Loop over utf8 values in string.
        for u in self.utf8 {
            
            let s = Int(u)
            var resultCharacter = Character(UnicodeScalar(s))
            if s <= lowerZ && s >= lowerA { // Between a and z.
                if s + 13 > lowerZ {
                    resultCharacter = Character(UnicodeScalar(s - 13))
                } else {
                    resultCharacter = Character(UnicodeScalar(s + 13))
                }
            } else if s <= upperZ && s >= upperA { // Between A and Z.
                if s + 13 > upperZ {
                    resultCharacter = Character(UnicodeScalar(s - 13))
                } else {
                    resultCharacter = Character(UnicodeScalar(s + 13))
                }
            }
            // Append to Character array.
            result.append(resultCharacter)
            
        }
        // Return String.
        return String(result)
    }
}