https://www.objc.io/blog/2015/01/26/functional-snippet-17-caesar-ciphers/

caeser-cipher-and-rot13.swift

// Updated to Swift 3 syntax.

func mapScalarValues(s: String, f: (UInt32) -> UInt32) -> String {
    let scalars = Array(s.unicodeScalars)
    let encrypted = scalars.map { x in
        Character(UnicodeScalar(f(x.value))!)
    }
    return String(encrypted)
}

func caesar(plainText: String) -> String {
    return mapScalarValues(s: plainText) { x in x + 7 }
}

func rot13(plainText: String) -> String {
    let A: UInt32 = 65
    let Z: UInt32 = 90
    return mapScalarValues(s: plainText) { x in
        if x >= A && x <= Z  {
            return A + ((x + 13) % 26)
        }
        return x
    }
}

let input = "AAA"
let output = rot13(plainText: input)
print(output) // output is "AAA".