Caesar cypher implementation in Swift

caesar.swift

extension String {
    var codePoint: UInt32? {
        guard self.unicodeScalars.count == 1 else { return nil }

        return self.unicodeScalars.first!.value
    }

    func mapCodePoints(fn: UInt32 -> UInt32) -> String {
        return String(self.unicodeScalars.map { Character(UnicodeScalar(fn($0.value))) })
    }
}

func rotate(code: UInt32, by offset: Int, withAdjustment adj: UInt32) -> UInt32 {
    let azLength = 26
    let normalizedOffset = UInt32((offset % azLength + azLength) % azLength)

    return adj + ((code - adj + normalizedOffset) % UInt32(azLength))
}

func caesarize(str: String, withOffset offset: Int) -> String {
    let A = "A".codePoint!, Z = "Z".codePoint!,
        a = "a".codePoint!, z = "z".codePoint!

    return str.mapCodePoints { code -> UInt32 in
        switch code {
        case A...Z:
            return rotate(code, by: offset, withAdjustment: A)
        case a...z:
            return rotate(code, by: offset, withAdjustment: a)
        default:
            return code
        }
    }
}

caesarize("Ave, Caesar!", withOffset: -1)