Caesar Encryption (Functional/Recursive Solution in Swift)

caesar.swift

// For uppercase letters
func encrypt(key: Int) -> ((String) -> String) {
    // 25 is the highest number for a possible shift
    let actualKey = key % 25
    // If the new index is bigger than 90 (which is an X), calculate the remainder and add 64 (which is A)
    var actualNewIndex: (Int) -> Int = { i in
        if i > 90 { return i % 90 + 64 }
        return i
    }
    
    return { str in
        // Return an empty string if empty
        if (str.isEmpty) { return String() }
        return String(format: "%c", actualNewIndex(str.first!.unicodeScalars.first!.hashValue + actualKey)) + encrypt(key: key)(String(str.dropFirst()))
    }
}