A simple example of Caesar Cipher made in swift

CaesarCipher.swift

struct CaesarCipher {
    enum Operation {
        case encode
        case decode
    }

    let charTable: [Character]

    private func crypt(text: String, shift: Int, operation: Operation) -> String {
        var result: [Character] = []
        let charTable = operation == .decode ? Array(charTable.reversed()) : charTable

        for character in text {
            guard let index = charTable.firstIndex(of: character) else {
                result.append(character)
                continue
            }

            let newIndex = (index + shift) % charTable.count
            result.append(charTable[newIndex])
        }

        return String(result)
    }

    func callAsFunction(text: String, shift: Int, operation: Operation) -> String {
        crypt(text: text, shift: shift, operation: operation)
    }
}

// ===================================
// USAGE
// ===================================

let charTable = "qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM1234567890 ,."
let cipher = CaesarCipher(charTable: Array(charTable))

let sourceText = "Yes, this is my source text."
let shift = 234
let encodedText = cipher(text: sourceText, shift: shift, operation: .encode)
let decodedText = cipher(text: encodedText, shift: shift, operation: .decode)

print("===== RESULT =====")
print("Source Text: \(sourceText)")
print("Encode Text: \(encodedText)")
print("Decode Text: \(decodedText)")