caesar_cipher.cpp

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#include <iostream>
#include <string>
#include <cassert>

namespace ciphers {
    namespace caesar {   
        namespace {        
            inline char get_char(const int x) {
                // By adding 65 we are scaling 0-25 to 65-90. 
                // Which are in fact ASCII values of A-Z. 
                return char(x + 65); 
            }  
            inline int get_value(const char c) {
                // A-Z have ASCII values in range 65-90.
                // Hence subtracting 65 will scale them to 0-25.
                return int(c - 65);
            }
        } // Unnamed namespace
        std::string encrypt (const std::string &text, const int &shift) {
            std::string encrypted_text = ""; // Empty string to store encrypted text
            for (char c : text) { // Going through each character
                int place_value = get_value(c); // Getting value of character (i.e. 0-25)
                place_value = (place_value + shift) % 26; // Applying encryption formula
                char new_char = get_char(place_value); // Getting new character from new value (i.e. A-Z)
                encrypted_text += new_char; // Appending encrypted character
            }
            return encrypted_text; // Returning encrypted text
        }
        
        std::string decrypt (const std::string &text, const int &shift) {
            std::string decrypted_text = ""; // Empty string to store decrypted text
            for (char c : text) { // Going through each character
                int place_value = get_value(c); // Getting value of character (i.e. 0-25)
                place_value = (place_value - shift) % 26;// Applying decryption formula
                if(place_value < 0) { // Handling case where remainder is negative 
                    place_value = place_value + 26;
                }
                char new_char = get_char(place_value); // Getting original character from decrypted value (i.e. A-Z)
                decrypted_text += new_char; // Appending decrypted character
            }
            return decrypted_text; // Returning decrypted text
        }
    } // namespace caesar
} // namespace ciphers

void test() {
    // Test 1
    std::string text1 = "ALANTURING";
    std::string encrypted1 = ciphers::caesar::encrypt(text1, 17);
    std::string decrypted1 = ciphers::caesar::decrypt(encrypted1, 17);
    assert(text1 == decrypted1);
    std::cout << "Original text : " << text1;
    std::cout << " , Encrypted text (with shift = 21) : " << encrypted1;
    std::cout << " , Decrypted text : "<< decrypted1 << std::endl;
    // Test 2
    std::string text2 = "HELLOWORLD";
    std::string encrypted2 = ciphers::caesar::encrypt(text2, 1729);
    std::string decrypted2 = ciphers::caesar::decrypt(encrypted2, 1729);
    assert(text2 == decrypted2);
    std::cout << "Original text : " << text2;
    std::cout << " , Encrypted text (with shift = 1729) : " << encrypted2;
    std::cout << " , Decrypted text : "<< decrypted2 << std::endl;
}

int main() {
    // Testing
    test();
    return 0;
}