feat: add Caesar Cipher (#957)

* Added caesar_cipher.cpp

* Added ranged based loops and made some args pass by ref

* Changed comment styling and added unnamed namespace

* Update ciphers/caesar_cipher.cpp

Co-authored-by: Krishna Vedala <7001608+kvedala@users.noreply.github.com>

Co-authored-by: Krishna Vedala <7001608+kvedala@users.noreply.github.com>

ciphers/caesar_cipher.cpp

@@ -0,0 +1,124 @@
+/**
+ * @file caesar_cipher.cpp
+ * @brief Implementation of [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher) algorithm.
+ *
+ * @details
+ * In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, 
+ * Caesar's code or Caesar shift, is one of the simplest and most widely known encryption 
+ * techniques. It is a type of substitution cipher in which each letter in the plaintext 
+ * is replaced by a letter some fixed number of positions down the alphabet. For example, 
+ * with a left shift of 3, D would be replaced by A, E would become B, and so on. 
+ * The method is named after Julius Caesar, who used it in his private correspondence.
+ *
+ * ### Algorithm
+ * The encryption can also be represented using modular arithmetic by first transforming 
+ * the letters into numbers, according to the scheme, A → 0, B → 1, ..., Z → 25.
+ * Encryption of a letter x by a shift n can be described mathematically as,
+ * \f[ E(x) = (x + n)\;\mbox{mod}\; 26\f]
+ * while decryption can be described as,
+ * \f[ D(x) = (x - n) \;\mbox{mod}\; 26\f]
+ * 
+ * \note This program implements caesar cipher for only uppercase English alphabet characters (i.e. A-Z). 
+ * 
+ * @author [Deep Raval](https://github.com/imdeep2905)
+ */
+#include <iostream>
+#include <string>
+#include <cassert>
+
+/** \namespace ciphers
+ * \brief Algorithms for encryption and decryption
+ */
+namespace ciphers {
+    /** \namespace caesar
+     * \brief Functions for [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher) algorithm.
+     */
+    namespace caesar {   
+        namespace {
+            /**
+             * This function finds character for given value (i.e.A-Z)
+             * @param x value for which we want character 
+             * @returns  corresponding character for perticular value
+             */        
+            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); 
+            }
+            /**
+             * This function finds value for given character (i.e.0-25)
+             * @param c character for which we want value
+             * @returns returns corresponding value for perticular character
+             */  
+            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
+        /**
+         * Encrypt given text using caesar cipher.
+         * @param text text to be encrypted
+         * @param shift number of shifts to be applied
+         * @returns new encrypted text
+         */
+        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
+        }
+        /**
+         * Decrypt given text using caesar cipher.
+         * @param text text to be decrypted
+         * @param shift number of shifts to be applied
+         * @returns new decrypted 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
+
+/**
+ * Function to test above algorithm
+ */
+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;
+}
+
+/** Driver Code */
+int main() {
+    // Testing
+    test();
+    return 0;
+}