Merge branch 'master' into master

DIRECTORY.md

@@ -19,6 +19,7 @@
   * [Hamming Distance](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/bit_manipulation/hamming_distance.cpp)
 
 ## Ciphers
+  * [Atbash Cipher](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/ciphers/atbash_cipher.cpp)
   * [Base64 Encoding](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/ciphers/base64_encoding.cpp)
   * [Caesar Cipher](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/ciphers/caesar_cipher.cpp)
   * [Elliptic Curve Key Exchange](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/ciphers/elliptic_curve_key_exchange.cpp)

ciphers/atbash_cipher.cpp

@@ -0,0 +1,83 @@
+/**
+ * @file
+ * @brief [Atbash Cipher](https://en.wikipedia.org/wiki/Atbash) implementation
+ * @details The Atbash cipher is a subsitution cipher where the letters of the
+ * alphabet are in reverse. For example, A is replaced with Z, B is replaced
+ * with Y, etc.
+ *
+ * ### Algorithm
+ * The algorithm takes a string, and looks up the corresponding reversed letter
+ * for each letter in the word and replaces it. Spaces are ignored and case is
+ * preserved.
+ *
+ * @author [Focusucof](https://github.com/Focusucof)
+ */
+#include <cassert>   /// for assert
+#include <iostream>  /// for IO operations
+#include <map>       /// for std::map
+#include <string>    /// for std::string
+
+/** \namespace ciphers
+ * \brief Algorithms for encryption and decryption
+ */
+namespace ciphers {
+/** \namespace atbash
+ * \brief Functions for the [Atbash Cipher](https://en.wikipedia.org/wiki/Atbash) implementation
+ */
+namespace atbash {
+std::map<char, char> atbash_cipher_map = {
+    {'a', 'z'}, {'b', 'y'}, {'c', 'x'}, {'d', 'w'}, {'e', 'v'}, {'f', 'u'},
+    {'g', 't'}, {'h', 's'}, {'i', 'r'}, {'j', 'q'}, {'k', 'p'}, {'l', 'o'},
+    {'m', 'n'}, {'n', 'm'}, {'o', 'l'}, {'p', 'k'}, {'q', 'j'}, {'r', 'i'},
+    {'s', 'h'}, {'t', 'g'}, {'u', 'f'}, {'v', 'e'}, {'w', 'd'}, {'x', 'c'},
+    {'y', 'b'}, {'z', 'a'}, {'A', 'Z'}, {'B', 'Y'}, {'C', 'X'}, {'D', 'W'},
+    {'E', 'V'}, {'F', 'U'}, {'G', 'T'}, {'H', 'S'}, {'I', 'R'}, {'J', 'Q'},
+    {'K', 'P'}, {'L', 'O'}, {'M', 'N'}, {'N', 'M'}, {'O', 'L'}, {'P', 'K'},
+    {'Q', 'J'}, {'R', 'I'}, {'S', 'H'}, {'T', 'G'}, {'U', 'F'}, {'V', 'E'},
+    {'W', 'D'}, {'X', 'C'}, {'Y', 'B'}, {'Z', 'A'}, {' ', ' '}
+
+};
+
+/**
+ * @brief atbash cipher encryption and decryption
+ * @param text Plaintext to be encrypted
+ * @returns encoded or decoded string
+ */
+std::string atbash_cipher(std::string text) {
+    std::string result;
+    for (char letter : text) {
+        result += atbash_cipher_map[letter];
+    }
+    return result;
+}
+
+}  // namespace atbash
+}  // namespace ciphers
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    // 1st test
+    std::string text = "Hello World";
+    std::string expected = "Svool Dliow";
+    std::string encrypted_text = ciphers::atbash::atbash_cipher(text);
+    std::string decrypted_text = ciphers::atbash::atbash_cipher(encrypted_text);
+    assert(expected == encrypted_text);
+    assert(text == decrypted_text);
+    std::cout << "Original text: " << text << std::endl;
+    std::cout << ", Expected text: " << expected << std::endl;
+    std::cout << ", Encrypted text: " << encrypted_text << std::endl;
+    std::cout << ", Decrypted text: " << decrypted_text << std::endl;
+    std::cout << "\nAll tests have successfully passed!\n";
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
+    return 0;
+}