chore: add Rot13 Cipher (#1008)

* Add ROT13 cipher.

* updating DIRECTORY.md

* Fix suggestions.

* Suggestions.

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <halfpacho@gmail.com>

misc/rot13.c

@@ -0,0 +1,60 @@
+/**
+ * @file
+ * @brief [ROT13](https://en.wikipedia.org/wiki/ROT13) is a simple letter 
+ * substitution cipher that replaces a letter with the 13th letter after it in 
+ * the alphabet.
+ * @details ROT13 transforms a piece of text by examining its alphabetic
+ * characters and replacing each one with the letter 13 places further along in
+ * the alphabet, wrapping back to the beginning if necessary. A becomes N, B
+ * becomes O, and so on up to M, which becomes Z, then the sequence continues at
+ * the beginning of the alphabet: N becomes A, O becomes B, and so on to Z,
+ * which becomes M.
+ * @author [Jeremias Moreira Gomes](https://github.com/j3r3mias)
+ */
+
+#include <stdio.h>     /// for IO operations
+#include <string.h>    /// for string operations
+#include <assert.h>    /// for assert
+
+/**
+ * @brief Apply the ROT13 cipher
+ * @param s contains the string to be processed
+ */
+void rot13(char *s) {
+    for (int i = 0; s[i]; i++) {
+        if (s[i] >= 'A' && s[i] <= 'Z') {
+            s[i] = 'A' + ((s[i] - 'A' + 13) % 26);
+        } else if (s[i] >= 'a' && s[i] <= 'z') {
+            s[i] = 'a' + ((s[i] - 'a' + 13) % 26);
+        }
+    } 
+}
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    char test_01[] = "The more I C, the less I see.";
+    rot13(test_01);
+    assert(strcmp(test_01, "Gur zber V P, gur yrff V frr.") == 0);
+
+    char test_02[] = "Which witch switched the Swiss wristwatches?";
+    rot13(test_02);
+    assert(strcmp(test_02, "Juvpu jvgpu fjvgpurq gur Fjvff jevfgjngpurf?") == 0);
+
+    char test_03[] = "Juvpu jvgpu fjvgpurq gur Fjvff jevfgjngpurf?";
+    rot13(test_03);
+    assert(strcmp(test_03, "Which witch switched the Swiss wristwatches?") == 0);
+
+    printf("All tests have successfully passed!\n");
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
+    return 0;
+}