Complete caesars cipher

Author: ahcode0919

src/fcc-course-projects/caesars_cipher.js

@@ -0,0 +1,15 @@
+export function rot13(str) {
+  // A = 0 , Z = 25
+  const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+  let decoded = [];
+  for (let i = 0; i < str.length; i += 1) {
+    if (/\W/.test(str[i])) {
+      decoded.push(str[i]);
+      continue;
+    }
+    const startIndex = alphabet.indexOf(str[i]);
+    const newIndex = startIndex + 13 <= 25 ? startIndex + 13 : (startIndex + 13) - 26;
+    decoded.push(alphabet[newIndex]);
+  }
+  return decoded.join('');
+}

src/fcc-course-projects/fcc_course_projects.md

@@ -51,3 +51,31 @@ export function convertToRoman(num) {
   return romanNumeral;
 }
 ```
+
+#### Caesars Cipher
+
+One of the simplest and most widely known ciphers is a Caesar cipher, also known as a shift cipher. In a shift cipher the meanings of the letters are shifted by some set amount.
+
+A common modern use is the ROT13 cipher, where the values of the letters are shifted by 13 places. Thus `'A' ↔ 'N', 'B' ↔ 'O'` and so on.
+
+Write a function which takes a ROT13 encoded string as input and returns a decoded string.
+
+All letters will be uppercase. Do not transform any non-alphabetic character (i.e. spaces, punctuation), but do pass them on.
+
+```javascript
+export function rot13(str) {
+  // A = 0 , Z = 25
+  const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+  let decoded = [];
+  for (let i = 0; i < str.length; i += 1) {
+    if (/\W/.test(str[i])) {
+      decoded.push(str[i]);
+      continue;
+    }
+    const startIndex = alphabet.indexOf(str[i]);
+    const newIndex = startIndex + 13 <= 25 ? startIndex + 13 : (startIndex + 13) - 26;
+    decoded.push(alphabet[newIndex]);
+  }
+  return decoded.join('');
+}
+```

tests/fcc-course-projects/caesars_cipher.test.js

@@ -0,0 +1,5 @@
+import { rot13 } from "../../src/fcc-course-projects/caesars_cipher";
+
+test('should decode caesars cipher', () => {
+  expect(rot13("SERR PBQR PNZC!")).toBe("FREE CODE CAMP!");
+});