Refactor a Caesar Cipher algorithm.

src/algorithms/cryptography/caesar-cipher/caesarCipher.js

@@ -0,0 +1,58 @@
+// Create alphabet array: ['a', 'b', 'c', ..., 'z'].
+const englishAlphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');
+
+/**
+ * Generates a cipher map out of the alphabet.
+ * Example with a shift 3: {'a': 'd', 'b': 'e', 'c': 'f', ...}
+ *
+ * @param {string[]} alphabet - i.e. ['a', 'b', 'c', ... , 'z']
+ * @param {number} shift - i.e. 3
+ * @return {Object} - i.e. {'a': 'd', 'b': 'e', 'c': 'f', ..., 'z': 'c'}
+ */
+const getCipherMap = (alphabet, shift) => {
+  return alphabet
+    .reduce((charsMap, currentChar, charIndex) => {
+      const charsMapClone = { ...charsMap };
+      // Making the shift to be cyclic (i.e. with a shift of 1 - 'z' would be mapped to 'a').
+      let encryptedCharIndex = (charIndex + shift) % alphabet.length;
+      // Support negative shifts for creating a map for decryption
+      // (i.e. with shift -1 - 'a' would be mapped to 'z').
+      if (encryptedCharIndex < 0) {
+        encryptedCharIndex += alphabet.length;
+      }
+      charsMapClone[currentChar] = alphabet[encryptedCharIndex];
+      return charsMapClone;
+    }, {});
+};
+
+/**
+ * @param {string} str
+ * @param {number} shift
+ * @param {string[]} alphabet
+ * @return {string}
+ */
+export const caesarCipherEncrypt = (str, shift, alphabet = englishAlphabet) => {
+  // Create a cipher map:
+  const cipherMap = getCipherMap(alphabet, shift);
+  return str
+    .toLowerCase()
+    .split('')
+    .map((char) => cipherMap[char] || char)
+    .join('');
+};
+
+/**
+ * @param {string} str
+ * @param {number} shift
+ * @param {string[]} alphabet
+ * @return {string}
+ */
+export const caesarCipherDecrypt = (str, shift, alphabet = englishAlphabet) => {
+  // Create a cipher map:
+  const cipherMap = getCipherMap(alphabet, -shift);
+  return str
+    .toLowerCase()
+    .split('')
+    .map((char) => cipherMap[char] || char)
+    .join('');
+};