Refactor a Caesar Cipher algorithm.

Author: trekhleb

README.md

@@ -135,6 +135,7 @@ a set of rules that precisely define a sequence of operations.
   * `A` [Travelling Salesman Problem](src/algorithms/graph/travelling-salesman) - shortest possible route that visits each city and returns to the origin city
 * **Cryptography**
   * `B` [Polynomial Hash](src/algorithms/cryptography/polynomial-hash) - rolling hash function based on polynomial
+  * `B` [Caesar Cipher](src/algorithms/cryptography/caesar-cipher) - simple substitution cipher
 * **Machine Learning**
   * `B` [NanoNeuron](https://github.com/trekhleb/nano-neuron) - 7 simple JS functions that illustrate how machines can actually learn (forward/backward propagation)
 * **Uncategorized**

src/algorithms/cryptography/caesar-cipher/README.md

@@ -0,0 +1,30 @@
+# Caesar Cipher Algorithm
+
+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.
+
+![Caesar Cipher Algorithm](https://upload.wikimedia.org/wikipedia/commons/4/4a/Caesar_cipher_left_shift_of_3.svg)
+
+## Example
+
+The transformation can be represented by aligning two alphabets; the cipher alphabet is the plain alphabet rotated left or right by some number of positions. For instance, here is a Caesar cipher using a left rotation of three places, equivalent to a right shift of 23 (the shift parameter is used as the key):
+
+```text
+Plain:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
+Cipher:   XYZABCDEFGHIJKLMNOPQRSTUVW
+```
+
+When encrypting, a person looks up each letter of the message in the "plain" line and writes down the corresponding letter in the "cipher" line.
+
+```text
+Plaintext:  THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
+Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
+```
+
+## Complexity
+
+- Time: `O(|n|)`
+- Space: `O(|n|)`
+
+## References
+
+- [Caesar cipher on Wikipedia](https://en.wikipedia.org/wiki/Caesar_cipher)

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

@@ -0,0 +1,40 @@
+import { caesarCipherEncrypt, caesarCipherDecrypt } from '../caesarCipher';
+
+describe('caesarCipher', () => {
+  it('should not change a string with zero shift', () => {
+    expect(caesarCipherEncrypt('abcd', 0)).toBe('abcd');
+    expect(caesarCipherDecrypt('abcd', 0)).toBe('abcd');
+  });
+
+  it('should cipher a string with different shifts', () => {
+    expect(caesarCipherEncrypt('abcde', 3)).toBe('defgh');
+    expect(caesarCipherDecrypt('defgh', 3)).toBe('abcde');
+
+    expect(caesarCipherEncrypt('abcde', 1)).toBe('bcdef');
+    expect(caesarCipherDecrypt('bcdef', 1)).toBe('abcde');
+
+    expect(caesarCipherEncrypt('xyz', 1)).toBe('yza');
+    expect(caesarCipherDecrypt('yza', 1)).toBe('xyz');
+  });
+
+  it('should be case insensitive', () => {
+    expect(caesarCipherEncrypt('ABCDE', 3)).toBe('defgh');
+  });
+
+  it('should correctly handle an empty strings', () => {
+    expect(caesarCipherEncrypt('', 3)).toBe('');
+  });
+
+  it('should not cipher unknown chars', () => {
+    expect(caesarCipherEncrypt('ab2cde', 3)).toBe('de2fgh');
+    expect(caesarCipherDecrypt('de2fgh', 3)).toBe('ab2cde');
+  });
+
+  it('should encrypt and decrypt full phrases', () => {
+    expect(caesarCipherEncrypt('THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG', 23))
+      .toBe('qeb nrfzh yoltk clu grjmp lsbo qeb ixwv ald');
+
+    expect(caesarCipherDecrypt('qeb nrfzh yoltk clu grjmp lsbo qeb ixwv ald', 23))
+      .toBe('the quick brown fox jumps over the lazy dog');
+  });
+});

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('');
+};