TheAlgorithms · raklaptudirm · Jul 27, 2022 · Jul 27, 2022 · Jul 27, 2022 · Jul 27, 2022
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+/**
+ * @description - The affine cipher is a type of monoalphabetic substitution cipher, where each letter in an alphabet is mapped to    its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter
+ * @see - [wiki](https://en.wikipedia.org/wiki/Affine_cipher)
+ */
+
+// Default key for Affine Cipher
+const key = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+
+/**
+ * Fix result for negative value in modulas equation
+ * @param {Number} n - Constant number
+ * @param {Number} m - Modulos value
+ * @return {Number} Return n mod m
+ */
+function mod (n, m) {
+  return ((n % m) + m) % m
+}
+
+/**
+ * Modular multiplicative inverse
+ * @param {Number} a - A coefficient
+ * @param {Number} m - Modulos value
+ * @return {Number} Return modular multiplicative inverse of coefficient a and modulos m
+ */
+function inverseMod (a, m) {
+  for (let x = 1; x < m; x++) {
+    if (mod(a * x, m) === 1) return x
+  }
+}
+
+/**
+ * Argument validation
+ * @param {String} str - String to be checked
+ * @param {Number} a - A coefficient to be checked
+ * @param {Number} b - B coefficient to be checked
+ * @return {Boolean} Result of the checking
+ */
+function isCorrectFormat (str, a, b) {
+  if (typeof a !== 'number' || typeof b !== 'number') {
+    throw new TypeError('Coefficient a, b should be number')
+  }
+
+  if (typeof str !== 'string') {
+    throw new TypeError('Argument str should be String')
+  }
+
+  return true
+}
+/**
+ * Find character index based on ASCII order
+ * @param {String} char - Character index to be found
+ * @return {Boolean} Character index
+ */
+function findCharIndex (char) {
+  return char.toUpperCase().charCodeAt(0) - ('A').charCodeAt(0)
+}
+
+/**
+ * Encrypt a Affine Cipher
+ * @param {String} str - String to be encrypted
+ * @param {Number} a - A coefficient
+ * @param {Number} b - B coefficient
+ * @return {String} result - Encrypted string
+ */
+function encrypt (str, a, b) {
+  let result = ''
+  if (isCorrectFormat(str, a, b)) {
+    for (let x = 0; x < str.length; x++) {
+      const charIndex = findCharIndex(str[x])
+      if (charIndex < 0) result += '-1' + ' '
+      else result += mod(a * charIndex + b, 26) + ' '
+    }
+  }
+  return result.trim()
+}
+/**
+ * Decrypt a Affine Cipher
+ * @param {String} str - String to be decrypted
+ * @param {Number} a - A coefficient
+ * @param {Number} b - B coefficient
+ * @return {String} result - Decrypted string
+ */
+function decrypt (str, a, b) {
+  let result = ''
+  if (isCorrectFormat(str, a, b)) {
+    str = str.split(' ')
+    for (let x = 0; x < str.length; x++) {
+      if (str[x] === '-1') result += ' '
+      else result += key[mod(inverseMod(a, 26) * (parseInt(str[x]) - b), 26)]
+    }
+  }
+  return result
+}
+
+export { encrypt, decrypt }
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+import { encrypt, decrypt } from '../AffineCipher'
+
+describe('Test Affine Cipher', () => {
+  it('Test - 1, Pass invalid input to encrypt function', () => {
+    expect(() => encrypt(null, null, null)).toThrow()
+    expect(() => encrypt('null', null, null)).toThrow()
+    expect(() => encrypt('null', 1, null)).toThrow()
+    expect(() => encrypt('null', null, 1)).toThrow()
+  })
+
+  it('Test - 2, Pass invalid input to decrypt function', () => {
+    expect(() => decrypt(null, null, null)).toThrow()
+    expect(() => decrypt('null', null, null)).toThrow()
+    expect(() => decrypt('null', 1, null)).toThrow()
+    expect(() => decrypt('null', null, 1)).toThrow()
+  })
+
+  it('Test - 3 Pass string value to encrypt and ecrypt function', () => {
+    const a = 5
+    const b = 8
+    expect(decrypt(encrypt('HELLO WORLD', a, b), a, b)).toBe('HELLO WORLD')
+    expect(decrypt(encrypt('ABC DEF', a, b), a, b)).toBe('ABC DEF')
+    expect(decrypt(encrypt('Brown fox jump over the fence', a, b), a, b)).toBe(
+      'BROWN FOX JUMP OVER THE FENCE'
+    )
+  })
+})