Add LuhnAlgorithm.js and LuhnAlgorithm.test.js in Maths folder

Maths/LuhnAlgorithm.js

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+/**
+ * Luhn algorithm, also known as the “Modulus 10” or “mod 10” algorithm.
+ * This algorithm is a simple checksum formula used to validate the authenticity of credit card numbers.
+ * The Luhn algorithm works by doubling every second digit from right to left, adding the resulting digits, and then checking if the total is divisible by 10.
+ * If it is, the credit card number is considered valid.
+ * The Luhn algorithm is widely used in credit card verification and is specified in ISO/IEC 7812-1.
+ * It is not intended to be a cryptographically secure hash function but rather a simple method to detect accidental errors, not malicious attacks.
+ * https://en.wikipedia.org/wiki/Luhn_algorithm
+ */
+
+// Valid credit card numbers
+export const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8]
+export const valid2 = [5, 5, 3, 5, 7, 6, 6, 7, 6, 8, 7, 5, 1, 4, 3, 9]
+export const valid3 = [6, 0, 1, 1, 1, 4, 4, 3, 4, 0, 6, 8, 2, 9, 0, 5]
+export const valid4 = [3, 7, 1, 6, 1, 2, 0, 1, 9, 9, 8, 5, 2, 3, 6]
+export const valid5 = [4, 5, 3, 9, 4, 0, 4, 9, 6, 7, 8, 6, 9, 6, 6, 6]
+
+// Invalid credit card numbers
+export const invalid1 = [4, 5, 3, 2, 7, 7, 8, 7, 7, 1, 0, 9, 1, 7, 9, 5]
+export const invalid2 = [5, 7, 9, 5, 5, 9, 3, 3, 9, 2, 1, 3, 4, 6, 4, 3]
+export const invalid3 = [3, 7, 5, 7, 9, 6, 0, 8, 4, 4, 5, 9, 9, 1, 4]
+export const invalid4 = [6, 0, 1, 1, 1, 2, 7, 9, 6, 1, 7, 7, 7, 9, 3, 5]
+export const invalid5 = [5, 3, 8, 2, 0, 1, 9, 7, 7, 2, 8, 8, 3, 8, 5, 4]
+
+// Mixed batch of credit card numbers
+export const mystery1 = [3, 4, 4, 8, 0, 1, 9, 6, 8, 3, 0, 5, 4, 1, 4]
+export const mystery2 = [5, 4, 6, 6, 1, 0, 0, 8, 6, 1, 6, 2, 0, 2, 3, 9]
+export const mystery3 = [
+  6, 0, 1, 1, 3, 7, 7, 0, 2, 0, 9, 6, 2, 6, 5, 6, 2, 0, 3
+]
+export const mystery4 = [4, 9, 2, 9, 8, 7, 7, 1, 6, 9, 2, 1, 7, 0, 9, 3]
+export const mystery5 = [4, 9, 1, 3, 5, 4, 0, 4, 6, 3, 0, 7, 2, 5, 2, 3]
+
+export const batch = [
+  valid1,
+  valid2,
+  valid3,
+  valid4,
+  valid5,
+  invalid1,
+  invalid2,
+  invalid3,
+  invalid4,
+  invalid5,
+  mystery1,
+  mystery2,
+  mystery3,
+  mystery4,
+  mystery5
+]
+
+/**
+ * Validates a credit card number using the Luhn Algorithm.
+ * @param {number[]} numArr - Array representing the credit card number.
+ * @returns {boolean} - True if the card is valid, false otherwise.
+ */
+export function validateCred(numArr) {
+  let total = 0
+  for (let i = numArr.length - 1; i >= 0; i--) {
+    let currValue = numArr[i]
+    // Double every other digit starting from the second-to-last
+    if ((numArr.length - 1 - i) % 2 === 1) {
+      currValue *= 2
+      if (currValue > 9) {
+        currValue -= 9
+      }
+    }
+    total += currValue
+  }
+  return total % 10 === 0
+}
+
+/**
+ * Finds invalid credit card numbers from a batch.
+ * @param {number[][]} cards - Array of credit card numbers.
+ * @returns {number[][]} - Array of invalid credit card numbers.
+ */
+export function findInvalidCards(cards) {
+  return cards.filter((card) => !validateCred(card))
+}
+
+/**
+ * Identifies credit card companies that issued invalid card numbers.
+ * @param {number[][]} invalidBatch - Array of invalid credit card numbers.
+ * @returns {string[]} - Array of company names.
+ */
+export function idInvalidCardCompanies(invalidBatch) {
+  const companies = new Set()
+  invalidBatch.forEach((card) => {
+    switch (card[0]) {
+      case 3:
+        companies.add('Amex')
+        break
+      case 4:
+        companies.add('Visa')
+        break
+      case 5:
+        companies.add('Mastercard')
+        break
+      case 6:
+        companies.add('Discover')
+        break
+      default:
+        console.warn('Company not found for card starting with:', card[0])
+    }
+  })
+  return Array.from(companies)
+}
+
+// Example Usage
+console.log(validateCred(valid1)) // Should print true
+console.log(validateCred(invalid1)) // Should print false
+console.log(findInvalidCards(batch)) // Find all invalid cards
+console.log(idInvalidCardCompanies(findInvalidCards(batch))) // Find companies of invalid cards

Maths/test/LuhnAlgorithm.test.js

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+// Import the functions to be tested
+import {
+  validateCred,
+  findInvalidCards,
+  idInvalidCardCompanies
+} from '../LuhnAlgorithm.js'
+// Import credit card numbers to be tested
+import {
+  valid1,
+  valid2,
+  valid3,
+  valid4,
+  valid5,
+  invalid1,
+  invalid2,
+  invalid3,
+  invalid4,
+  invalid5,
+  batch
+} from '../LuhnAlgorithm.js'
+
+describe('Luhn Algorithm Tests', () => {
+  // Test for validateCred function
+  describe('validateCred', () => {
+    test('should return true for valid credit cards', () => {
+      const validCards = [valid1, valid2, valid3, valid4, valid5]
+      validCards.forEach((card) => {
+        expect(validateCred(card)).toBe(true)
+      })
+    })
+
+    test('should return false for invalid credit cards', () => {
+      const invalidCards = [invalid1, invalid2, invalid3, invalid4, invalid5]
+      invalidCards.forEach((card) => {
+        expect(validateCred(card)).toBe(false)
+      })
+    })
+  })
+
+  // Test for findInvalidCards function
+  describe('findInvalidCards', () => {
+    test('should return only invalid credit cards from a batch', () => {
+      const invalidCards = findInvalidCards(batch)
+      const allInvalid = invalidCards.every((card) => !validateCred(card))
+      expect(allInvalid).toBe(true)
+    })
+
+    test('should return an empty array if no invalid cards are found', () => {
+      const validCards = [valid1, valid2, valid3, valid4, valid5]
+      const invalidCards = findInvalidCards(validCards)
+      expect(invalidCards).toEqual([])
+    })
+  })
+
+  // Test for idInvalidCardCompanies function
+  describe('idInvalidCardCompanies', () => {
+    test('should identify all companies that issued invalid cards', () => {
+      const invalidCards = findInvalidCards(batch)
+      const companies = idInvalidCardCompanies(invalidCards)
+
+      // Expect the companies array to include known companies
+      expect(companies).toEqual(
+        expect.arrayContaining(['Visa', 'Mastercard', 'Amex', 'Discover'])
+      )
+    })
+
+    test('should return an empty array if no invalid cards are found', () => {
+      const validCards = [valid1, valid2, valid3, valid4, valid5]
+      const invalidCards = findInvalidCards(validCards) // Ensure only invalid cards are processed
+      const companies = idInvalidCardCompanies(invalidCards)
+      expect(companies).toEqual([])
+    })
+
+    test('should handle unknown card companies gracefully', () => {
+      const unknownCard = [9, 3, 1, 8, 2, 0, 7, 4, 5, 6, 7, 8, 9]
+      const companies = idInvalidCardCompanies([unknownCard])
+      expect(companies).not.toContain('Unknown')
+    })
+  })
+})