Made Code Dojo 4: Palindromic Numbers

README.txt

SUCCESS

## http://projecteuler.net/problem=4

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.

PalindromeProduct.js

function Palindrome () { };

Palindrome.prototype.isPalindrome = function (candidate) {
  reversedCandidate = candidate.toString().split("").reverse().join("");

  return candidate.toString() == reversedCandidate;
};

Palindrome.prototype.product = function (number1, number2) {
  return number1 * number2;
};

Palindrome.prototype.highestNumberForNumberOfDigits = function (numberOfDigits) {
  return Math.pow(10, numberOfDigits) - 1;
};

Palindrome.prototype.lowestNumberForNumberOfDigits = function (numberOfDigits) {
  return Math.pow(10, numberOfDigits - 1);
};

Palindrome.prototype.highestProduct = function (numberOfDigits) {
  var highestNumber = this.highestNumberForNumberOfDigits(numberOfDigits),
      lowestNumber = this.lowestNumberForNumberOfDigits(numberOfDigits),
      digit1 = highestNumber,
      digit2 = highestNumber,
      product,
      highestPalindrome = 0;

  for (; digit1 >= lowestNumber; digit1--) {
    for (; digit2 >= lowestNumber; digit2--) {
      product = this.product(digit1, digit2);

      if (this.isPalindrome(product) && (product > highestPalindrome)) {
        highestPalindrome = product;
      }
    }

    digit2 = highestNumber;
  };

  return highestPalindrome;
};

Palindrome.prototype.highestProductFor2Digits = function () {
  return this.highestProduct(2);
};

Palindrome.prototype.highestProductFor3Digits = function () {
  return this.highestProduct(3);
};

PalindromeProductSpec.js

describe('Palindrome', function () {
  var palindrome = new Palindrome();

  describe('isPalindrome', function () {
    it('should return false for a non palindrome', function () {
      expect(palindrome.isPalindrome(54)).toBe(false);
    });

    it('should return true for a palindrome', function () {
      expect(palindrome.isPalindrome(9009)).toBe(true);
    });
  });

  describe('product', function () {
    it('should return the product of 2 numbers', function () {
      expect(palindrome.product(10, 10)).toBe(100);
    });
  });

  describe('highestProductFor2Digits', function () {
    it('should return the highest product of 2 digit numbers that is a palindrome', function () {
      expect(palindrome.highestProductFor2Digits()).toBe(9009);
    });
  });

  describe('highestProductFor3Digits', function () {
    it('should return the highest product of 3 digit numbers that is a palindrome', function () {
      expect(palindrome.highestProductFor3Digits()).toBe(906609);
    });
  });

  describe('highestNumberForNumberOfDigits', function () {
    it('should return highest number given number of digits', function () {
      expect(palindrome.highestNumberForNumberOfDigits(3)).toBe(999);
    });
  });

  describe('lowestNumberForNumberOfDigits', function () {
    it('should return lowest number given number of digits', function () {
      expect(palindrome.lowestNumberForNumberOfDigits(3)).toBe(100);
    });
  });
});