largest prime factor calculator (euler project challenges)

index.js

// The prime factors of 13195 are 5, 7, 13 and 29.
// What is the largest prime factor of the number 600851475143?

// 13195 / 5 = 2639
// 2639 / 7 = 377
// 377 / 13 = 29

// a prime number is a number that is divisible only by itself (and 1)

var target = 600851475143;



function findNextPrime(currentPrime) {
  if (currentPrime === 2) {
    currentPrime = 1;
  }

  var newPrime = currentPrime + 2,
      primeFound = false;

  while (!primeFound) {
    var i;

    for (i = 3; i <= newPrime; i = i + 2) {
      if (newPrime % i === 0) {
        if (i === newPrime) {
          primeFound = true;
        } else {
          break;
        }
      }
    }

    if (!primeFound) {
      newPrime = newPrime + 2; // primes are not even, ever, after 2;
    }
  }

  return newPrime;
}



var largestPrimeNumber = 2,
    current = target;

while (current !== largestPrimeNumber) {
  var currentDivision = current % largestPrimeNumber;

  if (currentDivision === 0) {
    // ok
    current = current / largestPrimeNumber;
  } else {
    largestPrimeNumber = findNextPrime(largestPrimeNumber);
  }

  // console.log('current division: ' + current);
  // console.log('current prime: ' + largestPrimeNumber);
}



console.log('largest prime factor: ' + largestPrimeNumber);