Question

I am trying to write a function in Java that will return the number of factors a specific number has.

The following restrictions should be taken into account.

1. It should be done with BigInteger
2. Storing the previously generated numbers is not allowed, thus more processing and less memory. (You can not use "Sieve of Atkin" like in this)
3. Negative numbers can be ignored.

This is what I have so far, but it is extremely slow.

public static int getNumberOfFactors(BigInteger number) {
    // If the number is 1
    int numberOfFactors = 1;
    
    if (number.compareTo(BigInteger.ONE) <= 0)  {
        return numberOfFactors;
    }
    
    BigInteger boundry = number.divide(new BigInteger("2"));
    BigInteger counter = new BigInteger("2");
    
    while (counter.compareTo(boundry) <= 0) {
        if (number.mod(counter).compareTo(BigInteger.ZERO) == 0) {
            numberOfFactors++;
        }
        
        counter = counter.add(BigInteger.ONE);
    }
    
    // For the number it self
    numberOfFactors++;
    
    return numberOfFactors;
}

Answer 1

I can propose faster solution, though I have a feeling that it will not be fast enough yet. Your solution runs in O(n) and mine will work in O(sqrt(n)).

I am going to use the fact that if n = x_i1^p1 * x_i2^p2 * x_i3^p3 * … x_ik^pk is the prime factorization of n (i.e. x_ij are all distinct primes) then n has (p1 + 1) * (p2 + 1) * … * (pk + 1) factors in total.

Now here goes the solution:

BigInteger x = new BigInteger("2");
long totalFactors = 1;
while (x.multiply(x).compareTo(number) <= 0) {
    int power = 0;
    while (number.mod(x).equals(BigInteger.ZERO)) {
        power++;
        number = number.divide(x);
    }
    totalFactors *= (power + 1);
    x = x.add(BigInteger.ONE);
}
if (!number.equals(BigInteger.ONE)) {
    totalFactors *= 2;
}
System.out.println("The total number of factors is: " + totalFactors);

This can be further optimized if you consider the case of 2 separately and then have the step for x equal to 2 not 1 (iterating only the odd numbers).

Also note that in my code I modify number, you might find it more suitable to keep number and have another variable equal to number to iterate over.

I suppose that this code will run reasonably fast for numbers not greater than 2⁶⁴.

EDIT I will add the measures of reasonably fast to the answer for completeness. As it can be seen in the comments below I made several measurements on the performance of the proposed algorithm for the test case 100000007², which was proposed by Betlista:

• If the algorithm is used as is the time taken is 57 seconds on my machine.
• If I consider only the odd numbers the time is reduced to 28 seconds
• If I change the check for the end condition of the while to comparing with the square root of number which I find using binary search the time taken reduces to 22 second.
• Finally when I tried switching all the BigIntegers with long the time was reduced to 2 seconds. As the proposed algorithm will not run fast enough for number larger than the range of long it might make sense to switch the implementation to long