/*
* For the given int, return the list of its prime factors. Will include itself if it is prime.
* Will not compress the list (i.e. the list may naturally return duplicates)
*/
public static List <long> GetPrimeFactors(long num)
{
List <long> primeFactors = new List <long>();
// Some safety - don't evaluate numbers < 2, instead immediately return the empty list
if (num < 2)
{
return(primeFactors);
}
LowestAndHighestFactor factorPair = GetLowestAndHighestFactor(num);
long lowestFactor = factorPair.lowest;
long highestFactor = factorPair.highest;
// If the highest factor is itself, it is prime
if (highestFactor == num)
{
primeFactors.Add(highestFactor);
return(primeFactors);
}
primeFactors.AddRange(GetPrimeFactors(highestFactor));
primeFactors.Add(lowestFactor);
return(primeFactors);
}
/*
* For the given int, determine the lowest and highest factor (not necessarily prime, and not including 1) and return a LowestAndHighestFactor object.
* E.g. for 24, this method would return 2 and 12; 19 would return 19 and 19.
*/
public static LowestAndHighestFactor GetLowestAndHighestFactor(long num)
{
// Some safety - don't evaluate numbers < 2, instead immediately return null
if (num < 2)
{
throw new InvalidOperationException("Don't call getLowestAndHighestFactor() with a number less than 2");
}
bool found = false;
long lowest = 2;
long highest = num / lowest;
while (lowest <= highest)
{
if (num % lowest == 0)
{
found = true;
break;
}
else
{
lowest++;
}
}
if (found)
{
highest = num / lowest;
}
else
{
highest = num;
lowest = num;
}
LowestAndHighestFactor pair = new LowestAndHighestFactor(lowest, highest);
return(pair);
}