largestPrime.c

#include <string.h>
#include <stdio.h>
#include <math.h>
#include <inttypes.h>

int64_t divide_into_largest_factor(int64_t input) {
  int64_t divisor = 2;
  if (2 < input) {
    while (divisor < (sqrt(input)+1)) {
      if (input % divisor == 0) {
        return (input / divisor);
      } else {
        ++divisor;
      }
    }
  }
  return input;
}

int main() {
  int64_t input = 600851475143;
  int64_t temp = input;
  int64_t lastTemp = 0;
  while (temp != lastTemp) {
    lastTemp = temp;
    temp = divide_into_largest_factor(temp);
  }
  printf("outta input %lld; largest prime is: %lld\n", input, temp);
  return 0;
}