Calculate maximum prime multiplier of number

maxprimemultiplier.cpp

#include <iostream>
#include <inttypes.h>
#include <math.h>
using namespace std;

uint64_t maxPrimeMul (uint64_t num);
bool isPrime (uint64_t num);

int main () {
    uint64_t num; // = 600851475143;
    cin >> num;
    uint64_t mul = maxPrimeMul(num);
    cout << mul << endl;
    return 0;
}

// Search for maximum prime multiplier of number
uint64_t maxPrimeMul (uint64_t num) {
    // In the case if num is prime number itself…
    if (isPrime(num))
        return num; // Yes, it's the answer!
    // Otherwise, we need to search for it
    uint64_t top = num/2, max = 1;
    for (uint64_t i = 1; i < top; i++) {
        if (isPrime(i) && !(num%i)) {
            max = i;
            cerr << "POSSIBLE: " << i << endl; // Uncomment for debug purposes
        }
    }
    return max;
} // Thank you for your patience

// Check number to be a prime one
bool isPrime (uint64_t num) {
    if ( !(num%2) )
        return false;
    uint64_t top = (uint64_t) sqrt((double)num);
    for (uint64_t i = 3; i <= top; i += 2) {
        if ( !(num%i) )
            return false;
    }
    return true;
}