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I made this algorithm to compute Euler's totient function for large numbers. A sieve is used.

#include <iostream> 
#include <cstdint> 
#include <vector> 
#include <cmath> 
 
typedef uint64_t integer; 
 
integer euler_totient(integer n) { 
 integer phi_n=n;
 integer len_SIEVE=(integer) std::sqrt(n)+1;
 std::vector<char> SIEVE(len_SIEVE, true); 
 for (integer p=2; p<len_SIEVE; p++) 
 if (SIEVE[p]==true) 
 if (n%p==0) {
 for (integer m=p; m<len_SIEVE; m+=p) 
 SIEVE[m]=false;
 phi_n/=p;
 phi_n*=p-1;
 }
 return phi_n; 
}
int main() {
 std::cout << euler_totient(100000000) << std::endl;
 return 0;
}

Is it a good solution?

Can it be improved in any way?

EDIT: I ​​changed the algorithm because I realized that the second part was useless.