欧拉函数

https://codeforces.com/contest/1295/problem/D

不理解为啥这样做

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1e6+10;
const int mod=1e9+7;
ll gcd(ll a,ll b) {return !b?a:gcd(b,a%b);}
ll euler(ll x)
{
    ll ans=x;
    for(ll i=2;i*i<=x;i++)
    {
        if(x%i==0)
        {
            ans=ans/i*(i-1);
            while(x%i==0) x/=i;
        }
    }
    if(x>1) ans=ans/x*(x-1);
    return ans;
}
int main()
{
    int T;
    scanf("%d",&T);
    while(T--){
        ll a,m;
        scanf("%lld %lld",&a,&m);
        ll g=__gcd(a,m);
        printf("%lld\n",euler(m/g));
    }
    return 0;
}