a^b % mod

modpower.c

#define mod 1000000007LL
typedef long long ll;

ll power(ll a, ll b)
{
  ll result = 1;
 
  while (b){
    if (b&1){                  // if(b%2 == 1)
      result = ((result % mod) * (a % mod)) % mod;
    }
    b >>=1 ;              // b = b/2 
    a = (a*a) % mod;
  }
 
  return result;
}