Sum of Geometric Progression

// Source :  //https://discuss.codechef.com/t/sum-of-gp-mod-m/20998/2
#define int long long
const int mod = 1e9 + 7;
int mul(int x, int y) {
	return (x * y) % mod;
}

int add(int x, int y) {
	return (x + y) % mod;
}
// a is common ratio
// series always starts from 1
int GeometricProg(int a, int n) {
	if (!n) {
		return 1;
	}
	if (n & 1) {
    int x =  gp(mul(a, a), n / 2);
		return mul((1 + a), x);
	}
	else {
	  int x = gp(mul(a, a), n / 2 - 1);
		return add(1, mul(a, mul((a+1), x)));
	}
}