Uva10518-how many CILS? (Matrix fast power)

Question Link

Evaluate the recursion times of the nth Fibonacci number mod B

Idea: Use a matrix to quickly calculate the Fibonacci sequence, and then create a table to find out the regular number of recursion: F (n) = 2 * F (N)-1 (f (N) is the Fibonacci number ).

Code:

#include <iostream>#include <cstdio>#include <cstring>#include <cmath>#include <algorithm>typedef long long ll;using namespace std;ll n, b;struct mat{    ll s[2][2];    mat(ll a = 0, ll b = 0, ll c = 0, ll d = 0) {        s[0][0] = a;         s[0][1] = b;         s[1][0] = c;         s[1][1] = d;     }}c(1, 1, 1, 0), tmp(1, 0, 0, 1);mat operator * (const mat& p, const mat& q) {    mat state;    for (int i = 0; i < 2; i++)        for (int j = 0; j < 2; j++)            state.s[i][j] = (p.s[i][0] * q.s[0][j] + p.s[i][1] * q.s[1][j]) % b;    return state;}mat pow_mod(ll k) {    if (k == 0)        return tmp;    else if (k == 1)        return c;    mat a = pow_mod(k / 2);    a = a * a;    if (k % 2)        a = a * c;    return a;}int main() {    int t = 1;    while (scanf("%lld %lld", &n, &b) && (n || b)) {        mat temp = pow_mod(n);         ll ans = temp.s[0][0];        printf("Case %d: %lld %lld %lld\n", t++, n, b, (2 * ans - 1 + b) % b);     }        return 0;}

Uva10518-how many CILS? (Matrix fast power)