math - Matrix multiplication simple

using Mat = vector<vector<ll>>;
Mat mul(Mat &A, Mat &B) {
    int r = (int)A.size(), c = (int)B[0].size();
    Mat C(r, vector<long long>(c));
    for (int i = 0; i < r; i++)
        for (int j = 0; j < c; j++) {
            ll v = 0;
            for (int k = 0; k < c; k++) v = (v + A[i][k] * B[k][j]) % mod;
            C[i][j] = v;
        }
    return C;
}

Mat exp(Mat &A, long long e) {
    int r = (int)A.size(), c = (int)A[0].size();
    Mat C(r, vector<long long>(c));
    for(int i = 0; i < r; i ++) C[i][i] = 1;

    Mat B = A;
    while (true) {
        if (e & 1) C = mul(C, B);

        e >>= 1;
        if (e == 0) break;
        B = mul(B, B);
    }
    return C;
}