math - Matrix Exponentiation

/*
    Change MRows,MCols
    1. Call init(row,col)
    2. Build Initial Matrix
    3. Expo

*/
const int MRows = 2;
const int MCols = 2;
typedef long long  m_t;
long long MOD;
struct Matrix {

    int r, c;
    m_t m[MRows][MCols];

    void init(int R, int C) {
        memset(m, 0, sizeof m);
        r=R; c=C;
    }
    void iden() {
        for(int i = 0 ; i < r; i ++) m[i][i] = 1;
    }
    void print() {

        puts("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
        for (int i = 0; i < r; ++i) {
            for (int j = 0; j < c; ++j) printf("%4d  ", m[i][j]);
            printf("\n");
        }
        puts("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
    }

    void mul(const Matrix &y, Matrix &z) const {
        // value Stored in Z
        z.r = r, z.c = y.c; m_t v;

        for (int i = 0; i < z.r; ++i)
            for (int j = 0; j < z.c; ++j) {
                v = 0;
                for (int k = 0; k < c; ++k){
                    v += m[i][k] * y.m[k][j];
                    v %= MOD;

                }

                z.m[i][j] = v;
            }
    }
    void add(const Matrix &B , Matrix &c) {
        c.r = c.c = r;
        for(int i = 0 ; i < c.r ; i ++ ){
            for(int j = 0 ; j <c.c ; j ++ ) {
                c.m[i][j] = m[i][j] + B.m[i][j];
            }
        }
    }
    void exp(long long e, Matrix &z) {
        //value stored in Z
        z.init(r, c); z.iden();
        Matrix x, b = *this;
        while (true) {
            if (e & 1) { z.mul(b, x); z = x; }
            e >>= 1;
            if (e == 0) break;
            b.mul(b, x);
            b = x;
        }
    }
};