please, help me to find a bug, or give me some tests

Here is my code.

#include <iostream>
#include <vector>
#include <string>
#include <math.h>
#include <algorithm>
#include <string.h>
#include <stdio.h>

#define MAX_VAL 100

using namespace std;

int res = -1;

vector<int> mem;

void go(bool a[4][4],int cur) {
    long hex = 0,step = 1;
    for (int i = 0; i < 4; i++) {
        for (int j = 0; j < 4; j++) {
            if (a[i][j]==1) hex+=step;
            step*=2;
        }
    }

    if (hex==65535) {
        if (cur<res||res==-1) res = cur;
        return;
    }

    if (cur<mem[hex]) {
        mem[hex] = cur;

        int mov[4][2];

        mov[0][0] = 1; mov[0][1] = 0;
        mov[1][0] = -1;mov[1][1] = 0;
        mov[2][0] = 0; mov[2][1] = 1;
        mov[3][0] = 0; mov[3][1] = -1;


        for (int x = 0; x < 4; x++) {
            for (int y = 0; y < 4; y++) {
                bool next[4][4];

                for (int w1 = 0; w1 < 4; w1++) for (int w2 = 0; w2 < 4; w2++) next[w1][w2] = a[w1][w2];

                for (int k = 0; k < 4; k++) {
                    int nx = x+mov[k][0],ny = y+mov[k][1];

                    if (nx>=0&&ny>=0&&nx<4&&ny<4)
                        next[nx][ny] = 1-next[nx][ny];// flip
                }
                next[x][y] = 1-next[x][y];

                go(next,cur+1);
            }
        }
    }
}

int main() {
    bool a[4][4],f = true;

    string s;
    for (int i = 0; i < 4; i++) {
        cin >> s;
        for (int j = 0; j < 4; j++) {
            if (s[j]=='w') a[i][j] = 0;
            else {
                a[i][j] = 1;
                f = false;
            }
        }
    }
    if (f) {
        cout << "0";
        return 0;
    }

    for (long x = 0; x < 1<<16+1; x++) mem.push_back(MAX_VAL);

    go(a,0);

    if (res==-1) cout << "Impossible";
    else cout << res;

    return 0;
}