F - Prime Path

#include <iostream>
#include <queue>
#include <cstdio>
#include <cstring>
using namespace std;

struct node{
    int cur, step;
}now, Next;
int vis[10001], star, finish, prime[10001] = { 0, 0, 1 };

void init(){
    for (int i = 2; i < 10001; i++){
        if (!prime[i]){
            for (int j = 2; i*j < 10001; j++)
                prime[i*j] = 1;
        }
    }
}

int bfs(){
    queue<node> Q;
    vis[star] = 1;
    now.cur = star, now.step = 0;
    Q.push(now);
    while (!Q.empty()){
        int i, j;
        char num[5];
        now = Q.front();
        Q.pop();
        if (now.cur == finish) return now.step;
        for (i = 0; i < 4; i++){
            sprintf(num, "%d", now.cur);
            for (j = 0; j < 10; j++){
                if (j == 0 && i == 0)
                    continue;
                if (i == 0)
                    Next.cur = j * 1000 + (num[1] - '0') * 100 + (num[2] - '0') * 10 + (num[3] - '0');
                else if (i == 1)
                    Next.cur = j * 100 + (num[0] - '0') * 1000 + (num[2] - '0') * 10 + (num[3] - '0');
                else if (i == 2)
                    Next.cur = j * 10 + (num[0] - '0') * 1000 + (num[1] - '0') * 100 + (num[3] - '0');
                else if (i == 3)
                    Next.cur = j + (num[0] - '0') * 1000 + (num[1] - '0') * 100 + (num[2] - '0') * 10;
                if (!prime[Next.cur] && !vis[Next.cur])
                {
                    Next.step = now.step + 1;
                    vis[Next.cur] = 1;
                    Q.push(Next);
                }
            }
        }
    }
    return -1;
}

int main(){
    int t, ans;
    cin >> t;
    init();
    while (t--){
        cin >> star >> finish;
        memset(vis, 0, sizeof(vis));
        ans = bfs();
        if (ans == -1) cout << "Impossible\n";
        else cout << ans << endl;
    }
    return 0;
}