understanding kmp

vector<int> cal_next(string s) {
    vector<int> next(s.size(), 0);
    
    int j = 0; 
    for (int i = 1; i < s.size(); ++i) {
        while (j > 0 && s[i] != s[j]) j = next[j-1];
        if (s[i] == s[j]) { j++; } 
        next[i] = j;
    }
    return next;
}