c_cpp 提取出现在源字符串中的目标子字符串的所有组合。即：目标：“abc”，来源：“abcdefgbcahijkacb12df”，

Posted 2021-05-14

/*
Extract all the the combinations of a target substring that appear in a source string. I.e: target: 
"abc", source:"abcdefgbcahijkacb12df", solution: {abc, bca, acb}
*/

// compute a hash value for a string. this function may be only suitable for letters from 'a' to 'z'
long long hash_fun(const string &t) {
    if(t.empty()) return 0;
    long long hash_val = 0;
    string s = t;
    sort(s.begin(), s.end());
    for(char c : s) {
        hash_val = 26*hash_val + c - 'a';
    }
    return hash_val;
}

// another hash function. can handle any char. 
long long hash_fun2(const string &t) {
    if(t.empty()) return 0;
    long long hash_val = 0;
    multiset<char> mset;                // note, multiset belongs to #include <set>
    for(char c : t) mset.insert(c);
    for(char c : mset) 
        hash_val = 26 * hash_val + c - 'a';
    return hash_val;
}

vector<string> extract_combinations(const string& s, const string& p) {
    vector<string> res;
    if(s.empty() || p.empty() || s.size() < p.size()) return res;
    long long hash_val = 0;
    int Ns = s.size(), Np = p.size();
    
    long long hash_val_p = hash_fun(p);
    
    for(int i=0; i<Ns - Np; i++) {
        string sub = s.substr(i, Np);
        if(hash_fun(sub) == hash_val_p)
            res.push_back(sub);
    }
    return res;
}

int main()
{
    string p = "abc";
   vector<string> vs = extract_combinations("abcdefgbcahijkacb12df", p);
   for(auto s : vs) cout << s << endl;
   return 0;
}