Спринт 10. Искусство хеш-функций. Задание 1.

#include <iostream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <sstream>

using namespace std;

template <typename Hash>
int FindCollisions(const Hash& hasher, istream& text) {
    string str;
    int collisions = 0;

    unordered_map<size_t, unordered_set<string>> coll_hash;

    while(text >> str) {
        const auto hash_ = hasher(str);

        if (const auto helper = coll_hash.find(hash_);
            helper != coll_hash.end()) {

            collisions += helper->second.insert(str).second;

        } else {
            coll_hash[hash_].insert(str);
        }
    }
    return collisions;
}

struct HasherDummy {
    size_t operator() (const string& str) const {
        size_t res = 0;
        for (char c : str) {
            res += static_cast<size_t>(c);
        }
        return res;
    }
};

struct DummyHash {
    size_t operator()(const string&) const {
        return 42;
    }
};

int main() {
    DummyHash dummy_hash;
    hash<string> good_hash;

    {
        istringstream stream("I love C++"s);
        cout << FindCollisions(dummy_hash, stream) << endl;
    }
    {
        istringstream stream("I love C++"s);
        cout << FindCollisions(good_hash, stream) << endl;
    }
}