2.1-2.2

#include <iostream>
#include <map>
#include <set>
#include <string>
#include <ctime>
#include <random>
using namespace std;

class Book
{
private:
    string first_name_a;
    string second_name_a;
    string titel;
    string publisher;
    int count;
    int year;
    int paper;
    int electronic;
    int audio;

public:

    Book(string const f = " ", string const s = " ", string const t = " ",
        string const p = " ", int c = 0, int y = 0, int ta = 0, int  tp = 0, int te = 0)
    {
        first_name_a = f;
        second_name_a = s;
        titel = t;
        publisher = p;
        count = c;
        year = y;
        paper = ta;
        electronic = tp;
        audio = te;
    }

    bool operator < (const Book& B) const
    {
        return ((count < B.count) + ((count != B.count) * (year < B.year)) +
            ((count != B.count) * (year != B.year) * (titel < B.titel)));
    }

    bool operator ==(const Book& B) const
    {
        return !(*this < B) && !(B < *this);
    }

    bool operator >(int P) const
    {
        return (count > P);
    }

    int count_() { return count; }
    int year_() { return year; }
    string titel_() { return titel; }

    void print() const
    {
        cout << "\nName authot: " << first_name_a << "\nSurname author: " << second_name_a <<
            "\nTitel of book: " << titel << "\nCount books: " << count << "\nPublisher: " << publisher <<
            "\nYear of publication: " << year << "\nNumber of paper books: " << paper << "\nNumber of electronic books:" <<
            electronic << "\nNumber of audio books:" << audio << '\n';
    }

    ~Book() = default;

    friend void fill(Book& V);
};

void fill(Book& V)
{
    srand(time(NULL));

    string names[5] = { "Alice", "Bob", "Carla", "David", "Eva" };
    string surnames[5] = { "Smith", "Johnson", "Brown", "Davis", "Wilson" };
    string publishers[5] = { "Penguin Random House", "HarperCollins", "Simon & Schuster", "Macmillan Publishers", "Hachette Livre" };
    string books[5] = { "To Kill a Mockingbird", "1984", "The Great Gatsby", "Pride and Prejudice", "The Catcher in the Rye" };

    int i1 = rand() % 5, i2 = rand() % 5, i3 = rand() % 5, i4 = rand() % 5,
        i5 = rand() % 5000, i6 = rand() % 2023, i7 = rand() % 1000, i8 = rand() % 1000, i9 = rand() % 1000;

    V.first_name_a = names[i1];
    V.second_name_a = surnames[i2];
    V.titel = books[i3];
    V.publisher = publishers[i4];
    V.count = i5;
    V.year = i6;
    V.paper = i7;
    V.electronic = i8;
    V.audio = i9;
}


bool book_count(Book B, int P)
{
    return (B > P);
}

template<class K, class V>
K find_key(const map<K, V>& m, V const& val)
{
    auto it = m.begin();
    for (it; it != m.end(); ++it)
        if (it->second == val)
            break;
    return it->first;
}

template <class K, class V>
V find_val(const map<K, V>& m, K const& key)
{
    auto it = m.begin();
    for (it; it != m.end(); ++it)
        if (it->first == key)
            break;
    return it->second;
}

template<class K, class V, class T>
map<K, V> filter(const map<K, V>& m, bool(*comp)(V,T), T P)
{
    if (comp == nullptr) cout << "nullptr";

    map<K, V> ans;
    for (auto it = m.begin(); it != m.end(); ++it)
        if (comp(it->second,P))
            ans.emplace(*it);

    return ans;
}

template<class K, class V>
set<V> all_values(const map<K, V>& m)
{
    set<V> s;

    for (auto it = m.begin(); it != m.end(); ++it)
        s.emplace(it->second);

    return s;
}


template<class K, class V>
void map_print(const map<K,V>& m)
{
    for (auto it = m.begin(); it != m.end(); ++it)
        cout << "Key: " << it->first << ", " << "Value: " << it->second << '\n';
}


///////////////////////////////////////////////////////////////////////////

template<class K, class V>
K find_key(const multimap<K, V>& m, V const& val)
{
    auto it = m.begin();
    for (it; it != m.end(); ++it)
        if (it->second == val)
            break;
    return it->first;
}

template <class K, class V>
V find_val(const multimap<K, V>& m, K const& key)
{
    auto it = m.begin();
    for (it; it != m.end(); ++it)
        if (it->first == key)
            break;
    return it->second;
}

template<class K, class V, class T>
multimap<K, V> filter(const multimap<K, V>& m, bool(*comp)(V, T), T P)
{
    if (comp == nullptr) cout << "nullptr";

    multimap<K, V> ans;
    for (auto it = m.begin(); it != m.end(); ++it)
        if (comp(it->second, P))
            ans.emplace(*it);

    return ans;
}

template<class K, class V>
set<V> all_values(const multimap<K, V>& m)
{
    set<V> s;

    for (auto it = m.begin(); it != m.end(); ++it)
        s.emplace(it->second);

    return s;
}


template<class K, class V>
void multimap_print(const multimap<K, V>& m)
{
    for (auto it = m.begin(); it != m.end(); ++it)
        cout << "Key: " << it->first << ", " << "Value: " << it->second << '\n';
}

int main()
{
    map<string, int> marks;

    marks["Petrov"] = 5;
    marks["Ivanov"] = 4;
    marks["Sidorov"] = 5;
    marks["Nikolaev"] = 3;
    marks["Abramov"] = 4;

    map_print(marks);

    auto a = find_key(marks, 5);
    cout << '\n' << a << '\n';

    set<int> s = all_values(marks);

    for (auto it = s.begin(); it != s.end(); ++it)
        cout << *it << " ";
    cout << '\n';

    map<int, Book> books;

    for (int i = 0; i < 5; ++i)
    {
        Book A;
        fill(A);
        books[i] = A;
    }
    bool(*ptr)(Book, int) = &book_count;
    map<int, Book> ans = filter(books, ptr, 2000);

    cout << books.size() << " " << ans.size();

    return 0;
}