#include <vector>
#include <unordered_map>
#include <algorithm>
#include <stdexcept>
#include <utility>
#include <functional>
#include <string>
using std::string;
using std::vector;
using std::function;
using std::unordered_map;
using std::pair;

template<typename T, typename H = std::hash<T>>
class Docum{
private:
	T initial_state;
	T current_state;
	H state_hash;
	vector<pair<function<void(T&)>, int>> operation;
	unordered_map<int, T> save_state;
public:
	Docum(const T &s) :initial_state(s), current_state(s), state_hash() {
		save_state[0] = initial_state;
	};
	const &T get() const { return current_state; }
	void checkout(int n){
		assert(abs(n) < operation.size());
		unsigned int step = 0;
		if (n < 0) step = operation.size() + n;
		else step = n;
		operation.resize(step);
		auto up = save_state.upper_bound(step);
		--up;
		current_state = up->second;
		for (int i = up->first; i < step){
			operation[i].first(current_state);
		}
		if (state_hash(current_state) != operation[step - 1].second){
			throw std::logic_error("Hash does not match!");
		}
	}
	void apply(const function<void(T&)> &f){
		operation.push_back({ f, state_hash(current_state) });
		f(current_state);
	}
	void save_current(){
		save_state[operation.size()] = current_state;
	}
};

template <typename T>
Docum<T> make_doc(const T& temp){
	return Docum<T>(temp);
}


void delete_symbol(string &s){
	s.pop_back();
}

int main(){

	Docum<string> doc("127fhfhe67df");
	for (int i = 0; i < 3; ++i){
		doc.apply(delete_symbol);
	}

	system("pause");
	return 0;
}