sugar 自動爲DP 加cache （or打表）

// from http://www.csdn.net/article/2015-12-03/2826381

#include <iostream>
#include <tuple>
#include <memory>
#include <map>
#include <functional>
#include <unordered_map>

template <class Function>
auto my_sugar(Function func) {
    static std::unordered_map<int, int> m;

    return [&func](int key) {    // why not auto key
        if (m.find(key) == m.end()) {
            m[key] = func(key);
        }

        return m[key];    // overhead is O(1)
    };
}

//template <typename R, typename...  Args>
//std::function<R(Args...)> cache(R(*func)(Args...)) {
//    return func;
//}

template <typename R, typename... Args>
std::function<R(Args...)> cache(R(*func) (Args...)) {
    auto result_map = std::make_shared<std::map<std::tuple<Args...>, R>>();    // key: cache map is in heap

    return [=](Args... args) {
        std::tuple<Args...> t(args...);
        std::cout << "result_map size is " << result_map->size() << std::endl;
        if (result_map->find(t) == result_map->end()) {
            (*result_map)[t] = func(args...);
        }

        return (*result_map)[t];
    };
}

template <typename R, typename...  Args>
std::function<R(Args...)> sugar(R(*func)(Args...), bool needClear = false) {
    using function_type = std::function<R(Args...)>;
    static std::unordered_map<decltype(func), function_type> functor_map;    // pointer => function

    if (needClear) {
        return functor_map[func] = cache(func);
    }

    if (functor_map.find(func) == functor_map.end()) {
        functor_map[func] = cache(func);
    }

    return functor_map[func];
}

int f(int n) {
    return n < 2 ? n : f(n - 1) + f(n - 2);
}

int f_my_sugar(int n) {
    return n < 2 ? n : my_sugar(f_my_sugar)(n - 1) + my_sugar(f_my_sugar)(n - 2);
}

int f_sugar(int n) {
    return n < 2 ? n : sugar(f_sugar)(n - 1) + sugar(f_sugar)(n - 2);
}

template <class Function>
void test(Function f) {
    int start = 34;
    int n = 40;
    for (int i = start; i <= n; ++i) {
        std::cout << i << ": " << f(i) << std::endl;
    }
}

int main() {
    std::cout << "f ---------------------" << std::endl;
    test(f);
    std::cout << "f_my_sugar ------------" << std::endl;
    test(f_my_sugar);
    std::cout << "f_sugar ---------------" << std::endl;
    test(f_sugar);

    return 0;
}