Knapsack 0-1 using hash map cache, C++ (gcc)

Language: Layout:

//g++  7.4.0

#include <iostream>
#include <vector>
#include <unordered_map>

struct IntPairHash {
    static_assert(sizeof(int) * 2 == sizeof(size_t));

size_t operator()(const std::pair<int, int>& p) const noexcept {
        return size_t(p.first) << 32 | p.second;
    }
};

std::unordered_map<std::pair<int, int>, std::pair<long, std::pair<int, int>>, IntPairHash> cache;

std::pair<long, std::pair<int, int>> knapsack01(const std::vector<int>& V, const std::vector<int>& W, int i, int w) {
    const auto key = std::make_pair(i, w);
    if (cache.count(key) > 0) return cache[key];
    // m[0, w]
    if (i == 0) return cache[key] = std::make_pair(0, key); // from == current nghĩa là kết thúc
    // m[i - 1, w]
    const auto [total1, from1] = knapsack01(V, W, i - 1, w);
    if (W[i] > w) return cache[key] = std::make_pair(total1, std::make_pair(i - 1, w));
    // m[i - 1, w - W[i]) + V[i]
    auto [total2, from2] = knapsack01(V, W, i - 1, w - W[i]);
    total2 += V[i];
    if (total1 < total2) return cache[key] = std::make_pair(total2, std::make_pair(i - 1, w - W[i]));
    return cache[key] = std::make_pair(total1, std::make_pair(i - 1, w));
}

std::vector<int> getChosenIds(const std::vector<int>& V, const std::vector<int>& W, int i, int w) {
    std::vector<int> res;
    while (1) {
        const auto [_, from] = knapsack01(V, W, i, w);
        const auto [i1, w1] = from;
        if (i1 == i && w1 == w) break;
        if (w != w1) res.push_back(i); // khác weight nghĩa là take current item
        i = i1;
        w = w1;
    }
    return res;
}

int main() {
    std::vector<int> V{0, 2, 4, 4, 5, 4, 7, 9, 11, 4, 7};
    std::vector<int> W{0, 134545, 316527, 583465, 738342, 1293487, 232423, 443500, 893445, 632545, 352347};
    int i = V.size() - 1;
    int w = 2000000;
    const auto [total, _] = knapsack01(V, W, i, w);
    std::cout << total << "\n";
    const auto chosenIds = getChosenIds(V, W, i, w);
    for (int i : chosenIds) std::cout << i << " ";
    std::cout << "\n";
    std::cout << "cache size = " << cache.size() << "\n";
}

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Compilation time: 0.62 sec, absolute running time: 0.16 sec, cpu time: 0.01 sec, memory peak: 6 Mb, absolute service time: 0,83 sec