unordered_map.cpp

#include<bits/stdc++.h>
using namespace std;
/// https://codeforces.com/blog/entry/62393
const int N = 2e5;
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        /// http://xorshift.di.unimi.it/splitmix64.c
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};
//struct custom_hash {
//    static uint64_t splitmix64(uint64_t x) {
//        // http://xorshift.di.unimi.it/splitmix64.c
//        x += 0x9e3779b97f4a7c15;
//        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
//        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
//        return x ^ (x >> 31);
//    }
//
//    size_t operator()(pair<uint64_t,uint64_t> x) const {                  ///For Pair
//        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
//        return splitmix64(x.first + FIXED_RANDOM)^(splitmix64(x.second + FIXED_RANDOM)>>1);
//    }
//};
void insert_numbers(long long x) {
    double start_time = clock();
    unordered_map<long long, int, custom_hash> numbers;

    for (int i = 1; i <= N; i++)
        numbers[i * x] = i;

    long long sum = 0;

    for (auto &entry : numbers)
        sum += (entry.first / x) * entry.second;

    printf("x = %lld : sum = %lld\n", x, sum);
    double end_time = clock();
    cerr<<"Time = "<<fixed<<setprecision(10)<<(end_time - start_time) / CLOCKS_PER_SEC<<'\n';
}

int main() {
    insert_numbers(107897); ///custom hash- 0.21second and without custom hash 10second;
    insert_numbers(126271);
}