unordered_map的hash函數

#include <ctime>
#include <iostream>
#include <unordered_map>
using namespace std;

const int N = 2e5;

void insert_numbers(long long x) {
    clock_t begin = clock();
    unordered_map<long long, int> 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: %.3lf seconds, sum = %lld\n", x, (double) (clock() - begin) / CLOCKS_PER_SEC, sum);
}

int main() {
    insert_numbers(107897);
    insert_numbers(126271);
}

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);
    }
};

unordered_map<long long, int, custom_hash> safe_map;
gp_hash_table<long long, int, custom_hash> safe_hash_table;

\(refer to:\) https://codeforces.com/blog/entry/62393。