a interesting benchmark

[hyb]

random Insert and erase begin is a quite interesting benchmark
I think you know why some hash maps maybe very slow and others are quite efficient.

#include "Map.h"
#include "bench.h"
#include "hex.h"
#include "sfc64.h"

#include <algorithm>
#include <sstream>

BENCHMARK(RandomInsertEraseBegin) {
    size_t max_n = 10000;
    using M = Map<uint64_t, uint32_t>;

    for (int i = 0; i < 6; ++i) {
#ifdef USE_POOL_ALLOCATOR
        Resource<uint64_t, uint32_t> resource;
        M map{0, M::hasher{}, M::key_equal{}, &resource};
#else
        M map;
#endif

        std::stringstream ss;
        ss << (max_n / 1000000.) << " M cycles";
        sfc64 rng(999 + i);

        // benchmark randomly inserting & erasing begin
        bench.beginMeasure(ss.str().c_str());
        for (size_t i = 0; i < max_n; ++i) 
            map.emplace(rng(), 0);

        for (size_t i = 0; i < 2 * max_n; ++i) {
	    map.erase(map.begin());
            map.emplace(rng(), 0);
        }

        bench.endMeasure(map.size(), map.size());
        max_n *= 3;
    }
}

[hyb]

random Insert and erase continue key is another benchmark
even it's not offten used by the other benchamrk.

#include "Map.h"
#include "bench.h"
#include "hex.h"
#include "sfc64.h"

#include <algorithm>
#include <sstream>

BENCHMARK(RandomInsertEraseContinue) {
    size_t max_n = 100000;

    using M = Map<uint32_t, uint32_t>;

    for (int i = 0; i < 4; ++i) {
#ifdef USE_POOL_ALLOCATOR
        Resource<uint32_t, uint32_t> resource;
        M map{0, M::hasher{}, M::key_equal{}, &resource};
#else
        M map;
#endif

        std::stringstream ss;
        ss << (max_n / 1000000.) << " M cycles";
        sfc64 rng(2023 + i);

        // benchmark randomly inserting & erasing begin
        bench.beginMeasure(ss.str().c_str());
        for (size_t i = 0; i < max_n / 2; ++i)
            map.emplace((uint32_t)rng(), 0);

        auto key = map.begin()->first;
        for (size_t i = max_n; i > 0; i--) {
            auto it = map.find(key);
            if (it == map.end()) {
                it = map.begin();
                key = it->first;
            }

            if constexpr(std::is_void_v<decltype(map.erase(it))>) {
                map.erase(it), key = (++it)->first;
            } else {
                it = map.erase(it), key = it->first;
            }

            map.emplace((uint32_t)rng(), 0);
        }

        bench.endMeasure(map.size(), map.size());
        max_n *= 7;
    }
}