robin_hood.h

//                 ______  _____                 ______                _________
//  ______________ ___  /_ ___(_)_______         ___  /_ ______ ______ ______  /
//  __  ___/_  __ \__  __ \__  / __  __ \        __  __ \_  __ \_  __ \_  __  /
//  _  /    / /_/ /_  /_/ /_  /  _  / / /        _  / / // /_/ // /_/ // /_/ /
//  /_/     \____/ /_.___/ /_/   /_/ /_/ ________/_/ /_/ \____/ \____/ \__,_/
//                                      _/_____/
//
// Fast & memory efficient hashtable based on robin hood hashing for C++11/14/17/20
// https://github.com/martinus/robin-hood-hashing
//
// Licensed under the MIT License <http://opensource.org/licenses/MIT>.
// SPDX-License-Identifier: MIT
// Copyright (c) 2018-2020 Martin Ankerl <http://martin.ankerl.com>
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#ifndef ROBIN_HOOD_H_INCLUDED
#define ROBIN_HOOD_H_INCLUDED

// see https://semver.org/
#define ROBIN_HOOD_VERSION_MAJOR 3 // for incompatible API changes
#define ROBIN_HOOD_VERSION_MINOR 9 // for adding functionality in a backwards-compatible manner
#define ROBIN_HOOD_VERSION_PATCH 1 // for backwards-compatible bug fixes

#include <algorithm>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <memory> // only to support hash of smart pointers
#include <stdexcept>
#include <string>
#include <type_traits>
#include <utility>
#if __cplusplus >= 201703L
#    include <string_view>
#endif

// #define ROBIN_HOOD_LOG_ENABLED
#ifdef ROBIN_HOOD_LOG_ENABLED
#    include <iostream>
#    define ROBIN_HOOD_LOG(...) \
        std::cout << __FUNCTION__ << "@" << __LINE__ << ": " << __VA_ARGS__ << std::endl;
#else
#    define ROBIN_HOOD_LOG(x)
#endif

// #define ROBIN_HOOD_TRACE_ENABLED
#ifdef ROBIN_HOOD_TRACE_ENABLED
#    include <iostream>
#    define ROBIN_HOOD_TRACE(...) \
        std::cout << __FUNCTION__ << "@" << __LINE__ << ": " << __VA_ARGS__ << std::endl;
#else
#    define ROBIN_HOOD_TRACE(x)
#endif

// #define ROBIN_HOOD_COUNT_ENABLED
#ifdef ROBIN_HOOD_COUNT_ENABLED
#    include <iostream>
#    define ROBIN_HOOD_COUNT(x) ++counts().x;
namespace robin_hood {
struct Counts {
    uint64_t shiftUp{};
    uint64_t shiftDown{};
};
inline std::ostream& operator<<(std::ostream& os, Counts const& c) {
    return os << c.shiftUp << " shiftUp" << std::endl << c.shiftDown << " shiftDown" << std::endl;
}

static Counts& counts() {
    static Counts counts{};
    return counts;
}
} // namespace robin_hood
#else
#    define ROBIN_HOOD_COUNT(x)
#endif

// all non-argument macros should use this facility. See
// https://www.fluentcpp.com/2019/05/28/better-macros-better-flags/
#define ROBIN_HOOD(x) ROBIN_HOOD_PRIVATE_DEFINITION_##x()

// mark unused members with this macro
#define ROBIN_HOOD_UNUSED(identifier)

// bitness
#if SIZE_MAX == UINT32_MAX
#    define ROBIN_HOOD_PRIVATE_DEFINITION_BITNESS() 32
#elif SIZE_MAX == UINT64_MAX
#    define ROBIN_HOOD_PRIVATE_DEFINITION_BITNESS() 64
#else
#    error Unsupported bitness
#endif

// endianess
#ifdef _MSC_VER
#    define ROBIN_HOOD_PRIVATE_DEFINITION_LITTLE_ENDIAN() 1
#    define ROBIN_HOOD_PRIVATE_DEFINITION_BIG_ENDIAN() 0
#else
#    define ROBIN_HOOD_PRIVATE_DEFINITION_LITTLE_ENDIAN() \
        (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
#    define ROBIN_HOOD_PRIVATE_DEFINITION_BIG_ENDIAN() (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
#endif

// inline
#ifdef _MSC_VER
#    define ROBIN_HOOD_PRIVATE_DEFINITION_NOINLINE() __declspec(noinline)
#else
#    define ROBIN_HOOD_PRIVATE_DEFINITION_NOINLINE() __attribute__((noinline))
#endif

// exceptions
#if !defined(__cpp_exceptions) && !defined(__EXCEPTIONS) && !defined(_CPPUNWIND)
#    define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_EXCEPTIONS() 0
#else
#    define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_EXCEPTIONS() 1
#endif

// count leading/trailing bits
#if !defined(ROBIN_HOOD_DISABLE_INTRINSICS)
#    ifdef _MSC_VER
#        if ROBIN_HOOD(BITNESS) == 32
#            define ROBIN_HOOD_PRIVATE_DEFINITION_BITSCANFORWARD() _BitScanForward
#        else
#            define ROBIN_HOOD_PRIVATE_DEFINITION_BITSCANFORWARD() _BitScanForward64
#        endif
#        include <intrin.h>
#        pragma intrinsic(ROBIN_HOOD(BITSCANFORWARD))
#        define ROBIN_HOOD_COUNT_TRAILING_ZEROES(x)                                       \
            [](size_t mask) noexcept -> int {                                             \
                unsigned long index;                                                      \
                return ROBIN_HOOD(BITSCANFORWARD)(&index, mask) ? static_cast<int>(index) \
                                                                : ROBIN_HOOD(BITNESS);    \
            }(x)
#    else
#        if ROBIN_HOOD(BITNESS) == 32
#            define ROBIN_HOOD_PRIVATE_DEFINITION_CTZ() __builtin_ctzl
#            define ROBIN_HOOD_PRIVATE_DEFINITION_CLZ() __builtin_clzl
#        else
#            define ROBIN_HOOD_PRIVATE_DEFINITION_CTZ() __builtin_ctzll
#            define ROBIN_HOOD_PRIVATE_DEFINITION_CLZ() __builtin_clzll
#        endif
#        define ROBIN_HOOD_COUNT_LEADING_ZEROES(x) ((x) ? ROBIN_HOOD(CLZ)(x) : ROBIN_HOOD(BITNESS))
#        define ROBIN_HOOD_COUNT_TRAILING_ZEROES(x) ((x) ? ROBIN_HOOD(CTZ)(x) : ROBIN_HOOD(BITNESS))
#    endif
#endif

// fallthrough
#ifndef __has_cpp_attribute // For backwards compatibility
#    define __has_cpp_attribute(x) 0
#endif
#if __has_cpp_attribute(clang::fallthrough)
#    define ROBIN_HOOD_PRIVATE_DEFINITION_FALLTHROUGH() [[clang::fallthrough]]
#elif __has_cpp_attribute(gnu::fallthrough)
#    define ROBIN_HOOD_PRIVATE_DEFINITION_FALLTHROUGH() [[gnu::fallthrough]]
#else
#    define ROBIN_HOOD_PRIVATE_DEFINITION_FALLTHROUGH()
#endif

// likely/unlikely
#ifdef _MSC_VER
#    define ROBIN_HOOD_LIKELY(condition) condition
#    define ROBIN_HOOD_UNLIKELY(condition) condition
#else
#    define ROBIN_HOOD_LIKELY(condition) __builtin_expect(condition, 1)
#    define ROBIN_HOOD_UNLIKELY(condition) __builtin_expect(condition, 0)
#endif

// detect if native wchar_t type is availiable in MSVC
#ifdef _MSC_VER
#    ifdef _NATIVE_WCHAR_T_DEFINED
#        define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_NATIVE_WCHART() 1
#    else
#        define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_NATIVE_WCHART() 0
#    endif
#else
#    define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_NATIVE_WCHART() 1
#endif

// workaround missing "is_trivially_copyable" in g++ < 5.0
// See https://stackoverflow.com/a/31798726/48181
#if defined(__GNUC__) && __GNUC__ < 5
#    define ROBIN_HOOD_IS_TRIVIALLY_COPYABLE(...) __has_trivial_copy(__VA_ARGS__)
#else
#    define ROBIN_HOOD_IS_TRIVIALLY_COPYABLE(...) std::is_trivially_copyable<__VA_ARGS__>::value
#endif

// helpers for C++ versions, see https://gcc.gnu.org/onlinedocs/cpp/Standard-Predefined-Macros.html
#define ROBIN_HOOD_PRIVATE_DEFINITION_CXX() __cplusplus
#define ROBIN_HOOD_PRIVATE_DEFINITION_CXX98() 199711L
#define ROBIN_HOOD_PRIVATE_DEFINITION_CXX11() 201103L
#define ROBIN_HOOD_PRIVATE_DEFINITION_CXX14() 201402L
#define ROBIN_HOOD_PRIVATE_DEFINITION_CXX17() 201703L

#if ROBIN_HOOD(CXX) >= ROBIN_HOOD(CXX17)
#    define ROBIN_HOOD_PRIVATE_DEFINITION_NODISCARD() [[nodiscard]]
#else
#    define ROBIN_HOOD_PRIVATE_DEFINITION_NODISCARD()
#endif

namespace robin_hood {

#if ROBIN_HOOD(CXX) >= ROBIN_HOOD(CXX14)
#    define ROBIN_HOOD_STD std
#else

    // c++11 compatibility layer
namespace ROBIN_HOOD_STD {
template <class T>
struct alignment_of
    : std::integral_constant<std::size_t, alignof(typename std::remove_all_extents<T>::type)> {};

template <class T, T... Ints>
class integer_sequence {
public:
    using value_type = T;
    static_assert(std::is_integral<value_type>::value, "not integral type");
    static constexpr std::size_t size() noexcept {
        return sizeof...(Ints);
    }
};
template <std::size_t... Inds>
using index_sequence = integer_sequence<std::size_t, Inds...>;

namespace detail_ {
template <class T, T Begin, T End, bool>
struct IntSeqImpl {
    using TValue = T;
    static_assert(std::is_integral<TValue>::value, "not integral type");
    static_assert(Begin >= 0 && Begin < End, "unexpected argument (Begin<0 || Begin<=End)");

    template <class, class>
    struct IntSeqCombiner;

    template <TValue... Inds0, TValue... Inds1>
    struct IntSeqCombiner<integer_sequence<TValue, Inds0...>, integer_sequence<TValue, Inds1...>> {
        using TResult = integer_sequence<TValue, Inds0..., Inds1...>;
    };

    using TResult =
        typename IntSeqCombiner<typename IntSeqImpl<TValue, Begin, Begin + (End - Begin) / 2,
                                                    (End - Begin) / 2 == 1>::TResult,
                                typename IntSeqImpl<TValue, Begin + (End - Begin) / 2, End,
                                                    (End - Begin + 1) / 2 == 1>::TResult>::TResult;
};

template <class T, T Begin>
struct IntSeqImpl<T, Begin, Begin, false> {
    using TValue = T;
    static_assert(std::is_integral<TValue>::value, "not integral type");
    static_assert(Begin >= 0, "unexpected argument (Begin<0)");
    using TResult = integer_sequence<TValue>;
};

template <class T, T Begin, T End>
struct IntSeqImpl<T, Begin, End, true> {
    using TValue = T;
    static_assert(std::is_integral<TValue>::value, "not integral type");
    static_assert(Begin >= 0, "unexpected argument (Begin<0)");
    using TResult = integer_sequence<TValue, Begin>;
};
} // namespace detail_

template <class T, T N>
using make_integer_sequence = typename detail_::IntSeqImpl<T, 0, N, (N - 0) == 1>::TResult;

template <std::size_t N>
using make_index_sequence = make_integer_sequence<std::size_t, N>;

template <class... T>
using index_sequence_for = make_index_sequence<sizeof...(T)>;

} // namespace ROBIN_HOOD_STD

#endif

    namespace detail {

// make sure we static_cast to the correct type for hash_int
#if ROBIN_HOOD(BITNESS) == 64
        using SizeT = uint64_t;
#else
        using SizeT = uint32_t;
#endif

        template <typename T>
        T rotr(T x, unsigned k) {
            return (x >> k) | (x << (8U * sizeof(T) - k));
        }

// This cast gets rid of warnings like "cast from 'uint8_t*' {aka 'unsigned char*'} to
// 'uint64_t*' {aka 'long unsigned int*'} increases required alignment of target type". Use with
// care!
        template <typename T>
        inline T reinterpret_cast_no_cast_align_warning(void* ptr) noexcept {
            return reinterpret_cast<T>(ptr);
        }

        template <typename T>
        inline T reinterpret_cast_no_cast_align_warning(void const* ptr) noexcept {
            return reinterpret_cast<T>(ptr);
        }

// make sure this is not inlined as it is slow and dramatically enlarges code, thus making other
// inlinings more difficult. Throws are also generally the slow path.
        template <typename E, typename... Args>
        [[noreturn]] ROBIN_HOOD(NOINLINE)
#if ROBIN_HOOD(HAS_EXCEPTIONS)
        void doThrow(Args&&... args) {
            // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
            throw E(std::forward<Args>(args)...);
        }
#else
        void doThrow(Args&&... ROBIN_HOOD_UNUSED(args) /*unused*/) {
    abort();
}
#endif

        template <typename E, typename T, typename... Args>
        T* assertNotNull(T* t, Args&&... args) {
            if (ROBIN_HOOD_UNLIKELY(nullptr == t)) {
                doThrow<E>(std::forward<Args>(args)...);
            }
            return t;
        }

        template <typename T>
        inline T unaligned_load(void const* ptr) noexcept {
            // using memcpy so we don't get into unaligned load problems.
            // compiler should optimize this very well anyways.
            T t;
            std::memcpy(&t, ptr, sizeof(T));
            return t;
        }

// Allocates bulks of memory for objects of type T. This deallocates the memory in the destructor,
// and keeps a linked list of the allocated memory around. Overhead per allocation is the size of a
// pointer.
        template <typename T, size_t MinNumAllocs = 4, size_t MaxNumAllocs = 256>
        class BulkPoolAllocator {
        public:
            BulkPoolAllocator() noexcept = default;

            // does not copy anything, just creates a new allocator.
            BulkPoolAllocator(const BulkPoolAllocator& ROBIN_HOOD_UNUSED(o) /*unused*/) noexcept
                    : mHead(nullptr)
                    , mListForFree(nullptr) {}

            BulkPoolAllocator(BulkPoolAllocator&& o) noexcept
                    : mHead(o.mHead)
                    , mListForFree(o.mListForFree) {
                o.mListForFree = nullptr;
                o.mHead = nullptr;
            }

            BulkPoolAllocator& operator=(BulkPoolAllocator&& o) noexcept {
                reset();
                mHead = o.mHead;
                mListForFree = o.mListForFree;
                o.mListForFree = nullptr;
                o.mHead = nullptr;
                return *this;
            }

            BulkPoolAllocator&
            // NOLINTNEXTLINE(bugprone-unhandled-self-assignment,cert-oop54-cpp)
            operator=(const BulkPoolAllocator& ROBIN_HOOD_UNUSED(o) /*unused*/) noexcept {
                // does not do anything
                return *this;
            }

            ~BulkPoolAllocator() noexcept {
                reset();
            }

            // Deallocates all allocated memory.
            void reset() noexcept {
                while (mListForFree) {
                    T* tmp = *mListForFree;
                    ROBIN_HOOD_LOG("std::free")
                    std::free(mListForFree);
                    mListForFree = reinterpret_cast_no_cast_align_warning<T**>(tmp);
                }
                mHead = nullptr;
            }

            // allocates, but does NOT initialize. Use in-place new constructor, e.g.
            //   T* obj = pool.allocate();
            //   ::new (static_cast<void*>(obj)) T();
            T* allocate() {
                T* tmp = mHead;
                if (!tmp) {
                    tmp = performAllocation();
                }

                mHead = *reinterpret_cast_no_cast_align_warning<T**>(tmp);
                return tmp;
            }

            // does not actually deallocate but puts it in store.
            // make sure you have already called the destructor! e.g. with
            //  obj->~T();
            //  pool.deallocate(obj);
            void deallocate(T* obj) noexcept {
                *reinterpret_cast_no_cast_align_warning<T**>(obj) = mHead;
                mHead = obj;
            }

            // Adds an already allocated block of memory to the allocator. This allocator is from now on
            // responsible for freeing the data (with free()). If the provided data is not large enough to
            // make use of, it is immediately freed. Otherwise it is reused and freed in the destructor.
            void addOrFree(void* ptr, const size_t numBytes) noexcept {
                // calculate number of available elements in ptr
                if (numBytes < ALIGNMENT + ALIGNED_SIZE) {
                    // not enough data for at least one element. Free and return.
                    ROBIN_HOOD_LOG("std::free")
                    std::free(ptr);
                } else {
                    ROBIN_HOOD_LOG("add to buffer")
                    add(ptr, numBytes);
                }
            }

            void swap(BulkPoolAllocator<T, MinNumAllocs, MaxNumAllocs>& other) noexcept {
                using std::swap;
                swap(mHead, other.mHead);
                swap(mListForFree, other.mListForFree);
            }

        private:
            // iterates the list of allocated memory to calculate how many to alloc next.
            // Recalculating this each time saves us a size_t member.
            // This ignores the fact that memory blocks might have been added manually with addOrFree. In
            // practice, this should not matter much.
            ROBIN_HOOD(NODISCARD) size_t calcNumElementsToAlloc() const noexcept {
                auto tmp = mListForFree;
                size_t numAllocs = MinNumAllocs;

                while (numAllocs * 2 <= MaxNumAllocs && tmp) {
                    auto x = reinterpret_cast<T***>(tmp);
                    tmp = *x;
                    numAllocs *= 2;
                }

                return numAllocs;
            }

            // WARNING: Underflow if numBytes < ALIGNMENT! This is guarded in addOrFree().
            void add(void* ptr, const size_t numBytes) noexcept {
                const size_t numElements = (numBytes - ALIGNMENT) / ALIGNED_SIZE;

                auto data = reinterpret_cast<T**>(ptr);

                // link free list
                auto x = reinterpret_cast<T***>(data);
                *x = mListForFree;
                mListForFree = data;

                // create linked list for newly allocated data
                auto* const headT =
                        reinterpret_cast_no_cast_align_warning<T*>(reinterpret_cast<char*>(ptr) + ALIGNMENT);

                auto* const head = reinterpret_cast<char*>(headT);

                // Visual Studio compiler automatically unrolls this loop, which is pretty cool
                for (size_t i = 0; i < numElements; ++i) {
                    *reinterpret_cast_no_cast_align_warning<char**>(head + i * ALIGNED_SIZE) =
                            head + (i + 1) * ALIGNED_SIZE;
                }

                // last one points to 0
                *reinterpret_cast_no_cast_align_warning<T**>(head + (numElements - 1) * ALIGNED_SIZE) =
                        mHead;
                mHead = headT;
            }

            // Called when no memory is available (mHead == 0).
            // Don't inline this slow path.
            ROBIN_HOOD(NOINLINE) T* performAllocation() {
                size_t const numElementsToAlloc = calcNumElementsToAlloc();

                // alloc new memory: [prev |T, T, ... T]
                size_t const bytes = ALIGNMENT + ALIGNED_SIZE * numElementsToAlloc;
                ROBIN_HOOD_LOG("std::malloc " << bytes << " = " << ALIGNMENT << " + " << ALIGNED_SIZE
                                              << " * " << numElementsToAlloc)
                add(assertNotNull<std::bad_alloc>(std::malloc(bytes)), bytes);
                return mHead;
            }

            // enforce byte alignment of the T's
#if ROBIN_HOOD(CXX) >= ROBIN_HOOD(CXX14)
            static constexpr size_t ALIGNMENT =
                    (std::max)(std::alignment_of<T>::value, std::alignment_of<T*>::value);
#else
            static const size_t ALIGNMENT =
        (ROBIN_HOOD_STD::alignment_of<T>::value > ROBIN_HOOD_STD::alignment_of<T*>::value)
            ? ROBIN_HOOD_STD::alignment_of<T>::value
            : +ROBIN_HOOD_STD::alignment_of<T*>::value; // the + is for walkarround
#endif

            static constexpr size_t ALIGNED_SIZE = ((sizeof(T) - 1) / ALIGNMENT + 1) * ALIGNMENT;

            static_assert(MinNumAllocs >= 1, "MinNumAllocs");
            static_assert(MaxNumAllocs >= MinNumAllocs, "MaxNumAllocs");
            static_assert(ALIGNED_SIZE >= sizeof(T*), "ALIGNED_SIZE");
            static_assert(0 == (ALIGNED_SIZE % sizeof(T*)), "ALIGNED_SIZE mod");
            static_assert(ALIGNMENT >= sizeof(T*), "ALIGNMENT");

            T* mHead{nullptr};
            T** mListForFree{nullptr};
        };

        template <typename T, size_t MinSize, size_t MaxSize, bool IsFlat>
        struct NodeAllocator;

// dummy allocator that does nothing
        template <typename T, size_t MinSize, size_t MaxSize>
        struct NodeAllocator<T, MinSize, MaxSize, true> {

            // we are not using the data, so just free it.
            void addOrFree(void* ptr, size_t ROBIN_HOOD_UNUSED(numBytes) /*unused*/) noexcept {
                ROBIN_HOOD_LOG("std::free")
                std::free(ptr);
            }
        };

        template <typename T, size_t MinSize, size_t MaxSize>
        struct NodeAllocator<T, MinSize, MaxSize, false> : public BulkPoolAllocator<T, MinSize, MaxSize> {};

// dummy hash, unsed as mixer when robin_hood::hash is already used
        template <typename T>
        struct identity_hash {
            constexpr size_t operator()(T const& obj) const noexcept {
                return static_cast<size_t>(obj);
            }
        };

// c++14 doesn't have is_nothrow_swappable, and clang++ 6.0.1 doesn't like it either, so I'm making
// my own here.
        namespace swappable {
#if ROBIN_HOOD(CXX) < ROBIN_HOOD(CXX17)
            using std::swap;
template <typename T>
struct nothrow {
    static const bool value = noexcept(swap(std::declval<T&>(), std::declval<T&>()));
};
#else
            template <typename T>
            struct nothrow {
                static const bool value = std::is_nothrow_swappable<T>::value;
            };
#endif
        } // namespace swappable

    } // namespace detail

    struct is_transparent_tag {};

// A custom pair implementation is used in the map because std::pair is not is_trivially_copyable,
// which means it would  not be allowed to be used in std::memcpy. This struct is copyable, which is
// also tested.
    template <typename T1, typename T2>
    struct pair {
        using first_type = T1;
        using second_type = T2;

        template <typename U1 = T1, typename U2 = T2,
                typename = typename std::enable_if<std::is_default_constructible<U1>::value &&
                                                   std::is_default_constructible<U2>::value>::type>
        constexpr pair() noexcept(noexcept(U1()) && noexcept(U2()))
                : first()
                , second() {}

        // pair constructors are explicit so we don't accidentally call this ctor when we don't have to.
        explicit constexpr pair(std::pair<T1, T2> const& o) noexcept(
        noexcept(T1(std::declval<T1 const&>())) && noexcept(T2(std::declval<T2 const&>())))
                : first(o.first)
                , second(o.second) {}

        // pair constructors are explicit so we don't accidentally call this ctor when we don't have to.
        explicit constexpr pair(std::pair<T1, T2>&& o) noexcept(noexcept(
                                                                        T1(std::move(std::declval<T1&&>()))) && noexcept(T2(std::move(std::declval<T2&&>()))))
                : first(std::move(o.first))
                , second(std::move(o.second)) {}

        constexpr pair(T1&& a, T2&& b) noexcept(noexcept(
                                                        T1(std::move(std::declval<T1&&>()))) && noexcept(T2(std::move(std::declval<T2&&>()))))
                : first(std::move(a))
                , second(std::move(b)) {}

        template <typename U1, typename U2>
        constexpr pair(U1&& a, U2&& b) noexcept(noexcept(T1(std::forward<U1>(
                std::declval<U1&&>()))) && noexcept(T2(std::forward<U2>(std::declval<U2&&>()))))
                : first(std::forward<U1>(a))
                , second(std::forward<U2>(b)) {}

        template <typename... U1, typename... U2>
        constexpr pair(
                std::piecewise_construct_t /*unused*/, std::tuple<U1...> a,
                std::tuple<U2...> b) noexcept(noexcept(pair(std::declval<std::tuple<U1...>&>(),
                                                            std::declval<std::tuple<U2...>&>(),
                                                            ROBIN_HOOD_STD::index_sequence_for<U1...>(),
                                                            ROBIN_HOOD_STD::index_sequence_for<U2...>())))
                : pair(a, b, ROBIN_HOOD_STD::index_sequence_for<U1...>(),
                       ROBIN_HOOD_STD::index_sequence_for<U2...>()) {}

        // constructor called from the std::piecewise_construct_t ctor
        template <typename... U1, size_t... I1, typename... U2, size_t... I2>
        pair(std::tuple<U1...>& a, std::tuple<U2...>& b, ROBIN_HOOD_STD::index_sequence<I1...> /*unused*/, ROBIN_HOOD_STD::index_sequence<I2...> /*unused*/) noexcept(
        noexcept(T1(std::forward<U1>(std::get<I1>(
                std::declval<std::tuple<
                        U1...>&>()))...)) && noexcept(T2(std::
                                                         forward<U2>(std::get<I2>(
                std::declval<std::tuple<U2...>&>()))...)))
                : first(std::forward<U1>(std::get<I1>(a))...)
                , second(std::forward<U2>(std::get<I2>(b))...) {
            // make visual studio compiler happy about warning about unused a & b.
            // Visual studio's pair implementation disables warning 4100.
            (void)a;
            (void)b;
        }

        void swap(pair<T1, T2>& o) noexcept((detail::swappable::nothrow<T1>::value) &&
                                            (detail::swappable::nothrow<T2>::value)) {
            using std::swap;
            swap(first, o.first);
            swap(second, o.second);
        }

        T1 first;  // NOLINT(misc-non-private-member-variables-in-classes)
        T2 second; // NOLINT(misc-non-private-member-variables-in-classes)
    };

    template <typename A, typename B>
    inline void swap(pair<A, B>& a, pair<A, B>& b) noexcept(
    noexcept(std::declval<pair<A, B>&>().swap(std::declval<pair<A, B>&>()))) {
        a.swap(b);
    }

    template <typename A, typename B>
    inline constexpr bool operator==(pair<A, B> const& x, pair<A, B> const& y) {
        return (x.first == y.first) && (x.second == y.second);
    }
    template <typename A, typename B>
    inline constexpr bool operator!=(pair<A, B> const& x, pair<A, B> const& y) {
        return !(x == y);
    }
    template <typename A, typename B>
    inline constexpr bool operator<(pair<A, B> const& x, pair<A, B> const& y) noexcept(noexcept(
                                                                                               std::declval<A const&>() < std::declval<A const&>()) && noexcept(std::declval<B const&>() <
                                                                                                                                                                std::declval<B const&>())) {
        return x.first < y.first || (!(y.first < x.first) && x.second < y.second);
    }
    template <typename A, typename B>
    inline constexpr bool operator>(pair<A, B> const& x, pair<A, B> const& y) {
        return y < x;
    }
    template <typename A, typename B>
    inline constexpr bool operator<=(pair<A, B> const& x, pair<A, B> const& y) {
        return !(x > y);
    }
    template <typename A, typename B>
    inline constexpr bool operator>=(pair<A, B> const& x, pair<A, B> const& y) {
        return !(x < y);
    }

    inline size_t hash_bytes(void const* ptr, size_t len) noexcept {
        static constexpr uint64_t m = UINT64_C(0xc6a4a7935bd1e995);
        static constexpr uint64_t seed = UINT64_C(0xe17a1465);
        static constexpr unsigned int r = 47;

        auto const* const data64 = static_cast<uint64_t const*>(ptr);
        uint64_t h = seed ^ (len * m);

        size_t const n_blocks = len / 8;
        for (size_t i = 0; i < n_blocks; ++i) {
            auto k = detail::unaligned_load<uint64_t>(data64 + i);

            k *= m;
            k ^= k >> r;
            k *= m;

            h ^= k;
            h *= m;
        }

        auto const* const data8 = reinterpret_cast<uint8_t const*>(data64 + n_blocks);
        switch (len & 7U) {
            case 7:
                h ^= static_cast<uint64_t>(data8[6]) << 48U;
                ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
            case 6:
                h ^= static_cast<uint64_t>(data8[5]) << 40U;
                ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
            case 5:
                h ^= static_cast<uint64_t>(data8[4]) << 32U;
                ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
            case 4:
                h ^= static_cast<uint64_t>(data8[3]) << 24U;
                ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
            case 3:
                h ^= static_cast<uint64_t>(data8[2]) << 16U;
                ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
            case 2:
                h ^= static_cast<uint64_t>(data8[1]) << 8U;
                ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
            case 1:
                h ^= static_cast<uint64_t>(data8[0]);
                h *= m;
                ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
            default:
                break;
        }

        h ^= h >> r;
        h *= m;
        h ^= h >> r;
        return static_cast<size_t>(h);
    }

    inline size_t hash_int(uint64_t x) noexcept {
        // inspired by lemire's strongly universal hashing
        // https://lemire.me/blog/2018/08/15/fast-strongly-universal-64-bit-hashing-everywhere/
        //
        // Instead of shifts, we use rotations so we don't lose any bits.
        //
        // Added a final multiplcation with a constant for more mixing. It is most important that
        // the lower bits are well mixed.
        auto h1 = x * UINT64_C(0xA24BAED4963EE407);
        auto h2 = detail::rotr(x, 32U) * UINT64_C(0x9FB21C651E98DF25);
        auto h = detail::rotr(h1 + h2, 32U);
        return static_cast<size_t>(h);
    }

// A thin wrapper around std::hash, performing an additional simple mixing step of the result.
    template <typename T, typename Enable = void>
    struct hash : public std::hash<T> {
        size_t operator()(T const& obj) const
        noexcept(noexcept(std::declval<std::hash<T>>().operator()(std::declval<T const&>()))) {
            // call base hash
            auto result = std::hash<T>::operator()(obj);
            // return mixed of that, to be save against identity has
            return hash_int(static_cast<detail::SizeT>(result));
        }
    };

    template <typename CharT>
    struct hash<std::basic_string<CharT>> {
        size_t operator()(std::basic_string<CharT> const& str) const noexcept {
            return hash_bytes(str.data(), sizeof(CharT) * str.size());
        }
    };

#if ROBIN_HOOD(CXX) >= ROBIN_HOOD(CXX17)
    template <typename CharT>
    struct hash<std::basic_string_view<CharT>> {
        size_t operator()(std::basic_string_view<CharT> const& sv) const noexcept {
            return hash_bytes(sv.data(), sizeof(CharT) * sv.size());
        }
    };
#endif

    template <class T>
    struct hash<T*> {
        size_t operator()(T* ptr) const noexcept {
            return hash_int(reinterpret_cast<detail::SizeT>(ptr));
        }
    };

    template <class T>
    struct hash<std::unique_ptr<T>> {
        size_t operator()(std::unique_ptr<T> const& ptr) const noexcept {
            return hash_int(reinterpret_cast<detail::SizeT>(ptr.get()));
        }
    };

    template <class T>
    struct hash<std::shared_ptr<T>> {
        size_t operator()(std::shared_ptr<T> const& ptr) const noexcept {
            return hash_int(reinterpret_cast<detail::SizeT>(ptr.get()));
        }
    };

    template <typename Enum>
    struct hash<Enum, typename std::enable_if<std::is_enum<Enum>::value>::type> {
        size_t operator()(Enum e) const noexcept {
            using Underlying = typename std::underlying_type<Enum>::type;
            return hash<Underlying>{}(static_cast<Underlying>(e));
        }
    };

#define ROBIN_HOOD_HASH_INT(T)                           \
    template <>                                          \
    struct hash<T> {                                     \
        size_t operator()(T const& obj) const noexcept { \
            return hash_int(static_cast<uint64_t>(obj)); \
        }                                                \
    }

#if defined(__GNUC__) && !defined(__clang__)
#    pragma GCC diagnostic push
#    pragma GCC diagnostic ignored "-Wuseless-cast"
#endif
// see https://en.cppreference.com/w/cpp/utility/hash
    ROBIN_HOOD_HASH_INT(bool);
    ROBIN_HOOD_HASH_INT(char);
    ROBIN_HOOD_HASH_INT(signed char);
    ROBIN_HOOD_HASH_INT(unsigned char);
    ROBIN_HOOD_HASH_INT(char16_t);
    ROBIN_HOOD_HASH_INT(char32_t);
#if ROBIN_HOOD(HAS_NATIVE_WCHART)
    ROBIN_HOOD_HASH_INT(wchar_t);
#endif
    ROBIN_HOOD_HASH_INT(short);
    ROBIN_HOOD_HASH_INT(unsigned short);
    ROBIN_HOOD_HASH_INT(int);
    ROBIN_HOOD_HASH_INT(unsigned int);
    ROBIN_HOOD_HASH_INT(long);
    ROBIN_HOOD_HASH_INT(long long);
    ROBIN_HOOD_HASH_INT(unsigned long);
    ROBIN_HOOD_HASH_INT(unsigned long long);
#if defined(__GNUC__) && !defined(__clang__)
#    pragma GCC diagnostic pop
#endif
    namespace detail {

        template <typename T>
        struct void_type {
            using type = void;
        };

        template <typename T, typename = void>
        struct has_is_transparent : public std::false_type {};

        template <typename T>
        struct has_is_transparent<T, typename void_type<typename T::is_transparent>::type>
                : public std::true_type {};

// using wrapper classes for hash and key_equal prevents the diamond problem when the same type
// is used. see https://stackoverflow.com/a/28771920/48181
        template <typename T>
        struct WrapHash : public T {
            WrapHash() = default;
            explicit WrapHash(T const& o) noexcept(noexcept(T(std::declval<T const&>())))
                    : T(o) {}
        };

        template <typename T>
        struct WrapKeyEqual : public T {
            WrapKeyEqual() = default;
            explicit WrapKeyEqual(T const& o) noexcept(noexcept(T(std::declval<T const&>())))
                    : T(o) {}
        };

// A highly optimized hashmap implementation, using the Robin Hood algorithm.
//
// In most cases, this map should be usable as a drop-in replacement for std::unordered_map, but
// be about 2x faster in most cases and require much less allocations.
//
// This implementation uses the following memory layout:
//
// [Node, Node, ... Node | info, info, ... infoSentinel ]
//
// * Node: either a DataNode that directly has the std::pair<key, val> as member,
//   or a DataNode with a pointer to std::pair<key,val>. Which DataNode representation to use
//   depends on how fast the swap() operation is. Heuristically, this is automatically choosen
//   based on sizeof(). there are always 2^n Nodes.
//
// * info: Each Node in the map has a corresponding info byte, so there are 2^n info bytes.
//   Each byte is initialized to 0, meaning the corresponding Node is empty. Set to 1 means the
//   corresponding node contains data. Set to 2 means the corresponding Node is filled, but it
//   actually belongs to the previous position and was pushed out because that place is already
//   taken.
//
// * infoSentinel: Sentinel byte set to 1, so that iterator's ++ can stop at end() without the
//   need for a idx variable.
//
// According to STL, order of templates has effect on throughput. That's why I've moved the
// boolean to the front.
// https://www.reddit.com/r/cpp/comments/ahp6iu/compile_time_binary_size_reductions_and_cs_future/eeguck4/
        template <bool IsFlat, size_t MaxLoadFactor100, typename Key, typename T, typename Hash,
                typename KeyEqual>
        class Table
                : public WrapHash<Hash>,
                  public WrapKeyEqual<KeyEqual>,
                  detail::NodeAllocator<
                          typename std::conditional<
                                  std::is_void<T>::value, Key,
                                  robin_hood::pair<typename std::conditional<IsFlat, Key, Key const>::type, T>>::type,
                          4, 16384, IsFlat> {
        public:
            static constexpr bool is_flat = IsFlat;
            static constexpr bool is_map = !std::is_void<T>::value;
            static constexpr bool is_set = !is_map;
            static constexpr bool is_transparent =
                    has_is_transparent<Hash>::value && has_is_transparent<KeyEqual>::value;

            using key_type = Key;
            using mapped_type = T;
            using value_type = typename std::conditional<
                    is_set, Key,
                    robin_hood::pair<typename std::conditional<is_flat, Key, Key const>::type, T>>::type;
            using size_type = size_t;
            using hasher = Hash;
            using key_equal = KeyEqual;
            using Self = Table<IsFlat, MaxLoadFactor100, key_type, mapped_type, hasher, key_equal>;

        private:
            static_assert(MaxLoadFactor100 > 10 && MaxLoadFactor100 < 100,
                          "MaxLoadFactor100 needs to be >10 && < 100");

            using WHash = WrapHash<Hash>;
            using WKeyEqual = WrapKeyEqual<KeyEqual>;

            // configuration defaults

            // make sure we have 8 elements, needed to quickly rehash mInfo
            static constexpr size_t InitialNumElements = sizeof(uint64_t);
            static constexpr uint32_t InitialInfoNumBits = 5;
            static constexpr uint8_t InitialInfoInc = 1U << InitialInfoNumBits;
            static constexpr size_t InfoMask = InitialInfoInc - 1U;
            static constexpr uint8_t InitialInfoHashShift = 0;
            using DataPool = detail::NodeAllocator<value_type, 4, 16384, IsFlat>;

            // type needs to be wider than uint8_t.
            using InfoType = uint32_t;

            // DataNode ////////////////////////////////////////////////////////

            // Primary template for the data node. We have special implementations for small and big
            // objects. For large objects it is assumed that swap() is fairly slow, so we allocate these
            // on the heap so swap merely swaps a pointer.
            template <typename M, bool>
            class DataNode {};

            // Small: just allocate on the stack.
            template <typename M>
            class DataNode<M, true> final {
            public:
                template <typename... Args>
                explicit DataNode(M& ROBIN_HOOD_UNUSED(map) /*unused*/, Args&&... args) noexcept(
                noexcept(value_type(std::forward<Args>(args)...)))
                        : mData(std::forward<Args>(args)...) {}

                DataNode(M& ROBIN_HOOD_UNUSED(map) /*unused*/, DataNode<M, true>&& n) noexcept(
                std::is_nothrow_move_constructible<value_type>::value)
                        : mData(std::move(n.mData)) {}

                // doesn't do anything
                void destroy(M& ROBIN_HOOD_UNUSED(map) /*unused*/) noexcept {}
                void destroyDoNotDeallocate() noexcept {}

                value_type const* operator->() const noexcept {
                    return &mData;
                }
                value_type* operator->() noexcept {
                    return &mData;
                }

                const value_type& operator*() const noexcept {
                    return mData;
                }

                value_type& operator*() noexcept {
                    return mData;
                }

                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_map, typename VT::first_type&>::type getFirst() noexcept {
                    return mData.first;
                }
                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_set, VT&>::type getFirst() noexcept {
                    return mData;
                }

                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_map, typename VT::first_type const&>::type
                getFirst() const noexcept {
                    return mData.first;
                }
                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_set, VT const&>::type getFirst() const noexcept {
                    return mData;
                }

                template <typename MT = mapped_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_map, MT&>::type getSecond() noexcept {
                    return mData.second;
                }

                template <typename MT = mapped_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_set, MT const&>::type getSecond() const noexcept {
                    return mData.second;
                }

                void swap(DataNode<M, true>& o) noexcept(
                noexcept(std::declval<value_type>().swap(std::declval<value_type>()))) {
                    mData.swap(o.mData);
                }

            private:
                value_type mData;
            };

            // big object: allocate on heap.
            template <typename M>
            class DataNode<M, false> {
            public:
                template <typename... Args>
                explicit DataNode(M& map, Args&&... args)
                        : mData(map.allocate()) {
                    ::new (static_cast<void*>(mData)) value_type(std::forward<Args>(args)...);
                }

                DataNode(M& ROBIN_HOOD_UNUSED(map) /*unused*/, DataNode<M, false>&& n) noexcept
                        : mData(std::move(n.mData)) {}

                void destroy(M& map) noexcept {
                    // don't deallocate, just put it into list of datapool.
                    mData->~value_type();
                    map.deallocate(mData);
                }

                void destroyDoNotDeallocate() noexcept {
                    mData->~value_type();
                }

                value_type const* operator->() const noexcept {
                    return mData;
                }

                value_type* operator->() noexcept {
                    return mData;
                }

                const value_type& operator*() const {
                    return *mData;
                }

                value_type& operator*() {
                    return *mData;
                }

                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_map, typename VT::first_type&>::type getFirst() noexcept {
                    return mData->first;
                }
                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_set, VT&>::type getFirst() noexcept {
                    return *mData;
                }

                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_map, typename VT::first_type const&>::type
                getFirst() const noexcept {
                    return mData->first;
                }
                template <typename VT = value_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_set, VT const&>::type getFirst() const noexcept {
                    return *mData;
                }

                template <typename MT = mapped_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_map, MT&>::type getSecond() noexcept {
                    return mData->second;
                }

                template <typename MT = mapped_type>
                ROBIN_HOOD(NODISCARD)
                typename std::enable_if<is_map, MT const&>::type getSecond() const noexcept {
                    return mData->second;
                }

                void swap(DataNode<M, false>& o) noexcept {
                    using std::swap;
                    swap(mData, o.mData);
                }

            private:
                value_type* mData;
            };

            using Node = DataNode<Self, IsFlat>;

            // helpers for doInsert: extract first entry (only const required)
            ROBIN_HOOD(NODISCARD) key_type const& getFirstConst(Node const& n) const noexcept {
                return n.getFirst();
            }

            // in case we have void mapped_type, we are not using a pair, thus we just route k through.
            // No need to disable this because it's just not used if not applicable.
            ROBIN_HOOD(NODISCARD) key_type const& getFirstConst(key_type const& k) const noexcept {
                return k;
            }

            // in case we have non-void mapped_type, we have a standard robin_hood::pair
            template <typename Q = mapped_type>
            ROBIN_HOOD(NODISCARD)
            typename std::enable_if<!std::is_void<Q>::value, key_type const&>::type
            getFirstConst(value_type const& vt) const noexcept {
                return vt.first;
            }

            // Cloner //////////////////////////////////////////////////////////

            template <typename M, bool UseMemcpy>
            struct Cloner;

            // fast path: Just copy data, without allocating anything.
            template <typename M>
            struct Cloner<M, true> {
                void operator()(M const& source, M& target) const {
                    auto const* const src = reinterpret_cast<char const*>(source.mKeyVals);
                    auto* tgt = reinterpret_cast<char*>(target.mKeyVals);
                    auto const numElementsWithBuffer = target.calcNumElementsWithBuffer(target.mMask + 1);
                    std::copy(src, src + target.calcNumBytesTotal(numElementsWithBuffer), tgt);
                }
            };

            template <typename M>
            struct Cloner<M, false> {
                void operator()(M const& s, M& t) const {
                    auto const numElementsWithBuffer = t.calcNumElementsWithBuffer(t.mMask + 1);
                    std::copy(s.mInfo, s.mInfo + t.calcNumBytesInfo(numElementsWithBuffer), t.mInfo);

                    for (size_t i = 0; i < numElementsWithBuffer; ++i) {
                        if (t.mInfo[i]) {
                            ::new (static_cast<void*>(t.mKeyVals + i)) Node(t, *s.mKeyVals[i]);
                        }
                    }
                }
            };

            // Destroyer ///////////////////////////////////////////////////////

            template <typename M, bool IsFlatAndTrivial>
            struct Destroyer {};

            template <typename M>
            struct Destroyer<M, true> {
                void nodes(M& m) const noexcept {
                    m.mNumElements = 0;
                }

                void nodesDoNotDeallocate(M& m) const noexcept {
                    m.mNumElements = 0;
                }
            };

            template <typename M>
            struct Destroyer<M, false> {
                void nodes(M& m) const noexcept {
                    m.mNumElements = 0;
                    // clear also resets mInfo to 0, that's sometimes not necessary.
                    auto const numElementsWithBuffer = m.calcNumElementsWithBuffer(m.mMask + 1);

                    for (size_t idx = 0; idx < numElementsWithBuffer; ++idx) {
                        if (0 != m.mInfo[idx]) {
                            Node& n = m.mKeyVals[idx];
                            n.destroy(m);
                            n.~Node();
                        }
                    }
                }

                void nodesDoNotDeallocate(M& m) const noexcept {
                    m.mNumElements = 0;
                    // clear also resets mInfo to 0, that's sometimes not necessary.
                    auto const numElementsWithBuffer = m.calcNumElementsWithBuffer(m.mMask + 1);
                    for (size_t idx = 0; idx < numElementsWithBuffer; ++idx) {
                        if (0 != m.mInfo[idx]) {
                            Node& n = m.mKeyVals[idx];
                            n.destroyDoNotDeallocate();
                            n.~Node();
                        }
                    }
                }
            };

            // Iter ////////////////////////////////////////////////////////////

            struct fast_forward_tag {};

            // generic iterator for both const_iterator and iterator.
            template <bool IsConst>
            // NOLINTNEXTLINE(hicpp-special-member-functions,cppcoreguidelines-special-member-functions)
            class Iter {
            private:
                using NodePtr = typename std::conditional<IsConst, Node const*, Node*>::type;

            public:
                using difference_type = std::ptrdiff_t;
                using value_type = typename Self::value_type;
                using reference = typename std::conditional<IsConst, value_type const&, value_type&>::type;
                using pointer = typename std::conditional<IsConst, value_type const*, value_type*>::type;
                using iterator_category = std::forward_iterator_tag;

                // default constructed iterator can be compared to itself, but WON'T return true when
                // compared to end().
                Iter() = default;

                // Rule of zero: nothing specified. The conversion constructor is only enabled for
                // iterator to const_iterator, so it doesn't accidentally work as a copy ctor.

                // Conversion constructor from iterator to const_iterator.
                template <bool OtherIsConst,
                        typename = typename std::enable_if<IsConst && !OtherIsConst>::type>
                // NOLINTNEXTLINE(hicpp-explicit-conversions)
                Iter(Iter<OtherIsConst> const& other) noexcept
                        : mKeyVals(other.mKeyVals)
                        , mInfo(other.mInfo) {}

                Iter(NodePtr valPtr, uint8_t const* infoPtr) noexcept
                        : mKeyVals(valPtr)
                        , mInfo(infoPtr) {}

                Iter(NodePtr valPtr, uint8_t const* infoPtr,
                     fast_forward_tag ROBIN_HOOD_UNUSED(tag) /*unused*/) noexcept
                        : mKeyVals(valPtr)
                        , mInfo(infoPtr) {
                    fastForward();
                }

                template <bool OtherIsConst,
                        typename = typename std::enable_if<IsConst && !OtherIsConst>::type>
                Iter& operator=(Iter<OtherIsConst> const& other) noexcept {
                    mKeyVals = other.mKeyVals;
                    mInfo = other.mInfo;
                    return *this;
                }

                // prefix increment. Undefined behavior if we are at end()!
                Iter& operator++() noexcept {
                    mInfo++;
                    mKeyVals++;
                    fastForward();
                    return *this;
                }

                Iter operator++(int) noexcept {
                    Iter tmp = *this;
                    ++(*this);
                    return tmp;
                }

                reference operator*() const {
                    return **mKeyVals;
                }

                pointer operator->() const {
                    return &**mKeyVals;
                }

                template <bool O>
                bool operator==(Iter<O> const& o) const noexcept {
                    return mKeyVals == o.mKeyVals;
                }

                template <bool O>
                bool operator!=(Iter<O> const& o) const noexcept {
                    return mKeyVals != o.mKeyVals;
                }

            private:
                // fast forward to the next non-free info byte
                // I've tried a few variants that don't depend on intrinsics, but unfortunately they are
                // quite a bit slower than this one. So I've reverted that change again. See map_benchmark.
                void fastForward() noexcept {
                    size_t n = 0;
                    while (0U == (n = detail::unaligned_load<size_t>(mInfo))) {
                        mInfo += sizeof(size_t);
                        mKeyVals += sizeof(size_t);
                    }
#if defined(ROBIN_HOOD_DISABLE_INTRINSICS)
                    // we know for certain that within the next 8 bytes we'll find a non-zero one.
            if (ROBIN_HOOD_UNLIKELY(0U == detail::unaligned_load<uint32_t>(mInfo))) {
                mInfo += 4;
                mKeyVals += 4;
            }
            if (ROBIN_HOOD_UNLIKELY(0U == detail::unaligned_load<uint16_t>(mInfo))) {
                mInfo += 2;
                mKeyVals += 2;
            }
            if (ROBIN_HOOD_UNLIKELY(0U == *mInfo)) {
                mInfo += 1;
                mKeyVals += 1;
            }
#else
#    if ROBIN_HOOD(LITTLE_ENDIAN)
                    auto inc = ROBIN_HOOD_COUNT_TRAILING_ZEROES(n) / 8;
#    else
                    auto inc = ROBIN_HOOD_COUNT_LEADING_ZEROES(n) / 8;
#    endif
                    mInfo += inc;
                    mKeyVals += inc;
#endif
                }

                friend class Table<IsFlat, MaxLoadFactor100, key_type, mapped_type, hasher, key_equal>;
                NodePtr mKeyVals{nullptr};
                uint8_t const* mInfo{nullptr};
            };

            ////////////////////////////////////////////////////////////////////

            // highly performance relevant code.
            // Lower bits are used for indexing into the array (2^n size)
            // The upper 1-5 bits need to be a reasonable good hash, to save comparisons.
            template <typename HashKey>
            void keyToIdx(HashKey&& key, size_t* idx, InfoType* info) const {
                // for a user-specified hash that is *not* robin_hood::hash, apply robin_hood::hash as
                // an additional mixing step. This serves as a bad hash prevention, if the given data is
                // badly mixed.
                using Mix =
                typename std::conditional<std::is_same<::robin_hood::hash<key_type>, hasher>::value,
                        ::robin_hood::detail::identity_hash<size_t>,
                        ::robin_hood::hash<size_t>>::type;

                // the lower InitialInfoNumBits are reserved for info.
                auto h = Mix{}(WHash::operator()(key));
                *info = mInfoInc + static_cast<InfoType>((h & InfoMask) >> mInfoHashShift);
                *idx = (h >> InitialInfoNumBits) & mMask;
            }

            // forwards the index by one, wrapping around at the end
            void next(InfoType* info, size_t* idx) const noexcept {
                *idx = *idx + 1;
                *info += mInfoInc;
            }

            void nextWhileLess(InfoType* info, size_t* idx) const noexcept {
                // unrolling this by hand did not bring any speedups.
                while (*info < mInfo[*idx]) {
                    next(info, idx);
                }
            }

            // Shift everything up by one element. Tries to move stuff around.
            void
            shiftUp(size_t startIdx,
                    size_t const insertion_idx) noexcept(std::is_nothrow_move_assignable<Node>::value) {
                auto idx = startIdx;
                ::new (static_cast<void*>(mKeyVals + idx)) Node(std::move(mKeyVals[idx - 1]));
                while (--idx != insertion_idx) {
                    mKeyVals[idx] = std::move(mKeyVals[idx - 1]);
                }

                idx = startIdx;
                while (idx != insertion_idx) {
                    ROBIN_HOOD_COUNT(shiftUp)
                    mInfo[idx] = static_cast<uint8_t>(mInfo[idx - 1] + mInfoInc);
                    if (ROBIN_HOOD_UNLIKELY(mInfo[idx] + mInfoInc > 0xFF)) {
                        mMaxNumElementsAllowed = 0;
                    }
                    --idx;
                }
            }

            void shiftDown(size_t idx) noexcept(std::is_nothrow_move_assignable<Node>::value) {
                // until we find one that is either empty or has zero offset.
                // TODO(martinus) we don't need to move everything, just the last one for the same
                // bucket.
                mKeyVals[idx].destroy(*this);

                // until we find one that is either empty or has zero offset.
                while (mInfo[idx + 1] >= 2 * mInfoInc) {
                    ROBIN_HOOD_COUNT(shiftDown)
                    mInfo[idx] = static_cast<uint8_t>(mInfo[idx + 1] - mInfoInc);
                    mKeyVals[idx] = std::move(mKeyVals[idx + 1]);
                    ++idx;
                }

                mInfo[idx] = 0;
                // don't destroy, we've moved it
                // mKeyVals[idx].destroy(*this);
                mKeyVals[idx].~Node();
            }

            // copy of find(), except that it returns iterator instead of const_iterator.
            template <typename Other>
            ROBIN_HOOD(NODISCARD)
            size_t findIdx(Other const& key) const {
                size_t idx{};
                InfoType info{};
                keyToIdx(key, &idx, &info);

                do {
                    // unrolling this twice gives a bit of a speedup. More unrolling did not help.
                    if (info == mInfo[idx] &&
                        ROBIN_HOOD_LIKELY(WKeyEqual::operator()(key, mKeyVals[idx].getFirst()))) {
                        return idx;
                    }
                    next(&info, &idx);
                    if (info == mInfo[idx] &&
                        ROBIN_HOOD_LIKELY(WKeyEqual::operator()(key, mKeyVals[idx].getFirst()))) {
                        return idx;
                    }
                    next(&info, &idx);
                } while (info <= mInfo[idx]);

                // nothing found!
                return mMask == 0 ? 0
                                  : static_cast<size_t>(std::distance(
                                mKeyVals, reinterpret_cast_no_cast_align_warning<Node*>(mInfo)));
            }

            void cloneData(const Table& o) {
                Cloner<Table, IsFlat && ROBIN_HOOD_IS_TRIVIALLY_COPYABLE(Node)>()(o, *this);
            }

            // inserts a keyval that is guaranteed to be new, e.g. when the hashmap is resized.
            // @return index where the element was created
            size_t insert_move(Node&& keyval) {
                // we don't retry, fail if overflowing
                // don't need to check max num elements
                if (0 == mMaxNumElementsAllowed && !try_increase_info()) {
                    throwOverflowError(); // impossible to reach LCOV_EXCL_LINE
                }

                size_t idx{};
                InfoType info{};
                keyToIdx(keyval.getFirst(), &idx, &info);

                // skip forward. Use <= because we are certain that the element is not there.
                while (info <= mInfo[idx]) {
                    idx = idx + 1;
                    info += mInfoInc;
                }

                // key not found, so we are now exactly where we want to insert it.
                auto const insertion_idx = idx;
                auto const insertion_info = static_cast<uint8_t>(info);
                if (ROBIN_HOOD_UNLIKELY(insertion_info + mInfoInc > 0xFF)) {
                    mMaxNumElementsAllowed = 0;
                }

                // find an empty spot
                while (0 != mInfo[idx]) {
                    next(&info, &idx);
                }

                auto& l = mKeyVals[insertion_idx];
                if (idx == insertion_idx) {
                    ::new (static_cast<void*>(&l)) Node(std::move(keyval));
                } else {
                    shiftUp(idx, insertion_idx);
                    l = std::move(keyval);
                }

                // put at empty spot
                mInfo[insertion_idx] = insertion_info;

                ++mNumElements;
                return insertion_idx;
            }

        public:
            using iterator = Iter<false>;
            using const_iterator = Iter<true>;

            Table() noexcept(noexcept(Hash()) && noexcept(KeyEqual()))
                    : WHash()
                    , WKeyEqual() {
                ROBIN_HOOD_TRACE(this)
            }

            // Creates an empty hash map. Nothing is allocated yet, this happens at the first insert.
            // This tremendously speeds up ctor & dtor of a map that never receives an element. The
            // penalty is payed at the first insert, and not before. Lookup of this empty map works
            // because everybody points to DummyInfoByte::b. parameter bucket_count is dictated by the
            // standard, but we can ignore it.
            explicit Table(
                    size_t ROBIN_HOOD_UNUSED(bucket_count) /*unused*/, const Hash& h = Hash{},
                    const KeyEqual& equal = KeyEqual{}) noexcept(noexcept(Hash(h)) && noexcept(KeyEqual(equal)))
                    : WHash(h)
                    , WKeyEqual(equal) {
                ROBIN_HOOD_TRACE(this)
            }

            template <typename Iter>
            Table(Iter first, Iter last, size_t ROBIN_HOOD_UNUSED(bucket_count) /*unused*/ = 0,
                  const Hash& h = Hash{}, const KeyEqual& equal = KeyEqual{})
                    : WHash(h)
                    , WKeyEqual(equal) {
                ROBIN_HOOD_TRACE(this)
                insert(first, last);
            }

            Table(std::initializer_list<value_type> initlist,
                  size_t ROBIN_HOOD_UNUSED(bucket_count) /*unused*/ = 0, const Hash& h = Hash{},
                  const KeyEqual& equal = KeyEqual{})
                    : WHash(h)
                    , WKeyEqual(equal) {
                ROBIN_HOOD_TRACE(this)
                insert(initlist.begin(), initlist.end());
            }

            Table(Table&& o) noexcept
                    : WHash(std::move(static_cast<WHash&>(o)))
                    , WKeyEqual(std::move(static_cast<WKeyEqual&>(o)))
                    , DataPool(std::move(static_cast<DataPool&>(o))) {
                ROBIN_HOOD_TRACE(this)
                if (o.mMask) {
                    mKeyVals = std::move(o.mKeyVals);
                    mInfo = std::move(o.mInfo);
                    mNumElements = std::move(o.mNumElements);
                    mMask = std::move(o.mMask);
                    mMaxNumElementsAllowed = std::move(o.mMaxNumElementsAllowed);
                    mInfoInc = std::move(o.mInfoInc);
                    mInfoHashShift = std::move(o.mInfoHashShift);
                    // set other's mask to 0 so its destructor won't do anything
                    o.init();
                }
            }

            Table& operator=(Table&& o) noexcept {
                ROBIN_HOOD_TRACE(this)
                if (&o != this) {
                    if (o.mMask) {
                        // only move stuff if the other map actually has some data
                        destroy();
                        mKeyVals = std::move(o.mKeyVals);
                        mInfo = std::move(o.mInfo);
                        mNumElements = std::move(o.mNumElements);
                        mMask = std::move(o.mMask);
                        mMaxNumElementsAllowed = std::move(o.mMaxNumElementsAllowed);
                        mInfoInc = std::move(o.mInfoInc);
                        mInfoHashShift = std::move(o.mInfoHashShift);
                        WHash::operator=(std::move(static_cast<WHash&>(o)));
                        WKeyEqual::operator=(std::move(static_cast<WKeyEqual&>(o)));
                        DataPool::operator=(std::move(static_cast<DataPool&>(o)));

                        o.init();

                    } else {
                        // nothing in the other map => just clear us.
                        clear();
                    }
                }
                return *this;
            }

            Table(const Table& o)
                    : WHash(static_cast<const WHash&>(o))
                    , WKeyEqual(static_cast<const WKeyEqual&>(o))
                    , DataPool(static_cast<const DataPool&>(o)) {
                ROBIN_HOOD_TRACE(this)
                if (!o.empty()) {
                    // not empty: create an exact copy. it is also possible to just iterate through all
                    // elements and insert them, but copying is probably faster.

                    auto const numElementsWithBuffer = calcNumElementsWithBuffer(o.mMask + 1);
                    auto const numBytesTotal = calcNumBytesTotal(numElementsWithBuffer);

                    ROBIN_HOOD_LOG("std::malloc " << numBytesTotal << " = calcNumBytesTotal("
                                                  << numElementsWithBuffer << ")")
                    mKeyVals = static_cast<Node*>(
                            detail::assertNotNull<std::bad_alloc>(std::malloc(numBytesTotal)));
                    // no need for calloc because clonData does memcpy
                    mInfo = reinterpret_cast<uint8_t*>(mKeyVals + numElementsWithBuffer);
                    mNumElements = o.mNumElements;
                    mMask = o.mMask;
                    mMaxNumElementsAllowed = o.mMaxNumElementsAllowed;
                    mInfoInc = o.mInfoInc;
                    mInfoHashShift = o.mInfoHashShift;
                    cloneData(o);
                }
            }

            // Creates a copy of the given map. Copy constructor of each entry is used.
            // Not sure why clang-tidy thinks this doesn't handle self assignment, it does
            // NOLINTNEXTLINE(bugprone-unhandled-self-assignment,cert-oop54-cpp)
            Table& operator=(Table const& o) {
                ROBIN_HOOD_TRACE(this)
                if (&o == this) {
                    // prevent assigning of itself
                    return *this;
                }

                // we keep using the old allocator and not assign the new one, because we want to keep
                // the memory available. when it is the same size.
                if (o.empty()) {
                    if (0 == mMask) {
                        // nothing to do, we are empty too
                        return *this;
                    }

                    // not empty: destroy what we have there
                    // clear also resets mInfo to 0, that's sometimes not necessary.
                    destroy();
                    init();
                    WHash::operator=(static_cast<const WHash&>(o));
                    WKeyEqual::operator=(static_cast<const WKeyEqual&>(o));
                    DataPool::operator=(static_cast<DataPool const&>(o));

                    return *this;
                }

                // clean up old stuff
                Destroyer<Self, IsFlat && std::is_trivially_destructible<Node>::value>{}.nodes(*this);

                if (mMask != o.mMask) {
                    // no luck: we don't have the same array size allocated, so we need to realloc.
                    if (0 != mMask) {
                        // only deallocate if we actually have data!
                        ROBIN_HOOD_LOG("std::free")
                        std::free(mKeyVals);
                    }

                    auto const numElementsWithBuffer = calcNumElementsWithBuffer(o.mMask + 1);
                    auto const numBytesTotal = calcNumBytesTotal(numElementsWithBuffer);
                    ROBIN_HOOD_LOG("std::malloc " << numBytesTotal << " = calcNumBytesTotal("
                                                  << numElementsWithBuffer << ")")
                    mKeyVals = static_cast<Node*>(
                            detail::assertNotNull<std::bad_alloc>(std::malloc(numBytesTotal)));

                    // no need for calloc here because cloneData performs a memcpy.
                    mInfo = reinterpret_cast<uint8_t*>(mKeyVals + numElementsWithBuffer);
                    // sentinel is set in cloneData
                }
                WHash::operator=(static_cast<const WHash&>(o));
                WKeyEqual::operator=(static_cast<const WKeyEqual&>(o));
                DataPool::operator=(static_cast<DataPool const&>(o));
                mNumElements = o.mNumElements;
                mMask = o.mMask;
                mMaxNumElementsAllowed = o.mMaxNumElementsAllowed;
                mInfoInc = o.mInfoInc;
                mInfoHashShift = o.mInfoHashShift;
                cloneData(o);

                return *this;
            }

            // Swaps everything between the two maps.
            void swap(Table& o) {
                ROBIN_HOOD_TRACE(this)
                using std::swap;
                swap(o, *this);
            }

            // Clears all data, without resizing.
            void clear() {
                ROBIN_HOOD_TRACE(this)
                if (empty()) {
                    // don't do anything! also important because we don't want to write to
                    // DummyInfoByte::b, even though we would just write 0 to it.
                    return;
                }

                Destroyer<Self, IsFlat && std::is_trivially_destructible<Node>::value>{}.nodes(*this);

                auto const numElementsWithBuffer = calcNumElementsWithBuffer(mMask + 1);
                // clear everything, then set the sentinel again
                uint8_t const z = 0;
                std::fill(mInfo, mInfo + calcNumBytesInfo(numElementsWithBuffer), z);
                mInfo[numElementsWithBuffer] = 1;

                mInfoInc = InitialInfoInc;
                mInfoHashShift = InitialInfoHashShift;
            }

            // Destroys the map and all it's contents.
            ~Table() {
                ROBIN_HOOD_TRACE(this)
                destroy();
            }

            // Checks if both tables contain the same entries. Order is irrelevant.
            bool operator==(const Table& other) const {
                ROBIN_HOOD_TRACE(this)
                if (other.size() != size()) {
                    return false;
                }
                for (auto const& otherEntry : other) {
                    if (!has(otherEntry)) {
                        return false;
                    }
                }

                return true;
            }

            bool operator!=(const Table& other) const {
                ROBIN_HOOD_TRACE(this)
                return !operator==(other);
            }

            template <typename Q = mapped_type>
            typename std::enable_if<!std::is_void<Q>::value, Q&>::type operator[](const key_type& key) {
                ROBIN_HOOD_TRACE(this)
                return doCreateByKey(key);
            }

            template <typename Q = mapped_type>
            typename std::enable_if<!std::is_void<Q>::value, Q&>::type operator[](key_type&& key) {
                ROBIN_HOOD_TRACE(this)
                return doCreateByKey(std::move(key));
            }

            template <typename Iter>
            void insert(Iter first, Iter last) {
                for (; first != last; ++first) {
                    // value_type ctor needed because this might be called with std::pair's
                    insert(value_type(*first));
                }
            }

            template <typename... Args>
            std::pair<iterator, bool> emplace(Args&&... args) {
                ROBIN_HOOD_TRACE(this)
                Node n{*this, std::forward<Args>(args)...};
                auto r = doInsert(std::move(n));
                if (!r.second) {
                    // insertion not possible: destroy node
                    // NOLINTNEXTLINE(bugprone-use-after-move)
                    n.destroy(*this);
                }
                return r;
            }

            template <typename... Args>
            std::pair<iterator, bool> try_emplace(const key_type& key, Args&&... args) {
                return try_emplace_impl(key, std::forward<Args>(args)...);
            }

            template <typename... Args>
            std::pair<iterator, bool> try_emplace(key_type&& key, Args&&... args) {
                return try_emplace_impl(std::move(key), std::forward<Args>(args)...);
            }

            template <typename... Args>
            std::pair<iterator, bool> try_emplace(const_iterator hint, const key_type& key,
                                                  Args&&... args) {
                (void)hint;
                return try_emplace_impl(key, std::forward<Args>(args)...);
            }

            template <typename... Args>
            std::pair<iterator, bool> try_emplace(const_iterator hint, key_type&& key, Args&&... args) {
                (void)hint;
                return try_emplace_impl(std::move(key), std::forward<Args>(args)...);
            }

            template <typename Mapped>
            std::pair<iterator, bool> insert_or_assign(const key_type& key, Mapped&& obj) {
                return insert_or_assign_impl(key, std::forward<Mapped>(obj));
            }

            template <typename Mapped>
            std::pair<iterator, bool> insert_or_assign(key_type&& key, Mapped&& obj) {
                return insert_or_assign_impl(std::move(key), std::forward<Mapped>(obj));
            }

            template <typename Mapped>
            std::pair<iterator, bool> insert_or_assign(const_iterator hint, const key_type& key,
                                                       Mapped&& obj) {
                (void)hint;
                return insert_or_assign_impl(key, std::forward<Mapped>(obj));
            }

            template <typename Mapped>
            std::pair<iterator, bool> insert_or_assign(const_iterator hint, key_type&& key, Mapped&& obj) {
                (void)hint;
                return insert_or_assign_impl(std::move(key), std::forward<Mapped>(obj));
            }

            std::pair<iterator, bool> insert(const value_type& keyval) {
                ROBIN_HOOD_TRACE(this)
                return doInsert(keyval);
            }

            std::pair<iterator, bool> insert(value_type&& keyval) {
                return doInsert(std::move(keyval));
            }

            // Returns 1 if key is found, 0 otherwise.
            size_t count(const key_type& key) const { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                auto kv = mKeyVals + findIdx(key);
                if (kv != reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
                    return 1;
                }
                return 0;
            }

            template <typename OtherKey, typename Self_ = Self>
            // NOLINTNEXTLINE(modernize-use-nodiscard)
            typename std::enable_if<Self_::is_transparent, size_t>::type count(const OtherKey& key) const {
                ROBIN_HOOD_TRACE(this)
                auto kv = mKeyVals + findIdx(key);
                if (kv != reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
                    return 1;
                }
                return 0;
            }

            bool contains(const key_type& key) const { // NOLINT(modernize-use-nodiscard)
                return 1U == count(key);
            }

            template <typename OtherKey, typename Self_ = Self>
            // NOLINTNEXTLINE(modernize-use-nodiscard)
            typename std::enable_if<Self_::is_transparent, bool>::type contains(const OtherKey& key) const {
                return 1U == count(key);
            }

            // Returns a reference to the value found for key.
            // Throws std::out_of_range if element cannot be found
            template <typename Q = mapped_type>
            // NOLINTNEXTLINE(modernize-use-nodiscard)
            typename std::enable_if<!std::is_void<Q>::value, Q&>::type at(key_type const& key) {
                ROBIN_HOOD_TRACE(this)
                auto kv = mKeyVals + findIdx(key);
                if (kv == reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
                    doThrow<std::out_of_range>("key not found");
                }
                return kv->getSecond();
            }

            // Returns a reference to the value found for key.
            // Throws std::out_of_range if element cannot be found
            template <typename Q = mapped_type>
            // NOLINTNEXTLINE(modernize-use-nodiscard)
            typename std::enable_if<!std::is_void<Q>::value, Q const&>::type at(key_type const& key) const {
                ROBIN_HOOD_TRACE(this)
                auto kv = mKeyVals + findIdx(key);
                if (kv == reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
                    doThrow<std::out_of_range>("key not found");
                }
                return kv->getSecond();
            }

            const_iterator find(const key_type& key) const { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                const size_t idx = findIdx(key);
                return const_iterator{mKeyVals + idx, mInfo + idx};
            }

            template <typename OtherKey>
            const_iterator find(const OtherKey& key, is_transparent_tag /*unused*/) const {
                ROBIN_HOOD_TRACE(this)
                const size_t idx = findIdx(key);
                return const_iterator{mKeyVals + idx, mInfo + idx};
            }

            template <typename OtherKey, typename Self_ = Self>
            typename std::enable_if<Self_::is_transparent, // NOLINT(modernize-use-nodiscard)
                    const_iterator>::type  // NOLINT(modernize-use-nodiscard)
            find(const OtherKey& key) const {              // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                const size_t idx = findIdx(key);
                return const_iterator{mKeyVals + idx, mInfo + idx};
            }

            iterator find(const key_type& key) {
                ROBIN_HOOD_TRACE(this)
                const size_t idx = findIdx(key);
                return iterator{mKeyVals + idx, mInfo + idx};
            }

            template <typename OtherKey>
            iterator find(const OtherKey& key, is_transparent_tag /*unused*/) {
                ROBIN_HOOD_TRACE(this)
                const size_t idx = findIdx(key);
                return iterator{mKeyVals + idx, mInfo + idx};
            }

            template <typename OtherKey, typename Self_ = Self>
            typename std::enable_if<Self_::is_transparent, iterator>::type find(const OtherKey& key) {
                ROBIN_HOOD_TRACE(this)
                const size_t idx = findIdx(key);
                return iterator{mKeyVals + idx, mInfo + idx};
            }

            iterator begin() {
                ROBIN_HOOD_TRACE(this)
                if (empty()) {
                    return end();
                }
                return iterator(mKeyVals, mInfo, fast_forward_tag{});
            }
            const_iterator begin() const { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                return cbegin();
            }
            const_iterator cbegin() const { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                if (empty()) {
                    return cend();
                }
                return const_iterator(mKeyVals, mInfo, fast_forward_tag{});
            }

            iterator end() {
                ROBIN_HOOD_TRACE(this)
                // no need to supply valid info pointer: end() must not be dereferenced, and only node
                // pointer is compared.
                return iterator{reinterpret_cast_no_cast_align_warning<Node*>(mInfo), nullptr};
            }
            const_iterator end() const { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                return cend();
            }
            const_iterator cend() const { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                return const_iterator{reinterpret_cast_no_cast_align_warning<Node*>(mInfo), nullptr};
            }

            iterator erase(const_iterator pos) {
                ROBIN_HOOD_TRACE(this)
                // its safe to perform const cast here
                // NOLINTNEXTLINE(cppcoreguidelines-pro-type-const-cast)
                return erase(iterator{const_cast<Node*>(pos.mKeyVals), const_cast<uint8_t*>(pos.mInfo)});
            }

            // Erases element at pos, returns iterator to the next element.
            iterator erase(iterator pos) {
                ROBIN_HOOD_TRACE(this)
                // we assume that pos always points to a valid entry, and not end().
                auto const idx = static_cast<size_t>(pos.mKeyVals - mKeyVals);

                shiftDown(idx);
                --mNumElements;

                if (*pos.mInfo) {
                    // we've backward shifted, return this again
                    return pos;
                }

                // no backward shift, return next element
                return ++pos;
            }

            size_t erase(const key_type& key) {
                ROBIN_HOOD_TRACE(this)
                size_t idx{};
                InfoType info{};
                keyToIdx(key, &idx, &info);

                // check while info matches with the source idx
                do {
                    if (info == mInfo[idx] && WKeyEqual::operator()(key, mKeyVals[idx].getFirst())) {
                        shiftDown(idx);
                        --mNumElements;
                        return 1;
                    }
                    next(&info, &idx);
                } while (info <= mInfo[idx]);

                // nothing found to delete
                return 0;
            }

            // reserves space for the specified number of elements. Makes sure the old data fits.
            // exactly the same as reserve(c).
            void rehash(size_t c) {
                // forces a reserve
                reserve(c, true);
            }

            // reserves space for the specified number of elements. Makes sure the old data fits.
            // Exactly the same as rehash(c). Use rehash(0) to shrink to fit.
            void reserve(size_t c) {
                // reserve, but don't force rehash
                reserve(c, false);
            }

            size_type size() const noexcept { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                return mNumElements;
            }

            size_type max_size() const noexcept { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                return static_cast<size_type>(-1);
            }

            ROBIN_HOOD(NODISCARD) bool empty() const noexcept {
                ROBIN_HOOD_TRACE(this)
                return 0 == mNumElements;
            }

            float max_load_factor() const noexcept { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                return MaxLoadFactor100 / 100.0F;
            }

            // Average number of elements per bucket. Since we allow only 1 per bucket
            float load_factor() const noexcept { // NOLINT(modernize-use-nodiscard)
                ROBIN_HOOD_TRACE(this)
                return static_cast<float>(size()) / static_cast<float>(mMask + 1);
            }

            ROBIN_HOOD(NODISCARD) size_t mask() const noexcept {
                ROBIN_HOOD_TRACE(this)
                return mMask;
            }

            ROBIN_HOOD(NODISCARD) size_t calcMaxNumElementsAllowed(size_t maxElements) const noexcept {
                if (ROBIN_HOOD_LIKELY(maxElements <= (std::numeric_limits<size_t>::max)() / 100)) {
                    return maxElements * MaxLoadFactor100 / 100;
                }

                // we might be a bit inprecise, but since maxElements is quite large that doesn't matter
                return (maxElements / 100) * MaxLoadFactor100;
            }

            ROBIN_HOOD(NODISCARD) size_t calcNumBytesInfo(size_t numElements) const noexcept {
                // we add a uint64_t, which houses the sentinel (first byte) and padding so we can load
                // 64bit types.
                return numElements + sizeof(uint64_t);
            }

            ROBIN_HOOD(NODISCARD)
            size_t calcNumElementsWithBuffer(size_t numElements) const noexcept {
                auto maxNumElementsAllowed = calcMaxNumElementsAllowed(numElements);
                return numElements + (std::min)(maxNumElementsAllowed, (static_cast<size_t>(0xFF)));
            }

            // calculation only allowed for 2^n values
            ROBIN_HOOD(NODISCARD) size_t calcNumBytesTotal(size_t numElements) const {
#if ROBIN_HOOD(BITNESS) == 64
                return numElements * sizeof(Node) + calcNumBytesInfo(numElements);
#else
                // make sure we're doing 64bit operations, so we are at least safe against 32bit overflows.
        auto const ne = static_cast<uint64_t>(numElements);
        auto const s = static_cast<uint64_t>(sizeof(Node));
        auto const infos = static_cast<uint64_t>(calcNumBytesInfo(numElements));

        auto const total64 = ne * s + infos;
        auto const total = static_cast<size_t>(total64);

        if (ROBIN_HOOD_UNLIKELY(static_cast<uint64_t>(total) != total64)) {
            throwOverflowError();
        }
        return total;
#endif
            }

        private:
            template <typename Q = mapped_type>
            ROBIN_HOOD(NODISCARD)
            typename std::enable_if<!std::is_void<Q>::value, bool>::type has(const value_type& e) const {
                ROBIN_HOOD_TRACE(this)
                auto it = find(e.first);
                return it != end() && it->second == e.second;
            }

            template <typename Q = mapped_type>
            ROBIN_HOOD(NODISCARD)
            typename std::enable_if<std::is_void<Q>::value, bool>::type has(const value_type& e) const {
                ROBIN_HOOD_TRACE(this)
                return find(e) != end();
            }

            void reserve(size_t c, bool forceRehash) {
                ROBIN_HOOD_TRACE(this)
                auto const minElementsAllowed = (std::max)(c, mNumElements);
                auto newSize = InitialNumElements;
                while (calcMaxNumElementsAllowed(newSize) < minElementsAllowed && newSize != 0) {
                    newSize *= 2;
                }
                if (ROBIN_HOOD_UNLIKELY(newSize == 0)) {
                    throwOverflowError();
                }

                ROBIN_HOOD_LOG("newSize > mMask + 1: " << newSize << " > " << mMask << " + 1")

                // only actually do anything when the new size is bigger than the old one. This prevents to
                // continuously allocate for each reserve() call.
                if (forceRehash || newSize > mMask + 1) {
                    rehashPowerOfTwo(newSize);
                }
            }

            // reserves space for at least the specified number of elements.
            // only works if numBuckets if power of two
            void rehashPowerOfTwo(size_t numBuckets) {
                ROBIN_HOOD_TRACE(this)

                Node* const oldKeyVals = mKeyVals;
                uint8_t const* const oldInfo = mInfo;

                const size_t oldMaxElementsWithBuffer = calcNumElementsWithBuffer(mMask + 1);

                // resize operation: move stuff
                init_data(numBuckets);
                if (oldMaxElementsWithBuffer > 1) {
                    for (size_t i = 0; i < oldMaxElementsWithBuffer; ++i) {
                        if (oldInfo[i] != 0) {
                            insert_move(std::move(oldKeyVals[i]));
                            // destroy the node but DON'T destroy the data.
                            oldKeyVals[i].~Node();
                        }
                    }

                    // this check is not necessary as it's guarded by the previous if, but it helps silence
                    // g++'s overeager "attempt to free a non-heap object 'map'
                    // [-Werror=free-nonheap-object]" warning.
                    if (oldKeyVals != reinterpret_cast_no_cast_align_warning<Node*>(&mMask)) {
                        // don't destroy old data: put it into the pool instead
                        DataPool::addOrFree(oldKeyVals, calcNumBytesTotal(oldMaxElementsWithBuffer));
                    }
                }
            }

            ROBIN_HOOD(NOINLINE) void throwOverflowError() const {
#if ROBIN_HOOD(HAS_EXCEPTIONS)
                throw std::overflow_error("robin_hood::map overflow");
#else
                abort();
#endif
            }

            template <typename OtherKey, typename... Args>
            std::pair<iterator, bool> try_emplace_impl(OtherKey&& key, Args&&... args) {
                ROBIN_HOOD_TRACE(this)
                auto it = find(key);
                if (it == end()) {
                    return emplace(std::piecewise_construct,
                                   std::forward_as_tuple(std::forward<OtherKey>(key)),
                                   std::forward_as_tuple(std::forward<Args>(args)...));
                }
                return {it, false};
            }

            template <typename OtherKey, typename Mapped>
            std::pair<iterator, bool> insert_or_assign_impl(OtherKey&& key, Mapped&& obj) {
                ROBIN_HOOD_TRACE(this)
                auto it = find(key);
                if (it == end()) {
                    return emplace(std::forward<OtherKey>(key), std::forward<Mapped>(obj));
                }
                it->second = std::forward<Mapped>(obj);
                return {it, false};
            }

            void init_data(size_t max_elements) {
                mNumElements = 0;
                mMask = max_elements - 1;
                mMaxNumElementsAllowed = calcMaxNumElementsAllowed(max_elements);

                auto const numElementsWithBuffer = calcNumElementsWithBuffer(max_elements);

                // calloc also zeroes everything
                auto const numBytesTotal = calcNumBytesTotal(numElementsWithBuffer);
                ROBIN_HOOD_LOG("std::calloc " << numBytesTotal << " = calcNumBytesTotal("
                                              << numElementsWithBuffer << ")")
                mKeyVals = reinterpret_cast<Node*>(
                        detail::assertNotNull<std::bad_alloc>(std::calloc(1, numBytesTotal)));
                mInfo = reinterpret_cast<uint8_t*>(mKeyVals + numElementsWithBuffer);

                // set sentinel
                mInfo[numElementsWithBuffer] = 1;

                mInfoInc = InitialInfoInc;
                mInfoHashShift = InitialInfoHashShift;
            }

            template <typename Arg, typename Q = mapped_type>
            typename std::enable_if<!std::is_void<Q>::value, Q&>::type doCreateByKey(Arg&& key) {
                while (true) {
                    size_t idx{};
                    InfoType info{};
                    keyToIdx(key, &idx, &info);
                    nextWhileLess(&info, &idx);

                    // while we potentially have a match. Can't do a do-while here because when mInfo is
                    // 0 we don't want to skip forward
                    while (info == mInfo[idx]) {
                        if (WKeyEqual::operator()(key, mKeyVals[idx].getFirst())) {
                            // key already exists, do not insert.
                            return mKeyVals[idx].getSecond();
                        }
                        next(&info, &idx);
                    }

                    // unlikely that this evaluates to true
                    if (ROBIN_HOOD_UNLIKELY(mNumElements >= mMaxNumElementsAllowed)) {
                        increase_size();
                        continue;
                    }

                    // key not found, so we are now exactly where we want to insert it.
                    auto const insertion_idx = idx;
                    auto const insertion_info = info;
                    if (ROBIN_HOOD_UNLIKELY(insertion_info + mInfoInc > 0xFF)) {
                        mMaxNumElementsAllowed = 0;
                    }

                    // find an empty spot
                    while (0 != mInfo[idx]) {
                        next(&info, &idx);
                    }

                    auto& l = mKeyVals[insertion_idx];
                    if (idx == insertion_idx) {
                        // put at empty spot. This forwards all arguments into the node where the object
                        // is constructed exactly where it is needed.
                        ::new (static_cast<void*>(&l))
                                Node(*this, std::piecewise_construct,
                                     std::forward_as_tuple(std::forward<Arg>(key)), std::forward_as_tuple());
                    } else {
                        shiftUp(idx, insertion_idx);
                        l = Node(*this, std::piecewise_construct,
                                 std::forward_as_tuple(std::forward<Arg>(key)), std::forward_as_tuple());
                    }

                    // mKeyVals[idx].getFirst() = std::move(key);
                    mInfo[insertion_idx] = static_cast<uint8_t>(insertion_info);

                    ++mNumElements;
                    return mKeyVals[insertion_idx].getSecond();
                }
            }

            // This is exactly the same code as operator[], except for the return values
            template <typename Arg>
            std::pair<iterator, bool> doInsert(Arg&& keyval) {
                while (true) {
                    size_t idx{};
                    InfoType info{};
                    keyToIdx(getFirstConst(keyval), &idx, &info);
                    nextWhileLess(&info, &idx);

                    // while we potentially have a match
                    while (info == mInfo[idx]) {
                        if (WKeyEqual::operator()(getFirstConst(keyval), mKeyVals[idx].getFirst())) {
                            // key already exists, do NOT insert.
                            // see http://en.cppreference.com/w/cpp/container/unordered_map/insert
                            return std::make_pair<iterator, bool>(iterator(mKeyVals + idx, mInfo + idx),
                                                                  false);
                        }
                        next(&info, &idx);
                    }

                    // unlikely that this evaluates to true
                    if (ROBIN_HOOD_UNLIKELY(mNumElements >= mMaxNumElementsAllowed)) {
                        increase_size();
                        continue;
                    }

                    // key not found, so we are now exactly where we want to insert it.
                    auto const insertion_idx = idx;
                    auto const insertion_info = info;
                    if (ROBIN_HOOD_UNLIKELY(insertion_info + mInfoInc > 0xFF)) {
                        mMaxNumElementsAllowed = 0;
                    }

                    // find an empty spot
                    while (0 != mInfo[idx]) {
                        next(&info, &idx);
                    }

                    auto& l = mKeyVals[insertion_idx];
                    if (idx == insertion_idx) {
                        ::new (static_cast<void*>(&l)) Node(*this, std::forward<Arg>(keyval));
                    } else {
                        shiftUp(idx, insertion_idx);
                        l = Node(*this, std::forward<Arg>(keyval));
                    }

                    // put at empty spot
                    mInfo[insertion_idx] = static_cast<uint8_t>(insertion_info);

                    ++mNumElements;
                    return std::make_pair(iterator(mKeyVals + insertion_idx, mInfo + insertion_idx), true);
                }
            }

            bool try_increase_info() {
                ROBIN_HOOD_LOG("mInfoInc=" << mInfoInc << ", numElements=" << mNumElements
                                           << ", maxNumElementsAllowed="
                                           << calcMaxNumElementsAllowed(mMask + 1))
                if (mInfoInc <= 2) {
                    // need to be > 2 so that shift works (otherwise undefined behavior!)
                    return false;
                }
                // we got space left, try to make info smaller
                mInfoInc = static_cast<uint8_t>(mInfoInc >> 1U);

                // remove one bit of the hash, leaving more space for the distance info.
                // This is extremely fast because we can operate on 8 bytes at once.
                ++mInfoHashShift;
                auto const numElementsWithBuffer = calcNumElementsWithBuffer(mMask + 1);

                for (size_t i = 0; i < numElementsWithBuffer; i += 8) {
                    auto val = unaligned_load<uint64_t>(mInfo + i);
                    val = (val >> 1U) & UINT64_C(0x7f7f7f7f7f7f7f7f);
                    std::memcpy(mInfo + i, &val, sizeof(val));
                }
                // update sentinel, which might have been cleared out!
                mInfo[numElementsWithBuffer] = 1;

                mMaxNumElementsAllowed = calcMaxNumElementsAllowed(mMask + 1);
                return true;
            }

            void increase_size() {
                // nothing allocated yet? just allocate InitialNumElements
                if (0 == mMask) {
                    init_data(InitialNumElements);
                    return;
                }

                auto const maxNumElementsAllowed = calcMaxNumElementsAllowed(mMask + 1);
                if (mNumElements < maxNumElementsAllowed && try_increase_info()) {
                    return;
                }

                ROBIN_HOOD_LOG("mNumElements=" << mNumElements << ", maxNumElementsAllowed="
                                               << maxNumElementsAllowed << ", load="
                                               << (static_cast<double>(mNumElements) * 100.0 /
                                                   (static_cast<double>(mMask) + 1)))
                // it seems we have a really bad hash function! don't try to resize again
                if (mNumElements * 2 < calcMaxNumElementsAllowed(mMask + 1)) {
                    throwOverflowError();
                }

                rehashPowerOfTwo((mMask + 1) * 2);
            }

            void destroy() {
                if (0 == mMask) {
                    // don't deallocate!
                    return;
                }

                Destroyer<Self, IsFlat && std::is_trivially_destructible<Node>::value>{}
                        .nodesDoNotDeallocate(*this);

                // This protection against not deleting mMask shouldn't be needed as it's sufficiently
                // protected with the 0==mMask check, but I have this anyways because g++ 7 otherwise
                // reports a compile error: attempt to free a non-heap object 'fm'
                // [-Werror=free-nonheap-object]
                if (mKeyVals != reinterpret_cast_no_cast_align_warning<Node*>(&mMask)) {
                    ROBIN_HOOD_LOG("std::free")
                    std::free(mKeyVals);
                }
            }

            void init() noexcept {
                mKeyVals = reinterpret_cast_no_cast_align_warning<Node*>(&mMask);
                mInfo = reinterpret_cast<uint8_t*>(&mMask);
                mNumElements = 0;
                mMask = 0;
                mMaxNumElementsAllowed = 0;
                mInfoInc = InitialInfoInc;
                mInfoHashShift = InitialInfoHashShift;
            }

            // members are sorted so no padding occurs
            Node* mKeyVals = reinterpret_cast_no_cast_align_warning<Node*>(&mMask); // 8 byte  8
            uint8_t* mInfo = reinterpret_cast<uint8_t*>(&mMask);                    // 8 byte 16
            size_t mNumElements = 0;                                                // 8 byte 24
            size_t mMask = 0;                                                       // 8 byte 32
            size_t mMaxNumElementsAllowed = 0;                                      // 8 byte 40
            InfoType mInfoInc = InitialInfoInc;                                     // 4 byte 44
            InfoType mInfoHashShift = InitialInfoHashShift;                         // 4 byte 48
            // 16 byte 56 if NodeAllocator
        };

    } // namespace detail

// map

    template <typename Key, typename T, typename Hash = hash<Key>,
            typename KeyEqual = std::equal_to<Key>, size_t MaxLoadFactor100 = 80>
    using unordered_flat_map = detail::Table<true, MaxLoadFactor100, Key, T, Hash, KeyEqual>;

    template <typename Key, typename T, typename Hash = hash<Key>,
            typename KeyEqual = std::equal_to<Key>, size_t MaxLoadFactor100 = 80>
    using unordered_node_map = detail::Table<false, MaxLoadFactor100, Key, T, Hash, KeyEqual>;

    template <typename Key, typename T, typename Hash = hash<Key>,
            typename KeyEqual = std::equal_to<Key>, size_t MaxLoadFactor100 = 80>
    using unordered_map =
    detail::Table<sizeof(robin_hood::pair<Key, T>) <= sizeof(size_t) * 6 &&
                  std::is_nothrow_move_constructible<robin_hood::pair<Key, T>>::value &&
                  std::is_nothrow_move_assignable<robin_hood::pair<Key, T>>::value,
            MaxLoadFactor100, Key, T, Hash, KeyEqual>;

// set

    template <typename Key, typename Hash = hash<Key>, typename KeyEqual = std::equal_to<Key>,
            size_t MaxLoadFactor100 = 80>
    using unordered_flat_set = detail::Table<true, MaxLoadFactor100, Key, void, Hash, KeyEqual>;

    template <typename Key, typename Hash = hash<Key>, typename KeyEqual = std::equal_to<Key>,
            size_t MaxLoadFactor100 = 80>
    using unordered_node_set = detail::Table<false, MaxLoadFactor100, Key, void, Hash, KeyEqual>;

    template <typename Key, typename Hash = hash<Key>, typename KeyEqual = std::equal_to<Key>,
            size_t MaxLoadFactor100 = 80>
    using unordered_set = detail::Table<sizeof(Key) <= sizeof(size_t) * 6 &&
                                        std::is_nothrow_move_constructible<Key>::value &&
                                        std::is_nothrow_move_assignable<Key>::value,
            MaxLoadFactor100, Key, void, Hash, KeyEqual>;

} // namespace robin_hood

#endif