rpcs3/3rdparty/robin_hood/include/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) 2018-2021 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 11 // for adding functionality in a backwards-compatible manner
#define ROBIN_HOOD_VERSION_PATCH 5  // for backwards-compatible bug fixes

#include <algorithm>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <limits>
#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

// detect if MSVC supports the pair(std::piecewise_construct_t,...) consructor being constexpr
#ifdef _MSC_VER
#    if _MSC_VER <= 1900
#        define ROBIN_HOOD_PRIVATE_DEFINITION_BROKEN_CONSTEXPR() 1
#    else
#        define ROBIN_HOOD_PRIVATE_DEFINITION_BROKEN_CONSTEXPR() 0
#    endif
#else
#    define ROBIN_HOOD_PRIVATE_DEFINITION_BROKEN_CONSTEXPR() 0
#endif

// workaround missing "is_trivially_copyable" in g++ < 5.0
// See https://stackoverflow.com/a/31798726/48181
#if defined(__GNUC__) && __GNUC__ < 5 && !defined(__clang__)
#    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> {};

		// 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>
		// MSVC 2015 produces error "C2476: ‘constexpr’ constructor does not initialize all members"
		// if this constructor is constexpr
#if !ROBIN_HOOD(BROKEN_CONSTEXPR)
		constexpr
#endif
			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;

		// not doing the final step here, because this will be done by keyToIdx anyways
		// h *= m;
		// h ^= h >> r;
		return static_cast<size_t>(h);
	}

	inline size_t hash_int(uint64_t x) noexcept {
		// tried lots of different hashes, let's stick with murmurhash3. It's simple, fast, well tested,
		// and doesn't need any special 128bit operations.
		x ^= x >> 33U;
		x *= UINT64_C(0xff51afd7ed558ccd);
		x ^= x >> 33U;

		// not doing the final step here, because this will be done by keyToIdx anyways
		// x *= UINT64_C(0xc4ceb9fe1a85ec53);
		// x ^= x >> 33U;
		return static_cast<size_t>(x);
	}

	// 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 insertKeyPrepareEmptySpot: 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 {
				// In addition to whatever hash is used, add another mul & shift so we get better hashing.
				// This serves as a bad hash prevention, if the given data is
				// badly mixed.
				auto h = static_cast<uint64_t>(WHash::operator()(key));

				h *= mHashMultiplier;
				h ^= h >> 33U;

				// the lower InitialInfoNumBits are reserved for info.
				*info = mInfoInc + static_cast<InfoType>((h & InfoMask) >> mInfoHashShift);
				*idx = (static_cast<size_t>(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 True on success, false if something went wrong
			void 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();
				}

				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;
			}

		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) {
						mHashMultiplier = std::move(o.mHashMultiplier);
						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();
							mHashMultiplier = std::move(o.mHashMultiplier);
							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 << ")")
							mHashMultiplier = o.mHashMultiplier;
						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));
				mHashMultiplier = o.mHashMultiplier;
				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)
					auto idxAndState = insertKeyPrepareEmptySpot(key);
				switch (idxAndState.second) {
				case InsertionState::key_found:
					break;

				case InsertionState::new_node:
					::new (static_cast<void*>(&mKeyVals[idxAndState.first]))
						Node(*this, std::piecewise_construct, std::forward_as_tuple(key),
							std::forward_as_tuple());
					break;

				case InsertionState::overwrite_node:
					mKeyVals[idxAndState.first] = Node(*this, std::piecewise_construct,
						std::forward_as_tuple(key), std::forward_as_tuple());
					break;

				case InsertionState::overflow_error:
					throwOverflowError();
				}

				return mKeyVals[idxAndState.first].getSecond();
			}

			template <typename Q = mapped_type>
			typename std::enable_if<!std::is_void<Q>::value, Q&>::type operator[](key_type&& key) {
				ROBIN_HOOD_TRACE(this)
					auto idxAndState = insertKeyPrepareEmptySpot(key);
				switch (idxAndState.second) {
				case InsertionState::key_found:
					break;

				case InsertionState::new_node:
					::new (static_cast<void*>(&mKeyVals[idxAndState.first]))
						Node(*this, std::piecewise_construct, std::forward_as_tuple(std::move(key)),
							std::forward_as_tuple());
					break;

				case InsertionState::overwrite_node:
					mKeyVals[idxAndState.first] =
						Node(*this, std::piecewise_construct, std::forward_as_tuple(std::move(key)),
							std::forward_as_tuple());
					break;

				case InsertionState::overflow_error:
					throwOverflowError();
				}

				return mKeyVals[idxAndState.first].getSecond();
			}

			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));
				}
			}

			void insert(std::initializer_list<value_type> ilist) {
				for (auto&& vt : ilist) {
					insert(std::move(vt));
				}
			}

			template <typename... Args>
			std::pair<iterator, bool> emplace(Args&&... args) {
				ROBIN_HOOD_TRACE(this)
					Node n {
					*this, std::forward<Args>(args)...
				};
				auto idxAndState = insertKeyPrepareEmptySpot(getFirstConst(n));
				switch (idxAndState.second) {
				case InsertionState::key_found:
					n.destroy(*this);
					break;

				case InsertionState::new_node:
					::new (static_cast<void*>(&mKeyVals[idxAndState.first])) Node(*this, std::move(n));
					break;

				case InsertionState::overwrite_node:
					mKeyVals[idxAndState.first] = std::move(n);
					break;

				case InsertionState::overflow_error:
					n.destroy(*this);
					throwOverflowError();
					break;
				}

				return std::make_pair(iterator(mKeyVals + idxAndState.first, mInfo + idxAndState.first),
					InsertionState::key_found != idxAndState.second);
			}

			template <typename... Args>
			iterator emplace_hint(const_iterator position, Args&&... args) {
				(void)position;
				return emplace(std::forward<Args>(args)...).first;
			}

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

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

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

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

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

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

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

			iterator insert(const_iterator hint, const value_type& keyval) {
				(void)hint;
				return emplace(keyval).first;
			}

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

			iterator insert(const_iterator hint, value_type&& keyval) {
				(void)hint;
				return emplace(std::move(keyval)).first;
			}

			// 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);
			}

			// If possible reallocates the map to a smaller one. This frees the underlying table.
			// Does not do anything if load_factor is too large for decreasing the table's size.
			void compact() {
				ROBIN_HOOD_TRACE(this)
					auto newSize = InitialNumElements;
				while (calcMaxNumElementsAllowed(newSize) < mNumElements && 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 (newSize < mMask + 1) {
						rehashPowerOfTwo(newSize, true);
					}
			}

			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, false);
					}
			}

			// reserves space for at least the specified number of elements.
			// only works if numBuckets if power of two
			// True on success, false otherwise
			void rehashPowerOfTwo(size_t numBuckets, bool forceFree) {
				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
				initData(numBuckets);
				if (oldMaxElementsWithBuffer > 1) {
					for (size_t i = 0; i < oldMaxElementsWithBuffer; ++i) {
						if (oldInfo[i] != 0) {
							// might throw an exception, which is really bad since we are in the middle of
							// moving stuff.
							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
						if (forceFree) {
							std::free(oldKeyVals);
						}
						else {
							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 idxAndState = insertKeyPrepareEmptySpot(key);
				switch (idxAndState.second) {
				case InsertionState::key_found:
					break;

				case InsertionState::new_node:
					::new (static_cast<void*>(&mKeyVals[idxAndState.first])) Node(
						*this, std::piecewise_construct, std::forward_as_tuple(std::forward<OtherKey>(key)),
						std::forward_as_tuple(std::forward<Args>(args)...));
					break;

				case InsertionState::overwrite_node:
					mKeyVals[idxAndState.first] = Node(*this, std::piecewise_construct,
						std::forward_as_tuple(std::forward<OtherKey>(key)),
						std::forward_as_tuple(std::forward<Args>(args)...));
					break;

				case InsertionState::overflow_error:
					throwOverflowError();
					break;
				}

				return std::make_pair(iterator(mKeyVals + idxAndState.first, mInfo + idxAndState.first),
					InsertionState::key_found != idxAndState.second);
			}

			template <typename OtherKey, typename Mapped>
			std::pair<iterator, bool> insertOrAssignImpl(OtherKey&& key, Mapped&& obj) {
				ROBIN_HOOD_TRACE(this)
					auto idxAndState = insertKeyPrepareEmptySpot(key);
				switch (idxAndState.second) {
				case InsertionState::key_found:
					mKeyVals[idxAndState.first].getSecond() = std::forward<Mapped>(obj);
					break;

				case InsertionState::new_node:
					::new (static_cast<void*>(&mKeyVals[idxAndState.first])) Node(
						*this, std::piecewise_construct, std::forward_as_tuple(std::forward<OtherKey>(key)),
						std::forward_as_tuple(std::forward<Mapped>(obj)));
					break;

				case InsertionState::overwrite_node:
					mKeyVals[idxAndState.first] = Node(*this, std::piecewise_construct,
						std::forward_as_tuple(std::forward<OtherKey>(key)),
						std::forward_as_tuple(std::forward<Mapped>(obj)));
					break;

				case InsertionState::overflow_error:
					throwOverflowError();
					break;
				}

				return std::make_pair(iterator(mKeyVals + idxAndState.first, mInfo + idxAndState.first),
					InsertionState::key_found != idxAndState.second);
			}

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

				auto const numElementsWithBuffer = calcNumElementsWithBuffer(max_elements);

				// malloc & zero mInfo. Faster than calloc everything.
				auto const numBytesTotal = calcNumBytesTotal(numElementsWithBuffer);
				ROBIN_HOOD_LOG("std::calloc " << numBytesTotal << " = calcNumBytesTotal("
					<< numElementsWithBuffer << ")")
					mKeyVals = reinterpret_cast<Node*>(
						detail::assertNotNull<std::bad_alloc>(std::malloc(numBytesTotal)));
				mInfo = reinterpret_cast<uint8_t*>(mKeyVals + numElementsWithBuffer);
				std::memset(mInfo, 0, numBytesTotal - numElementsWithBuffer * sizeof(Node));

				// set sentinel
				mInfo[numElementsWithBuffer] = 1;

				mInfoInc = InitialInfoInc;
				mInfoHashShift = InitialInfoHashShift;
			}

			enum class InsertionState { overflow_error, key_found, new_node, overwrite_node };

			// Finds key, and if not already present prepares a spot where to pot the key & value.
			// This potentially shifts nodes out of the way, updates mInfo and number of inserted
			// elements, so the only operation left to do is create/assign a new node at that spot.
			template <typename OtherKey>
			std::pair<size_t, InsertionState> insertKeyPrepareEmptySpot(OtherKey&& key) {
				for (int i = 0; i < 256; ++i) {
					size_t idx{};
					InfoType info{};
					keyToIdx(key, &idx, &info);
					nextWhileLess(&info, &idx);

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

					// unlikely that this evaluates to true
					if (ROBIN_HOOD_UNLIKELY(mNumElements >= mMaxNumElementsAllowed)) {
						if (!increase_size()) {
							return std::make_pair(size_t(0), InsertionState::overflow_error);
						}
						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);
					}

					if (idx != insertion_idx) {
						shiftUp(idx, insertion_idx);
					}
					// put at empty spot
					mInfo[insertion_idx] = static_cast<uint8_t>(insertion_info);
					++mNumElements;
					return std::make_pair(insertion_idx, idx == insertion_idx
						? InsertionState::new_node
						: InsertionState::overwrite_node);
				}

				// enough attempts failed, so finally give up.
				return std::make_pair(size_t(0), InsertionState::overflow_error);
			}

			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;
			}

			// True if resize was possible, false otherwise
			bool increase_size() {
				// nothing allocated yet? just allocate InitialNumElements
				if (0 == mMask) {
					initData(InitialNumElements);
					return true;
				}

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

				ROBIN_HOOD_LOG("mNumElements=" << mNumElements << ", maxNumElementsAllowed="
					<< maxNumElementsAllowed << ", load="
					<< (static_cast<double>(mNumElements) * 100.0 /
						(static_cast<double>(mMask) + 1)))

					if (mNumElements * 2 < calcMaxNumElementsAllowed(mMask + 1)) {
						// we have to resize, even though there would still be plenty of space left!
						// Try to rehash instead. Delete freed memory so we don't steadyily increase mem in case
						// we have to rehash a few times
						nextHashMultiplier();
						rehashPowerOfTwo(mMask + 1, true);
					}
					else {
						// we've reached the capacity of the map, so the hash seems to work nice. Keep using it.
						rehashPowerOfTwo((mMask + 1) * 2, false);
					}
				return true;
			}

			void nextHashMultiplier() {
				// adding an *even* number, so that the multiplier will always stay odd. This is necessary
				// so that the hash stays a mixing function (and thus doesn't have any information loss).
				mHashMultiplier += UINT64_C(0xc4ceb9fe1a85ec54);
			}

			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
			uint64_t mHashMultiplier = UINT64_C(0xc4ceb9fe1a85ec53);                // 8 byte  8
			Node* mKeyVals = reinterpret_cast_no_cast_align_warning<Node*>(&mMask); // 8 byte 16
			uint8_t* mInfo = reinterpret_cast<uint8_t*>(&mMask);                    // 8 byte 24
			size_t mNumElements = 0;                                                // 8 byte 32
			size_t mMask = 0;                                                       // 8 byte 40
			size_t mMaxNumElementsAllowed = 0;                                      // 8 byte 48
			InfoType mInfoInc = InitialInfoInc;                                     // 4 byte 52
			InfoType mInfoHashShift = InitialInfoHashShift;                         // 4 byte 56
			// 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