mariadb-columnstore-engine/utils/common/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.

#pragma once

// see https://semver.org/
#define ROBIN_HOOD_VERSION_MAJOR 3   // for incompatible API changes
#define ROBIN_HOOD_VERSION_MINOR 10  // for adding functionality in a backwards-compatible manner
#define ROBIN_HOOD_VERSION_PATCH 0   // 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
#define ROBIN_HOOD_PRIVATE_DEFINITION_LITTLE_ENDIAN() (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
#define ROBIN_HOOD_PRIVATE_DEFINITION_BIG_ENDIAN() (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)

// inline
#define ROBIN_HOOD_PRIVATE_DEFINITION_NOINLINE() __attribute__((noinline))

// 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)
#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

// 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
#define ROBIN_HOOD_LIKELY(condition) __builtin_expect(condition, 1)
#define ROBIN_HOOD_UNLIKELY(condition) __builtin_expect(condition, 0)

// detect if native wchar_t type is availiable in MSVC
#define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_NATIVE_WCHART() 1

#define ROBIN_HOOD_IS_TRIVIALLY_COPYABLE(...) std::is_trivially_copyable<__VA_ARGS__>::value

// 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);
  }

  // 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
  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
    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
        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 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, false);
  }

  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