//                 ______  _____                 ______                _________
//  ______________ ___  /_ ___(_)_______         ___  /_ ______ ______ ______  /
//  __  ___/_  __ \__  __ \__  / __  __ \        __  __ \_  __ \_  __ \_  __  /
//  _  /    / /_/ /_  /_/ /_  /  _  / / /        _  / / // /_/ // /_/ // /_/ /
//  /_/     \____/ /_.___/ /_/   /_/ /_/ ________/_/ /_/ \____/ \____/ \__,_/
//                                      _/_____/
//
// 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 1  // for backwards-compatible bug fixes

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

// 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
#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>
  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 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>
  std::pair<iterator, bool> insert_or_assign(const_iterator hint,
                                             const key_type &key,
                                             Mapped &&obj) {
    (void)hint;
    return insertOrAssignImpl(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 insertOrAssignImpl(std::move(key), std::forward<Mapped>(obj));
  }

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

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

    // 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;
  }

  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)))

    nextHashMultiplier();
    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
      rehashPowerOfTwo(mMask + 1, true);
    } else {
      // Each resize use a different hash so we don't so easily overflow.
      // Make sure we only have odd numbers, so that the multiplication is
      // reversible!
      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