/* Common base for Boost.Unordered open-addressing tables.
 *
 * Copyright 2022-2024 Joaquin M Lopez Munoz.
 * Copyright 2023 Christian Mazakas.
 * Copyright 2024 Braden Ganetsky.
 * Distributed under the Boost Software License, Version 1.0.
 * (See accompanying file LICENSE_1_0.txt or copy at
 * http://www.boost.org/LICENSE_1_0.txt)
 *
 * See https://www.boost.org/libs/unordered for library home page.
 */

#ifndef BOOST_UNORDERED_DETAIL_FOA_CORE_HPP
#define BOOST_UNORDERED_DETAIL_FOA_CORE_HPP

#include <boost/assert.hpp>
#include <boost/config.hpp>
#include <boost/config/workaround.hpp>
#include <boost/core/allocator_traits.hpp>
#include <boost/core/bit.hpp>
#include <boost/core/empty_value.hpp>
#include <boost/core/no_exceptions_support.hpp>
#include <boost/core/pointer_traits.hpp>
#include <boost/cstdint.hpp>
#include <boost/predef.h>
#include <boost/unordered/detail/allocator_constructed.hpp>
#include <boost/unordered/detail/narrow_cast.hpp>
#include <boost/unordered/detail/mulx.hpp>
#include <boost/unordered/detail/static_assert.hpp>
#include <boost/unordered/detail/type_traits.hpp>
#include <boost/unordered/hash_traits.hpp>
#include <climits>
#include <cmath>
#include <cstddef>
#include <cstring>
#include <limits>
#include <memory>
#include <new>
#include <tuple>
#include <type_traits>
#include <utility>

#if defined(BOOST_UNORDERED_ENABLE_STATS)
#include <boost/unordered/detail/foa/cumulative_stats.hpp>
#endif

#if !defined(BOOST_UNORDERED_DISABLE_SSE2)
#if defined(BOOST_UNORDERED_ENABLE_SSE2)|| \
    defined(__SSE2__)|| \
    defined(_M_X64)||(defined(_M_IX86_FP)&&_M_IX86_FP>=2)
#define BOOST_UNORDERED_SSE2
#endif
#endif

#if !defined(BOOST_UNORDERED_DISABLE_NEON)
#if defined(BOOST_UNORDERED_ENABLE_NEON)||\
    (defined(__ARM_NEON)&&!defined(__ARM_BIG_ENDIAN))
#define BOOST_UNORDERED_LITTLE_ENDIAN_NEON
#endif
#endif

#if defined(BOOST_UNORDERED_SSE2)
#include <emmintrin.h>
#elif defined(BOOST_UNORDERED_LITTLE_ENDIAN_NEON)
#include <arm_neon.h>
#endif

#ifdef __has_builtin
#define BOOST_UNORDERED_HAS_BUILTIN(x) __has_builtin(x)
#else
#define BOOST_UNORDERED_HAS_BUILTIN(x) 0
#endif

#if !defined(NDEBUG)
#define BOOST_UNORDERED_ASSUME(cond) BOOST_ASSERT(cond)
#elif BOOST_UNORDERED_HAS_BUILTIN(__builtin_assume)
#define BOOST_UNORDERED_ASSUME(cond) __builtin_assume(cond)
#elif defined(__GNUC__) || BOOST_UNORDERED_HAS_BUILTIN(__builtin_unreachable)
#define BOOST_UNORDERED_ASSUME(cond)    \
  do{                                   \
    if(!(cond))__builtin_unreachable(); \
  }while(0)
#elif defined(_MSC_VER)
#define BOOST_UNORDERED_ASSUME(cond) __assume(cond)
#else
#define BOOST_UNORDERED_ASSUME(cond)  \
  do{                                 \
    static_cast<void>(false&&(cond)); \
  }while(0)
#endif

/* We use BOOST_UNORDERED_PREFETCH[_ELEMENTS] macros rather than proper
 * functions because of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109985
 */

#if defined(BOOST_GCC)||defined(BOOST_CLANG)
#define BOOST_UNORDERED_PREFETCH(p) __builtin_prefetch((const char*)(p))
#elif defined(BOOST_UNORDERED_SSE2)
#define BOOST_UNORDERED_PREFETCH(p) _mm_prefetch((const char*)(p),_MM_HINT_T0)
#else
#define BOOST_UNORDERED_PREFETCH(p) ((void)(p))
#endif

/* We have experimentally confirmed that ARM architectures get a higher
 * speedup when around the first half of the element slots in a group are
 * prefetched, whereas for Intel just the first cache line is best.
 * Please report back if you find better tunings for some particular
 * architectures.
 */

#if BOOST_ARCH_ARM
/* Cache line size can't be known at compile time, so we settle on
 * the very frequent value of 64B.
 */

#define BOOST_UNORDERED_PREFETCH_ELEMENTS(p,N)                          \
  do{                                                                   \
    auto           BOOST_UNORDERED_P=(p);                               \
    constexpr int  cache_line=64;                                       \
    const char    *p0=reinterpret_cast<const char*>(BOOST_UNORDERED_P), \
                  *p1=p0+sizeof(*BOOST_UNORDERED_P)*(N)/2;              \
    for(;p0<p1;p0+=cache_line)BOOST_UNORDERED_PREFETCH(p0);             \
  }while(0)
#else
#define BOOST_UNORDERED_PREFETCH_ELEMENTS(p,N) BOOST_UNORDERED_PREFETCH(p)
#endif

#ifdef __has_feature
#define BOOST_UNORDERED_HAS_FEATURE(x) __has_feature(x)
#else
#define BOOST_UNORDERED_HAS_FEATURE(x) 0
#endif

#if BOOST_UNORDERED_HAS_FEATURE(thread_sanitizer)|| \
    defined(__SANITIZE_THREAD__)
#define BOOST_UNORDERED_THREAD_SANITIZER
#endif

#define BOOST_UNORDERED_STATIC_ASSERT_HASH_PRED(Hash, Pred)                    \
  static_assert(boost::unordered::detail::is_nothrow_swappable<Hash>::value,   \
    "Template parameter Hash is required to be nothrow Swappable.");           \
  static_assert(boost::unordered::detail::is_nothrow_swappable<Pred>::value,   \
    "Template parameter Pred is required to be nothrow Swappable");

namespace boost{
namespace unordered{
namespace detail{
namespace foa{

static constexpr std::size_t default_bucket_count=0;

/* foa::table_core is the common base of foa::table and foa::concurrent_table,
 * which in their turn serve as the foundational core of
 * boost::unordered_(flat|node)_(map|set) and boost::concurrent_flat_(map|set),
 * respectively. Its main internal design aspects are:
 * 
 *   - Element slots are logically split into groups of size N=15. The number
 *     of groups is always a power of two, so the number of allocated slots
       is of the form (N*2^n)-1 (final slot reserved for a sentinel mark).
 *   - Positioning is done at the group level rather than the slot level, that
 *     is, for any given element its hash value is used to locate a group and
 *     insertion is performed on the first available element of that group;
 *     if the group is full (overflow), further groups are tried using
 *     quadratic probing.
 *   - Each group has an associated 16B metadata word holding reduced hash
 *     values and overflow information. Reduced hash values are used to
 *     accelerate lookup within the group by using 128-bit SIMD or 64-bit word
 *     operations.
 */

/* group15 controls metadata information of a group of N=15 element slots.
 * The 16B metadata word is organized as follows (LSB depicted rightmost):
 *
 *   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 *   |ofw|h14|h13|h13|h11|h10|h09|h08|h07|h06|h05|h04|h03|h02|h01|h00|
 *   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 *
 * hi is 0 if the i-th element slot is avalaible, 1 to mark a sentinel and,
 * when the slot is occupied, a value in the range [2,255] obtained from the
 * element's original hash value.
 * ofw is the so-called overflow byte. If insertion of an element with hash
 * value h is tried on a full group, then the (h%8)-th bit of the overflow
 * byte is set to 1 and a further group is probed. Having an overflow byte
 * brings two advantages:
 * 
 *   - There's no need to reserve a special value of hi to mark tombstone
 *     slots; each reduced hash value keeps then log2(254)=7.99 bits of the
 *     original hash (alternative approaches reserve one full bit to mark
 *     if the slot is available/deleted, so their reduced hash values are 7 bit
 *     strong only).
 *   - When doing an unsuccessful lookup (i.e. the element is not present in
 *     the table), probing stops at the first non-overflowed group. Having 8
 *     bits for signalling overflow makes it very likely that we stop at the
 *     current group (this happens when no element with the same (h%8) value
 *     has overflowed in the group), saving us an additional group check even
 *     under high-load/high-erase conditions. It is critical that hash
 *     reduction is invariant under modulo 8 (see maybe_caused_overflow).
 *
 * When looking for an element with hash value h, match(h) returns a bitmask
 * signalling which slots have the same reduced hash value. If available,
 * match uses SSE2 or (little endian) Neon 128-bit SIMD operations. On non-SIMD
 * scenarios, the logical layout described above is physically mapped to two
 * 64-bit words with *bit interleaving*, i.e. the least significant 16 bits of
 * the first 64-bit word contain the least significant bits of each byte in the
 * "logical" 128-bit word, and so forth. With this layout, match can be
 * implemented with 4 ANDs, 3 shifts, 2 XORs, 1 OR and 1 NOT.
 * 
 * IntegralWrapper<Integral> is used to implement group15's underlying
 * metadata: it behaves as a plain integral for foa::table or introduces
 * atomic ops for foa::concurrent_table. If IntegralWrapper<...> is trivially
 * constructible, so is group15, in which case it can be initialized via memset
 * etc. Where needed, group15::initialize resets the metadata to the all
 * zeros (default state).
 */

#if defined(BOOST_UNORDERED_SSE2)

template<template<typename> class IntegralWrapper>
struct group15
{
  static constexpr std::size_t N=15;
  static constexpr bool        regular_layout=true;

  struct dummy_group_type
  {
    alignas(16) unsigned char storage[N+1]={0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0};
  };

  inline void initialize()
  {
    _mm_store_si128(
      reinterpret_cast<__m128i*>(m),_mm_setzero_si128());
  }

  inline void set(std::size_t pos,std::size_t hash)
  {
    BOOST_ASSERT(pos<N);
    at(pos)=reduced_hash(hash);
  }

  inline void set_sentinel()
  {
    at(N-1)=sentinel_;
  }

  inline bool is_sentinel(std::size_t pos)const
  {
    BOOST_ASSERT(pos<N);
    return at(pos)==sentinel_;
  }

  static inline bool is_sentinel(unsigned char* pc)noexcept
  {
    return *pc==sentinel_;
  }

  inline void reset(std::size_t pos)
  {
    BOOST_ASSERT(pos<N);
    at(pos)=available_;
  }

  static inline void reset(unsigned char* pc)
  {
    *reinterpret_cast<slot_type*>(pc)=available_;
  }

  inline int match(std::size_t hash)const
  {
    return _mm_movemask_epi8(
      _mm_cmpeq_epi8(load_metadata(),_mm_set1_epi32(match_word(hash))))&0x7FFF;
  }

  inline bool is_not_overflowed(std::size_t hash)const
  {
    static constexpr unsigned char shift[]={1,2,4,8,16,32,64,128};

    return !(overflow()&shift[hash%8]);
  }

  inline void mark_overflow(std::size_t hash)
  {
    overflow()|=static_cast<unsigned char>(1<<(hash%8));
  }

  static inline bool maybe_caused_overflow(unsigned char* pc)
  {
    std::size_t pos=reinterpret_cast<uintptr_t>(pc)%sizeof(group15);
    group15    *pg=reinterpret_cast<group15*>(pc-pos);
    return !pg->is_not_overflowed(*pc);
  }

  inline int match_available()const
  {
    return _mm_movemask_epi8(
      _mm_cmpeq_epi8(load_metadata(),_mm_setzero_si128()))&0x7FFF;
  }

  inline bool is_occupied(std::size_t pos)const
  {
    BOOST_ASSERT(pos<N);
    return at(pos)!=available_;
  }

  static inline bool is_occupied(unsigned char* pc)noexcept
  {
    return *reinterpret_cast<slot_type*>(pc)!=available_;
  }

  inline int match_occupied()const
  {
    return (~match_available())&0x7FFF;
  }

private:
  using slot_type=IntegralWrapper<unsigned char>;
  BOOST_UNORDERED_STATIC_ASSERT(sizeof(slot_type)==1);

  static constexpr unsigned char available_=0,
                                 sentinel_=1;

  inline __m128i load_metadata()const
  {
#if defined(BOOST_UNORDERED_THREAD_SANITIZER)
    /* ThreadSanitizer complains on 1-byte atomic writes combined with
     * 16-byte atomic reads.
     */

    return _mm_set_epi8(
      (char)m[15],(char)m[14],(char)m[13],(char)m[12],
      (char)m[11],(char)m[10],(char)m[ 9],(char)m[ 8],
      (char)m[ 7],(char)m[ 6],(char)m[ 5],(char)m[ 4],
      (char)m[ 3],(char)m[ 2],(char)m[ 1],(char)m[ 0]);
#else
    return _mm_load_si128(reinterpret_cast<const __m128i*>(m));
#endif
  }

  inline static int match_word(std::size_t hash)
  {
    static constexpr boost::uint32_t word[]=
    {
      0x08080808u,0x09090909u,0x02020202u,0x03030303u,0x04040404u,0x05050505u,
      0x06060606u,0x07070707u,0x08080808u,0x09090909u,0x0A0A0A0Au,0x0B0B0B0Bu,
      0x0C0C0C0Cu,0x0D0D0D0Du,0x0E0E0E0Eu,0x0F0F0F0Fu,0x10101010u,0x11111111u,
      0x12121212u,0x13131313u,0x14141414u,0x15151515u,0x16161616u,0x17171717u,
      0x18181818u,0x19191919u,0x1A1A1A1Au,0x1B1B1B1Bu,0x1C1C1C1Cu,0x1D1D1D1Du,
      0x1E1E1E1Eu,0x1F1F1F1Fu,0x20202020u,0x21212121u,0x22222222u,0x23232323u,
      0x24242424u,0x25252525u,0x26262626u,0x27272727u,0x28282828u,0x29292929u,
      0x2A2A2A2Au,0x2B2B2B2Bu,0x2C2C2C2Cu,0x2D2D2D2Du,0x2E2E2E2Eu,0x2F2F2F2Fu,
      0x30303030u,0x31313131u,0x32323232u,0x33333333u,0x34343434u,0x35353535u,
      0x36363636u,0x37373737u,0x38383838u,0x39393939u,0x3A3A3A3Au,0x3B3B3B3Bu,
      0x3C3C3C3Cu,0x3D3D3D3Du,0x3E3E3E3Eu,0x3F3F3F3Fu,0x40404040u,0x41414141u,
      0x42424242u,0x43434343u,0x44444444u,0x45454545u,0x46464646u,0x47474747u,
      0x48484848u,0x49494949u,0x4A4A4A4Au,0x4B4B4B4Bu,0x4C4C4C4Cu,0x4D4D4D4Du,
      0x4E4E4E4Eu,0x4F4F4F4Fu,0x50505050u,0x51515151u,0x52525252u,0x53535353u,
      0x54545454u,0x55555555u,0x56565656u,0x57575757u,0x58585858u,0x59595959u,
      0x5A5A5A5Au,0x5B5B5B5Bu,0x5C5C5C5Cu,0x5D5D5D5Du,0x5E5E5E5Eu,0x5F5F5F5Fu,
      0x60606060u,0x61616161u,0x62626262u,0x63636363u,0x64646464u,0x65656565u,
      0x66666666u,0x67676767u,0x68686868u,0x69696969u,0x6A6A6A6Au,0x6B6B6B6Bu,
      0x6C6C6C6Cu,0x6D6D6D6Du,0x6E6E6E6Eu,0x6F6F6F6Fu,0x70707070u,0x71717171u,
      0x72727272u,0x73737373u,0x74747474u,0x75757575u,0x76767676u,0x77777777u,
      0x78787878u,0x79797979u,0x7A7A7A7Au,0x7B7B7B7Bu,0x7C7C7C7Cu,0x7D7D7D7Du,
      0x7E7E7E7Eu,0x7F7F7F7Fu,0x80808080u,0x81818181u,0x82828282u,0x83838383u,
      0x84848484u,0x85858585u,0x86868686u,0x87878787u,0x88888888u,0x89898989u,
      0x8A8A8A8Au,0x8B8B8B8Bu,0x8C8C8C8Cu,0x8D8D8D8Du,0x8E8E8E8Eu,0x8F8F8F8Fu,
      0x90909090u,0x91919191u,0x92929292u,0x93939393u,0x94949494u,0x95959595u,
      0x96969696u,0x97979797u,0x98989898u,0x99999999u,0x9A9A9A9Au,0x9B9B9B9Bu,
      0x9C9C9C9Cu,0x9D9D9D9Du,0x9E9E9E9Eu,0x9F9F9F9Fu,0xA0A0A0A0u,0xA1A1A1A1u,
      0xA2A2A2A2u,0xA3A3A3A3u,0xA4A4A4A4u,0xA5A5A5A5u,0xA6A6A6A6u,0xA7A7A7A7u,
      0xA8A8A8A8u,0xA9A9A9A9u,0xAAAAAAAAu,0xABABABABu,0xACACACACu,0xADADADADu,
      0xAEAEAEAEu,0xAFAFAFAFu,0xB0B0B0B0u,0xB1B1B1B1u,0xB2B2B2B2u,0xB3B3B3B3u,
      0xB4B4B4B4u,0xB5B5B5B5u,0xB6B6B6B6u,0xB7B7B7B7u,0xB8B8B8B8u,0xB9B9B9B9u,
      0xBABABABAu,0xBBBBBBBBu,0xBCBCBCBCu,0xBDBDBDBDu,0xBEBEBEBEu,0xBFBFBFBFu,
      0xC0C0C0C0u,0xC1C1C1C1u,0xC2C2C2C2u,0xC3C3C3C3u,0xC4C4C4C4u,0xC5C5C5C5u,
      0xC6C6C6C6u,0xC7C7C7C7u,0xC8C8C8C8u,0xC9C9C9C9u,0xCACACACAu,0xCBCBCBCBu,
      0xCCCCCCCCu,0xCDCDCDCDu,0xCECECECEu,0xCFCFCFCFu,0xD0D0D0D0u,0xD1D1D1D1u,
      0xD2D2D2D2u,0xD3D3D3D3u,0xD4D4D4D4u,0xD5D5D5D5u,0xD6D6D6D6u,0xD7D7D7D7u,
      0xD8D8D8D8u,0xD9D9D9D9u,0xDADADADAu,0xDBDBDBDBu,0xDCDCDCDCu,0xDDDDDDDDu,
      0xDEDEDEDEu,0xDFDFDFDFu,0xE0E0E0E0u,0xE1E1E1E1u,0xE2E2E2E2u,0xE3E3E3E3u,
      0xE4E4E4E4u,0xE5E5E5E5u,0xE6E6E6E6u,0xE7E7E7E7u,0xE8E8E8E8u,0xE9E9E9E9u,
      0xEAEAEAEAu,0xEBEBEBEBu,0xECECECECu,0xEDEDEDEDu,0xEEEEEEEEu,0xEFEFEFEFu,
      0xF0F0F0F0u,0xF1F1F1F1u,0xF2F2F2F2u,0xF3F3F3F3u,0xF4F4F4F4u,0xF5F5F5F5u,
      0xF6F6F6F6u,0xF7F7F7F7u,0xF8F8F8F8u,0xF9F9F9F9u,0xFAFAFAFAu,0xFBFBFBFBu,
      0xFCFCFCFCu,0xFDFDFDFDu,0xFEFEFEFEu,0xFFFFFFFFu,
    };

    return (int)word[narrow_cast<unsigned char>(hash)];
  }

  inline static unsigned char reduced_hash(std::size_t hash)
  {
    return narrow_cast<unsigned char>(match_word(hash));
  }

  inline slot_type& at(std::size_t pos)
  {
    return m[pos];
  }

  inline const slot_type& at(std::size_t pos)const
  {
    return m[pos];
  }

  inline slot_type& overflow()
  {
    return at(N);
  }

  inline const slot_type& overflow()const
  {
    return at(N);
  }

  alignas(16) slot_type m[16];
};

#elif defined(BOOST_UNORDERED_LITTLE_ENDIAN_NEON)

template<template<typename> class IntegralWrapper>
struct group15
{
  static constexpr std::size_t N=15;
  static constexpr bool        regular_layout=true;

  struct dummy_group_type
  {
    alignas(16) unsigned char storage[N+1]={0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0};
  };

  inline void initialize()
  {
    vst1q_u8(reinterpret_cast<uint8_t*>(m),vdupq_n_u8(0));
  }

  inline void set(std::size_t pos,std::size_t hash)
  {
    BOOST_ASSERT(pos<N);
    at(pos)=reduced_hash(hash);
  }

  inline void set_sentinel()
  {
    at(N-1)=sentinel_;
  }

  inline bool is_sentinel(std::size_t pos)const
  {
    BOOST_ASSERT(pos<N);
    return pos==N-1&&at(N-1)==sentinel_;
  }

  static inline bool is_sentinel(unsigned char* pc)noexcept
  {
    return *reinterpret_cast<slot_type*>(pc)==sentinel_;
  }

  inline void reset(std::size_t pos)
  {
    BOOST_ASSERT(pos<N);
    at(pos)=available_;
  }

  static inline void reset(unsigned char* pc)
  {
    *reinterpret_cast<slot_type*>(pc)=available_;
  }

  inline int match(std::size_t hash)const
  {
    return simde_mm_movemask_epi8(vceqq_u8(
      load_metadata(),vdupq_n_u8(reduced_hash(hash))))&0x7FFF;
  }

  inline bool is_not_overflowed(std::size_t hash)const
  {
    static constexpr unsigned char shift[]={1,2,4,8,16,32,64,128};

    return !(overflow()&shift[hash%8]);
  }

  inline void mark_overflow(std::size_t hash)
  {
    overflow()|=static_cast<unsigned char>(1<<(hash%8));
  }

  static inline bool maybe_caused_overflow(unsigned char* pc)
  {
    std::size_t pos=reinterpret_cast<uintptr_t>(pc)%sizeof(group15);
    group15    *pg=reinterpret_cast<group15*>(pc-pos);
    return !pg->is_not_overflowed(*pc);
  };

  inline int match_available()const
  {
    return simde_mm_movemask_epi8(vceqq_u8(
      load_metadata(),vdupq_n_u8(0)))&0x7FFF;
  }

  inline bool is_occupied(std::size_t pos)const
  {
    BOOST_ASSERT(pos<N);
    return at(pos)!=available_;
  }

  static inline bool is_occupied(unsigned char* pc)noexcept
  {
    return *reinterpret_cast<slot_type*>(pc)!=available_;
  }

  inline int match_occupied()const
  {
    return simde_mm_movemask_epi8(vcgtq_u8(
      load_metadata(),vdupq_n_u8(0)))&0x7FFF;
  }

private:
  using slot_type=IntegralWrapper<unsigned char>;
  BOOST_UNORDERED_STATIC_ASSERT(sizeof(slot_type)==1);

  static constexpr unsigned char available_=0,
                                 sentinel_=1;

  inline uint8x16_t load_metadata()const
  {
#if defined(BOOST_UNORDERED_THREAD_SANITIZER)
    /* ThreadSanitizer complains on 1-byte atomic writes combined with
     * 16-byte atomic reads.
     */

    alignas(16) uint8_t data[16]={
      m[ 0],m[ 1],m[ 2],m[ 3],m[ 4],m[ 5],m[ 6],m[ 7],
      m[ 8],m[ 9],m[10],m[11],m[12],m[13],m[14],m[15]};
    return vld1q_u8(data);
#else
    return vld1q_u8(reinterpret_cast<const uint8_t*>(m));
#endif
  }

  inline static unsigned char reduced_hash(std::size_t hash)
  {
    static constexpr unsigned char table[]={
      8,9,2,3,4,5,6,7,8,9,10,11,12,13,14,15,
      16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,
      32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,
      48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,
      64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,
      80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,
      96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,
      112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,
      128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,
      144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,
      160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,
      176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,
      192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,
      208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,
      224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,
      240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,
    };
    
    return table[(unsigned char)hash];
  }

  /* Copied from 
   * https://github.com/simd-everywhere/simde/blob/master/simde/x86/
   * sse2.h#L3763
   */

  static inline int simde_mm_movemask_epi8(uint8x16_t a)
  {
    static constexpr uint8_t md[16]={
      1 << 0, 1 << 1, 1 << 2, 1 << 3,
      1 << 4, 1 << 5, 1 << 6, 1 << 7,
      1 << 0, 1 << 1, 1 << 2, 1 << 3,
      1 << 4, 1 << 5, 1 << 6, 1 << 7,
    };

    uint8x16_t  masked=vandq_u8(vld1q_u8(md),a);
    uint8x8x2_t tmp=vzip_u8(vget_low_u8(masked),vget_high_u8(masked));
    uint16x8_t  x=vreinterpretq_u16_u8(vcombine_u8(tmp.val[0],tmp.val[1]));

#if defined(__ARM_ARCH_ISA_A64)
    return vaddvq_u16(x);
#else
    uint64x2_t t64=vpaddlq_u32(vpaddlq_u16(x));
    return int(vgetq_lane_u64(t64,0))+int(vgetq_lane_u64(t64,1));
#endif
  }

  inline slot_type& at(std::size_t pos)
  {
    return m[pos];
  }

  inline const slot_type& at(std::size_t pos)const
  {
    return m[pos];
  }

  inline slot_type& overflow()
  {
    return at(N);
  }

  inline const slot_type& overflow()const
  {
    return at(N);
  }

  alignas(16) slot_type m[16];
};

#else /* non-SIMD */

template<template<typename> class IntegralWrapper>
struct group15
{
  static constexpr std::size_t N=15;
  static constexpr bool        regular_layout=false;

  struct dummy_group_type
  {
    alignas(16) boost::uint64_t m[2]=
      {0x0000000000004000ull,0x0000000000000000ull};
  };

  inline void initialize(){m[0]=0;m[1]=0;}

  inline void set(std::size_t pos,std::size_t hash)
  {
    BOOST_ASSERT(pos<N);
    set_impl(pos,reduced_hash(hash));
  }

  inline void set_sentinel()
  {
    set_impl(N-1,sentinel_);
  }

  inline bool is_sentinel(std::size_t pos)const
  {
    BOOST_ASSERT(pos<N);
    return 
      pos==N-1&&
      (m[0] & boost::uint64_t(0x4000400040004000ull))==
        boost::uint64_t(0x4000ull)&&
      (m[1] & boost::uint64_t(0x4000400040004000ull))==0;
  }

  inline void reset(std::size_t pos)
  {
    BOOST_ASSERT(pos<N);
    set_impl(pos,available_);
  }

  static inline void reset(unsigned char* pc)
  {
    std::size_t pos=reinterpret_cast<uintptr_t>(pc)%sizeof(group15);
    pc-=pos;
    reinterpret_cast<group15*>(pc)->reset(pos);
  }

  inline int match(std::size_t hash)const
  {
    return match_impl(reduced_hash(hash));
  }

  inline bool is_not_overflowed(std::size_t hash)const
  {
    return !(reinterpret_cast<const boost::uint16_t*>(m)[hash%8] & 0x8000u);
  }

  inline void mark_overflow(std::size_t hash)
  {
    reinterpret_cast<boost::uint16_t*>(m)[hash%8]|=0x8000u;
  }

  static inline bool maybe_caused_overflow(unsigned char* pc)
  {
    std::size_t     pos=reinterpret_cast<uintptr_t>(pc)%sizeof(group15);
    group15        *pg=reinterpret_cast<group15*>(pc-pos);
    boost::uint64_t x=((pg->m[0])>>pos)&0x000100010001ull;
    boost::uint32_t y=narrow_cast<boost::uint32_t>(x|(x>>15)|(x>>30));
    return !pg->is_not_overflowed(y);
  };

  inline int match_available()const
  {
    boost::uint64_t x=~(m[0]|m[1]);
    boost::uint32_t y=static_cast<boost::uint32_t>(x&(x>>32));
    y&=y>>16;
    return y&0x7FFF;
  }

  inline bool is_occupied(std::size_t pos)const
  {
    BOOST_ASSERT(pos<N);
    boost::uint64_t x=m[0]|m[1];
    return (x&(0x0001000100010001ull<<pos))!=0;
  }

  inline int match_occupied()const
  {
    boost::uint64_t x=m[0]|m[1];
    boost::uint32_t y=narrow_cast<boost::uint32_t>(x|(x>>32));
    y|=y>>16;
    return y&0x7FFF;
  }

private:
  using word_type=IntegralWrapper<uint64_t>;
  BOOST_UNORDERED_STATIC_ASSERT(sizeof(word_type)==8);

  static constexpr unsigned char available_=0,
                                 sentinel_=1;

  inline static unsigned char reduced_hash(std::size_t hash)
  {
    static constexpr unsigned char table[]={
      8,9,2,3,4,5,6,7,8,9,10,11,12,13,14,15,
      16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,
      32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,
      48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,
      64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,
      80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,
      96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,
      112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,
      128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,
      144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,
      160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,
      176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,
      192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,
      208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,
      224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,
      240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,
    };
    
    return table[narrow_cast<unsigned char>(hash)];
  }

  inline void set_impl(std::size_t pos,std::size_t n)
  {
    BOOST_ASSERT(n<256);
    set_impl(m[0],pos,n&0xFu);
    set_impl(m[1],pos,n>>4);
  }

  static inline void set_impl(word_type& x,std::size_t pos,std::size_t n)
  {
    static constexpr boost::uint64_t mask[]=
    {
      0x0000000000000000ull,0x0000000000000001ull,0x0000000000010000ull,
      0x0000000000010001ull,0x0000000100000000ull,0x0000000100000001ull,
      0x0000000100010000ull,0x0000000100010001ull,0x0001000000000000ull,
      0x0001000000000001ull,0x0001000000010000ull,0x0001000000010001ull,
      0x0001000100000000ull,0x0001000100000001ull,0x0001000100010000ull,
      0x0001000100010001ull,
    };
    static constexpr boost::uint64_t imask[]=
    {
      0x0001000100010001ull,0x0001000100010000ull,0x0001000100000001ull,
      0x0001000100000000ull,0x0001000000010001ull,0x0001000000010000ull,
      0x0001000000000001ull,0x0001000000000000ull,0x0000000100010001ull,
      0x0000000100010000ull,0x0000000100000001ull,0x0000000100000000ull,
      0x0000000000010001ull,0x0000000000010000ull,0x0000000000000001ull,
      0x0000000000000000ull,
    };

    BOOST_ASSERT(pos<16&&n<16);
    x|=   mask[n]<<pos;
    x&=~(imask[n]<<pos);
  }

  inline int match_impl(std::size_t n)const
  {
    static constexpr boost::uint64_t mask[]=
    {
      0x0000000000000000ull,0x000000000000ffffull,0x00000000ffff0000ull,
      0x00000000ffffffffull,0x0000ffff00000000ull,0x0000ffff0000ffffull,
      0x0000ffffffff0000ull,0x0000ffffffffffffull,0xffff000000000000ull,
      0xffff00000000ffffull,0xffff0000ffff0000ull,0xffff0000ffffffffull,
      0xffffffff00000000ull,0xffffffff0000ffffull,0xffffffffffff0000ull,
      0xffffffffffffffffull,
    };

    BOOST_ASSERT(n<256);
    boost::uint64_t x=m[0]^mask[n&0xFu];
                    x=~((m[1]^mask[n>>4])|x);
    boost::uint32_t y=static_cast<boost::uint32_t>(x&(x>>32));
                    y&=y>>16;
    return          y&0x7FFF;
  }

  alignas(16) word_type m[2];
};

#endif

/* foa::table_core uses a size policy to obtain the permissible sizes of the
 * group array (and, by implication, the element array) and to do the
 * hash->group mapping.
 * 
 *   - size_index(n) returns an unspecified "index" number used in other policy
 *     operations.
 *   - size(size_index_) returns the number of groups for the given index. It
 *     is guaranteed that size(size_index(n)) >= n.
 *   - min_size() is the minimum number of groups permissible, i.e.
 *     size(size_index(0)).
 *   - position(hash,size_index_) maps hash to a position in the range
 *     [0,size(size_index_)).
 * 
 * The reason we're introducing the intermediate index value for calculating
 * sizes and positions is that it allows us to optimize the implementation of
 * position, which is in the hot path of lookup and insertion operations:
 * pow2_size_policy, the actual size policy used by foa::table, returns 2^n
 * (n>0) as permissible sizes and returns the n most significant bits
 * of the hash value as the position in the group array; using a size index
 * defined as i = (bits in std::size_t) - n, we have an unbeatable
 * implementation of position(hash) as hash>>i.
 * There's a twofold reason for choosing the high bits of hash for positioning:
 *   - Multiplication-based mixing tends to yield better entropy in the high
 *     part of its result.
 *   - group15 reduced-hash values take the *low* bits of hash, and we want
 *     these values and positioning to be as uncorrelated as possible.
 */

struct pow2_size_policy
{
  static inline std::size_t size_index(std::size_t n)
  {
    // TODO: min size is 2, see if we can bring it down to 1 without loss
    // of performance

    return sizeof(std::size_t)*CHAR_BIT-
      (n<=2?1:((std::size_t)(boost::core::bit_width(n-1))));
  }

  static inline std::size_t size(std::size_t size_index_)
  {
     return std::size_t(1)<<(sizeof(std::size_t)*CHAR_BIT-size_index_);  
  }
    
  static constexpr std::size_t min_size(){return 2;}

  static inline std::size_t position(std::size_t hash,std::size_t size_index_)
  {
    return hash>>size_index_;
  }
};

/* size index of a group array for a given *element* capacity */

template<typename Group,typename SizePolicy>
static inline std::size_t size_index_for(std::size_t n)
{
  /* n/N+1 == ceil((n+1)/N) (extra +1 for the sentinel) */
  return SizePolicy::size_index(n/Group::N+1);
}

/* Quadratic prober over a power-of-two range using triangular numbers.
 * mask in next(mask) must be the range size minus one (and since size is 2^n,
 * mask has exactly its n first bits set to 1).
 */

struct pow2_quadratic_prober
{
  pow2_quadratic_prober(std::size_t pos_):pos{pos_}{}

  inline std::size_t get()const{return pos;}
  inline std::size_t length()const{return step+1;}

  /* next returns false when the whole array has been traversed, which ends
   * probing (in practice, full-table probing will only happen with very small
   * arrays).
   */

  inline bool next(std::size_t mask)
  {
    step+=1;
    pos=(pos+step)&mask;
    return step<=mask;
  }

private:
  std::size_t pos,step=0;
};

/* Mixing policies: no_mix is the identity function, and mulx_mix
 * uses the mulx function from <boost/unordered/detail/mulx.hpp>.
 *
 * foa::table_core mixes hash results with mulx_mix unless the hash is marked
 * as avalanching, i.e. of good quality
 * (see <boost/unordered/hash_traits.hpp>).
 */

struct no_mix
{
  template<typename Hash,typename T>
  static inline std::size_t mix(const Hash& h,const T& x)
  {
    return h(x);
  }
};

struct mulx_mix
{
  template<typename Hash,typename T>
  static inline std::size_t mix(const Hash& h,const T& x)
  {
    return mulx(h(x));
  }
};

/* boost::core::countr_zero has a potentially costly check for
 * the case x==0.
 */

inline unsigned int unchecked_countr_zero(int x)
{
#if defined(BOOST_MSVC)
  unsigned long r;
  _BitScanForward(&r,(unsigned long)x);
  return (unsigned int)r;
#else
  BOOST_UNORDERED_ASSUME(x!=0);
  return (unsigned int)boost::core::countr_zero((unsigned int)x);
#endif
}

/* table_arrays controls allocation, initialization and deallocation of
 * paired arrays of groups and element slots. Only one chunk of memory is
 * allocated to place both arrays: this is not done for efficiency reasons,
 * but in order to be able to properly align the group array without storing
 * additional offset information --the alignment required (16B) is usually
 * greater than alignof(std::max_align_t) and thus not guaranteed by
 * allocators.
 */

template<typename Group,std::size_t Size>
Group* dummy_groups()
{
  /* Dummy storage initialized as if in an empty container (actually, each
   * of its groups is initialized like a separate empty container).
   * We make table_arrays::groups point to this when capacity()==0, so that
   * we are not allocating any dynamic memory and yet lookup can be implemented
   * without checking for groups==nullptr. This space won't ever be used for
   * insertion as the container's capacity is precisely zero.
   */

  static constexpr typename Group::dummy_group_type
  storage[Size]={typename Group::dummy_group_type(),};

  return reinterpret_cast<Group*>(
    const_cast<typename Group::dummy_group_type*>(storage));
}

template<
  typename Ptr,typename Ptr2,
  typename std::enable_if<!std::is_same<Ptr,Ptr2>::value>::type* = nullptr
>
Ptr to_pointer(Ptr2 p)
{
  if(!p){return nullptr;}
  return boost::pointer_traits<Ptr>::pointer_to(*p);
}

template<typename Ptr>
Ptr to_pointer(Ptr p)
{
  return p;
}

template<typename Arrays,typename Allocator>
struct arrays_holder
{
  arrays_holder(const Arrays& arrays,const Allocator& al):
    arrays_{arrays},al_{al}
  {}
  
  /* not defined but VS in pre-C++17 mode needs to see it for RVO */
  arrays_holder(arrays_holder const&);
  arrays_holder& operator=(arrays_holder const&)=delete;

  ~arrays_holder()
  {
    if(!released_){
      arrays_.delete_(typename Arrays::allocator_type(al_),arrays_);
    }
  }

  const Arrays& release()
  {
    released_=true;
    return arrays_;
  }

private:
  Arrays    arrays_;
  Allocator al_;
  bool      released_=false;
};

template<typename Value,typename Group,typename SizePolicy,typename Allocator>
struct table_arrays
{
  using allocator_type=typename boost::allocator_rebind<Allocator,Value>::type;

  using value_type=Value;
  using group_type=Group;
  static constexpr auto N=group_type::N;
  using size_policy=SizePolicy;
  using value_type_pointer=
    typename boost::allocator_pointer<allocator_type>::type;
  using group_type_pointer=
    typename boost::pointer_traits<value_type_pointer>::template
      rebind<group_type>;
  using group_type_pointer_traits=boost::pointer_traits<group_type_pointer>;

  table_arrays(
    std::size_t gsi,std::size_t gsm,
    group_type_pointer pg,value_type_pointer pe):
    groups_size_index{gsi},groups_size_mask{gsm},groups_{pg},elements_{pe}{}

  value_type* elements()const noexcept{return boost::to_address(elements_);}
  group_type* groups()const noexcept{return boost::to_address(groups_);}

  static void set_arrays(table_arrays& arrays,allocator_type al,std::size_t n)
  {
    return set_arrays(
      arrays,al,n,std::is_same<group_type*,group_type_pointer>{});
  }

  static void set_arrays(
    table_arrays& arrays,allocator_type al,std::size_t,
    std::false_type /* always allocate */)
  {
    using storage_traits=boost::allocator_traits<allocator_type>;
    auto groups_size_index=arrays.groups_size_index;
    auto groups_size=size_policy::size(groups_size_index);

    auto sal=allocator_type(al);
    arrays.elements_=storage_traits::allocate(sal,buffer_size(groups_size));
    
    /* Align arrays.groups to sizeof(group_type). table_iterator critically
      * depends on such alignment for its increment operation.
      */

    auto p=reinterpret_cast<unsigned char*>(arrays.elements()+groups_size*N-1);
    p+=(uintptr_t(sizeof(group_type))-
        reinterpret_cast<uintptr_t>(p))%sizeof(group_type);
    arrays.groups_=
      group_type_pointer_traits::pointer_to(*reinterpret_cast<group_type*>(p));

    initialize_groups(
      arrays.groups(),groups_size,
      is_trivially_default_constructible<group_type>{});
    arrays.groups()[groups_size-1].set_sentinel();
  }

  static void set_arrays(
    table_arrays& arrays,allocator_type al,std::size_t n,
    std::true_type /* optimize for n==0*/)
  {
    if(!n){
      arrays.groups_=dummy_groups<group_type,size_policy::min_size()>();
    }
    else{
      set_arrays(arrays,al,n,std::false_type{});
    }
  }

  static table_arrays new_(allocator_type al,std::size_t n)
  {
    auto         groups_size_index=size_index_for<group_type,size_policy>(n);
    auto         groups_size=size_policy::size(groups_size_index);
    table_arrays arrays{groups_size_index,groups_size-1,nullptr,nullptr};

    set_arrays(arrays,al,n);
    return arrays;
  }

  static void delete_(allocator_type al,table_arrays& arrays)noexcept
  {
    using storage_traits=boost::allocator_traits<allocator_type>;

    auto sal=allocator_type(al);
    if(arrays.elements()){
      storage_traits::deallocate(
        sal,arrays.elements_,buffer_size(arrays.groups_size_mask+1));
    }
  }

  /* combined space for elements and groups measured in sizeof(value_type)s */

  static std::size_t buffer_size(std::size_t groups_size)
  {
    auto buffer_bytes=
      /* space for elements (we subtract 1 because of the sentinel) */
      sizeof(value_type)*(groups_size*N-1)+
      /* space for groups + padding for group alignment */
      sizeof(group_type)*(groups_size+1)-1;

    /* ceil(buffer_bytes/sizeof(value_type)) */
    return (buffer_bytes+sizeof(value_type)-1)/sizeof(value_type);
  }

  static void initialize_groups(
    group_type* pg,std::size_t size,std::true_type /* memset */)
  {
    /* memset faster/not slower than manual, assumes all zeros is group_type's
     * default layout.
     * reinterpret_cast: GCC may complain about group_type not being trivially
     * copy-assignable when we're relying on trivial copy constructibility.
     */

    std::memset(
      reinterpret_cast<unsigned char*>(pg),0,sizeof(group_type)*size);
  }

  static void initialize_groups(
    group_type* pg,std::size_t size,std::false_type /* manual */)
  {
    while(size--!=0)::new (pg++) group_type();
  }

  std::size_t        groups_size_index;
  std::size_t        groups_size_mask;
  group_type_pointer groups_;
  value_type_pointer elements_;
};

#if defined(BOOST_UNORDERED_ENABLE_STATS)
/* stats support */

struct table_core_cumulative_stats
{
  concurrent_cumulative_stats<1> insertion;
  concurrent_cumulative_stats<2> successful_lookup,
                                 unsuccessful_lookup;
};

struct table_core_insertion_stats
{
  std::size_t            count;
  sequence_stats_summary probe_length;
};

struct table_core_lookup_stats
{
  std::size_t            count;
  sequence_stats_summary probe_length;
  sequence_stats_summary num_comparisons;
};

struct table_core_stats
{
  table_core_insertion_stats insertion;
  table_core_lookup_stats    successful_lookup,
                             unsuccessful_lookup;
};

#define BOOST_UNORDERED_ADD_STATS(stats,args) stats.add args
#define BOOST_UNORDERED_SWAP_STATS(stats1,stats2) std::swap(stats1,stats2)
#define BOOST_UNORDERED_COPY_STATS(stats1,stats2) stats1=stats2
#define BOOST_UNORDERED_RESET_STATS_OF(x) x.reset_stats()
#define BOOST_UNORDERED_STATS_COUNTER(name) std::size_t name=0
#define BOOST_UNORDERED_INCREMENT_STATS_COUNTER(name) ++name

#else

#define BOOST_UNORDERED_ADD_STATS(stats,args) ((void)0)
#define BOOST_UNORDERED_SWAP_STATS(stats1,stats2) ((void)0)
#define BOOST_UNORDERED_COPY_STATS(stats1,stats2) ((void)0)
#define BOOST_UNORDERED_RESET_STATS_OF(x) ((void)0)
#define BOOST_UNORDERED_STATS_COUNTER(name) ((void)0)
#define BOOST_UNORDERED_INCREMENT_STATS_COUNTER(name) ((void)0)

#endif

struct if_constexpr_void_else{void operator()()const{}};

template<bool B,typename F,typename G=if_constexpr_void_else>
void if_constexpr(F f,G g={})
{
  std::get<B?0:1>(std::forward_as_tuple(f,g))();
}

template<bool B,typename T,typename std::enable_if<B>::type* =nullptr>
void copy_assign_if(T& x,const T& y){x=y;}

template<bool B,typename T,typename std::enable_if<!B>::type* =nullptr>
void copy_assign_if(T&,const T&){}

template<bool B,typename T,typename std::enable_if<B>::type* =nullptr>
void move_assign_if(T& x,T& y){x=std::move(y);}

template<bool B,typename T,typename std::enable_if<!B>::type* =nullptr>
void move_assign_if(T&,T&){}

template<bool B,typename T,typename std::enable_if<B>::type* =nullptr>
void swap_if(T& x,T& y){using std::swap; swap(x,y);}

template<bool B,typename T,typename std::enable_if<!B>::type* =nullptr>
void swap_if(T&,T&){}

template<typename Allocator>
struct is_std_allocator:std::false_type{};

template<typename T>
struct is_std_allocator<std::allocator<T>>:std::true_type{};

/* std::allocator::construct marked as deprecated */
#if defined(_LIBCPP_SUPPRESS_DEPRECATED_PUSH)
_LIBCPP_SUPPRESS_DEPRECATED_PUSH
#elif defined(_STL_DISABLE_DEPRECATED_WARNING)
_STL_DISABLE_DEPRECATED_WARNING
#elif defined(_MSC_VER)
#pragma warning(push)
#pragma warning(disable:4996)
#endif

template<typename Allocator,typename Ptr,typename... Args>
struct alloc_has_construct
{
private:
  template<typename Allocator2>
  static decltype(
    std::declval<Allocator2&>().construct(
      std::declval<Ptr>(),std::declval<Args&&>()...),
    std::true_type{}
  ) check(int);

  template<typename> static std::false_type check(...);

public:
  static constexpr bool value=decltype(check<Allocator>(0))::value;
};

#if defined(_LIBCPP_SUPPRESS_DEPRECATED_POP)
_LIBCPP_SUPPRESS_DEPRECATED_POP
#elif defined(_STL_RESTORE_DEPRECATED_WARNING)
_STL_RESTORE_DEPRECATED_WARNING
#elif defined(_MSC_VER)
#pragma warning(pop)
#endif

/* We expose the hard-coded max load factor so that tests can use it without
 * needing to pull it from an instantiated class template such as the table
 * class.
 */
static constexpr float mlf=0.875f;

template<typename Group,typename Element>
struct table_locator
{
  table_locator()=default;
  table_locator(Group* pg_,unsigned int n_,Element* p_):pg{pg_},n{n_},p{p_}{}

  explicit operator bool()const noexcept{return p!=nullptr;}

  Group        *pg=nullptr;
  unsigned int  n=0;
  Element      *p=nullptr;
};

struct try_emplace_args_t{};

template<typename TypePolicy,typename Allocator,typename... Args>
class alloc_cted_insert_type
{
  using emplace_type=typename std::conditional<
    std::is_constructible<typename TypePolicy::init_type,Args...>::value,
    typename TypePolicy::init_type,
    typename TypePolicy::value_type
  >::type;

  using insert_type=typename std::conditional<
    std::is_constructible<typename TypePolicy::value_type,emplace_type>::value,
    emplace_type,typename TypePolicy::element_type
  >::type;

  using alloc_cted = allocator_constructed<Allocator, insert_type, TypePolicy>;
  alloc_cted val;

public:
  alloc_cted_insert_type(const Allocator& al_,Args&&... args):val{al_,std::forward<Args>(args)...}
  {
  }

  insert_type& value(){return val.value();}
};

template<typename TypePolicy,typename Allocator,typename... Args>
alloc_cted_insert_type<TypePolicy,Allocator,Args...>
alloc_make_insert_type(const Allocator& al,Args&&... args)
{
  return {al,std::forward<Args>(args)...};
}

template <typename TypePolicy, typename Allocator, typename KFwdRef,
  typename = void>
class alloc_cted_or_fwded_key_type
{
  using key_type = typename TypePolicy::key_type;
  allocator_constructed<Allocator, key_type, TypePolicy> val;

public:
  alloc_cted_or_fwded_key_type(const Allocator& al_, KFwdRef k)
      : val(al_, std::forward<KFwdRef>(k))
  {
  }

  key_type&& move_or_fwd() { return std::move(val.value()); }
};

template <typename TypePolicy, typename Allocator, typename KFwdRef>
class alloc_cted_or_fwded_key_type<TypePolicy, Allocator, KFwdRef,
  typename std::enable_if<
    is_similar<KFwdRef, typename TypePolicy::key_type>::value>::type>
{
  // This specialization acts as a forwarding-reference wrapper
  BOOST_UNORDERED_STATIC_ASSERT(std::is_reference<KFwdRef>::value);
  KFwdRef ref;

public:
  alloc_cted_or_fwded_key_type(const Allocator&, KFwdRef k)
      : ref(std::forward<KFwdRef>(k))
  {
  }

  KFwdRef move_or_fwd() { return std::forward<KFwdRef>(ref); }
};

template <typename Container>
using is_map =
  std::integral_constant<bool, !std::is_same<typename Container::key_type,
                                 typename Container::value_type>::value>;

template <typename Container, typename K>
using is_emplace_kv_able = std::integral_constant<bool,
  is_map<Container>::value &&
    (is_similar<K, typename Container::key_type>::value ||
      is_complete_and_move_constructible<typename Container::key_type>::value)>;

/* table_core. The TypePolicy template parameter is used to generate
 * instantiations suitable for either maps or sets, and introduces non-standard
 * init_type and element_type:
 *
 *   - TypePolicy::key_type and TypePolicy::value_type have the obvious
 *     meaning. TypePolicy::mapped_type is expected to be provided as well
 *     when key_type and value_type are not the same.
 *
 *   - TypePolicy::init_type is the type implicitly converted to when
 *     writing x.insert({...}). For maps, this is std::pair<Key,T> rather
 *     than std::pair<const Key,T> so that, for instance, x.insert({"hello",0})
 *     produces a cheaply moveable std::string&& ("hello") rather than
 *     a copyable const std::string&&. foa::table::insert is extended to accept
 *     both init_type and value_type references.
 *
 *   - TypePolicy::construct and TypePolicy::destroy are used for the
 *     construction and destruction of the internal types: value_type,
 *     init_type, element_type, and key_type.
 * 
 *   - TypePolicy::move is used to provide move semantics for the internal
 *     types used by the container during rehashing and emplace. These types
 *     are init_type, value_type and emplace_type. During insertion, a
 *     stack-local type will be created based on the constructibility of the
 *     value_type and the supplied arguments. TypePolicy::move is used here
 *     for transfer of ownership. Similarly, TypePolicy::move is also used
 *     during rehashing when elements are moved to the new table.
 *
 *   - TypePolicy::extract returns a const reference to the key part of
 *     a value of type value_type, init_type, element_type or
 *     decltype(TypePolicy::move(...)).
 *
 *   - TypePolicy::element_type is the type that table_arrays uses when
 *     allocating buckets, which allows us to have flat and node container.
 *     For flat containers, element_type is value_type. For node
 *     containers, it is a strong typedef to value_type*.
 *
 *   - TypePolicy::value_from returns a mutable reference to value_type from
 *     a given element_type. This is used when elements of the table themselves
 *     need to be moved, such as during move construction/assignment when
 *     allocators are unequal and there is no propagation. For all other cases,
 *     the element_type itself is moved.
 */

#include <boost/unordered/detail/foa/ignore_wshadow.hpp>

#if defined(BOOST_MSVC)
#pragma warning(push)
#pragma warning(disable:4714) /* marked as __forceinline not inlined */
#endif

#if BOOST_WORKAROUND(BOOST_MSVC,<=1900)
/* VS2015 marks as unreachable generic catch clauses around non-throwing
 * code.
 */
#pragma warning(push)
#pragma warning(disable:4702)
#endif

template<
  typename TypePolicy,typename Group,template<typename...> class Arrays,
  typename SizeControl,typename Hash,typename Pred,typename Allocator
>
class 

#if defined(_MSC_VER)&&_MSC_FULL_VER>=190023918
__declspec(empty_bases) /* activate EBO with multiple inheritance */
#endif

table_core:empty_value<Hash,0>,empty_value<Pred,1>,empty_value<Allocator,2>
{
public:
  using type_policy=TypePolicy;
  using group_type=Group;
  static constexpr auto N=group_type::N;
  using size_policy=pow2_size_policy;
  using prober=pow2_quadratic_prober;
  using mix_policy=typename std::conditional<
    hash_is_avalanching<Hash>::value,
    no_mix,
    mulx_mix
  >::type;
  using alloc_traits=boost::allocator_traits<Allocator>;
  using element_type=typename type_policy::element_type;
  using arrays_type=Arrays<element_type,group_type,size_policy,Allocator>;
  using size_ctrl_type=SizeControl;
  static constexpr auto uses_fancy_pointers=!std::is_same<
    typename alloc_traits::pointer,
    typename alloc_traits::value_type*
  >::value;

  using key_type=typename type_policy::key_type;
  using init_type=typename type_policy::init_type;
  using value_type=typename type_policy::value_type;
  using hasher=Hash;
  using key_equal=Pred;
  using allocator_type=Allocator;
  using pointer=value_type*;
  using const_pointer=const value_type*;
  using reference=value_type&;
  using const_reference=const value_type&;
  using size_type=std::size_t;
  using difference_type=std::ptrdiff_t;
  using locator=table_locator<group_type,element_type>;
  using arrays_holder_type=arrays_holder<arrays_type,Allocator>;

#if defined(BOOST_UNORDERED_ENABLE_STATS)
  using cumulative_stats=table_core_cumulative_stats;
  using stats=table_core_stats;
#endif

  table_core(
    std::size_t n=default_bucket_count,const Hash& h_=Hash(),
    const Pred& pred_=Pred(),const Allocator& al_=Allocator()):
    hash_base{empty_init,h_},pred_base{empty_init,pred_},
    allocator_base{empty_init,al_},arrays(new_arrays(n)),
    size_ctrl{initial_max_load(),0}
    {}

  /* genericize on an ArraysFn so that we can do things like delay an
   * allocation for the group_access data required by cfoa after the move
   * constructors of Hash, Pred have been invoked
   */
  template<typename ArraysFn>
  table_core(
    Hash&& h_,Pred&& pred_,Allocator&& al_,
    ArraysFn arrays_fn,const size_ctrl_type& size_ctrl_):
    hash_base{empty_init,std::move(h_)},
    pred_base{empty_init,std::move(pred_)},
    allocator_base{empty_init,std::move(al_)},
    arrays(arrays_fn()),size_ctrl(size_ctrl_)
  {}

  table_core(const table_core& x):
    table_core{x,alloc_traits::select_on_container_copy_construction(x.al())}{}

  template<typename ArraysFn>
  table_core(table_core&& x,arrays_holder_type&& ah,ArraysFn arrays_fn):
    table_core(
      std::move(x.h()),std::move(x.pred()),std::move(x.al()),
      arrays_fn,x.size_ctrl)
  {
    x.arrays=ah.release();
    x.size_ctrl.ml=x.initial_max_load();
    x.size_ctrl.size=0;
    BOOST_UNORDERED_SWAP_STATS(cstats,x.cstats);
  }

  table_core(table_core&& x)
    noexcept(
      std::is_nothrow_move_constructible<Hash>::value&&
      std::is_nothrow_move_constructible<Pred>::value&&
      std::is_nothrow_move_constructible<Allocator>::value&&
      !uses_fancy_pointers):
    table_core{
      std::move(x),x.make_empty_arrays(),[&x]{return x.arrays;}}
  {}

  table_core(const table_core& x,const Allocator& al_):
    table_core{std::size_t(std::ceil(float(x.size())/mlf)),x.h(),x.pred(),al_}
  {
    copy_elements_from(x);
  }

  table_core(table_core&& x,const Allocator& al_):
    table_core{std::move(x.h()),std::move(x.pred()),al_}
  {
    if(al()==x.al()){
      using std::swap;
      swap(arrays,x.arrays);
      swap(size_ctrl,x.size_ctrl);
      BOOST_UNORDERED_SWAP_STATS(cstats,x.cstats);
    }
    else{
      reserve(x.size());
      clear_on_exit c{x};
      (void)c; /* unused var warning */
      BOOST_UNORDERED_RESET_STATS_OF(x);

      /* This works because subsequent x.clear() does not depend on the
       * elements' values.
       */
      x.for_all_elements([this](element_type* p){
        unchecked_insert(type_policy::move(type_policy::value_from(*p)));
      });
    }
  }

  ~table_core()noexcept
  {
    for_all_elements([this](element_type* p){
      destroy_element(p);
    });
    delete_arrays(arrays);
  }

  std::size_t initial_max_load()const
  {
    static constexpr std::size_t small_capacity=2*N-1;

    auto capacity_=capacity();
    if(capacity_<=small_capacity){
      return capacity_; /* we allow 100% usage */
    }
    else{
      return (std::size_t)(mlf*(float)(capacity_));
    }
  }

  arrays_holder_type make_empty_arrays()const
  {
    return make_arrays(0);
  }

  table_core& operator=(const table_core& x)
  {
    BOOST_UNORDERED_STATIC_ASSERT_HASH_PRED(Hash, Pred)

    static constexpr auto pocca=
      alloc_traits::propagate_on_container_copy_assignment::value;

    if(this!=std::addressof(x)){
      /* If copy construction here winds up throwing, the container is still
       * left intact so we perform these operations first.
       */
      hasher    tmp_h=x.h();
      key_equal tmp_p=x.pred();

      clear();

      /* Because we've asserted at compile-time that Hash and Pred are nothrow
       * swappable, we can safely mutate our source container and maintain
       * consistency between the Hash, Pred compatibility.
       */
      using std::swap;
      swap(h(),tmp_h);
      swap(pred(),tmp_p);

      if_constexpr<pocca>([&,this]{
        if(al()!=x.al()){
          auto ah=x.make_arrays(std::size_t(std::ceil(float(x.size())/mlf)));
          delete_arrays(arrays);
          arrays=ah.release();
          size_ctrl.ml=initial_max_load();
        }
        copy_assign_if<pocca>(al(),x.al());
      });
      /* noshrink: favor memory reuse over tightness */
      noshrink_reserve(x.size());
      copy_elements_from(x);
    }
    return *this;
  }

#if defined(BOOST_MSVC)
#pragma warning(push)
#pragma warning(disable:4127) /* conditional expression is constant */
#endif

  table_core& operator=(table_core&& x)
    noexcept(
      (alloc_traits::propagate_on_container_move_assignment::value||
      alloc_traits::is_always_equal::value)&&!uses_fancy_pointers)
  {
    BOOST_UNORDERED_STATIC_ASSERT_HASH_PRED(Hash, Pred)

    static constexpr auto pocma=
      alloc_traits::propagate_on_container_move_assignment::value;

    if(this!=std::addressof(x)){
      /* Given ambiguity in implementation strategies briefly discussed here:
       * https://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#2227
       *
       * we opt into requiring nothrow swappability and eschew the move
       * operations associated with Hash, Pred.
       *
       * To this end, we ensure that the user never has to consider the
       * moved-from state of their Hash, Pred objects
       */

      using std::swap;

      clear();

      if(pocma||al()==x.al()){
        auto ah=x.make_empty_arrays();
        swap(h(),x.h());
        swap(pred(),x.pred());
        delete_arrays(arrays);
        move_assign_if<pocma>(al(),x.al());
        arrays=x.arrays;
        size_ctrl.ml=std::size_t(x.size_ctrl.ml);
        size_ctrl.size=std::size_t(x.size_ctrl.size);
        BOOST_UNORDERED_COPY_STATS(cstats,x.cstats);
        x.arrays=ah.release();
        x.size_ctrl.ml=x.initial_max_load();
        x.size_ctrl.size=0;
        BOOST_UNORDERED_RESET_STATS_OF(x);
      }
      else{
        swap(h(),x.h());
        swap(pred(),x.pred());

        /* noshrink: favor memory reuse over tightness */
        noshrink_reserve(x.size());
        clear_on_exit c{x};
        (void)c; /* unused var warning */
        BOOST_UNORDERED_RESET_STATS_OF(x);

        /* This works because subsequent x.clear() does not depend on the
         * elements' values.
         */
        x.for_all_elements([this](element_type* p){
          unchecked_insert(type_policy::move(type_policy::value_from(*p)));
        });
      }
    }
    return *this;
  }

#if defined(BOOST_MSVC)
#pragma warning(pop) /* C4127 */
#endif

  allocator_type get_allocator()const noexcept{return al();}

  bool        empty()const noexcept{return size()==0;}
  std::size_t size()const noexcept{return size_ctrl.size;}
  std::size_t max_size()const noexcept{return SIZE_MAX;}

  BOOST_FORCEINLINE
  void erase(group_type* pg,unsigned int pos,element_type* p)noexcept
  {
    destroy_element(p);
    recover_slot(pg,pos);
  }

  BOOST_FORCEINLINE
  void erase(unsigned char* pc,element_type* p)noexcept
  {
    destroy_element(p);
    recover_slot(pc);
  }

  template<typename Key>
  BOOST_FORCEINLINE locator find(const Key& x)const
  {
    auto hash=hash_for(x);
    return find(x,position_for(hash),hash);
  }

#if defined(BOOST_MSVC)
/* warning: forcing value to bool 'true' or 'false' in bool(pred()...) */
#pragma warning(push)
#pragma warning(disable:4800)
#endif

  template<typename Key>
  BOOST_FORCEINLINE locator find(
    const Key& x,std::size_t pos0,std::size_t hash)const
  {    
    BOOST_UNORDERED_STATS_COUNTER(num_cmps);
    prober pb(pos0);
    do{
      auto pos=pb.get();
      auto pg=arrays.groups()+pos;
      auto mask=pg->match(hash);
      if(mask){
        auto elements=arrays.elements();
        BOOST_UNORDERED_ASSUME(elements!=nullptr);
        auto p=elements+pos*N;
        BOOST_UNORDERED_PREFETCH_ELEMENTS(p,N);
        do{
          BOOST_UNORDERED_INCREMENT_STATS_COUNTER(num_cmps);
          auto n=unchecked_countr_zero(mask);
          if(BOOST_LIKELY(bool(pred()(x,key_from(p[n]))))){
            BOOST_UNORDERED_ADD_STATS(
              cstats.successful_lookup,(pb.length(),num_cmps));
            return {pg,n,p+n};
          }
          mask&=mask-1;
        }while(mask);
      }
      if(BOOST_LIKELY(pg->is_not_overflowed(hash))){
        BOOST_UNORDERED_ADD_STATS(
          cstats.unsuccessful_lookup,(pb.length(),num_cmps));
        return {};
      }
    }
    while(BOOST_LIKELY(pb.next(arrays.groups_size_mask)));
    BOOST_UNORDERED_ADD_STATS(
      cstats.unsuccessful_lookup,(pb.length(),num_cmps));
    return {};
  }

#if defined(BOOST_MSVC)
#pragma warning(pop) /* C4800 */
#endif

  void swap(table_core& x)
    noexcept(
      alloc_traits::propagate_on_container_swap::value||
      alloc_traits::is_always_equal::value)
  {
    BOOST_UNORDERED_STATIC_ASSERT_HASH_PRED(Hash, Pred)

    static constexpr auto pocs=
      alloc_traits::propagate_on_container_swap::value;

    using std::swap;
    if_constexpr<pocs>([&,this]{
      swap_if<pocs>(al(),x.al());
    },
    [&,this]{ /* else */
      BOOST_ASSERT(al()==x.al());
      (void)this; /* makes sure captured this is used */
    });

    swap(h(),x.h());
    swap(pred(),x.pred());
    swap(arrays,x.arrays);
    swap(size_ctrl,x.size_ctrl);
  }

  void clear()noexcept
  {
    auto p=arrays.elements();
    if(p){
      for(auto pg=arrays.groups(),last=pg+arrays.groups_size_mask+1;
          pg!=last;++pg,p+=N){
        auto mask=match_really_occupied(pg,last);
        while(mask){
          destroy_element(p+unchecked_countr_zero(mask));
          mask&=mask-1;
        }
        /* we wipe the entire metadata to reset the overflow byte as well */
        pg->initialize();
      }
      arrays.groups()[arrays.groups_size_mask].set_sentinel();
      size_ctrl.ml=initial_max_load();
      size_ctrl.size=0;
    }
  }

  hasher hash_function()const{return h();}
  key_equal key_eq()const{return pred();}

  std::size_t capacity()const noexcept
  {
    return arrays.elements()?(arrays.groups_size_mask+1)*N-1:0;
  }
  
  float load_factor()const noexcept
  {
    if(capacity()==0)return 0;
    else             return float(size())/float(capacity());
  }

  float max_load_factor()const noexcept{return mlf;}

  std::size_t max_load()const noexcept{return size_ctrl.ml;}

  void rehash(std::size_t n)
  {
    auto m=size_t(std::ceil(float(size())/mlf));
    if(m>n)n=m;
    if(n)n=capacity_for(n); /* exact resulting capacity */

    if(n!=capacity())unchecked_rehash(n);
  }

  void reserve(std::size_t n)
  {
    rehash(std::size_t(std::ceil(float(n)/mlf)));
  }

#if defined(BOOST_UNORDERED_ENABLE_STATS)
  stats get_stats()const
  {
    auto insertion=cstats.insertion.get_summary();
    auto successful_lookup=cstats.successful_lookup.get_summary();
    auto unsuccessful_lookup=cstats.unsuccessful_lookup.get_summary();
    return{
      {
        insertion.count,
        insertion.sequence_summary[0]
      },
      {
        successful_lookup.count,
        successful_lookup.sequence_summary[0],
        successful_lookup.sequence_summary[1]
      },
      {
        unsuccessful_lookup.count,
        unsuccessful_lookup.sequence_summary[0],
        unsuccessful_lookup.sequence_summary[1]
      },
    };
  }

  void reset_stats()noexcept
  {
    cstats.insertion.reset();
    cstats.successful_lookup.reset();
    cstats.unsuccessful_lookup.reset();
  }
#endif

  friend bool operator==(const table_core& x,const table_core& y)
  {
    return
      x.size()==y.size()&&
      x.for_all_elements_while([&](element_type* p){
        auto loc=y.find(key_from(*p));
        return loc&&
          const_cast<const value_type&>(type_policy::value_from(*p))==
          const_cast<const value_type&>(type_policy::value_from(*loc.p));
      });
  }

  friend bool operator!=(const table_core& x,const table_core& y)
  {
    return !(x==y);
  }

  struct clear_on_exit
  {
    ~clear_on_exit(){x.clear();}
    table_core& x;
  };

  Hash&            h(){return hash_base::get();}
  const Hash&      h()const{return hash_base::get();}
  Pred&            pred(){return pred_base::get();}
  const Pred&      pred()const{return pred_base::get();}
  Allocator&       al(){return allocator_base::get();}
  const Allocator& al()const{return allocator_base::get();}

  template<typename... Args>
  void construct_element(element_type* p,Args&&... args)
  {
    type_policy::construct(al(),p,std::forward<Args>(args)...);
  }

  template<typename... Args>
  void construct_element(element_type* p,try_emplace_args_t,Args&&... args)
  {
    construct_element_from_try_emplace_args(
      p,
      std::integral_constant<bool,std::is_same<key_type,value_type>::value>{},
      std::forward<Args>(args)...);
  }

  void destroy_element(element_type* p)noexcept
  {
    type_policy::destroy(al(),p);
  }

  struct destroy_element_on_exit
  {
    ~destroy_element_on_exit(){this_->destroy_element(p);}
    table_core   *this_;
    element_type *p;
  };

  template<typename T>
  static inline auto key_from(const T& x)
    ->decltype(type_policy::extract(x))
  {
    return type_policy::extract(x);
  }

  template<typename Key,typename... Args>
  static inline const Key& key_from(
    try_emplace_args_t,const Key& x,const Args&...)
  {
    return x;
  }

  template<typename Key>
  inline std::size_t hash_for(const Key& x)const
  {
    return mix_policy::mix(h(),x);
  }

  inline std::size_t position_for(std::size_t hash)const
  {
    return position_for(hash,arrays);
  }

  static inline std::size_t position_for(
    std::size_t hash,const arrays_type& arrays_)
  {
    return size_policy::position(hash,arrays_.groups_size_index);
  }

  static inline int match_really_occupied(group_type* pg,group_type* last)
  {
    /* excluding the sentinel */
    return pg->match_occupied()&~(int(pg==last-1)<<(N-1));
  }

  template<typename... Args>
  locator unchecked_emplace_at(
    std::size_t pos0,std::size_t hash,Args&&... args)
  {
    auto res=nosize_unchecked_emplace_at(
      arrays,pos0,hash,std::forward<Args>(args)...);
    ++size_ctrl.size;
    return res;
  }

  BOOST_NOINLINE void unchecked_rehash_for_growth()
  {
    auto new_arrays_=new_arrays_for_growth();
    unchecked_rehash(new_arrays_);
  }

  template<typename... Args>
  BOOST_NOINLINE locator
  unchecked_emplace_with_rehash(std::size_t hash,Args&&... args)
  {
    auto    new_arrays_=new_arrays_for_growth();
    locator it;
    BOOST_TRY{
      /* strong exception guarantee -> try insertion before rehash */
      it=nosize_unchecked_emplace_at(
        new_arrays_,position_for(hash,new_arrays_),
        hash,std::forward<Args>(args)...);
    }
    BOOST_CATCH(...){
      delete_arrays(new_arrays_);
      BOOST_RETHROW
    }
    BOOST_CATCH_END

    /* new_arrays_ lifetime taken care of by unchecked_rehash */
    unchecked_rehash(new_arrays_);
    ++size_ctrl.size;
    return it;
  }

  void noshrink_reserve(std::size_t n)
  {
    /* used only on assignment after element clearance */
    BOOST_ASSERT(empty());

    if(n){
      n=std::size_t(std::ceil(float(n)/mlf)); /* elements -> slots */
      n=capacity_for(n); /* exact resulting capacity */

      if(n>capacity()){
        auto new_arrays_=new_arrays(n);
        delete_arrays(arrays);
        arrays=new_arrays_;
        size_ctrl.ml=initial_max_load();
      }
    }
  }

  template<typename F>
  void for_all_elements(F f)const
  {
    for_all_elements(arrays,f);
  }

  template<typename F>
  static auto for_all_elements(const arrays_type& arrays_,F f)
    ->decltype(f(nullptr),void())
  {
    for_all_elements_while(arrays_,[&](element_type* p){f(p);return true;});
  }

  template<typename F>
  static auto for_all_elements(const arrays_type& arrays_,F f)
    ->decltype(f(nullptr,0,nullptr),void())
  {
    for_all_elements_while(
      arrays_,[&](group_type* pg,unsigned int n,element_type* p)
        {f(pg,n,p);return true;});
  }

  template<typename F>
  bool for_all_elements_while(F f)const
  {
    return for_all_elements_while(arrays,f);
  }

  template<typename F>
  static auto for_all_elements_while(const arrays_type& arrays_,F f)
    ->decltype(f(nullptr),bool())
  {
    return for_all_elements_while(
      arrays_,[&](group_type*,unsigned int,element_type* p){return f(p);});
  }

  template<typename F>
  static auto for_all_elements_while(const arrays_type& arrays_,F f)
    ->decltype(f(nullptr,0,nullptr),bool())
  {
    auto p=arrays_.elements();
    if(p){
      for(auto pg=arrays_.groups(),last=pg+arrays_.groups_size_mask+1;
          pg!=last;++pg,p+=N){
        auto mask=match_really_occupied(pg,last);
        while(mask){
          auto n=unchecked_countr_zero(mask);
          if(!f(pg,n,p+n))return false;
          mask&=mask-1;
        }
      }
    }
    return true;
  }

  arrays_type              arrays;
  size_ctrl_type           size_ctrl;

#if defined(BOOST_UNORDERED_ENABLE_STATS)
  mutable cumulative_stats cstats;
#endif

private:
  template<
    typename,typename,template<typename...> class,
    typename,typename,typename,typename
  >
  friend class table_core;

  using hash_base=empty_value<Hash,0>;
  using pred_base=empty_value<Pred,1>;
  using allocator_base=empty_value<Allocator,2>;

  /* used by allocator-extended move ctor */

  table_core(Hash&& h_,Pred&& pred_,const Allocator& al_):
    hash_base{empty_init,std::move(h_)},
    pred_base{empty_init,std::move(pred_)},
    allocator_base{empty_init,al_},arrays(new_arrays(0)),
    size_ctrl{initial_max_load(),0}
  {
  }

  arrays_type new_arrays(std::size_t n)const
  {
    return arrays_type::new_(typename arrays_type::allocator_type(al()),n);
  }

  arrays_type new_arrays_for_growth()const
  {
    /* Due to the anti-drift mechanism (see recover_slot), the new arrays may
     * be of the same size as the old arrays; in the limit, erasing one
     * element at full load and then inserting could bring us back to the same
     * capacity after a costly rehash. To avoid this, we jump to the next
     * capacity level when the number of erased elements is <= 10% of total
     * elements at full load, which is implemented by requesting additional
     * F*size elements, with F = P * 10% / (1 - P * 10%), where P is the
     * probability of an element having caused overflow; P has been measured as
     * ~0.162 under ideal conditions, yielding F ~ 0.0165 ~ 1/61.
     */
    return new_arrays(std::size_t(
      std::ceil(static_cast<float>(size()+size()/61+1)/mlf)));
  }

  void delete_arrays(arrays_type& arrays_)noexcept
  {
    arrays_type::delete_(typename arrays_type::allocator_type(al()),arrays_);
  }

  arrays_holder_type make_arrays(std::size_t n)const
  {
    return {new_arrays(n),al()};
  }

  template<typename Key,typename... Args>
  void construct_element_from_try_emplace_args(
    element_type* p,std::false_type,Key&& x,Args&&... args)
  {
    type_policy::construct(
      this->al(),p,
      std::piecewise_construct,
      std::forward_as_tuple(std::forward<Key>(x)),
      std::forward_as_tuple(std::forward<Args>(args)...));
  }

  /* This overload allows boost::unordered_flat_set to internally use
   * try_emplace to implement heterogeneous insert (P2363).
   */

  template<typename Key>
  void construct_element_from_try_emplace_args(
    element_type* p,std::true_type,Key&& x)
  {
    type_policy::construct(this->al(),p,std::forward<Key>(x));
  }

  void copy_elements_from(const table_core& x)
  {
    BOOST_ASSERT(empty());
    BOOST_ASSERT(this!=std::addressof(x));
    if(arrays.groups_size_mask==x.arrays.groups_size_mask){
      fast_copy_elements_from(x);
    }
    else{
      x.for_all_elements([this](const element_type* p){
        unchecked_insert(*p);
      });
    }
  }

  void fast_copy_elements_from(const table_core& x)
  {
    if(arrays.elements()&&x.arrays.elements()){
      copy_elements_array_from(x);
      copy_groups_array_from(x);
      size_ctrl.ml=std::size_t(x.size_ctrl.ml);
      size_ctrl.size=std::size_t(x.size_ctrl.size);
    }
  }

  void copy_elements_array_from(const table_core& x)
  {
    copy_elements_array_from(
      x,
      std::integral_constant<
        bool,
        is_trivially_copy_constructible<element_type>::value&&(
          is_std_allocator<Allocator>::value||
          !alloc_has_construct<Allocator,value_type*,const value_type&>::value)
      >{}
    );
  }

  void copy_elements_array_from(
    const table_core& x,std::true_type /* -> memcpy */)
  {
    /* reinterpret_cast: GCC may complain about value_type not being trivially
     * copy-assignable when we're relying on trivial copy constructibility.
     */
    std::memcpy(
      reinterpret_cast<unsigned char*>(arrays.elements()),
      reinterpret_cast<unsigned char*>(x.arrays.elements()),
      x.capacity()*sizeof(value_type));
  }

  void copy_elements_array_from(
    const table_core& x,std::false_type /* -> manual */)
  {
    std::size_t num_constructed=0;
    BOOST_TRY{
      x.for_all_elements([&,this](const element_type* p){
        construct_element(arrays.elements()+(p-x.arrays.elements()),*p);
        ++num_constructed;
      });
    }
    BOOST_CATCH(...){
      if(num_constructed){
        x.for_all_elements_while([&,this](const element_type* p){
          destroy_element(arrays.elements()+(p-x.arrays.elements()));
          return --num_constructed!=0;
        });
      }
      BOOST_RETHROW
    }
    BOOST_CATCH_END
  }

  void copy_groups_array_from(const table_core& x) {
    copy_groups_array_from(x,is_trivially_copy_assignable<group_type>{});
  }

  void copy_groups_array_from(
    const table_core& x, std::true_type /* -> memcpy */)
  {
    std::memcpy(
      arrays.groups(),x.arrays.groups(),
      (arrays.groups_size_mask+1)*sizeof(group_type));
  }

  void copy_groups_array_from(
    const table_core& x, std::false_type /* -> manual */) 
  {
    auto pg=arrays.groups();
    auto xpg=x.arrays.groups();
    for(std::size_t i=0;i<arrays.groups_size_mask+1;++i){
      pg[i]=xpg[i];
    }
  }

  void recover_slot(unsigned char* pc)
  {
    /* If this slot potentially caused overflow, we decrease the maximum load
     * so that average probe length won't increase unboundedly in repeated
     * insert/erase cycles (drift).
     */
    size_ctrl.ml-=group_type::maybe_caused_overflow(pc);
    group_type::reset(pc);
    --size_ctrl.size;
  }

  void recover_slot(group_type* pg,std::size_t pos)
  {
    recover_slot(reinterpret_cast<unsigned char*>(pg)+pos);
  }

  static std::size_t capacity_for(std::size_t n)
  {
    return size_policy::size(size_index_for<group_type,size_policy>(n))*N-1;
  }

  BOOST_NOINLINE void unchecked_rehash(std::size_t n)
  {
    auto new_arrays_=new_arrays(n);
    unchecked_rehash(new_arrays_);
  }

  BOOST_NOINLINE void unchecked_rehash(arrays_type& new_arrays_)
  {
    std::size_t num_destroyed=0;
    BOOST_TRY{
      for_all_elements([&,this](element_type* p){
        nosize_transfer_element(p,new_arrays_,num_destroyed);
      });
    }
    BOOST_CATCH(...){
      if(num_destroyed){
        for_all_elements_while(
          [&,this](group_type* pg,unsigned int n,element_type*){
            recover_slot(pg,n);
            return --num_destroyed!=0;
          }
        );
      }
      for_all_elements(new_arrays_,[this](element_type* p){
        destroy_element(p);
      });
      delete_arrays(new_arrays_);
      BOOST_RETHROW
    }
    BOOST_CATCH_END

    /* either all moved and destroyed or all copied */
    BOOST_ASSERT(num_destroyed==size()||num_destroyed==0);
    if(num_destroyed!=size()){
      for_all_elements([this](element_type* p){
        destroy_element(p);
      });
    }
    delete_arrays(arrays);
    arrays=new_arrays_;
    size_ctrl.ml=initial_max_load();
  }

  template<typename Value>
  void unchecked_insert(Value&& x)
  {
    auto hash=hash_for(key_from(x));
    unchecked_emplace_at(position_for(hash),hash,std::forward<Value>(x));
  }

  void nosize_transfer_element(
    element_type* p,const arrays_type& arrays_,std::size_t& num_destroyed)
  {
    nosize_transfer_element(
      p,hash_for(key_from(*p)),arrays_,num_destroyed,
      std::integral_constant< /* std::move_if_noexcept semantics */
        bool,
        std::is_nothrow_move_constructible<init_type>::value||
        !std::is_same<element_type,value_type>::value||
        !std::is_copy_constructible<element_type>::value>{});
  }

  void nosize_transfer_element(
    element_type* p,std::size_t hash,const arrays_type& arrays_,
    std::size_t& num_destroyed,std::true_type /* ->move */)
  {
    /* Destroy p even if an an exception is thrown in the middle of move
     * construction, which could leave the source half-moved.
     */
    ++num_destroyed;
    destroy_element_on_exit d{this,p};
    (void)d; /* unused var warning */
    nosize_unchecked_emplace_at(
      arrays_,position_for(hash,arrays_),hash,type_policy::move(*p));
  }

  void nosize_transfer_element(
    element_type* p,std::size_t hash,const arrays_type& arrays_,
    std::size_t& /*num_destroyed*/,std::false_type /* ->copy */)
  {
    nosize_unchecked_emplace_at(
      arrays_,position_for(hash,arrays_),hash,
      const_cast<const element_type&>(*p));
  }

  template<typename... Args>
  locator nosize_unchecked_emplace_at(
    const arrays_type& arrays_,std::size_t pos0,std::size_t hash,
    Args&&... args)
  {
    for(prober pb(pos0);;pb.next(arrays_.groups_size_mask)){
      auto pos=pb.get();
      auto pg=arrays_.groups()+pos;
      auto mask=pg->match_available();
      if(BOOST_LIKELY(mask!=0)){
        auto n=unchecked_countr_zero(mask);
        auto p=arrays_.elements()+pos*N+n;
        construct_element(p,std::forward<Args>(args)...);
        pg->set(n,hash);
        BOOST_UNORDERED_ADD_STATS(cstats.insertion,(pb.length()));
        return {pg,n,p};
      }
      else pg->mark_overflow(hash);
    }
  }
};

#if BOOST_WORKAROUND(BOOST_MSVC,<=1900)
#pragma warning(pop) /* C4702 */
#endif

#if defined(BOOST_MSVC)
#pragma warning(pop) /* C4714 */
#endif

#include <boost/unordered/detail/foa/restore_wshadow.hpp>

} /* namespace foa */
} /* namespace detail */
} /* namespace unordered */
} /* namespace boost */

#undef BOOST_UNORDERED_STATIC_ASSERT_HASH_PRED
#undef BOOST_UNORDERED_HAS_FEATURE
#undef BOOST_UNORDERED_HAS_BUILTIN
#endif