robin_hash.h

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/**
 * MIT License
 *
 * Copyright (c) 2017 Tessil
 *
 * 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 TSL_ROBIN_HASH_H
#define TSL_ROBIN_HASH_H

#include "robin_growth_policy.h"
#include <algorithm>
#include <cassert>
#include <cmath>
#include <cstddef>
#include <cstdint>
#include <exception>
#include <iterator>
#include <limits>
#include <memory>
#include <stdexcept>
#include <tuple>
#include <type_traits>
#include <utility>
#include <vector>

namespace tsl {

  namespace detail_robin_hash {

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

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

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

    template <typename U> struct is_power_of_two_policy : std::false_type
    {
    };

    template <std::size_t GrowthFactor>
    struct is_power_of_two_policy<tsl::rh::power_of_two_growth_policy<GrowthFactor>>
        : std::true_type
    {
    };

    using truncated_hash_type = std::uint_least32_t;

    /**
     * Helper class that store a truncated hash if StoreHash is true and nothing otherwise.
     */
    template <bool StoreHash> class bucket_entry_hash
    {
    public:
      bool bucket_hash_equal(std::size_t /*hash*/) const noexcept { return true; }

      truncated_hash_type truncated_hash() const noexcept { return 0; }

    protected:
      void set_hash(truncated_hash_type /*hash*/) noexcept {}
    };

    template <> class bucket_entry_hash<true>
    {
    public:
      bool bucket_hash_equal(std::size_t hash) const noexcept
      {
        return m_hash == truncated_hash_type(hash);
      }

      truncated_hash_type truncated_hash() const noexcept { return m_hash; }

    protected:
      void set_hash(truncated_hash_type hash) noexcept { m_hash = truncated_hash_type(hash); }

    private:
      truncated_hash_type m_hash;
    };

    /**
     * Each bucket entry has:
     * - A value of type `ValueType`.
     * - An integer to store how far the value of the bucket, if any, is from its ideal bucket
     *   (ex: if the current bucket 5 has the value 'foo' and `hash('foo') % nb_buckets` == 3,
     *        `dist_from_ideal_bucket()` will return 2 as the current value of the bucket is two
     *        buckets away from its ideal bucket)
     *   If there is no value in the bucket (i.e. `empty()` is true) `dist_from_ideal_bucket()` will
     * be < 0.
     * - A marker which tells us if the bucket is the last bucket of the bucket array (useful for
     * the iterator of the hash table).
     * - If `StoreHash` is true, 32 bits of the hash of the value, if any, are also stored in the
     * bucket. If the size of the hash is more than 32 bits, it is truncated. We don't store the
     * full hash as storing the hash is a potential opportunity to use the unused space due to the
     * alignement of the bucket_entry structure. We can thus potentially store the hash without any
     * extra space (which would not be possible with 64 bits of the hash).
     */
    template <typename ValueType, bool StoreHash>
    class bucket_entry : public bucket_entry_hash<StoreHash>
    {
      using bucket_hash = bucket_entry_hash<StoreHash>;

    public:
      using value_type    = ValueType;
      using distance_type = std::int_least16_t;

      bucket_entry() noexcept
          : bucket_hash(), m_dist_from_ideal_bucket(EMPTY_MARKER_DIST_FROM_IDEAL_BUCKET),
            m_last_bucket(false)
      {
        tsl_assert(empty());
      }

      bucket_entry(bool last_bucket_) noexcept
          : bucket_hash(), m_dist_from_ideal_bucket(EMPTY_MARKER_DIST_FROM_IDEAL_BUCKET),
            m_last_bucket(last_bucket_)
      {
        tsl_assert(empty());
      }

      bucket_entry(const bucket_entry &other) noexcept(
          std::is_nothrow_copy_constructible<value_type>::value)
          : bucket_hash(other), m_dist_from_ideal_bucket(EMPTY_MARKER_DIST_FROM_IDEAL_BUCKET),
            m_last_bucket(other.m_last_bucket)
      {
        if (!other.empty()) {
          ::new (static_cast<void *>(std::addressof(m_value))) value_type(other.value());
          m_dist_from_ideal_bucket = other.m_dist_from_ideal_bucket;
        }
      }

      /**
       * Never really used, but still necessary as we must call resize on an empty
       * `std::vector<bucket_entry>`. and we need to support move-only types. See robin_hash
       * constructor for details.
       */
      bucket_entry(bucket_entry &&other) noexcept(
          std::is_nothrow_move_constructible<value_type>::value)
          : bucket_hash(std::move(other)),
            m_dist_from_ideal_bucket(EMPTY_MARKER_DIST_FROM_IDEAL_BUCKET),
            m_last_bucket(other.m_last_bucket)
      {
        if (!other.empty()) {
          ::new (static_cast<void *>(std::addressof(m_value))) value_type(std::move(other.value()));
          m_dist_from_ideal_bucket = other.m_dist_from_ideal_bucket;
        }
      }

      bucket_entry &operator=(const bucket_entry &other) noexcept(
          std::is_nothrow_copy_constructible<value_type>::value)
      {
        if (this != &other) {
          clear();

          bucket_hash::operator=(other);
          if (!other.empty()) {
            ::new (static_cast<void *>(std::addressof(m_value))) value_type(other.value());
          }

          m_dist_from_ideal_bucket = other.m_dist_from_ideal_bucket;
          m_last_bucket            = other.m_last_bucket;
        }

        return *this;
      }

      bucket_entry &operator=(bucket_entry &&) = delete;

      ~bucket_entry() noexcept { clear(); }

      void clear() noexcept
      {
        if (!empty()) {
          destroy_value();
          m_dist_from_ideal_bucket = EMPTY_MARKER_DIST_FROM_IDEAL_BUCKET;
        }
      }

      bool empty() const noexcept
      {
        return m_dist_from_ideal_bucket == EMPTY_MARKER_DIST_FROM_IDEAL_BUCKET;
      }

      value_type &value() noexcept
      {
        tsl_assert(!empty());
        return *reinterpret_cast<value_type *>(std::addressof(m_value));
      }

      const value_type &value() const noexcept
      {
        tsl_assert(!empty());
        return *reinterpret_cast<const value_type *>(std::addressof(m_value));
      }

      distance_type dist_from_ideal_bucket() const noexcept { return m_dist_from_ideal_bucket; }

      bool last_bucket() const noexcept { return m_last_bucket; }

      void set_as_last_bucket() noexcept { m_last_bucket = true; }

      template <typename... Args>
      void set_value_of_empty_bucket(distance_type       dist_from_ideal_bucket_,
                                     truncated_hash_type hash, Args &&... value_type_args)
      {
        tsl_assert(dist_from_ideal_bucket_ >= 0);
        tsl_assert(empty());

        ::new (static_cast<void *>(std::addressof(m_value)))
            value_type(std::forward<Args>(value_type_args)...);
        this->set_hash(hash);
        m_dist_from_ideal_bucket = dist_from_ideal_bucket_;

        tsl_assert(!empty());
      }

      void swap_with_value_in_bucket(distance_type &      dist_from_ideal_bucket_,
                                     truncated_hash_type &hash, value_type &value_)
      {
        tsl_assert(!empty());

        using std::swap;
        swap(value_, this->value());
        swap(dist_from_ideal_bucket_, m_dist_from_ideal_bucket);

        // Avoid warning of unused variable if StoreHash is false
        (void)hash;
        if (StoreHash) {
          const truncated_hash_type tmp_hash = this->truncated_hash();
          this->set_hash(hash);
          hash = tmp_hash;
        }
      }

      static truncated_hash_type truncate_hash(std::size_t hash) noexcept
      {
        return truncated_hash_type(hash);
      }

    private:
      void destroy_value() noexcept
      {
        tsl_assert(!empty());
        value().~value_type();
      }

    private:
      using storage = typename std::aligned_storage<sizeof(value_type), alignof(value_type)>::type;

      static const distance_type EMPTY_MARKER_DIST_FROM_IDEAL_BUCKET = -1;

      distance_type m_dist_from_ideal_bucket;
      bool          m_last_bucket;
      storage       m_value;
    };

    /**
     * Internal common class used by `robin_map` and `robin_set`.
     *
     * ValueType is what will be stored by `robin_hash` (usually `std::pair<Key, T>` for map and
     * `Key` for set).
     *
     * `KeySelect` should be a `FunctionObject` which takes a `ValueType` in parameter and returns a
     *  reference to the key.
     *
     * `ValueSelect` should be a `FunctionObject` which takes a `ValueType` in parameter and returns
     * a reference to the value. `ValueSelect` should be void if there is no value (in a set for
     * example).
     *
     * The strong exception guarantee only holds if the expression
     * `std::is_nothrow_swappable<ValueType>::value &&
     * std::is_nothrow_move_constructible<ValueType>::value` is true.
     *
     * Behaviour is undefined if the destructor of `ValueType` throws.
     */
    template <class ValueType, class KeySelect, class ValueSelect, class Hash, class KeyEqual,
              class Allocator, bool StoreHash, class GrowthPolicy>
    class robin_hash : private Hash, private KeyEqual, private GrowthPolicy
    {
    private:
      template <typename U>
      using has_mapped_type = typename std::integral_constant<bool, !std::is_same<U, void>::value>;

      static_assert(noexcept(std::declval<GrowthPolicy>().bucket_for_hash(std::size_t(0))),
                    "GrowthPolicy::bucket_for_hash must be noexcept.");
      static_assert(noexcept(std::declval<GrowthPolicy>().clear()),
                    "GrowthPolicy::clear must be noexcept.");

    public:
      template <bool IsConst> class robin_iterator;

      using key_type        = typename KeySelect::key_type;
      using value_type      = ValueType;
      using size_type       = std::size_t;
      using difference_type = std::ptrdiff_t;
      using hasher          = Hash;
      using key_equal       = KeyEqual;
      using allocator_type  = Allocator;
      using reference       = value_type &;
      using const_reference = const value_type &;
      using pointer         = value_type *;
      using const_pointer   = const value_type *;
      using iterator        = robin_iterator<false>;
      using const_iterator  = robin_iterator<true>;

    private:
      /**
       * Either store the hash because we are asked by the `StoreHash` template parameter
       * or store the hash because it doesn't cost us anything in size and can be used to speed up
       * rehash.
       */
      static constexpr bool STORE_HASH =
          StoreHash || ((sizeof(tsl::detail_robin_hash::bucket_entry<value_type, true>) ==
                         sizeof(tsl::detail_robin_hash::bucket_entry<value_type, false>)) &&
                        (sizeof(std::size_t) == sizeof(truncated_hash_type) ||
                         is_power_of_two_policy<GrowthPolicy>::value) &&
                        // Don't store the hash for primitive types with default hash.
                        (!std::is_arithmetic<key_type>::value ||
                         !std::is_same<Hash, std::hash<key_type>>::value));

      /**
       * Only use the stored hash on lookup if we are explictly asked. We are not sure how slow
       * the KeyEqual operation is. An extra comparison may slow things down with a fast KeyEqual.
       */
      static constexpr bool USE_STORED_HASH_ON_LOOKUP = StoreHash;

      /**
       * We can only use the hash on rehash if the size of the hash type is the same as the stored
       * one or if we use a power of two modulo. In the case of the power of two modulo, we just
       * mask the least significant bytes, we just have to check that the truncated_hash_type didn't
       * truncated more bytes.
       */
      static bool USE_STORED_HASH_ON_REHASH(size_type bucket_count)
      {
        (void)bucket_count;
        if (STORE_HASH && sizeof(std::size_t) == sizeof(truncated_hash_type)) {
          return true;
        }
        else if (STORE_HASH && is_power_of_two_policy<GrowthPolicy>::value) {
          tsl_assert(bucket_count > 0);
          return (bucket_count - 1) <= std::numeric_limits<truncated_hash_type>::max();
        }
        else {
          return false;
        }
      }

      using bucket_entry  = tsl::detail_robin_hash::bucket_entry<value_type, STORE_HASH>;
      using distance_type = typename bucket_entry::distance_type;

      using buckets_allocator =
          typename std::allocator_traits<allocator_type>::template rebind_alloc<bucket_entry>;
      using buckets_container_type = std::vector<bucket_entry, buckets_allocator>;

    public:
      /**
       * The 'operator*()' and 'operator->()' methods return a const reference and const pointer
       * respectively to the stored value type.
       *
       * In case of a map, to get a mutable reference to the value associated to a key (the
       * '.second' in the stored pair), you have to call 'value()'.
       *
       * The main reason for this is that if we returned a `std::pair<Key, T>&` instead
       * of a `const std::pair<Key, T>&`, the user may modify the key which will put the map in a
       * undefined state.
       */
      template <bool IsConst> class robin_iterator
      {
        friend class robin_hash;

      private:
        using iterator_bucket =
            typename std::conditional<IsConst, typename buckets_container_type::const_iterator,
                                      typename buckets_container_type::iterator>::type;

        robin_iterator(iterator_bucket it) noexcept : m_iterator(it) {}

      public:
        using iterator_category = std::forward_iterator_tag;
        using value_type        = const typename robin_hash::value_type;
        using difference_type   = std::ptrdiff_t;
        using reference         = value_type &;
        using pointer           = value_type *;

        robin_iterator() noexcept {}

        robin_iterator(const robin_iterator<false> &other) noexcept : m_iterator(other.m_iterator)
        {
        }

        const typename robin_hash::key_type &key() const
        {
          return KeySelect()(m_iterator->value());
        }

        template <class U = ValueSelect,
                  typename std::enable_if<has_mapped_type<U>::value && IsConst>::type * = nullptr>
        const typename U::value_type &value() const
        {
          return U()(m_iterator->value());
        }

        template <class U = ValueSelect,
                  typename std::enable_if<has_mapped_type<U>::value && !IsConst>::type * = nullptr>
        typename U::value_type &value()
        {
          return U()(m_iterator->value());
        }

        reference operator*() const { return m_iterator->value(); }

        pointer operator->() const { return std::addressof(m_iterator->value()); }

        robin_iterator &operator++()
        {
          while (true) {
            if (m_iterator->last_bucket()) {
              ++m_iterator;
              return *this;
            }

            ++m_iterator;
            if (!m_iterator->empty()) {
              return *this;
            }
          }
        }

        robin_iterator operator++(int)
        {
          robin_iterator tmp(*this);
          ++*this;

          return tmp;
        }

        friend bool operator==(const robin_iterator &lhs, const robin_iterator &rhs)
        {
          return lhs.m_iterator == rhs.m_iterator;
        }

        friend bool operator!=(const robin_iterator &lhs, const robin_iterator &rhs)
        {
          return !(lhs == rhs);
        }

      private:
        iterator_bucket m_iterator;
      };

    public:
      robin_hash(size_type bucket_count_, const Hash &hash, const KeyEqual &equal,
                 const Allocator &alloc, float max_load_factor_)
          : Hash(hash), KeyEqual(equal), GrowthPolicy(bucket_count_), m_buckets(alloc),
            m_first_or_empty_bucket(static_empty_bucket_ptr()), m_bucket_count(bucket_count_),
            m_nb_elements(0), m_grow_on_next_insert(false)
      {
        if (bucket_count_ > max_bucket_count()) {
          TSL_THROW_OR_TERMINATE(std::length_error, "The map exceeds its maxmimum size.");
        }

        if (m_bucket_count > 0) {
          /*
           * We can't use the `vector(size_type count, const Allocator& alloc)` constructor
           * as it's only available in C++14 and we need to support C++11. We thus must resize after
           * using the `vector(const Allocator& alloc)` constructor.
           *
           * We can't use `vector(size_type count, const T& value, const Allocator& alloc)` as it
           * requires the value T to be copyable.
           */
          m_buckets.resize(m_bucket_count);
          m_first_or_empty_bucket = m_buckets.data();

          tsl_assert(!m_buckets.empty());
          m_buckets.back().set_as_last_bucket();
        }

        this->max_load_factor(max_load_factor_);
      }

      robin_hash(const robin_hash &other)
          : Hash(other), KeyEqual(other), GrowthPolicy(other), m_buckets(other.m_buckets),
            m_first_or_empty_bucket(m_buckets.empty() ? static_empty_bucket_ptr()
                                                      : m_buckets.data()),
            m_bucket_count(other.m_bucket_count), m_nb_elements(other.m_nb_elements),
            m_load_threshold(other.m_load_threshold), m_max_load_factor(other.m_max_load_factor),
            m_grow_on_next_insert(other.m_grow_on_next_insert)
      {
      }

      robin_hash(robin_hash &&other) noexcept(
          std::is_nothrow_move_constructible<Hash>::value &&std::is_nothrow_move_constructible<
              KeyEqual>::value &&std::is_nothrow_move_constructible<GrowthPolicy>::value
              &&                 std::is_nothrow_move_constructible<buckets_container_type>::value)
          : Hash(std::move(static_cast<Hash &>(other))),
            KeyEqual(std::move(static_cast<KeyEqual &>(other))),
            GrowthPolicy(std::move(static_cast<GrowthPolicy &>(other))),
            m_buckets(std::move(other.m_buckets)),
            m_first_or_empty_bucket(m_buckets.empty() ? static_empty_bucket_ptr()
                                                      : m_buckets.data()),
            m_bucket_count(other.m_bucket_count), m_nb_elements(other.m_nb_elements),
            m_load_threshold(other.m_load_threshold), m_max_load_factor(other.m_max_load_factor),
            m_grow_on_next_insert(other.m_grow_on_next_insert)
      {
        other.GrowthPolicy::clear();
        other.m_buckets.clear();
        other.m_first_or_empty_bucket = static_empty_bucket_ptr();
        other.m_bucket_count          = 0;
        other.m_nb_elements           = 0;
        other.m_load_threshold        = 0;
        other.m_grow_on_next_insert   = false;
      }

      robin_hash &operator=(const robin_hash &other)
      {
        if (&other != this) {
          Hash::        operator=(other);
          KeyEqual::    operator=(other);
          GrowthPolicy::operator=(other);

          m_buckets = other.m_buckets;
          m_first_or_empty_bucket =
              m_buckets.empty() ? static_empty_bucket_ptr() : m_buckets.data();
          m_bucket_count        = other.m_bucket_count;
          m_nb_elements         = other.m_nb_elements;
          m_load_threshold      = other.m_load_threshold;
          m_max_load_factor     = other.m_max_load_factor;
          m_grow_on_next_insert = other.m_grow_on_next_insert;
        }

        return *this;
      }

      robin_hash &operator=(robin_hash &&other)
      {
        other.swap(*this);
        other.clear();

        return *this;
      }

      allocator_type get_allocator() const { return m_buckets.get_allocator(); }

      /*
       * Iterators
       */
      iterator begin() noexcept
      {
        auto begin_ = m_buckets.begin();
        while (begin_ != m_buckets.end() && begin_->empty()) {
          ++begin_;
        }

        return iterator(begin_);
      }

      const_iterator begin() const noexcept { return cbegin(); }

      const_iterator cbegin() const noexcept
      {
        auto begin_ = m_buckets.cbegin();
        while (begin_ != m_buckets.cend() && begin_->empty()) {
          ++begin_;
        }

        return const_iterator(begin_);
      }

      iterator end() noexcept { return iterator(m_buckets.end()); }

      const_iterator end() const noexcept { return cend(); }

      const_iterator cend() const noexcept { return const_iterator(m_buckets.cend()); }

      /*
       * Capacity
       */
      bool empty() const noexcept { return m_nb_elements == 0; }

      size_type size() const noexcept { return m_nb_elements; }

      size_type max_size() const noexcept { return m_buckets.max_size(); }

      /*
       * Modifiers
       */
      void clear() noexcept
      {
        for (auto &bucket : m_buckets) {
          bucket.clear();
        }

        m_nb_elements         = 0;
        m_grow_on_next_insert = false;
      }

      template <typename P> std::pair<iterator, bool> insert(P &&value)
      {
        return insert_impl(KeySelect()(value), std::forward<P>(value));
      }

      template <typename P> iterator insert(const_iterator hint, P &&value)
      {
        if (hint != cend() && compare_keys(KeySelect()(*hint), KeySelect()(value))) {
          return mutable_iterator(hint);
        }

        return insert(std::forward<P>(value)).first;
      }

      template <class InputIt> void insert(InputIt first, InputIt last)
      {
        if (std::is_base_of<std::forward_iterator_tag,
                            typename std::iterator_traits<InputIt>::iterator_category>::value) {
          const auto      nb_elements_insert = std::distance(first, last);
          const size_type nb_free_buckets    = m_load_threshold - size();
          tsl_assert(m_load_threshold >= size());

          if (nb_elements_insert > 0 && nb_free_buckets < size_type(nb_elements_insert)) {
            reserve(size() + size_type(nb_elements_insert));
          }
        }

        for (; first != last; ++first) {
          insert(*first);
        }
      }

      template <class K, class M> std::pair<iterator, bool> insert_or_assign(K &&key, M &&obj)
      {
        auto it = try_emplace(std::forward<K>(key), std::forward<M>(obj));
        if (!it.second) {
          it.first.value() = std::forward<M>(obj);
        }

        return it;
      }

      template <class K, class M> iterator insert_or_assign(const_iterator hint, K &&key, M &&obj)
      {
        if (hint != cend() && compare_keys(KeySelect()(*hint), key)) {
          auto it    = mutable_iterator(hint);
          it.value() = std::forward<M>(obj);

          return it;
        }

        return insert_or_assign(std::forward<K>(key), std::forward<M>(obj)).first;
      }

      template <class... Args> std::pair<iterator, bool> emplace(Args &&... args)
      {
        return insert(value_type(std::forward<Args>(args)...));
      }

      template <class... Args> iterator emplace_hint(const_iterator hint, Args &&... args)
      {
        return insert(hint, value_type(std::forward<Args>(args)...));
      }

      template <class K, class... Args>
      std::pair<iterator, bool> try_emplace(K &&key, Args &&... args)
      {
        return insert_impl(key, std::piecewise_construct,
                           std::forward_as_tuple(std::forward<K>(key)),
                           std::forward_as_tuple(std::forward<Args>(args)...));
      }

      template <class K, class... Args>
      iterator try_emplace(const_iterator hint, K &&key, Args &&... args)
      {
        if (hint != cend() && compare_keys(KeySelect()(*hint), key)) {
          return mutable_iterator(hint);
        }

        return try_emplace(std::forward<K>(key), std::forward<Args>(args)...).first;
      }

      /**
       * Here to avoid `template<class K> size_type erase(const K& key)` being used when
       * we use an `iterator` instead of a `const_iterator`.
       */
      iterator erase(iterator pos)
      {
        erase_from_bucket(pos);

        /**
         * Erase bucket used a backward shift after clearing the bucket.
         * Check if there is a new value in the bucket, if not get the next non-empty.
         */
        if (pos.m_iterator->empty()) {
          ++pos;
        }

        return pos;
      }

      iterator erase(const_iterator pos) { return erase(mutable_iterator(pos)); }

      iterator erase(const_iterator first, const_iterator last)
      {
        if (first == last) {
          return mutable_iterator(first);
        }

        auto first_mutable = mutable_iterator(first);
        auto last_mutable  = mutable_iterator(last);
        for (auto it = first_mutable.m_iterator; it != last_mutable.m_iterator; ++it) {
          if (!it->empty()) {
            it->clear();
            m_nb_elements--;
          }
        }

        if (last_mutable == end()) {
          return end();
        }

        /*
         * Backward shift on the values which come after the deleted values.
         * We try to move the values closer to their ideal bucket.
         */
        std::size_t icloser_bucket =
            std::size_t(std::distance(m_buckets.begin(), first_mutable.m_iterator));
        std::size_t ito_move_closer_value =
            std::size_t(std::distance(m_buckets.begin(), last_mutable.m_iterator));
        tsl_assert(ito_move_closer_value > icloser_bucket);

        const std::size_t ireturn_bucket =
            ito_move_closer_value -
            std::min(ito_move_closer_value - icloser_bucket,
                     std::size_t(m_buckets[ito_move_closer_value].dist_from_ideal_bucket()));

        while (ito_move_closer_value < m_buckets.size() &&
               m_buckets[ito_move_closer_value].dist_from_ideal_bucket() > 0) {
          icloser_bucket =
              ito_move_closer_value -
              std::min(ito_move_closer_value - icloser_bucket,
                       std::size_t(m_buckets[ito_move_closer_value].dist_from_ideal_bucket()));

          tsl_assert(m_buckets[icloser_bucket].empty());
          const distance_type new_distance =
              distance_type(m_buckets[ito_move_closer_value].dist_from_ideal_bucket() -
                            (ito_move_closer_value - icloser_bucket));
          m_buckets[icloser_bucket].set_value_of_empty_bucket(
              new_distance, m_buckets[ito_move_closer_value].truncated_hash(),
              std::move(m_buckets[ito_move_closer_value].value()));
          m_buckets[ito_move_closer_value].clear();

          ++icloser_bucket;
          ++ito_move_closer_value;
        }

        return iterator(m_buckets.begin() + ireturn_bucket);
      }

      template <class K> size_type erase(const K &key) { return erase(key, hash_key(key)); }

      template <class K> size_type erase(const K &key, std::size_t hash)
      {
        auto it = find(key, hash);
        if (it != end()) {
          erase_from_bucket(it);

          return 1;
        }
        else {
          return 0;
        }
      }

      void swap(robin_hash &other)
      {
        using std::swap;

        swap(static_cast<Hash &>(*this), static_cast<Hash &>(other));
        swap(static_cast<KeyEqual &>(*this), static_cast<KeyEqual &>(other));
        swap(static_cast<GrowthPolicy &>(*this), static_cast<GrowthPolicy &>(other));
        swap(m_buckets, other.m_buckets);
        swap(m_first_or_empty_bucket, other.m_first_or_empty_bucket);
        swap(m_bucket_count, other.m_bucket_count);
        swap(m_nb_elements, other.m_nb_elements);
        swap(m_load_threshold, other.m_load_threshold);
        swap(m_max_load_factor, other.m_max_load_factor);
        swap(m_grow_on_next_insert, other.m_grow_on_next_insert);
      }

      /*
       * Lookup
       */
      template <class K, class U = ValueSelect,
                typename std::enable_if<has_mapped_type<U>::value>::type * = nullptr>
      typename U::value_type &at(const K &key)
      {
        return at(key, hash_key(key));
      }

      template <class K, class U = ValueSelect,
                typename std::enable_if<has_mapped_type<U>::value>::type * = nullptr>
      typename U::value_type &at(const K &key, std::size_t hash)
      {
        return const_cast<typename U::value_type &>(
            static_cast<const robin_hash *>(this)->at(key, hash));
      }

      template <class K, class U = ValueSelect,
                typename std::enable_if<has_mapped_type<U>::value>::type * = nullptr>
      const typename U::value_type &at(const K &key) const
      {
        return at(key, hash_key(key));
      }

      template <class K, class U = ValueSelect,
                typename std::enable_if<has_mapped_type<U>::value>::type * = nullptr>
      const typename U::value_type &at(const K &key, std::size_t hash) const
      {
        auto it = find(key, hash);
        if (it != cend()) {
          return it.value();
        }
        else {
          TSL_THROW_OR_TERMINATE(std::out_of_range, "Couldn't find key.");
        }
      }

      template <class K, class U = ValueSelect,
                typename std::enable_if<has_mapped_type<U>::value>::type * = nullptr>
      typename U::value_type &operator[](K &&key)
      {
        return try_emplace(std::forward<K>(key)).first.value();
      }

      template <class K> size_type count(const K &key) const { return count(key, hash_key(key)); }

      template <class K> size_type count(const K &key, std::size_t hash) const
      {
        if (find(key, hash) != cend()) {
          return 1;
        }
        else {
          return 0;
        }
      }

      template <class K> iterator find(const K &key) { return find_impl(key, hash_key(key)); }

      template <class K> iterator find(const K &key, std::size_t hash)
      {
        return find_impl(key, hash);
      }

      template <class K> const_iterator find(const K &key) const
      {
        return find_impl(key, hash_key(key));
      }

      template <class K> const_iterator find(const K &key, std::size_t hash) const
      {
        return find_impl(key, hash);
      }

      template <class K> std::pair<iterator, iterator> equal_range(const K &key)
      {
        return equal_range(key, hash_key(key));
      }

      template <class K> std::pair<iterator, iterator> equal_range(const K &key, std::size_t hash)
      {
        iterator it = find(key, hash);
        return std::make_pair(it, (it == end()) ? it : std::next(it));
      }

      template <class K> std::pair<const_iterator, const_iterator> equal_range(const K &key) const
      {
        return equal_range(key, hash_key(key));
      }

      template <class K>
      std::pair<const_iterator, const_iterator> equal_range(const K &key, std::size_t hash) const
      {
        const_iterator it = find(key, hash);
        return std::make_pair(it, (it == cend()) ? it : std::next(it));
      }

      /*
       * Bucket interface
       */
      size_type bucket_count() const { return m_bucket_count; }

      size_type max_bucket_count() const
      {
        return std::min(GrowthPolicy::max_bucket_count(), m_buckets.max_size());
      }

      /*
       * Hash policy
       */
      float load_factor() const
      {
        if (bucket_count() == 0) {
          return 0;
        }

        return float(m_nb_elements) / float(bucket_count());
      }

      float max_load_factor() const { return m_max_load_factor; }

      void max_load_factor(float ml)
      {
        m_max_load_factor = std::max(0.1f, std::min(ml, 0.95f));
        m_load_threshold  = size_type(float(bucket_count()) * m_max_load_factor);
      }

      void rehash(size_type count_)
      {
        count_ = std::max(count_, size_type(std::ceil(float(size()) / max_load_factor())));
        rehash_impl(count_);
      }

      void reserve(size_type count_)
      {
        rehash(size_type(std::ceil(float(count_) / max_load_factor())));
      }

      /*
       * Observers
       */
      hasher hash_function() const { return static_cast<const Hash &>(*this); }

      key_equal key_eq() const { return static_cast<const KeyEqual &>(*this); }

      /*
       * Other
       */
      iterator mutable_iterator(const_iterator pos)
      {
        return iterator(m_buckets.begin() + std::distance(m_buckets.cbegin(), pos.m_iterator));
      }

    private:
      template <class K> std::size_t hash_key(const K &key) const { return Hash::operator()(key); }

      template <class K1, class K2> bool compare_keys(const K1 &key1, const K2 &key2) const
      {
        return KeyEqual::operator()(key1, key2);
      }

      std::size_t bucket_for_hash(std::size_t hash) const
      {
        const std::size_t bucket = GrowthPolicy::bucket_for_hash(hash);
        tsl_assert(bucket < m_buckets.size() || (bucket == 0 && m_buckets.empty()));

        return bucket;
      }

      template <class U                                                           = GrowthPolicy,
                typename std::enable_if<is_power_of_two_policy<U>::value>::type * = nullptr>
      std::size_t next_bucket(std::size_t index) const noexcept
      {
        tsl_assert(index < bucket_count());

        return (index + 1) & this->m_mask;
      }

      template <class U                                                            = GrowthPolicy,
                typename std::enable_if<!is_power_of_two_policy<U>::value>::type * = nullptr>
      std::size_t next_bucket(std::size_t index) const noexcept
      {
        tsl_assert(index < bucket_count());

        index++;
        return (index != bucket_count()) ? index : 0;
      }

      template <class K> iterator find_impl(const K &key, std::size_t hash)
      {
        return mutable_iterator(static_cast<const robin_hash *>(this)->find(key, hash));
      }

      template <class K> const_iterator find_impl(const K &key, std::size_t hash) const
      {
        std::size_t   ibucket                 = bucket_for_hash(hash);
        distance_type dist_from_ideal_bucket_ = 0;

        while (dist_from_ideal_bucket_ <=
               (m_first_or_empty_bucket + ibucket)->dist_from_ideal_bucket()) {
          if (TSL_LIKELY(
                  (!USE_STORED_HASH_ON_LOOKUP ||
                   (m_first_or_empty_bucket + ibucket)->bucket_hash_equal(hash)) &&
                  compare_keys(KeySelect()((m_first_or_empty_bucket + ibucket)->value()), key))) {
            return const_iterator(m_buckets.begin() + ibucket);
          }

          ibucket = next_bucket(ibucket);
          dist_from_ideal_bucket_++;
        }

        return cend();
      }

      void erase_from_bucket(iterator pos)
      {
        pos.m_iterator->clear();
        m_nb_elements--;

        /**
         * Backward shift, swap the empty bucket, previous_ibucket, with the values on its right,
         * ibucket, until we cross another empty bucket or if the other bucket has a
         * distance_from_ideal_bucket == 0.
         *
         * We try to move the values closer to their ideal bucket.
         */
        std::size_t previous_ibucket =
            std::size_t(std::distance(m_buckets.begin(), pos.m_iterator));
        std::size_t ibucket = next_bucket(previous_ibucket);

        while (m_buckets[ibucket].dist_from_ideal_bucket() > 0) {
          tsl_assert(m_buckets[previous_ibucket].empty());

          const distance_type new_distance =
              distance_type(m_buckets[ibucket].dist_from_ideal_bucket() - 1);
          m_buckets[previous_ibucket].set_value_of_empty_bucket(
              new_distance, m_buckets[ibucket].truncated_hash(),
              std::move(m_buckets[ibucket].value()));
          m_buckets[ibucket].clear();

          previous_ibucket = ibucket;
          ibucket          = next_bucket(ibucket);
        }
      }

      template <class K, class... Args>
      std::pair<iterator, bool> insert_impl(const K &key, Args &&... value_type_args)
      {
        const std::size_t hash = hash_key(key);

        std::size_t   ibucket                 = bucket_for_hash(hash);
        distance_type dist_from_ideal_bucket_ = 0;

        while (dist_from_ideal_bucket_ <=
               (m_first_or_empty_bucket + ibucket)->dist_from_ideal_bucket()) {
          if ((!USE_STORED_HASH_ON_LOOKUP ||
               (m_first_or_empty_bucket + ibucket)->bucket_hash_equal(hash)) &&
              compare_keys(KeySelect()((m_first_or_empty_bucket + ibucket)->value()), key)) {
            return std::make_pair(iterator(m_buckets.begin() + ibucket), false);
          }

          ibucket = next_bucket(ibucket);
          dist_from_ideal_bucket_++;
        }

        if (grow_on_high_load()) {
          ibucket                 = bucket_for_hash(hash);
          dist_from_ideal_bucket_ = 0;

          while (dist_from_ideal_bucket_ <=
                 (m_first_or_empty_bucket + ibucket)->dist_from_ideal_bucket()) {
            ibucket = next_bucket(ibucket);
            dist_from_ideal_bucket_++;
          }
        }

        if ((m_first_or_empty_bucket + ibucket)->empty()) {
          (m_first_or_empty_bucket + ibucket)
              ->set_value_of_empty_bucket(dist_from_ideal_bucket_,
                                          bucket_entry::truncate_hash(hash),
                                          std::forward<Args>(value_type_args)...);
        }
        else {
          insert_value(ibucket, dist_from_ideal_bucket_, bucket_entry::truncate_hash(hash),
                       std::forward<Args>(value_type_args)...);
        }

        m_nb_elements++;
        /*
         * The value will be inserted in ibucket in any case, either because it was
         * empty or by stealing the bucket (robin hood).
         */
        return std::make_pair(iterator(m_buckets.begin() + ibucket), true);
      }

      template <class... Args>
      void insert_value(std::size_t ibucket, distance_type dist_from_ideal_bucket_,
                        truncated_hash_type hash, Args &&... value_type_args)
      {
        insert_value(ibucket, dist_from_ideal_bucket_, hash,
                     value_type(std::forward<Args>(value_type_args)...));
      }

      void insert_value(std::size_t ibucket, distance_type dist_from_ideal_bucket_,
                        truncated_hash_type hash, value_type &&value)
      {
        m_buckets[ibucket].swap_with_value_in_bucket(dist_from_ideal_bucket_, hash, value);
        ibucket = next_bucket(ibucket);
        dist_from_ideal_bucket_++;

        while (!m_buckets[ibucket].empty()) {
          if (dist_from_ideal_bucket_ > m_buckets[ibucket].dist_from_ideal_bucket()) {
            if (dist_from_ideal_bucket_ >= REHASH_ON_HIGH_NB_PROBES__NPROBES &&
                load_factor() >= REHASH_ON_HIGH_NB_PROBES__MIN_LOAD_FACTOR) {
              /**
               * The number of probes is really high, rehash the map on the next insert.
               * Difficult to do now as rehash may throw an exception.
               */
              m_grow_on_next_insert = true;
            }

            m_buckets[ibucket].swap_with_value_in_bucket(dist_from_ideal_bucket_, hash, value);
          }

          ibucket = next_bucket(ibucket);
          dist_from_ideal_bucket_++;
        }

        m_buckets[ibucket].set_value_of_empty_bucket(dist_from_ideal_bucket_, hash,
                                                     std::move(value));
      }

      void rehash_impl(size_type count_)
      {
        robin_hash new_table(count_, static_cast<Hash &>(*this), static_cast<KeyEqual &>(*this),
                             get_allocator(), m_max_load_factor);

        const bool use_stored_hash = USE_STORED_HASH_ON_REHASH(new_table.bucket_count());
        for (auto &bucket : m_buckets) {
          if (bucket.empty()) {
            continue;
          }

          const std::size_t hash = use_stored_hash
                                       ? bucket.truncated_hash()
                                       : new_table.hash_key(KeySelect()(bucket.value()));

          new_table.insert_value_on_rehash(new_table.bucket_for_hash(hash), 0,
                                           bucket_entry::truncate_hash(hash),
                                           std::move(bucket.value()));
        }

        new_table.m_nb_elements = m_nb_elements;
        new_table.swap(*this);
      }

      void insert_value_on_rehash(std::size_t ibucket, distance_type dist_from_ideal_bucket_,
                                  truncated_hash_type hash, value_type &&value)
      {
        while (true) {
          if (dist_from_ideal_bucket_ > m_buckets[ibucket].dist_from_ideal_bucket()) {
            if (m_buckets[ibucket].empty()) {
              m_buckets[ibucket].set_value_of_empty_bucket(dist_from_ideal_bucket_, hash,
                                                           std::move(value));
              return;
            }
            else {
              m_buckets[ibucket].swap_with_value_in_bucket(dist_from_ideal_bucket_, hash, value);
            }
          }

          dist_from_ideal_bucket_++;
          ibucket = next_bucket(ibucket);
        }
      }

      /**
       * Return true if the map has been rehashed.
       */
      bool grow_on_high_load()
      {
        if (m_grow_on_next_insert || size() >= m_load_threshold) {
          rehash_impl(GrowthPolicy::next_bucket_count());
          m_grow_on_next_insert = false;

          return true;
        }

        return false;
      }

    public:
      static const size_type DEFAULT_INIT_BUCKETS_SIZE = 16;
      static constexpr float DEFAULT_MAX_LOAD_FACTOR   = 0.5f;

    private:
      static const distance_type REHASH_ON_HIGH_NB_PROBES__NPROBES         = 128;
      static constexpr float     REHASH_ON_HIGH_NB_PROBES__MIN_LOAD_FACTOR = 0.15f;

      /**
       * Return an always valid pointer to an static empty bucket_entry with last_bucket() == true.
       */
      bucket_entry *static_empty_bucket_ptr()
      {
        static bucket_entry empty_bucket(true);
        return &empty_bucket;
      }

    private:
      buckets_container_type m_buckets;

      /**
       * Points to m_buckets.data() if !m_buckets.empty() otherwise points to
       * static_empty_bucket_ptr. This variable is useful to avoid the cost of checking if m_buckets
       * is empty when trying to find an element.
       */
      bucket_entry *m_first_or_empty_bucket;

      /**
       * Used a lot in find, avoid the call to m_buckets.size() which is a bit slower.
       */
      size_type m_bucket_count;

      size_type m_nb_elements;

      size_type m_load_threshold;
      float     m_max_load_factor;

      bool m_grow_on_next_insert;
    };

  } // namespace detail_robin_hash

} // namespace tsl

#endif