text: #pragma once
  
  #include <Common/HashTable/Hash.h>
  #include <Common/HashTable/HashTable.h>
  #include <Common/HashTable/HashTableAllocator.h>
  
  
  /** NOTE HashMap could only be used for memmoveable (position independent) types.
    * Example: std::string is not position independent in libstdc++ with C++11 ABI or in libc++.
    * Also, key in hash table must be of type, that zero bytes is compared equals to zero key.
    */
  
  
  struct NoInitTag
  {
  };
  
  /// A pair that does not initialize the elements, if not needed.
  template <typename First, typename Second>
  struct PairNoInit
  {
      First first;
      Second second;
  
      PairNoInit() {}
  
      template <typename First_>
      PairNoInit(First_ && first_, NoInitTag) : first(std::forward<First_>(first_))
      {
      }
  
      template <typename First_, typename Second_>
      PairNoInit(First_ && first_, Second_ && second_) : first(std::forward<First_>(first_)), second(std::forward<Second_>(second_))
      {
      }
  };
  
  
  template <typename Key, typename TMapped, typename Hash, typename TState = HashTableNoState>
  struct HashMapCell
  {
      using Mapped = TMapped;
      using State = TState;
  
      using value_type = PairNoInit<Key, Mapped>;
      using mapped_type = Mapped;
      using key_type = Key;
  
      value_type value;
  
      HashMapCell() {}
      HashMapCell(const Key & key_, const State &) : value(key_, NoInitTag()) {}
      HashMapCell(const value_type & value_, const State &) : value(value_) {}
  
      /// Get the key (externally).
      const Key & getKey() const { return value.first; }
      Mapped & getMapped() { return value.second; }
      const Mapped & getMapped() const { return value.second; }
      const value_type & getValue() const { return value; }
  
      /// Get the key (internally).
      static const Key & getKey(const value_type & value) { return value.first; }
  
      bool keyEquals(const Key & key_) const { return value.first == key_; }
      bool keyEquals(const Key & key_, size_t /*hash_*/) const { return value.first == key_; }
      bool keyEquals(const Key & key_, size_t /*hash_*/, const State & /*state*/) const { return value.first == key_; }
  
      void setHash(size_t /*hash_value*/) {}
      size_t getHash(const Hash & hash) const { return hash(value.first); }
  
      bool isZero(const State & state) const { return isZero(value.first, state); }
      static bool isZero(const Key & key, const State & /*state*/) { return ZeroTraits::check(key); }
  
      /// Set the key value to zero.
      void setZero() { ZeroTraits::set(value.first); }
  
      /// Do I need to store the zero key separately (that is, can a zero key be inserted into the hash table).
      static constexpr bool need_zero_value_storage = true;
  
      /// Whether the cell was deleted.
      bool isDeleted() const { return false; }
  
      void setMapped(const value_type & value_) { value.second = value_.second; }
  
      /// Serialization, in binary and text form.
      void write(DB::WriteBuffer & wb) const
      {
          DB::writeBinary(value.first, wb);
          DB::writeBinary(value.second, wb);
      }
  
      void writeText(DB::WriteBuffer & wb) const
      {
          DB::writeDoubleQuoted(value.first, wb);
          DB::writeChar(',', wb);
          DB::writeDoubleQuoted(value.second, wb);
      }
  
      /// Deserialization, in binary and text form.
      void read(DB::ReadBuffer & rb)
      {
          DB::readBinary(value.first, rb);
          DB::readBinary(value.second, rb);
      }
  
      void readText(DB::ReadBuffer & rb)
      {
          DB::readDoubleQuoted(value.first, rb);
          DB::assertChar(',', rb);
          DB::readDoubleQuoted(value.second, rb);
      }
  };
  
  template <typename Key, typename TMapped, typename Hash, typename TState = HashTableNoState>
  struct HashMapCellWithSavedHash : public HashMapCell<Key, TMapped, Hash, TState>
  {
      using Base = HashMapCell<Key, TMapped, Hash, TState>;
  
      size_t saved_hash;
  
      using Base::Base;
  
      bool keyEquals(const Key & key_) const { return this->value.first == key_; }
      bool keyEquals(const Key & key_, size_t hash_) const { return saved_hash == hash_ && this->value.first == key_; }
      bool keyEquals(const Key & key_, size_t hash_, const typename Base::State &) const { return keyEquals(key_, hash_); }
  
      void setHash(size_t hash_value) { saved_hash = hash_value; }
      size_t getHash(const Hash & /*hash_function*/) const { return saved_hash; }
  };
  
  template <
      typename Key,
      typename Cell,
      typename Hash = DefaultHash<Key>,
      typename Grower = HashTableGrower<>,
      typename Allocator = HashTableAllocator>
  class HashMapTable : public HashTable<Key, Cell, Hash, Grower, Allocator>
  {
  public:
      using Self = HashMapTable;
      using Base = HashTable<Key, Cell, Hash, Grower, Allocator>;
      using LookupResult = typename Base::LookupResult;
  
      using Base::Base;
  
      /// Merge every cell's value of current map into the destination map via emplace.
      ///  Func should have signature void(Mapped & dst, Mapped & src, bool emplaced).
      ///  Each filled cell in current map will invoke func once. If that map doesn't
      ///  have a key equals to the given cell, a new cell gets emplaced into that map,
      ///  and func is invoked with the third argument emplaced set to true. Otherwise
      ///  emplaced is set to false.
      template <typename Func>
      void ALWAYS_INLINE mergeToViaEmplace(Self & that, Func && func)
      {
          for (auto it = this->begin(), end = this->end(); it != end; ++it)
          {
              typename Self::LookupResult res_it;
              bool inserted;
              that.emplace(Cell::getKey(it->getValue()), res_it, inserted, it.getHash());
              func(res_it->getMapped(), it->getMapped(), inserted);
          }
      }
  
      /// Merge every cell's value of current map into the destination map via find.
      ///  Func should have signature void(Mapped & dst, Mapped & src, bool exist).
      ///  Each filled cell in current map will invoke func once. If that map doesn't
      ///  have a key equals to the given cell, func is invoked with the third argument
      ///  exist set to false. Otherwise exist is set to true.
      template <typename Func>
      void ALWAYS_INLINE mergeToViaFind(Self & that, Func && func)
      {
          for (auto it = this->begin(), end = this->end(); it != end; ++it)
          {
              auto res_it = that.find(Cell::getKey(it->getValue()), it.getHash());
              if (!res_it)
                  func(it->getMapped(), it->getMapped(), false);
              else
                  func(res_it->getMapped(), it->getMapped(), true);
          }
      }
  
      /// Call func(const Key &, Mapped &) for each hash map element.
      template <typename Func>
      void forEachValue(Func && func)
      {
          for (auto & v : *this)
              func(v.getKey(), v.getMapped());
      }
  
      /// Call func(Mapped &) for each hash map element.
      template <typename Func>
      void forEachMapped(Func && func)
      {
          for (auto & v : *this)
              func(v.getMapped());
      }
  
      typename Cell::Mapped & ALWAYS_INLINE operator[](const Key & x)
      {
          LookupResult it;
          bool inserted;
          this->emplace(x, it, inserted);
  
          /** It may seem that initialization is not necessary for POD-types (or __has_trivial_constructor),
            *  since the hash table memory is initially initialized with zeros.
            * But, in fact, an empty cell may not be initialized with zeros in the following cases:
            * - ZeroValueStorage (it only zeros the key);
            * - after resizing and moving a part of the cells to the new half of the hash table, the old cells also have only the key to zero.
            *
            * On performance, there is almost always no difference, due to the fact that it->second is usually assigned immediately
            *  after calling `operator[]`, and since `operator[]` is inlined, the compiler removes unnecessary initialization.
            *
            * Sometimes due to initialization, the performance even grows. This occurs in code like `++map[key]`.
            * When we do the initialization, for new cells, it's enough to make `store 1` right away.
            * And if we did not initialize, then even though there was zero in the cell,
            *  the compiler can not guess about this, and generates the `load`, `increment`, `store` code.
            */
          if (inserted)
              new (&it->getMapped()) typename Cell::Mapped();
  
          return it->getMapped();
      }
  };
  
  
  template <
      typename Key,
      typename Mapped,
      typename Hash = DefaultHash<Key>,
      typename Grower = HashTableGrower<>,
      typename Allocator = HashTableAllocator>
  using HashMap = HashMapTable<Key, HashMapCell<Key, Mapped, Hash>, Hash, Grower, Allocator>;
  
  
  template <
      typename Key,
      typename Mapped,
      typename Hash = DefaultHash<Key>,
      typename Grower = HashTableGrower<>,
      typename Allocator = HashTableAllocator>
  using HashMapWithSavedHash = HashMapTable<Key, HashMapCellWithSavedHash<Key, Mapped, Hash>, Hash, Grower, Allocator>;
  