text: /*
   * Copyright (C) 2005, 2006, 2007, 2008, 2011, 2012 Apple Inc. All rights reserved.
   *
   * This library is free software; you can redistribute it and/or
   * modify it under the terms of the GNU Library General Public
   * License as published by the Free Software Foundation; either
   * version 2 of the License, or (at your option) any later version.
   *
   * This library is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   * Library General Public License for more details.
   *
   * You should have received a copy of the GNU Library General Public License
   * along with this library; see the file COPYING.LIB.  If not, write to
   * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
   * Boston, MA 02110-1301, USA.
   *
   */
  
  #ifndef WTF_HashTraits_h
  #define WTF_HashTraits_h
  
  #include <limits>
  #include <utility>
  #include <wtf/HashFunctions.h>
  #include <wtf/KeyValuePair.h>
  #include <wtf/Optional.h>
  #include <wtf/StdLibExtras.h>
  
  namespace WTF {
  
  class String;
  
  template<typename T> struct HashTraits;
  
  template<bool isInteger, typename T> struct GenericHashTraitsBase;
  
  template<typename T> struct GenericHashTraitsBase<false, T> {
      // The emptyValueIsZero flag is used to optimize allocation of empty hash tables with zeroed memory.
      static const bool emptyValueIsZero = false;
      
      // The hasIsEmptyValueFunction flag allows the hash table to automatically generate code to check
      // for the empty value when it can be done with the equality operator, but allows custom functions
      // for cases like String that need them.
      static const bool hasIsEmptyValueFunction = false;
  
      // The starting table size. Can be overridden when we know beforehand that
      // a hash table will have at least N entries.
      static const unsigned minimumTableSize = 8;
  };
  
  // Default integer traits disallow both 0 and -1 as keys (max value instead of -1 for unsigned).
  template<typename T> struct GenericHashTraitsBase<true, T> : GenericHashTraitsBase<false, T> {
      static const bool emptyValueIsZero = true;
      static void constructDeletedValue(T& slot) { slot = static_cast<T>(-1); }
      static bool isDeletedValue(T value) { return value == static_cast<T>(-1); }
  };
  
  template<typename T> struct GenericHashTraits : GenericHashTraitsBase<std::is_integral<T>::value, T> {
      typedef T TraitType;
      typedef T EmptyValueType;
  
      static T emptyValue() { return T(); }
  
      template<typename U, typename V> 
      static void assignToEmpty(U& emptyValue, V&& value)
      { 
          emptyValue = std::forward<V>(value);
      }
  
      // Type for return value of functions that do not transfer ownership, such as get.
      typedef T PeekType;
      template<typename U> static U&& peek(U&& value) { return std::forward<U>(value); }
  
      typedef T TakeType;
      template<typename U> static TakeType take(U&& value) { return std::forward<U>(value); }
  };
  
  template<typename T> struct HashTraits : GenericHashTraits<T> { };
  
  template<typename T> struct FloatHashTraits : GenericHashTraits<T> {
      static T emptyValue() { return std::numeric_limits<T>::infinity(); }
      static void constructDeletedValue(T& slot) { slot = -std::numeric_limits<T>::infinity(); }
      static bool isDeletedValue(T value) { return value == -std::numeric_limits<T>::infinity(); }
  };
  
  template<> struct HashTraits<float> : FloatHashTraits<float> { };
  template<> struct HashTraits<double> : FloatHashTraits<double> { };
  
  // Default unsigned traits disallow both 0 and max as keys -- use these traits to allow zero and disallow max - 1.
  template<typename T> struct UnsignedWithZeroKeyHashTraits : GenericHashTraits<T> {
      static const bool emptyValueIsZero = false;
      static T emptyValue() { return std::numeric_limits<T>::max(); }
      static void constructDeletedValue(T& slot) { slot = std::numeric_limits<T>::max() - 1; }
      static bool isDeletedValue(T value) { return value == std::numeric_limits<T>::max() - 1; }
  };
  
  template<typename T> struct SignedWithZeroKeyHashTraits : GenericHashTraits<T> {
      static const bool emptyValueIsZero = false;
      static T emptyValue() { return std::numeric_limits<T>::min(); }
      static void constructDeletedValue(T& slot) { slot = std::numeric_limits<T>::max(); }
      static bool isDeletedValue(T value) { return value == std::numeric_limits<T>::max(); }
  };
  
  // Can be used with strong enums, allows zero as key.
  template<typename T> struct StrongEnumHashTraits : GenericHashTraits<T> {
      using UnderlyingType = typename std::underlying_type<T>::type;
      static const bool emptyValueIsZero = false;
      static T emptyValue() { return static_cast<T>(std::numeric_limits<UnderlyingType>::max()); }
      static void constructDeletedValue(T& slot) { slot = static_cast<T>(std::numeric_limits<UnderlyingType>::max() - 1); }
      static bool isDeletedValue(T value) { return value == static_cast<T>(std::numeric_limits<UnderlyingType>::max() - 1); }
  };
  
  template<typename P> struct HashTraits<P*> : GenericHashTraits<P*> {
      static const bool emptyValueIsZero = true;
      static void constructDeletedValue(P*& slot) { slot = reinterpret_cast<P*>(-1); }
      static bool isDeletedValue(P* value) { return value == reinterpret_cast<P*>(-1); }
  };
  
  template<typename T> struct SimpleClassHashTraits : GenericHashTraits<T> {
      static const bool emptyValueIsZero = true;
      static void constructDeletedValue(T& slot) { new (NotNull, std::addressof(slot)) T(HashTableDeletedValue); }
      static bool isDeletedValue(const T& value) { return value.isHashTableDeletedValue(); }
  };
  
  template<typename T, typename Deleter> struct HashTraits<std::unique_ptr<T, Deleter>> : SimpleClassHashTraits<std::unique_ptr<T, Deleter>> {
      typedef std::nullptr_t EmptyValueType;
      static EmptyValueType emptyValue() { return nullptr; }
  
      static void constructDeletedValue(std::unique_ptr<T, Deleter>& slot) { new (NotNull, std::addressof(slot)) std::unique_ptr<T, Deleter> { reinterpret_cast<T*>(-1) }; }
      static bool isDeletedValue(const std::unique_ptr<T, Deleter>& value) { return value.get() == reinterpret_cast<T*>(-1); }
  
      typedef T* PeekType;
      static T* peek(const std::unique_ptr<T, Deleter>& value) { return value.get(); }
      static T* peek(std::nullptr_t) { return nullptr; }
  
      static void customDeleteBucket(std::unique_ptr<T, Deleter>& value)
      {
          // The custom delete function exists to avoid a dead store before the value is destructed.
          // The normal destruction sequence of a bucket would be:
          // 1) Call the destructor of unique_ptr.
          // 2) unique_ptr store a zero for its internal pointer.
          // 3) unique_ptr destroys its value.
          // 4) Call constructDeletedValue() to set the bucket as destructed.
          //
          // The problem is the call in (3) prevents the compile from eliminating the dead store in (2)
          // becase a side effect of free() could be observing the value.
          //
          // This version of deleteBucket() ensures the dead 2 stores changing "value"
          // are on the same side of the function call.
          ASSERT(!isDeletedValue(value));
          T* pointer = value.release();
          constructDeletedValue(value);
  
          // The null case happens if a caller uses std::move() to remove the pointer before calling remove()
          // with an iterator. This is very uncommon.
          if (LIKELY(pointer))
              Deleter()(pointer);
      }
  };
  
  template<typename P> struct HashTraits<RefPtr<P>> : SimpleClassHashTraits<RefPtr<P>> {
      static P* emptyValue() { return nullptr; }
  
      typedef P* PeekType;
      static PeekType peek(const RefPtr<P>& value) { return value.get(); }
      static PeekType peek(P* value) { return value; }
  
      static void customDeleteBucket(RefPtr<P>& value)
      {
          // See unique_ptr's customDeleteBucket() for an explanation.
          ASSERT(!SimpleClassHashTraits<RefPtr<P>>::isDeletedValue(value));
          auto valueToBeDestroyed = WTFMove(value);
          SimpleClassHashTraits<RefPtr<P>>::constructDeletedValue(value);
      }
  };
  
  template<typename P> struct HashTraits<Ref<P>> : SimpleClassHashTraits<Ref<P>> {
      static const bool emptyValueIsZero = true;
      static Ref<P> emptyValue() { return HashTableEmptyValue; }
  
      static const bool hasIsEmptyValueFunction = true;
      static bool isEmptyValue(const Ref<P>& value) { return value.isHashTableEmptyValue(); }
  
      static void assignToEmpty(Ref<P>& emptyValue, Ref<P>&& newValue) { ASSERT(isEmptyValue(emptyValue)); emptyValue.assignToHashTableEmptyValue(WTFMove(newValue)); }
  
      typedef P* PeekType;
      static PeekType peek(const Ref<P>& value) { return const_cast<PeekType>(value.ptrAllowingHashTableEmptyValue()); }
      static PeekType peek(P* value) { return value; }
  
      typedef std::optional<Ref<P>> TakeType;
      static TakeType take(Ref<P>&& value) { return isEmptyValue(value) ? std::nullopt : std::optional<Ref<P>>(WTFMove(value)); }
  };
  
  template<> struct HashTraits<String> : SimpleClassHashTraits<String> {
      static const bool hasIsEmptyValueFunction = true;
      static bool isEmptyValue(const String&);
  
      static void customDeleteBucket(String&);
  };
  
  // This struct template is an implementation detail of the isHashTraitsEmptyValue function,
  // which selects either the emptyValue function or the isEmptyValue function to check for empty values.
  template<typename Traits, bool hasEmptyValueFunction> struct HashTraitsEmptyValueChecker;
  template<typename Traits> struct HashTraitsEmptyValueChecker<Traits, true> {
      template<typename T> static bool isEmptyValue(const T& value) { return Traits::isEmptyValue(value); }
  };
  template<typename Traits> struct HashTraitsEmptyValueChecker<Traits, false> {
      template<typename T> static bool isEmptyValue(const T& value) { return value == Traits::emptyValue(); }
  };
  template<typename Traits, typename T> inline bool isHashTraitsEmptyValue(const T& value)
  {
      return HashTraitsEmptyValueChecker<Traits, Traits::hasIsEmptyValueFunction>::isEmptyValue(value);
  }
  
  template<typename Traits, typename T>
  struct HashTraitHasCustomDelete {
      static T& bucketArg;
      template<typename X> static std::true_type TestHasCustomDelete(X*, decltype(X::customDeleteBucket(bucketArg))* = nullptr);
      static std::false_type TestHasCustomDelete(...);
      typedef decltype(TestHasCustomDelete(static_cast<Traits*>(nullptr))) ResultType;
      static const bool value = ResultType::value;
  };
  
  template<typename Traits, typename T>
  typename std::enable_if<HashTraitHasCustomDelete<Traits, T>::value>::type
  hashTraitsDeleteBucket(T& value)
  {
      Traits::customDeleteBucket(value);
  }
  
  template<typename Traits, typename T>
  typename std::enable_if<!HashTraitHasCustomDelete<Traits, T>::value>::type
  hashTraitsDeleteBucket(T& value)
  {
      value.~T();
      Traits::constructDeletedValue(value);
  }
  
  template<typename FirstTraitsArg, typename SecondTraitsArg>
  struct PairHashTraits : GenericHashTraits<std::pair<typename FirstTraitsArg::TraitType, typename SecondTraitsArg::TraitType>> {
      typedef FirstTraitsArg FirstTraits;
      typedef SecondTraitsArg SecondTraits;
      typedef std::pair<typename FirstTraits::TraitType, typename SecondTraits::TraitType> TraitType;
      typedef std::pair<typename FirstTraits::EmptyValueType, typename SecondTraits::EmptyValueType> EmptyValueType;
  
      static const bool emptyValueIsZero = FirstTraits::emptyValueIsZero && SecondTraits::emptyValueIsZero;
      static EmptyValueType emptyValue() { return std::make_pair(FirstTraits::emptyValue(), SecondTraits::emptyValue()); }
  
      static const unsigned minimumTableSize = FirstTraits::minimumTableSize;
  
      static void constructDeletedValue(TraitType& slot) { FirstTraits::constructDeletedValue(slot.first); }
      static bool isDeletedValue(const TraitType& value) { return FirstTraits::isDeletedValue(value.first); }
  };
  
  template<typename First, typename Second>
  struct HashTraits<std::pair<First, Second>> : public PairHashTraits<HashTraits<First>, HashTraits<Second>> { };
  
  template<typename FirstTrait, typename... Traits>
  struct TupleHashTraits : GenericHashTraits<std::tuple<typename FirstTrait::TraitType, typename Traits::TraitType...>> {
      typedef std::tuple<typename FirstTrait::TraitType, typename Traits::TraitType...> TraitType;
      typedef std::tuple<typename FirstTrait::EmptyValueType, typename Traits::EmptyValueType...> EmptyValueType;
  
      // We should use emptyValueIsZero = Traits::emptyValueIsZero &&... whenever we switch to C++17. We can't do anything
      // better here right now because GCC can't do C++.
      template<typename BoolType>
      static constexpr bool allTrue(BoolType value) { return value; }
      template<typename BoolType, typename... BoolTypes>
      static constexpr bool allTrue(BoolType value, BoolTypes... values) { return value && allTrue(values...); }
      static const bool emptyValueIsZero = allTrue(FirstTrait::emptyValueIsZero, Traits::emptyValueIsZero...);
      static EmptyValueType emptyValue() { return std::make_tuple(FirstTrait::emptyValue(), Traits::emptyValue()...); }
  
      static const unsigned minimumTableSize = FirstTrait::minimumTableSize;
  
      static void constructDeletedValue(TraitType& slot) { FirstTrait::constructDeletedValue(std::get<0>(slot)); }
      static bool isDeletedValue(const TraitType& value) { return FirstTrait::isDeletedValue(std::get<0>(value)); }
  };
  
  template<typename... Traits>
  struct HashTraits<std::tuple<Traits...>> : public TupleHashTraits<HashTraits<Traits>...> { };
  
  template<typename KeyTraitsArg, typename ValueTraitsArg>
  struct KeyValuePairHashTraits : GenericHashTraits<KeyValuePair<typename KeyTraitsArg::TraitType, typename ValueTraitsArg::TraitType>> {
      typedef KeyTraitsArg KeyTraits;
      typedef ValueTraitsArg ValueTraits;
      typedef KeyValuePair<typename KeyTraits::TraitType, typename ValueTraits::TraitType> TraitType;
      typedef KeyValuePair<typename KeyTraits::EmptyValueType, typename ValueTraits::EmptyValueType> EmptyValueType;
      typedef typename ValueTraitsArg::TraitType ValueType;
  
      static const bool emptyValueIsZero = KeyTraits::emptyValueIsZero && ValueTraits::emptyValueIsZero;
      static EmptyValueType emptyValue() { return KeyValuePair<typename KeyTraits::EmptyValueType, typename ValueTraits::EmptyValueType>(KeyTraits::emptyValue(), ValueTraits::emptyValue()); }
  
      static const unsigned minimumTableSize = KeyTraits::minimumTableSize;
  
      static void constructDeletedValue(TraitType& slot) { KeyTraits::constructDeletedValue(slot.key); }
      static bool isDeletedValue(const TraitType& value) { return KeyTraits::isDeletedValue(value.key); }
  
      static void customDeleteBucket(TraitType& value)
      {
          static_assert(std::is_trivially_destructible<KeyValuePair<int, int>>::value,
              "The wrapper itself has to be trivially destructible for customDeleteBucket() to make sense, since we do not destruct the wrapper itself.");
  
          hashTraitsDeleteBucket<KeyTraits>(value.key);
          value.value.~ValueType();
      }
  };
  
  template<typename Key, typename Value>
  struct HashTraits<KeyValuePair<Key, Value>> : public KeyValuePairHashTraits<HashTraits<Key>, HashTraits<Value>> { };
  
  template<typename T>
  struct NullableHashTraits : public HashTraits<T> {
      static const bool emptyValueIsZero = false;
      static T emptyValue() { return reinterpret_cast<T>(1); }
  };
  
  // Useful for classes that want complete control over what is empty and what is deleted,
  // and how to construct both.
  template<typename T>
  struct CustomHashTraits : public GenericHashTraits<T> {
      static const bool emptyValueIsZero = false;
      static const bool hasIsEmptyValueFunction = true;
      
      static void constructDeletedValue(T& slot)
      {
          new (NotNull, std::addressof(slot)) T(T::DeletedValue);
      }
      
      static bool isDeletedValue(const T& value)
      {
          return value.isDeletedValue();
      }
      
      static T emptyValue()
      {
          return T(T::EmptyValue);
      }
      
      static bool isEmptyValue(const T& value)
      {
          return value.isEmptyValue();
      }
  };
  
  } // namespace WTF
  
  using WTF::HashTraits;
  using WTF::KeyValuePair;
  using WTF::PairHashTraits;
  using WTF::NullableHashTraits;
  using WTF::SimpleClassHashTraits;
  
  #endif // WTF_HashTraits_h
  