libartbase/base/hash_set.h - LeafOS-Project/android_art - Gitiles

 /*
  * Copyright (C) 2014 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #ifndef ART_LIBARTBASE_BASE_HASH_SET_H_
 #define ART_LIBARTBASE_BASE_HASH_SET_H_

 #include <stdint.h>

 #include <functional>
 #include <iterator>
 #include <memory>
 #include <type_traits>
 #include <utility>

 #include <android-base/logging.h>

 #include "bit_utils.h"
 #include "macros.h"

 namespace art {

 // Returns true if an item is empty.
 template <class T>
 class DefaultEmptyFn {
  public:
   void MakeEmpty(T& item) const {
     item = T();
   }
   bool IsEmpty(const T& item) const {
     return item == T();
   }
 };

 template <class T>
 class DefaultEmptyFn<T*> {
  public:
   void MakeEmpty(T*& item) const {
     item = nullptr;
   }
   bool IsEmpty(T* const& item) const {
     return item == nullptr;
   }
 };

 // Low memory version of a hash set, uses less memory than std::unordered_set since elements aren't
 // boxed. Uses linear probing to resolve collisions.
 // EmptyFn needs to implement two functions MakeEmpty(T& item) and IsEmpty(const T& item).
 // TODO: We could get rid of this requirement by using a bitmap, though maybe this would be slower
 // and more complicated.
 template <class T, class EmptyFn = DefaultEmptyFn<T>, class HashFn = std::hash<T>,
     class Pred = std::equal_to<T>, class Alloc = std::allocator<T>>
 class HashSet {
   template <class Elem, class HashSetType>
   class BaseIterator : std::iterator<std::forward_iterator_tag, Elem> {
    public:
     BaseIterator(const BaseIterator&) = default;
     BaseIterator(BaseIterator&&) = default;
     BaseIterator(HashSetType* hash_set, size_t index) : index_(index), hash_set_(hash_set) {
     }
     BaseIterator& operator=(const BaseIterator&) = default;
     BaseIterator& operator=(BaseIterator&&) = default;

     bool operator==(const BaseIterator& other) const {
       return hash_set_ == other.hash_set_ && this->index_ == other.index_;
     }

     bool operator!=(const BaseIterator& other) const {
       return !(*this == other);
     }

     BaseIterator operator++() {  // Value after modification.
       this->index_ = this->NextNonEmptySlot(this->index_, hash_set_);
       return *this;
     }

     BaseIterator operator++(int) {
       BaseIterator temp = *this;
       this->index_ = this->NextNonEmptySlot(this->index_, hash_set_);
       return temp;
     }

     Elem& operator*() const {
       DCHECK(!hash_set_->IsFreeSlot(this->index_));
       return hash_set_->ElementForIndex(this->index_);
     }

     Elem* operator->() const {
       return &**this;
     }

     // TODO: Operator -- --(int)  (and use std::bidirectional_iterator_tag)

    private:
     size_t index_;
     HashSetType* hash_set_;

     size_t NextNonEmptySlot(size_t index, const HashSet* hash_set) const {
       const size_t num_buckets = hash_set->NumBuckets();
       DCHECK_LT(index, num_buckets);
       do {
         ++index;
       } while (index < num_buckets && hash_set->IsFreeSlot(index));
       return index;
     }

     friend class HashSet;
   };

  public:
   using value_type = T;
   using allocator_type = Alloc;
   using reference = T&;
   using const_reference = const T&;
   using pointer = T*;
   using const_pointer = const T*;
   using iterator = BaseIterator<T, HashSet>;
   using const_iterator = BaseIterator<const T, const HashSet>;
   using size_type = size_t;
   using difference_type = ptrdiff_t;

   static constexpr double kDefaultMinLoadFactor = 0.4;
   static constexpr double kDefaultMaxLoadFactor = 0.7;
   static constexpr size_t kMinBuckets = 1000;

   // If we don't own the data, this will create a new array which owns the data.
   void Clear() {
     DeallocateStorage();
     num_elements_ = 0;
     elements_until_expand_ = 0;
   }

   HashSet() : HashSet(kDefaultMinLoadFactor, kDefaultMaxLoadFactor) {}

   HashSet(double min_load_factor, double max_load_factor) noexcept
       : num_elements_(0u),
         num_buckets_(0u),
         elements_until_expand_(0u),
         owns_data_(false),
         data_(nullptr),
         min_load_factor_(min_load_factor),
         max_load_factor_(max_load_factor) {
     DCHECK_GT(min_load_factor, 0.0);
     DCHECK_LT(max_load_factor, 1.0);
   }

   explicit HashSet(const allocator_type& alloc) noexcept
       : allocfn_(alloc),
         hashfn_(),
         emptyfn_(),
         pred_(),
         num_elements_(0u),
         num_buckets_(0u),
         elements_until_expand_(0u),
         owns_data_(false),
         data_(nullptr),
         min_load_factor_(kDefaultMinLoadFactor),
         max_load_factor_(kDefaultMaxLoadFactor) {
   }

   HashSet(const HashSet& other) noexcept
       : allocfn_(other.allocfn_),
         hashfn_(other.hashfn_),
         emptyfn_(other.emptyfn_),
         pred_(other.pred_),
         num_elements_(other.num_elements_),
         num_buckets_(0),
         elements_until_expand_(other.elements_until_expand_),
         owns_data_(false),
         data_(nullptr),
         min_load_factor_(other.min_load_factor_),
         max_load_factor_(other.max_load_factor_) {
     AllocateStorage(other.NumBuckets());
     for (size_t i = 0; i < num_buckets_; ++i) {
       ElementForIndex(i) = other.data_[i];
     }
   }

   // noexcept required so that the move constructor is used instead of copy constructor.
   // b/27860101
   HashSet(HashSet&& other) noexcept
       : allocfn_(std::move(other.allocfn_)),
         hashfn_(std::move(other.hashfn_)),
         emptyfn_(std::move(other.emptyfn_)),
         pred_(std::move(other.pred_)),
         num_elements_(other.num_elements_),
         num_buckets_(other.num_buckets_),
         elements_until_expand_(other.elements_until_expand_),
         owns_data_(other.owns_data_),
         data_(other.data_),
         min_load_factor_(other.min_load_factor_),
         max_load_factor_(other.max_load_factor_) {
     other.num_elements_ = 0u;
     other.num_buckets_ = 0u;
     other.elements_until_expand_ = 0u;
     other.owns_data_ = false;
     other.data_ = nullptr;
   }

   // Construct from existing data.
   // Read from a block of memory, if make_copy_of_data is false, then data_ points to within the
   // passed in ptr_.
   HashSet(const uint8_t* ptr, bool make_copy_of_data, size_t* read_count) noexcept {
     uint64_t temp;
     size_t offset = 0;
     offset = ReadFromBytes(ptr, offset, &temp);
     num_elements_ = static_cast<uint64_t>(temp);
     offset = ReadFromBytes(ptr, offset, &temp);
     num_buckets_ = static_cast<uint64_t>(temp);
     CHECK_LE(num_elements_, num_buckets_);
     offset = ReadFromBytes(ptr, offset, &temp);
     elements_until_expand_ = static_cast<uint64_t>(temp);
     offset = ReadFromBytes(ptr, offset, &min_load_factor_);
     offset = ReadFromBytes(ptr, offset, &max_load_factor_);
     if (!make_copy_of_data) {
       owns_data_ = false;
       data_ = const_cast<T*>(reinterpret_cast<const T*>(ptr + offset));
       offset += sizeof(*data_) * num_buckets_;
     } else {
       AllocateStorage(num_buckets_);
       // Write elements, not that this may not be safe for cross compilation if the elements are
       // pointer sized.
       for (size_t i = 0; i < num_buckets_; ++i) {
         offset = ReadFromBytes(ptr, offset, &data_[i]);
       }
     }
     // Caller responsible for aligning.
     *read_count = offset;
   }

   // Returns how large the table is after being written. If target is null, then no writing happens
   // but the size is still returned. Target must be 8 byte aligned.
   size_t WriteToMemory(uint8_t* ptr) const {
     size_t offset = 0;
     offset = WriteToBytes(ptr, offset, static_cast<uint64_t>(num_elements_));
     offset = WriteToBytes(ptr, offset, static_cast<uint64_t>(num_buckets_));
     offset = WriteToBytes(ptr, offset, static_cast<uint64_t>(elements_until_expand_));
     offset = WriteToBytes(ptr, offset, min_load_factor_);
     offset = WriteToBytes(ptr, offset, max_load_factor_);
     // Write elements, not that this may not be safe for cross compilation if the elements are
     // pointer sized.
     for (size_t i = 0; i < num_buckets_; ++i) {
       offset = WriteToBytes(ptr, offset, data_[i]);
     }
     // Caller responsible for aligning.
     return offset;
   }

   ~HashSet() {
     DeallocateStorage();
   }

   HashSet& operator=(HashSet&& other) noexcept {
     HashSet(std::move(other)).swap(*this);  // NOLINT [runtime/explicit] [5]
     return *this;
   }

   HashSet& operator=(const HashSet& other) noexcept {
     HashSet(other).swap(*this);  // NOLINT(runtime/explicit) - a case of lint gone mad.
     return *this;
   }

   // Lower case for c++11 for each.
   iterator begin() {
     iterator ret(this, 0);
     if (num_buckets_ != 0 && IsFreeSlot(ret.index_)) {
       ++ret;  // Skip all the empty slots.
     }
     return ret;
   }

   // Lower case for c++11 for each. const version.
   const_iterator begin() const {
     const_iterator ret(this, 0);
     if (num_buckets_ != 0 && IsFreeSlot(ret.index_)) {
       ++ret;  // Skip all the empty slots.
     }
     return ret;
   }

   // Lower case for c++11 for each.
   iterator end() {
     return iterator(this, NumBuckets());
   }

   // Lower case for c++11 for each. const version.
   const_iterator end() const {
     return const_iterator(this, NumBuckets());
   }

   bool Empty() const {
     return Size() == 0;
   }

   // Return true if the hash set has ownership of the underlying data.
   bool OwnsData() const {
     return owns_data_;
   }

   // Erase algorithm:
   // Make an empty slot where the iterator is pointing.
   // Scan forwards until we hit another empty slot.
   // If an element in between doesn't rehash to the range from the current empty slot to the
   // iterator. It must be before the empty slot, in that case we can move it to the empty slot
   // and set the empty slot to be the location we just moved from.
   // Relies on maintaining the invariant that there's no empty slots from the 'ideal' index of an
   // element to its actual location/index.
   iterator Erase(iterator it) {
     // empty_index is the index that will become empty.
     size_t empty_index = it.index_;
     DCHECK(!IsFreeSlot(empty_index));
     size_t next_index = empty_index;
     bool filled = false;  // True if we filled the empty index.
     while (true) {
       next_index = NextIndex(next_index);
       T& next_element = ElementForIndex(next_index);
       // If the next element is empty, we are done. Make sure to clear the current empty index.
       if (emptyfn_.IsEmpty(next_element)) {
         emptyfn_.MakeEmpty(ElementForIndex(empty_index));
         break;
       }
       // Otherwise try to see if the next element can fill the current empty index.
       const size_t next_hash = hashfn_(next_element);
       // Calculate the ideal index, if it is within empty_index + 1 to next_index then there is
       // nothing we can do.
       size_t next_ideal_index = IndexForHash(next_hash);
       // Loop around if needed for our check.
       size_t unwrapped_next_index = next_index;
       if (unwrapped_next_index < empty_index) {
         unwrapped_next_index += NumBuckets();
       }
       // Loop around if needed for our check.
       size_t unwrapped_next_ideal_index = next_ideal_index;
       if (unwrapped_next_ideal_index < empty_index) {
         unwrapped_next_ideal_index += NumBuckets();
       }
       if (unwrapped_next_ideal_index <= empty_index ||
           unwrapped_next_ideal_index > unwrapped_next_index) {
         // If the target index isn't within our current range it must have been probed from before
         // the empty index.
         ElementForIndex(empty_index) = std::move(next_element);
         filled = true;  // TODO: Optimize
         empty_index = next_index;
       }
     }
     --num_elements_;
     // If we didn't fill the slot then we need go to the next non free slot.
     if (!filled) {
       ++it;
     }
     return it;
   }

   // Find an element, returns end() if not found.
   // Allows custom key (K) types, example of when this is useful:
   // Set of Class* sorted by name, want to find a class with a name but can't allocate a dummy
   // object in the heap for performance solution.
   template <typename K>
   iterator Find(const K& key) {
     return FindWithHash(key, hashfn_(key));
   }

   template <typename K>
   const_iterator Find(const K& key) const {
     return FindWithHash(key, hashfn_(key));
   }

   template <typename K>
   iterator FindWithHash(const K& key, size_t hash) {
     return iterator(this, FindIndex(key, hash));
   }

   template <typename K>
   const_iterator FindWithHash(const K& key, size_t hash) const {
     return const_iterator(this, FindIndex(key, hash));
   }

   // Insert an element, allows duplicates.
   template <typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type>
   void Insert(U&& element) {
     InsertWithHash(std::forward<U>(element), hashfn_(element));
   }

   template <typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type>
   void InsertWithHash(U&& element, size_t hash) {
     DCHECK_EQ(hash, hashfn_(element));
     if (num_elements_ >= elements_until_expand_) {
       Expand();
       DCHECK_LT(num_elements_, elements_until_expand_);
     }
     const size_t index = FirstAvailableSlot(IndexForHash(hash));
     data_[index] = std::forward<U>(element);
     ++num_elements_;
   }

   size_t Size() const {
     return num_elements_;
   }

   void swap(HashSet& other) {
     // Use argument-dependent lookup with fall-back to std::swap() for function objects.
     using std::swap;
     swap(allocfn_, other.allocfn_);
     swap(hashfn_, other.hashfn_);
     swap(emptyfn_, other.emptyfn_);
     swap(pred_, other.pred_);
     std::swap(data_, other.data_);
     std::swap(num_buckets_, other.num_buckets_);
     std::swap(num_elements_, other.num_elements_);
     std::swap(elements_until_expand_, other.elements_until_expand_);
     std::swap(min_load_factor_, other.min_load_factor_);
     std::swap(max_load_factor_, other.max_load_factor_);
     std::swap(owns_data_, other.owns_data_);
   }

   allocator_type get_allocator() const {
     return allocfn_;
   }

   void ShrinkToMaximumLoad() {
     Resize(Size() / max_load_factor_);
   }

   // Reserve enough room to insert until Size() == num_elements without requiring to grow the hash
   // set. No-op if the hash set is already large enough to do this.
   void Reserve(size_t num_elements) {
     size_t num_buckets = num_elements / max_load_factor_;
     // Deal with rounding errors. Add one for rounding.
     while (static_cast<size_t>(num_buckets * max_load_factor_) <= num_elements + 1u) {
       ++num_buckets;
     }
     if (num_buckets > NumBuckets()) {
       Resize(num_buckets);
     }
   }

   // To distance that inserted elements were probed. Used for measuring how good hash functions
   // are.
   size_t TotalProbeDistance() const {
     size_t total = 0;
     for (size_t i = 0; i < NumBuckets(); ++i) {
       const T& element = ElementForIndex(i);
       if (!emptyfn_.IsEmpty(element)) {
         size_t ideal_location = IndexForHash(hashfn_(element));
         if (ideal_location > i) {
           total += i + NumBuckets() - ideal_location;
         } else {
           total += i - ideal_location;
         }
       }
     }
     return total;
   }

   // Calculate the current load factor and return it.
   double CalculateLoadFactor() const {
     return static_cast<double>(Size()) / static_cast<double>(NumBuckets());
   }

   // Make sure that everything reinserts in the right spot. Returns the number of errors.
   size_t Verify() NO_THREAD_SAFETY_ANALYSIS {
     size_t errors = 0;
     for (size_t i = 0; i < num_buckets_; ++i) {
       T& element = data_[i];
       if (!emptyfn_.IsEmpty(element)) {
         T temp;
         emptyfn_.MakeEmpty(temp);
         std::swap(temp, element);
         size_t first_slot = FirstAvailableSlot(IndexForHash(hashfn_(temp)));
         if (i != first_slot) {
           LOG(ERROR) << "Element " << i << " should be in slot " << first_slot;
           ++errors;
         }
         std::swap(temp, element);
       }
     }
     return errors;
   }

   double GetMinLoadFactor() const {
     return min_load_factor_;
   }

   double GetMaxLoadFactor() const {
     return max_load_factor_;
   }

   // Change the load factor of the hash set. If the current load factor is greater than the max
   // specified, then we resize the hash table storage.
   void SetLoadFactor(double min_load_factor, double max_load_factor) {
     DCHECK_LT(min_load_factor, max_load_factor);
     DCHECK_GT(min_load_factor, 0.0);
     DCHECK_LT(max_load_factor, 1.0);
     min_load_factor_ = min_load_factor;
     max_load_factor_ = max_load_factor;
     elements_until_expand_ = NumBuckets() * max_load_factor_;
     // If the current load factor isn't in the range, then resize to the mean of the minimum and
     // maximum load factor.
     const double load_factor = CalculateLoadFactor();
     if (load_factor > max_load_factor_) {
       Resize(Size() / ((min_load_factor_ + max_load_factor_) * 0.5));
     }
   }

   // The hash set expands when Size() reaches ElementsUntilExpand().
   size_t ElementsUntilExpand() const {
     return elements_until_expand_;
   }

   size_t NumBuckets() const {
     return num_buckets_;
   }

  private:
   T& ElementForIndex(size_t index) {
     DCHECK_LT(index, NumBuckets());
     DCHECK(data_ != nullptr);
     return data_[index];
   }

   const T& ElementForIndex(size_t index) const {
     DCHECK_LT(index, NumBuckets());
     DCHECK(data_ != nullptr);
     return data_[index];
   }

   size_t IndexForHash(size_t hash) const {
     // Protect against undefined behavior (division by zero).
     if (UNLIKELY(num_buckets_ == 0)) {
       return 0;
     }
     return hash % num_buckets_;
   }

   size_t NextIndex(size_t index) const {
     if (UNLIKELY(++index >= num_buckets_)) {
       DCHECK_EQ(index, NumBuckets());
       return 0;
     }
     return index;
   }

   // Find the hash table slot for an element, or return NumBuckets() if not found.
   // This value for not found is important so that iterator(this, FindIndex(...)) == end().
   template <typename K>
   size_t FindIndex(const K& element, size_t hash) const {
     // Guard against failing to get an element for a non-existing index.
     if (UNLIKELY(NumBuckets() == 0)) {
       return 0;
     }
     DCHECK_EQ(hashfn_(element), hash);
     size_t index = IndexForHash(hash);
     while (true) {
       const T& slot = ElementForIndex(index);
       if (emptyfn_.IsEmpty(slot)) {
         return NumBuckets();
       }
       if (pred_(slot, element)) {
         return index;
       }
       index = NextIndex(index);
     }
   }

   bool IsFreeSlot(size_t index) const {
     return emptyfn_.IsEmpty(ElementForIndex(index));
   }

   // Allocate a number of buckets.
   void AllocateStorage(size_t num_buckets) {
     num_buckets_ = num_buckets;
     data_ = allocfn_.allocate(num_buckets_);
     owns_data_ = true;
     for (size_t i = 0; i < num_buckets_; ++i) {
       allocfn_.construct(allocfn_.address(data_[i]));
       emptyfn_.MakeEmpty(data_[i]);
     }
   }

   void DeallocateStorage() {
     if (owns_data_) {
       for (size_t i = 0; i < NumBuckets(); ++i) {
         allocfn_.destroy(allocfn_.address(data_[i]));
       }
       if (data_ != nullptr) {
         allocfn_.deallocate(data_, NumBuckets());
       }
       owns_data_ = false;
     }
     data_ = nullptr;
     num_buckets_ = 0;
   }

   // Expand the set based on the load factors.
   void Expand() {
     size_t min_index = static_cast<size_t>(Size() / min_load_factor_);
     // Resize based on the minimum load factor.
     Resize(min_index);
   }

   // Expand / shrink the table to the new specified size.
   void Resize(size_t new_size) {
     if (new_size < kMinBuckets) {
       new_size = kMinBuckets;
     }
     DCHECK_GE(new_size, Size());
     T* const old_data = data_;
     size_t old_num_buckets = num_buckets_;
     // Reinsert all of the old elements.
     const bool owned_data = owns_data_;
     AllocateStorage(new_size);
     for (size_t i = 0; i < old_num_buckets; ++i) {
       T& element = old_data[i];
       if (!emptyfn_.IsEmpty(element)) {
         data_[FirstAvailableSlot(IndexForHash(hashfn_(element)))] = std::move(element);
       }
       if (owned_data) {
         allocfn_.destroy(allocfn_.address(element));
       }
     }
     if (owned_data) {
       allocfn_.deallocate(old_data, old_num_buckets);
     }

     // When we hit elements_until_expand_, we are at the max load factor and must expand again.
     elements_until_expand_ = NumBuckets() * max_load_factor_;
   }

   ALWAYS_INLINE size_t FirstAvailableSlot(size_t index) const {
     DCHECK_LT(index, NumBuckets());  // Don't try to get a slot out of range.
     size_t non_empty_count = 0;
     while (!emptyfn_.IsEmpty(data_[index])) {
       index = NextIndex(index);
       non_empty_count++;
       DCHECK_LE(non_empty_count, NumBuckets());  // Don't loop forever.
     }
     return index;
   }

   // Return new offset.
   template <typename Elem>
   static size_t WriteToBytes(uint8_t* ptr, size_t offset, Elem n) {
     DCHECK_ALIGNED(ptr + offset, sizeof(n));
     if (ptr != nullptr) {
       *reinterpret_cast<Elem*>(ptr + offset) = n;
     }
     return offset + sizeof(n);
   }

   template <typename Elem>
   static size_t ReadFromBytes(const uint8_t* ptr, size_t offset, Elem* out) {
     DCHECK(ptr != nullptr);
     DCHECK_ALIGNED(ptr + offset, sizeof(*out));
     *out = *reinterpret_cast<const Elem*>(ptr + offset);
     return offset + sizeof(*out);
   }

   Alloc allocfn_;  // Allocator function.
   HashFn hashfn_;  // Hashing function.
   EmptyFn emptyfn_;  // IsEmpty/SetEmpty function.
   Pred pred_;  // Equals function.
   size_t num_elements_;  // Number of inserted elements.
   size_t num_buckets_;  // Number of hash table buckets.
   size_t elements_until_expand_;  // Maximum number of elements until we expand the table.
   bool owns_data_;  // If we own data_ and are responsible for freeing it.
   T* data_;  // Backing storage.
   double min_load_factor_;
   double max_load_factor_;

   ART_FRIEND_TEST(InternTableTest, CrossHash);
 };

 template <class T, class EmptyFn, class HashFn, class Pred, class Alloc>
 void swap(HashSet<T, EmptyFn, HashFn, Pred, Alloc>& lhs,
           HashSet<T, EmptyFn, HashFn, Pred, Alloc>& rhs) {
   lhs.swap(rhs);
 }

 }  // namespace art

 #endif  // ART_LIBARTBASE_BASE_HASH_SET_H_
	/*
	* Copyright (C) 2014 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#ifndef ART_LIBARTBASE_BASE_HASH_SET_H_
	#define ART_LIBARTBASE_BASE_HASH_SET_H_

	#include <stdint.h>

	#include <functional>
	#include <iterator>
	#include <memory>
	#include <type_traits>
	#include <utility>

	#include <android-base/logging.h>

	#include "bit_utils.h"
	#include "macros.h"

	namespace art {

	// Returns true if an item is empty.
	template <class T>
	class DefaultEmptyFn {
	public:
	void MakeEmpty(T& item) const {
	item = T();
	}
	bool IsEmpty(const T& item) const {
	return item == T();
	}
	};

	template <class T>
	class DefaultEmptyFn<T*> {
	public:
	void MakeEmpty(T*& item) const {
	item = nullptr;
	}
	bool IsEmpty(T* const& item) const {
	return item == nullptr;
	}
	};

	// Low memory version of a hash set, uses less memory than std::unordered_set since elements aren't
	// boxed. Uses linear probing to resolve collisions.
	// EmptyFn needs to implement two functions MakeEmpty(T& item) and IsEmpty(const T& item).
	// TODO: We could get rid of this requirement by using a bitmap, though maybe this would be slower
	// and more complicated.
	template <class T, class EmptyFn = DefaultEmptyFn<T>, class HashFn = std::hash<T>,
	class Pred = std::equal_to<T>, class Alloc = std::allocator<T>>
	class HashSet {
	template <class Elem, class HashSetType>
	class BaseIterator : std::iterator<std::forward_iterator_tag, Elem> {
	public:
	BaseIterator(const BaseIterator&) = default;
	BaseIterator(BaseIterator&&) = default;
	BaseIterator(HashSetType* hash_set, size_t index) : index_(index), hash_set_(hash_set) {
	}
	BaseIterator& operator=(const BaseIterator&) = default;
	BaseIterator& operator=(BaseIterator&&) = default;

	bool operator==(const BaseIterator& other) const {
	return hash_set_ == other.hash_set_ && this->index_ == other.index_;
	}

	bool operator!=(const BaseIterator& other) const {
	return !(*this == other);
	}

	BaseIterator operator++() { // Value after modification.
	this->index_ = this->NextNonEmptySlot(this->index_, hash_set_);
	return *this;
	}

	BaseIterator operator++(int) {
	BaseIterator temp = *this;
	this->index_ = this->NextNonEmptySlot(this->index_, hash_set_);
	return temp;
	}

	Elem& operator*() const {
	DCHECK(!hash_set_->IsFreeSlot(this->index_));
	return hash_set_->ElementForIndex(this->index_);
	}

	Elem* operator->() const {
	return &**this;
	}

	// TODO: Operator -- --(int) (and use std::bidirectional_iterator_tag)

	private:
	size_t index_;
	HashSetType* hash_set_;

	size_t NextNonEmptySlot(size_t index, const HashSet* hash_set) const {
	const size_t num_buckets = hash_set->NumBuckets();
	DCHECK_LT(index, num_buckets);
	do {
	++index;
	} while (index < num_buckets && hash_set->IsFreeSlot(index));
	return index;
	}

	friend class HashSet;
	};

	public:
	using value_type = T;
	using allocator_type = Alloc;
	using reference = T&;
	using const_reference = const T&;
	using pointer = T*;
	using const_pointer = const T*;
	using iterator = BaseIterator<T, HashSet>;
	using const_iterator = BaseIterator<const T, const HashSet>;
	using size_type = size_t;
	using difference_type = ptrdiff_t;

	static constexpr double kDefaultMinLoadFactor = 0.4;
	static constexpr double kDefaultMaxLoadFactor = 0.7;
	static constexpr size_t kMinBuckets = 1000;

	// If we don't own the data, this will create a new array which owns the data.
	void Clear() {
	DeallocateStorage();
	num_elements_ = 0;
	elements_until_expand_ = 0;
	}

	HashSet() : HashSet(kDefaultMinLoadFactor, kDefaultMaxLoadFactor) {}

	HashSet(double min_load_factor, double max_load_factor) noexcept
	: num_elements_(0u),
	num_buckets_(0u),
	elements_until_expand_(0u),
	owns_data_(false),
	data_(nullptr),
	min_load_factor_(min_load_factor),
	max_load_factor_(max_load_factor) {
	DCHECK_GT(min_load_factor, 0.0);
	DCHECK_LT(max_load_factor, 1.0);
	}

	explicit HashSet(const allocator_type& alloc) noexcept
	: allocfn_(alloc),
	hashfn_(),
	emptyfn_(),
	pred_(),
	num_elements_(0u),
	num_buckets_(0u),
	elements_until_expand_(0u),
	owns_data_(false),
	data_(nullptr),
	min_load_factor_(kDefaultMinLoadFactor),
	max_load_factor_(kDefaultMaxLoadFactor) {
	}

	HashSet(const HashSet& other) noexcept
	: allocfn_(other.allocfn_),
	hashfn_(other.hashfn_),
	emptyfn_(other.emptyfn_),
	pred_(other.pred_),
	num_elements_(other.num_elements_),
	num_buckets_(0),
	elements_until_expand_(other.elements_until_expand_),
	owns_data_(false),
	data_(nullptr),
	min_load_factor_(other.min_load_factor_),
	max_load_factor_(other.max_load_factor_) {
	AllocateStorage(other.NumBuckets());
	for (size_t i = 0; i < num_buckets_; ++i) {
	ElementForIndex(i) = other.data_[i];
	}
	}

	// noexcept required so that the move constructor is used instead of copy constructor.
	// b/27860101
	HashSet(HashSet&& other) noexcept
	: allocfn_(std::move(other.allocfn_)),
	hashfn_(std::move(other.hashfn_)),
	emptyfn_(std::move(other.emptyfn_)),
	pred_(std::move(other.pred_)),
	num_elements_(other.num_elements_),
	num_buckets_(other.num_buckets_),
	elements_until_expand_(other.elements_until_expand_),
	owns_data_(other.owns_data_),
	data_(other.data_),
	min_load_factor_(other.min_load_factor_),
	max_load_factor_(other.max_load_factor_) {
	other.num_elements_ = 0u;
	other.num_buckets_ = 0u;
	other.elements_until_expand_ = 0u;
	other.owns_data_ = false;
	other.data_ = nullptr;
	}

	// Construct from existing data.
	// Read from a block of memory, if make_copy_of_data is false, then data_ points to within the
	// passed in ptr_.
	HashSet(const uint8_t* ptr, bool make_copy_of_data, size_t* read_count) noexcept {
	uint64_t temp;
	size_t offset = 0;
	offset = ReadFromBytes(ptr, offset, &temp);
	num_elements_ = static_cast<uint64_t>(temp);
	offset = ReadFromBytes(ptr, offset, &temp);
	num_buckets_ = static_cast<uint64_t>(temp);
	CHECK_LE(num_elements_, num_buckets_);
	offset = ReadFromBytes(ptr, offset, &temp);
	elements_until_expand_ = static_cast<uint64_t>(temp);
	offset = ReadFromBytes(ptr, offset, &min_load_factor_);
	offset = ReadFromBytes(ptr, offset, &max_load_factor_);
	if (!make_copy_of_data) {
	owns_data_ = false;
	data_ = const_cast<T>(reinterpret_cast<const T>(ptr + offset));
	offset += sizeof(data_) num_buckets_;
	} else {
	AllocateStorage(num_buckets_);
	// Write elements, not that this may not be safe for cross compilation if the elements are
	// pointer sized.
	for (size_t i = 0; i < num_buckets_; ++i) {
	offset = ReadFromBytes(ptr, offset, &data_[i]);
	}
	}
	// Caller responsible for aligning.
	*read_count = offset;
	}

	// Returns how large the table is after being written. If target is null, then no writing happens
	// but the size is still returned. Target must be 8 byte aligned.
	size_t WriteToMemory(uint8_t* ptr) const {
	size_t offset = 0;
	offset = WriteToBytes(ptr, offset, static_cast<uint64_t>(num_elements_));
	offset = WriteToBytes(ptr, offset, static_cast<uint64_t>(num_buckets_));
	offset = WriteToBytes(ptr, offset, static_cast<uint64_t>(elements_until_expand_));
	offset = WriteToBytes(ptr, offset, min_load_factor_);
	offset = WriteToBytes(ptr, offset, max_load_factor_);
	// Write elements, not that this may not be safe for cross compilation if the elements are
	// pointer sized.
	for (size_t i = 0; i < num_buckets_; ++i) {
	offset = WriteToBytes(ptr, offset, data_[i]);
	}
	// Caller responsible for aligning.
	return offset;
	}

	~HashSet() {
	DeallocateStorage();
	}

	HashSet& operator=(HashSet&& other) noexcept {
	HashSet(std::move(other)).swap(*this); // NOLINT [runtime/explicit] [5]
	return *this;
	}

	HashSet& operator=(const HashSet& other) noexcept {
	HashSet(other).swap(*this); // NOLINT(runtime/explicit) - a case of lint gone mad.
	return *this;
	}

	// Lower case for c++11 for each.
	iterator begin() {
	iterator ret(this, 0);
	if (num_buckets_ != 0 && IsFreeSlot(ret.index_)) {
	++ret; // Skip all the empty slots.
	}
	return ret;
	}

	// Lower case for c++11 for each. const version.
	const_iterator begin() const {
	const_iterator ret(this, 0);
	if (num_buckets_ != 0 && IsFreeSlot(ret.index_)) {
	++ret; // Skip all the empty slots.
	}
	return ret;
	}

	// Lower case for c++11 for each.
	iterator end() {
	return iterator(this, NumBuckets());
	}

	// Lower case for c++11 for each. const version.
	const_iterator end() const {
	return const_iterator(this, NumBuckets());
	}

	bool Empty() const {
	return Size() == 0;
	}

	// Return true if the hash set has ownership of the underlying data.
	bool OwnsData() const {
	return owns_data_;
	}

	// Erase algorithm:
	// Make an empty slot where the iterator is pointing.
	// Scan forwards until we hit another empty slot.
	// If an element in between doesn't rehash to the range from the current empty slot to the
	// iterator. It must be before the empty slot, in that case we can move it to the empty slot
	// and set the empty slot to be the location we just moved from.
	// Relies on maintaining the invariant that there's no empty slots from the 'ideal' index of an
	// element to its actual location/index.
	iterator Erase(iterator it) {
	// empty_index is the index that will become empty.
	size_t empty_index = it.index_;
	DCHECK(!IsFreeSlot(empty_index));
	size_t next_index = empty_index;
	bool filled = false; // True if we filled the empty index.
	while (true) {
	next_index = NextIndex(next_index);
	T& next_element = ElementForIndex(next_index);
	// If the next element is empty, we are done. Make sure to clear the current empty index.
	if (emptyfn_.IsEmpty(next_element)) {
	emptyfn_.MakeEmpty(ElementForIndex(empty_index));
	break;
	}
	// Otherwise try to see if the next element can fill the current empty index.
	const size_t next_hash = hashfn_(next_element);
	// Calculate the ideal index, if it is within empty_index + 1 to next_index then there is
	// nothing we can do.
	size_t next_ideal_index = IndexForHash(next_hash);
	// Loop around if needed for our check.
	size_t unwrapped_next_index = next_index;
	if (unwrapped_next_index < empty_index) {
	unwrapped_next_index += NumBuckets();
	}
	// Loop around if needed for our check.
	size_t unwrapped_next_ideal_index = next_ideal_index;
	if (unwrapped_next_ideal_index < empty_index) {
	unwrapped_next_ideal_index += NumBuckets();
	}
	if (unwrapped_next_ideal_index <= empty_index \|\|
	unwrapped_next_ideal_index > unwrapped_next_index) {
	// If the target index isn't within our current range it must have been probed from before
	// the empty index.
	ElementForIndex(empty_index) = std::move(next_element);
	filled = true; // TODO: Optimize
	empty_index = next_index;
	}
	}
	--num_elements_;
	// If we didn't fill the slot then we need go to the next non free slot.
	if (!filled) {
	++it;
	}
	return it;
	}

	// Find an element, returns end() if not found.
	// Allows custom key (K) types, example of when this is useful:
	// Set of Class* sorted by name, want to find a class with a name but can't allocate a dummy
	// object in the heap for performance solution.
	template <typename K>
	iterator Find(const K& key) {
	return FindWithHash(key, hashfn_(key));
	}

	template <typename K>
	const_iterator Find(const K& key) const {
	return FindWithHash(key, hashfn_(key));
	}

	template <typename K>
	iterator FindWithHash(const K& key, size_t hash) {
	return iterator(this, FindIndex(key, hash));
	}

	template <typename K>
	const_iterator FindWithHash(const K& key, size_t hash) const {
	return const_iterator(this, FindIndex(key, hash));
	}

	// Insert an element, allows duplicates.
	template <typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type>
	void Insert(U&& element) {
	InsertWithHash(std::forward<U>(element), hashfn_(element));
	}

	template <typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type>
	void InsertWithHash(U&& element, size_t hash) {
	DCHECK_EQ(hash, hashfn_(element));
	if (num_elements_ >= elements_until_expand_) {
	Expand();
	DCHECK_LT(num_elements_, elements_until_expand_);
	}
	const size_t index = FirstAvailableSlot(IndexForHash(hash));
	data_[index] = std::forward<U>(element);
	++num_elements_;
	}

	size_t Size() const {
	return num_elements_;
	}

	void swap(HashSet& other) {
	// Use argument-dependent lookup with fall-back to std::swap() for function objects.
	using std::swap;
	swap(allocfn_, other.allocfn_);
	swap(hashfn_, other.hashfn_);
	swap(emptyfn_, other.emptyfn_);
	swap(pred_, other.pred_);
	std::swap(data_, other.data_);
	std::swap(num_buckets_, other.num_buckets_);
	std::swap(num_elements_, other.num_elements_);
	std::swap(elements_until_expand_, other.elements_until_expand_);
	std::swap(min_load_factor_, other.min_load_factor_);
	std::swap(max_load_factor_, other.max_load_factor_);
	std::swap(owns_data_, other.owns_data_);
	}

	allocator_type get_allocator() const {
	return allocfn_;
	}

	void ShrinkToMaximumLoad() {
	Resize(Size() / max_load_factor_);
	}

	// Reserve enough room to insert until Size() == num_elements without requiring to grow the hash
	// set. No-op if the hash set is already large enough to do this.
	void Reserve(size_t num_elements) {
	size_t num_buckets = num_elements / max_load_factor_;
	// Deal with rounding errors. Add one for rounding.
	while (static_cast<size_t>(num_buckets * max_load_factor_) <= num_elements + 1u) {
	++num_buckets;
	}
	if (num_buckets > NumBuckets()) {
	Resize(num_buckets);
	}
	}

	// To distance that inserted elements were probed. Used for measuring how good hash functions
	// are.
	size_t TotalProbeDistance() const {
	size_t total = 0;
	for (size_t i = 0; i < NumBuckets(); ++i) {
	const T& element = ElementForIndex(i);
	if (!emptyfn_.IsEmpty(element)) {
	size_t ideal_location = IndexForHash(hashfn_(element));
	if (ideal_location > i) {
	total += i + NumBuckets() - ideal_location;
	} else {
	total += i - ideal_location;
	}
	}
	}
	return total;
	}

	// Calculate the current load factor and return it.
	double CalculateLoadFactor() const {
	return static_cast<double>(Size()) / static_cast<double>(NumBuckets());
	}

	// Make sure that everything reinserts in the right spot. Returns the number of errors.
	size_t Verify() NO_THREAD_SAFETY_ANALYSIS {
	size_t errors = 0;
	for (size_t i = 0; i < num_buckets_; ++i) {
	T& element = data_[i];
	if (!emptyfn_.IsEmpty(element)) {
	T temp;
	emptyfn_.MakeEmpty(temp);
	std::swap(temp, element);
	size_t first_slot = FirstAvailableSlot(IndexForHash(hashfn_(temp)));
	if (i != first_slot) {
	LOG(ERROR) << "Element " << i << " should be in slot " << first_slot;
	++errors;
	}
	std::swap(temp, element);
	}
	}
	return errors;
	}

	double GetMinLoadFactor() const {
	return min_load_factor_;
	}

	double GetMaxLoadFactor() const {
	return max_load_factor_;
	}

	// Change the load factor of the hash set. If the current load factor is greater than the max
	// specified, then we resize the hash table storage.
	void SetLoadFactor(double min_load_factor, double max_load_factor) {
	DCHECK_LT(min_load_factor, max_load_factor);
	DCHECK_GT(min_load_factor, 0.0);
	DCHECK_LT(max_load_factor, 1.0);
	min_load_factor_ = min_load_factor;
	max_load_factor_ = max_load_factor;
	elements_until_expand_ = NumBuckets() * max_load_factor_;
	// If the current load factor isn't in the range, then resize to the mean of the minimum and
	// maximum load factor.
	const double load_factor = CalculateLoadFactor();
	if (load_factor > max_load_factor_) {
	Resize(Size() / ((min_load_factor_ + max_load_factor_) * 0.5));
	}
	}

	// The hash set expands when Size() reaches ElementsUntilExpand().
	size_t ElementsUntilExpand() const {
	return elements_until_expand_;
	}

	size_t NumBuckets() const {
	return num_buckets_;
	}

	private:
	T& ElementForIndex(size_t index) {
	DCHECK_LT(index, NumBuckets());
	DCHECK(data_ != nullptr);
	return data_[index];
	}

	const T& ElementForIndex(size_t index) const {
	DCHECK_LT(index, NumBuckets());
	DCHECK(data_ != nullptr);
	return data_[index];
	}

	size_t IndexForHash(size_t hash) const {
	// Protect against undefined behavior (division by zero).
	if (UNLIKELY(num_buckets_ == 0)) {
	return 0;
	}
	return hash % num_buckets_;
	}

	size_t NextIndex(size_t index) const {
	if (UNLIKELY(++index >= num_buckets_)) {
	DCHECK_EQ(index, NumBuckets());
	return 0;
	}
	return index;
	}

	// Find the hash table slot for an element, or return NumBuckets() if not found.
	// This value for not found is important so that iterator(this, FindIndex(...)) == end().
	template <typename K>
	size_t FindIndex(const K& element, size_t hash) const {
	// Guard against failing to get an element for a non-existing index.
	if (UNLIKELY(NumBuckets() == 0)) {
	return 0;
	}
	DCHECK_EQ(hashfn_(element), hash);
	size_t index = IndexForHash(hash);
	while (true) {
	const T& slot = ElementForIndex(index);
	if (emptyfn_.IsEmpty(slot)) {
	return NumBuckets();
	}
	if (pred_(slot, element)) {
	return index;
	}
	index = NextIndex(index);
	}
	}

	bool IsFreeSlot(size_t index) const {
	return emptyfn_.IsEmpty(ElementForIndex(index));
	}

	// Allocate a number of buckets.
	void AllocateStorage(size_t num_buckets) {
	num_buckets_ = num_buckets;
	data_ = allocfn_.allocate(num_buckets_);
	owns_data_ = true;
	for (size_t i = 0; i < num_buckets_; ++i) {
	allocfn_.construct(allocfn_.address(data_[i]));
	emptyfn_.MakeEmpty(data_[i]);
	}
	}

	void DeallocateStorage() {
	if (owns_data_) {
	for (size_t i = 0; i < NumBuckets(); ++i) {
	allocfn_.destroy(allocfn_.address(data_[i]));
	}
	if (data_ != nullptr) {
	allocfn_.deallocate(data_, NumBuckets());
	}
	owns_data_ = false;
	}
	data_ = nullptr;
	num_buckets_ = 0;
	}

	// Expand the set based on the load factors.
	void Expand() {
	size_t min_index = static_cast<size_t>(Size() / min_load_factor_);
	// Resize based on the minimum load factor.
	Resize(min_index);
	}

	// Expand / shrink the table to the new specified size.
	void Resize(size_t new_size) {
	if (new_size < kMinBuckets) {
	new_size = kMinBuckets;
	}
	DCHECK_GE(new_size, Size());
	T* const old_data = data_;
	size_t old_num_buckets = num_buckets_;
	// Reinsert all of the old elements.
	const bool owned_data = owns_data_;
	AllocateStorage(new_size);
	for (size_t i = 0; i < old_num_buckets; ++i) {
	T& element = old_data[i];
	if (!emptyfn_.IsEmpty(element)) {
	data_[FirstAvailableSlot(IndexForHash(hashfn_(element)))] = std::move(element);
	}
	if (owned_data) {
	allocfn_.destroy(allocfn_.address(element));
	}
	}
	if (owned_data) {
	allocfn_.deallocate(old_data, old_num_buckets);
	}

	// When we hit elements_until_expand_, we are at the max load factor and must expand again.
	elements_until_expand_ = NumBuckets() * max_load_factor_;
	}

	ALWAYS_INLINE size_t FirstAvailableSlot(size_t index) const {
	DCHECK_LT(index, NumBuckets()); // Don't try to get a slot out of range.
	size_t non_empty_count = 0;
	while (!emptyfn_.IsEmpty(data_[index])) {
	index = NextIndex(index);
	non_empty_count++;
	DCHECK_LE(non_empty_count, NumBuckets()); // Don't loop forever.
	}
	return index;
	}

	// Return new offset.
	template <typename Elem>
	static size_t WriteToBytes(uint8_t* ptr, size_t offset, Elem n) {
	DCHECK_ALIGNED(ptr + offset, sizeof(n));
	if (ptr != nullptr) {
	reinterpret_cast<Elem>(ptr + offset) = n;
	}
	return offset + sizeof(n);
	}

	template <typename Elem>
	static size_t ReadFromBytes(const uint8_t* ptr, size_t offset, Elem* out) {
	DCHECK(ptr != nullptr);
	DCHECK_ALIGNED(ptr + offset, sizeof(*out));
	out = reinterpret_cast<const Elem*>(ptr + offset);
	return offset + sizeof(*out);
	}

	Alloc allocfn_; // Allocator function.
	HashFn hashfn_; // Hashing function.
	EmptyFn emptyfn_; // IsEmpty/SetEmpty function.
	Pred pred_; // Equals function.
	size_t num_elements_; // Number of inserted elements.
	size_t num_buckets_; // Number of hash table buckets.
	size_t elements_until_expand_; // Maximum number of elements until we expand the table.
	bool owns_data_; // If we own data_ and are responsible for freeing it.
	T* data_; // Backing storage.
	double min_load_factor_;
	double max_load_factor_;

	ART_FRIEND_TEST(InternTableTest, CrossHash);
	};

	template <class T, class EmptyFn, class HashFn, class Pred, class Alloc>
	void swap(HashSet<T, EmptyFn, HashFn, Pred, Alloc>& lhs,
	HashSet<T, EmptyFn, HashFn, Pred, Alloc>& rhs) {
	lhs.swap(rhs);
	}

	} // namespace art

	#endif // ART_LIBARTBASE_BASE_HASH_SET_H_