/* SPDX-License-Identifier: GPL-2.0-or-later */

#pragma once

/** \file
 * \ingroup bli
 *
 * A `blender::Set<Key>` is an unordered container for unique elements of type `Key`. It is
 * designed to be a more convenient and efficient replacement for `std::unordered_set`. All core
 * operations (add, remove and contains) can be done in O(1) amortized expected time.
 *
 * In most cases, your default choice for a hash set in Blender should be `blender::Set`.
 *
 * blender::Set is implemented using open addressing in a slot array with a power-of-two size.
 * Every slot is in one of three states: empty, occupied or removed. If a slot is occupied, it
 * contains an instance of the key type.
 *
 * Bench-marking and comparing hash tables is hard, because many factors influence the result. The
 * performance of a hash table depends on the combination of the hash function, probing strategy,
 * max load factor, key type, slot type and the data distribution. This implementation is designed
 * to be relatively fast by default in all cases. However, it also offers many customization
 * points that allow it to be optimized for a specific use case.
 *
 * A rudimentary benchmark can be found in BLI_set_test.cc. The results of that benchmark are
 * there as well. The numbers show that in this specific case blender::Set outperforms
 * std::unordered_set consistently by a good amount.
 *
 * Some noteworthy information:
 * - Key must be a movable type.
 * - Pointers to keys might be invalidated when the set is changed or moved.
 * - The hash function can be customized. See BLI_hash.hh for details.
 * - The probing strategy can be customized. See BLI_probing_stragies.hh for details.
 * - The slot type can be customized. See BLI_set_slots.hh for details.
 * - Small buffer optimization is enabled by default, if the key is not too large.
 * - The methods `add_new` and `remove_contained` should be used instead of `add` and `remove`
 *   whenever appropriate. Assumptions and intention are described better this way.
 * - Lookups can be performed using types other than Key without conversion. For that use the
 *   methods ending with `_as`. The template parameters Hash and #IsEqual have to support the other
 *   key type. This can greatly improve performance when the set contains strings.
 * - The default constructor is cheap, even when a large #InlineBufferCapacity is used. A large
 *   slot array will only be initialized when the first key is added.
 * - The `print_stats` method can be used to get information about the distribution of keys and
 *   memory usage of the set.
 * - The method names don't follow the std::unordered_set names in many cases. Searching for such
 *   names in this file will usually let you discover the new name.
 * - There is a #StdUnorderedSetWrapper class, that wraps std::unordered_set and gives it the same
 *   interface as blender::Set. This is useful for bench-marking.
 *
 * Possible Improvements:
 * - Use a branch-less loop over slots in grow function (measured ~10% performance improvement when
 *   the distribution of occupied slots is sufficiently random).
 * - Support max load factor customization.
 * - Improve performance with large data sets through software prefetching. I got fairly
 *   significant improvements in simple tests (~30% faster). It still needs to be investigated how
 *   to make a nice interface for this functionality.
 */

#include <unordered_set>

#include "BLI_array.hh"
#include "BLI_hash.hh"
#include "BLI_hash_tables.hh"
#include "BLI_probing_strategies.hh"
#include "BLI_set_slots.hh"

namespace blender {

template<
    /** Type of the elements that are stored in this set. It has to be movable. Furthermore, the
     * hash and is-equal functions have to support it.
     */
    typename Key,
    /**
     * The minimum number of elements that can be stored in this Set without doing a heap
     * allocation. This is useful when you expect to have many small sets. However, keep in mind
     * that (unlike vector) initializing a set has a O(n) cost in the number of slots.
     */
    int64_t InlineBufferCapacity = default_inline_buffer_capacity(sizeof(Key)),
    /**
     * The strategy used to deal with collisions. They are defined in BLI_probing_strategies.hh.
     */
    typename ProbingStrategy = DefaultProbingStrategy,
    /**
     * The hash function used to hash the keys. There is a default for many types. See BLI_hash.hh
     * for examples on how to define a custom hash function.
     */
    typename Hash = DefaultHash<Key>,
    /**
     * The equality operator used to compare keys. By default it will simply compare keys using the
     * `==` operator.
     */
    typename IsEqual = DefaultEquality,
    /**
     * This is what will actually be stored in the hash table array. At a minimum a slot has to
     * be able to hold a key and information about whether the slot is empty, occupied or removed.
     * Using a non-standard slot type can improve performance or reduce the memory footprint. For
     * example, a hash can be stored in the slot, to make inequality checks more efficient. Some
     * types have special values that can represent an empty or removed state, eliminating the need
     * for an additional variable. Also see BLI_set_slots.hh.
     */
    typename Slot = typename DefaultSetSlot<Key>::type,
    /**
     * The allocator used by this set. Should rarely be changed, except when you don't want that
     * MEM_* is used internally.
     */
    typename Allocator = GuardedAllocator>
class Set {
 public:
  class Iterator;
  using value_type = Key;
  using pointer = Key *;
  using const_pointer = const Key *;
  using reference = Key &;
  using const_reference = const Key &;
  using iterator = Iterator;
  using size_type = int64_t;

 private:
  /**
   * Slots are either empty, occupied or removed. The number of occupied slots can be computed by
   * subtracting the removed slots from the occupied-and-removed slots.
   */
  int64_t removed_slots_;
  int64_t occupied_and_removed_slots_;

  /**
   * The maximum number of slots that can be used (either occupied or removed) until the set has to
   * grow. This is the total number of slots times the max load factor.
   */
  int64_t usable_slots_;

  /**
   * The number of slots minus one. This is a bit mask that can be used to turn any integer into a
   * valid slot index efficiently.
   */
  uint64_t slot_mask_;

  /** This is called to hash incoming keys. */
  BLI_NO_UNIQUE_ADDRESS Hash hash_;

  /** This is called to check equality of two keys. */
  BLI_NO_UNIQUE_ADDRESS IsEqual is_equal_;

  /** The max load factor is 1/2 = 50% by default. */
#define LOAD_FACTOR 1, 2
  LoadFactor max_load_factor_ = LoadFactor(LOAD_FACTOR);
  using SlotArray =
      Array<Slot, LoadFactor::compute_total_slots(InlineBufferCapacity, LOAD_FACTOR), Allocator>;
#undef LOAD_FACTOR

  /**
   * This is the array that contains the actual slots. There is always at least one empty slot and
   * the size of the array is a power of two.
   */
  SlotArray slots_;

  /** Iterate over a slot index sequence for a given hash. */
#define SET_SLOT_PROBING_BEGIN(HASH, R_SLOT) \
  SLOT_PROBING_BEGIN (ProbingStrategy, HASH, slot_mask_, SLOT_INDEX) \
    auto &R_SLOT = slots_[SLOT_INDEX];
#define SET_SLOT_PROBING_END() SLOT_PROBING_END()

 public:
  /**
   * Initialize an empty set. This is a cheap operation no matter how large the inline buffer
   * is. This is necessary to avoid a high cost when no elements are added at all. An optimized
   * grow operation is performed on the first insertion.
   */
  Set(Allocator allocator = {}) noexcept
      : removed_slots_(0),
        occupied_and_removed_slots_(0),
        usable_slots_(0),
        slot_mask_(0),
        slots_(1, allocator)
  {
  }

  Set(NoExceptConstructor, Allocator allocator = {}) noexcept : Set(allocator)
  {
  }

  Set(Span<Key> values, Allocator allocator = {}) : Set(NoExceptConstructor(), allocator)
  {
    this->add_multiple(values);
  }

  /**
   * Construct a set that contains the given keys. Duplicates will be removed automatically.
   */
  Set(const std::initializer_list<Key> &values) : Set(Span<Key>(values))
  {
  }

  ~Set() = default;

  Set(const Set &other) = default;

  Set(Set &&other) noexcept(std::is_nothrow_move_constructible_v<SlotArray>)
      : Set(NoExceptConstructor(), other.slots_.allocator())

  {
    if constexpr (std::is_nothrow_move_constructible_v<SlotArray>) {
      slots_ = std::move(other.slots_);
    }
    else {
      try {
        slots_ = std::move(other.slots_);
      }
      catch (...) {
        other.noexcept_reset();
        throw;
      }
    }
    removed_slots_ = other.removed_slots_;
    occupied_and_removed_slots_ = other.occupied_and_removed_slots_;
    usable_slots_ = other.usable_slots_;
    slot_mask_ = other.slot_mask_;
    hash_ = std::move(other.hash_);
    is_equal_ = std::move(other.is_equal_);
    other.noexcept_reset();
  }

  Set &operator=(const Set &other)
  {
    return copy_assign_container(*this, other);
  }

  Set &operator=(Set &&other)
  {
    return move_assign_container(*this, std::move(other));
  }

  /**
   * Add a new key to the set. This invokes undefined behavior when the key is in the set already.
   * When you know for certain that a key is not in the set yet, use this method for better
   * performance. This also expresses the intent better.
   */
  void add_new(const Key &key)
  {
    this->add_new__impl(key, hash_(key));
  }
  void add_new(Key &&key)
  {
    this->add_new__impl(std::move(key), hash_(key));
  }

  /**
   * Add a key to the set. If the key exists in the set already, nothing is done. The return value
   * is true if the key was newly added to the set.
   *
   * This is similar to std::unordered_set::insert.
   */
  bool add(const Key &key)
  {
    return this->add_as(key);
  }
  bool add(Key &&key)
  {
    return this->add_as(std::move(key));
  }
  template<typename ForwardKey> bool add_as(ForwardKey &&key)
  {
    return this->add__impl(std::forward<ForwardKey>(key), hash_(key));
  }

  /**
   * Convenience function to add many keys to the set at once. Duplicates are removed
   * automatically.
   *
   * We might be able to make this faster than sequentially adding all keys, but that is not
   * implemented yet.
   */
  void add_multiple(Span<Key> keys)
  {
    for (const Key &key : keys) {
      this->add(key);
    }
  }

  /**
   * Convenience function to add many new keys to the set at once. The keys must not exist in the
   * set before and there must not be duplicates in the array.
   */
  void add_multiple_new(Span<Key> keys)
  {
    for (const Key &key : keys) {
      this->add_new(key);
    }
  }

  /**
   * Returns true if the key is in the set.
   *
   * This is similar to std::unordered_set::find() != std::unordered_set::end().
   */
  bool contains(const Key &key) const
  {
    return this->contains_as(key);
  }
  template<typename ForwardKey> bool contains_as(const ForwardKey &key) const
  {
    return this->contains__impl(key, hash_(key));
  }

  /**
   * Returns the key that is stored in the set that compares equal to the given key. This invokes
   * undefined behavior when the key is not in the set.
   */
  const Key &lookup_key(const Key &key) const
  {
    return this->lookup_key_as(key);
  }
  template<typename ForwardKey> const Key &lookup_key_as(const ForwardKey &key) const
  {
    return this->lookup_key__impl(key, hash_(key));
  }

  /**
   * Returns the key that is stored in the set that compares equal to the given key. If the key is
   * not in the set, the given default value is returned instead.
   */
  const Key &lookup_key_default(const Key &key, const Key &default_value) const
  {
    return this->lookup_key_default_as(key, default_value);
  }
  template<typename ForwardKey>
  const Key &lookup_key_default_as(const ForwardKey &key, const Key &default_key) const
  {
    const Key *ptr = this->lookup_key_ptr__impl(key, hash_(key));
    if (ptr == nullptr) {
      return default_key;
    }
    return *ptr;
  }

  /**
   * Returns a pointer to the key that is stored in the set that compares equal to the given key.
   * If the key is not in the set, nullptr is returned instead.
   */
  const Key *lookup_key_ptr(const Key &key) const
  {
    return this->lookup_key_ptr_as(key);
  }
  template<typename ForwardKey> const Key *lookup_key_ptr_as(const ForwardKey &key) const
  {
    return this->lookup_key_ptr__impl(key, hash_(key));
  }

  /**
   * Returns the key in the set that compares equal to the given key. If it does not exist, the key
   * is newly added.
   */
  const Key &lookup_key_or_add(const Key &key)
  {
    return this->lookup_key_or_add_as(key);
  }
  const Key &lookup_key_or_add(Key &&key)
  {
    return this->lookup_key_or_add_as(std::move(key));
  }
  template<typename ForwardKey> const Key &lookup_key_or_add_as(ForwardKey &&key)
  {
    return this->lookup_key_or_add__impl(std::forward<ForwardKey>(key), hash_(key));
  }

  /**
   * Deletes the key from the set. Returns true when the key did exist beforehand, otherwise false.
   *
   * This is similar to std::unordered_set::erase.
   */
  bool remove(const Key &key)
  {
    return this->remove_as(key);
  }
  template<typename ForwardKey> bool remove_as(const ForwardKey &key)
  {
    return this->remove__impl(key, hash_(key));
  }

  /**
   * Deletes the key from the set. This invokes undefined behavior when the key is not in the map.
   */
  void remove_contained(const Key &key)
  {
    this->remove_contained_as(key);
  }
  template<typename ForwardKey> void remove_contained_as(const ForwardKey &key)
  {
    this->remove_contained__impl(key, hash_(key));
  }

  /**
   * An iterator that can iterate over all keys in the set. The iterator is invalidated when the
   * set is moved or when it is grown.
   *
   * Keys returned by this iterator are always const. They should not change, because this might
   * also change their hash.
   */
  class Iterator {
   public:
    using iterator_category = std::forward_iterator_tag;
    using value_type = Key;
    using pointer = const Key *;
    using reference = const Key &;
    using difference_type = std::ptrdiff_t;

   private:
    const Slot *slots_;
    int64_t total_slots_;
    int64_t current_slot_;

    friend Set;

   public:
    Iterator(const Slot *slots, int64_t total_slots, int64_t current_slot)
        : slots_(slots), total_slots_(total_slots), current_slot_(current_slot)
    {
    }

    Iterator &operator++()
    {
      while (++current_slot_ < total_slots_) {
        if (slots_[current_slot_].is_occupied()) {
          break;
        }
      }
      return *this;
    }

    Iterator operator++(int)
    {
      Iterator copied_iterator = *this;
      ++(*this);
      return copied_iterator;
    }

    const Key &operator*() const
    {
      return *slots_[current_slot_].key();
    }

    const Key *operator->() const
    {
      return slots_[current_slot_].key();
    }

    friend bool operator!=(const Iterator &a, const Iterator &b)
    {
      BLI_assert(a.slots_ == b.slots_);
      BLI_assert(a.total_slots_ == b.total_slots_);
      return a.current_slot_ != b.current_slot_;
    }

    friend bool operator==(const Iterator &a, const Iterator &b)
    {
      return !(a != b);
    }

   protected:
    const Slot &current_slot() const
    {
      return slots_[current_slot_];
    }
  };

  Iterator begin() const
  {
    for (int64_t i = 0; i < slots_.size(); i++) {
      if (slots_[i].is_occupied()) {
        return Iterator(slots_.data(), slots_.size(), i);
      }
    }
    return this->end();
  }

  Iterator end() const
  {
    return Iterator(slots_.data(), slots_.size(), slots_.size());
  }

  /**
   * Remove the key that the iterator is currently pointing at. It is valid to call this method
   * while iterating over the set. However, after this method has been called, the removed element
   * must not be accessed anymore.
   */
  void remove(const Iterator &it)
  {
    /* The const cast is valid because this method itself is not const. */
    Slot &slot = const_cast<Slot &>(it.current_slot());
    BLI_assert(slot.is_occupied());
    slot.remove();
    removed_slots_++;
  }

  /**
   * Remove all values for which the given predicate is true.
   *
   * This is similar to std::erase_if.
   */
  template<typename Predicate> void remove_if(Predicate &&predicate)
  {
    for (Slot &slot : slots_) {
      if (slot.is_occupied()) {
        const Key &key = *slot.key();
        if (predicate(key)) {
          slot.remove();
          removed_slots_++;
        }
      }
    }
  }

  /**
   * Print common statistics like size and collision count. This is useful for debugging purposes.
   */
  void print_stats(StringRef name = "") const
  {
    HashTableStats stats(*this, *this);
    stats.print(name);
  }

  /**
   * Get the number of collisions that the probing strategy has to go through to find the key or
   * determine that it is not in the set.
   */
  int64_t count_collisions(const Key &key) const
  {
    return this->count_collisions__impl(key, hash_(key));
  }

  /**
   * Remove all elements from the set.
   */
  void clear()
  {
    for (Slot &slot : slots_) {
      slot.~Slot();
      new (&slot) Slot();
    }

    removed_slots_ = 0;
    occupied_and_removed_slots_ = 0;
  }

  /**
   * Creates a new slot array and reinserts all keys inside of that. This method can be used to get
   * rid of removed slots. Also this is useful for benchmarking the grow function.
   */
  void rehash()
  {
    this->realloc_and_reinsert(this->size());
  }

  /**
   * Returns the number of keys stored in the set.
   */
  int64_t size() const
  {
    return occupied_and_removed_slots_ - removed_slots_;
  }

  /**
   * Returns true if no keys are stored.
   */
  bool is_empty() const
  {
    return occupied_and_removed_slots_ == removed_slots_;
  }

  /**
   * Returns the number of available slots. This is mostly for debugging purposes.
   */
  int64_t capacity() const
  {
    return slots_.size();
  }

  /**
   * Returns the amount of removed slots in the set. This is mostly for debugging purposes.
   */
  int64_t removed_amount() const
  {
    return removed_slots_;
  }

  /**
   * Returns the bytes required per element. This is mostly for debugging purposes.
   */
  int64_t size_per_element() const
  {
    return sizeof(Slot);
  }

  /**
   * Returns the approximate memory requirements of the set in bytes. This is more correct for
   * larger sets.
   */
  int64_t size_in_bytes() const
  {
    return sizeof(Slot) * slots_.size();
  }

  /**
   * Potentially resize the set such that it can hold the specified number of keys without another
   * grow operation.
   */
  void reserve(const int64_t n)
  {
    if (usable_slots_ < n) {
      this->realloc_and_reinsert(n);
    }
  }

  /**
   * Returns true if there is a key that exists in both sets.
   */
  static bool Intersects(const Set &a, const Set &b)
  {
    /* Make sure we iterate over the shorter set. */
    if (a.size() > b.size()) {
      return Intersects(b, a);
    }

    for (const Key &key : a) {
      if (b.contains(key)) {
        return true;
      }
    }
    return false;
  }

  /**
   * Returns true if no key from a is also in b and vice versa.
   */
  static bool Disjoint(const Set &a, const Set &b)
  {
    return !Intersects(a, b);
  }

 private:
  BLI_NOINLINE void realloc_and_reinsert(const int64_t min_usable_slots)
  {
    int64_t total_slots, usable_slots;
    max_load_factor_.compute_total_and_usable_slots(
        SlotArray::inline_buffer_capacity(), min_usable_slots, &total_slots, &usable_slots);
    BLI_assert(total_slots >= 1);
    const uint64_t new_slot_mask = uint64_t(total_slots) - 1;

    /**
     * Optimize the case when the set was empty beforehand. We can avoid some copies here.
     */
    if (this->size() == 0) {
      try {
        slots_.reinitialize(total_slots);
      }
      catch (...) {
        this->noexcept_reset();
        throw;
      }
      removed_slots_ = 0;
      occupied_and_removed_slots_ = 0;
      usable_slots_ = usable_slots;
      slot_mask_ = new_slot_mask;
      return;
    }

    /* The grown array that we insert the keys into. */
    SlotArray new_slots(total_slots);

    try {
      for (Slot &slot : slots_) {
        if (slot.is_occupied()) {
          this->add_after_grow(slot, new_slots, new_slot_mask);
          slot.remove();
        }
      }
      slots_ = std::move(new_slots);
    }
    catch (...) {
      this->noexcept_reset();
      throw;
    }

    occupied_and_removed_slots_ -= removed_slots_;
    usable_slots_ = usable_slots;
    removed_slots_ = 0;
    slot_mask_ = new_slot_mask;
  }

  void add_after_grow(Slot &old_slot, SlotArray &new_slots, const uint64_t new_slot_mask)
  {
    const uint64_t hash = old_slot.get_hash(Hash());

    SLOT_PROBING_BEGIN (ProbingStrategy, hash, new_slot_mask, slot_index) {
      Slot &slot = new_slots[slot_index];
      if (slot.is_empty()) {
        slot.occupy(std::move(*old_slot.key()), hash);
        return;
      }
    }
    SLOT_PROBING_END();
  }

  /**
   * In some cases when exceptions are thrown, it's best to just reset the entire container to make
   * sure that invariants are maintained. This should happen very rarely in practice.
   */
  void noexcept_reset() noexcept
  {
    Allocator allocator = slots_.allocator();
    this->~Set();
    new (this) Set(NoExceptConstructor(), allocator);
  }

  template<typename ForwardKey>
  bool contains__impl(const ForwardKey &key, const uint64_t hash) const
  {
    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.is_empty()) {
        return false;
      }
      if (slot.contains(key, is_equal_, hash)) {
        return true;
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey>
  const Key &lookup_key__impl(const ForwardKey &key, const uint64_t hash) const
  {
    BLI_assert(this->contains_as(key));

    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.contains(key, is_equal_, hash)) {
        return *slot.key();
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey>
  const Key *lookup_key_ptr__impl(const ForwardKey &key, const uint64_t hash) const
  {
    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.contains(key, is_equal_, hash)) {
        return slot.key();
      }
      if (slot.is_empty()) {
        return nullptr;
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey> void add_new__impl(ForwardKey &&key, const uint64_t hash)
  {
    BLI_assert(!this->contains_as(key));

    this->ensure_can_add();

    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.is_empty()) {
        slot.occupy(std::forward<ForwardKey>(key), hash);
        occupied_and_removed_slots_++;
        return;
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey> bool add__impl(ForwardKey &&key, const uint64_t hash)
  {
    this->ensure_can_add();

    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.is_empty()) {
        slot.occupy(std::forward<ForwardKey>(key), hash);
        occupied_and_removed_slots_++;
        return true;
      }
      if (slot.contains(key, is_equal_, hash)) {
        return false;
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey> bool remove__impl(const ForwardKey &key, const uint64_t hash)
  {
    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.contains(key, is_equal_, hash)) {
        slot.remove();
        removed_slots_++;
        return true;
      }
      if (slot.is_empty()) {
        return false;
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey>
  void remove_contained__impl(const ForwardKey &key, const uint64_t hash)
  {
    BLI_assert(this->contains_as(key));

    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.contains(key, is_equal_, hash)) {
        slot.remove();
        removed_slots_++;
        return;
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey>
  const Key &lookup_key_or_add__impl(ForwardKey &&key, const uint64_t hash)
  {
    this->ensure_can_add();

    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.contains(key, is_equal_, hash)) {
        return *slot.key();
      }
      if (slot.is_empty()) {
        slot.occupy(std::forward<ForwardKey>(key), hash);
        occupied_and_removed_slots_++;
        return *slot.key();
      }
    }
    SET_SLOT_PROBING_END();
  }

  template<typename ForwardKey>
  int64_t count_collisions__impl(const ForwardKey &key, const uint64_t hash) const
  {
    int64_t collisions = 0;

    SET_SLOT_PROBING_BEGIN (hash, slot) {
      if (slot.contains(key, is_equal_, hash)) {
        return collisions;
      }
      if (slot.is_empty()) {
        return collisions;
      }
      collisions++;
    }
    SET_SLOT_PROBING_END();
  }

  void ensure_can_add()
  {
    if (occupied_and_removed_slots_ >= usable_slots_) {
      this->realloc_and_reinsert(this->size() + 1);
      BLI_assert(occupied_and_removed_slots_ < usable_slots_);
    }
  }
};

/**
 * A wrapper for std::unordered_set with the API of blender::Set. This can be used for
 * benchmarking.
 */
template<typename Key> class StdUnorderedSetWrapper {
 private:
  using SetType = std::unordered_set<Key, blender::DefaultHash<Key>>;
  SetType set_;

 public:
  int64_t size() const
  {
    return int64_t(set_.size());
  }

  bool is_empty() const
  {
    return set_.empty();
  }

  void reserve(int64_t n)
  {
    set_.reserve(n);
  }

  void add_new(const Key &key)
  {
    set_.insert(key);
  }
  void add_new(Key &&key)
  {
    set_.insert(std::move(key));
  }

  bool add(const Key &key)
  {
    return set_.insert(key).second;
  }
  bool add(Key &&key)
  {
    return set_.insert(std::move(key)).second;
  }

  void add_multiple(Span<Key> keys)
  {
    for (const Key &key : keys) {
      set_.insert(key);
    }
  }

  bool contains(const Key &key) const
  {
    return set_.find(key) != set_.end();
  }

  bool remove(const Key &key)
  {
    return bool(set_.erase(key));
  }

  void remove_contained(const Key &key)
  {
    return set_.erase(key);
  }

  void clear()
  {
    set_.clear();
  }

  typename SetType::iterator begin() const
  {
    return set_.begin();
  }

  typename SetType::iterator end() const
  {
    return set_.end();
  }
};

/**
 * Same as a normal Set, but does not use Blender's guarded allocator. This is useful when
 * allocating memory with static storage duration.
 */
template<typename Key,
         int64_t InlineBufferCapacity = default_inline_buffer_capacity(sizeof(Key)),
         typename ProbingStrategy = DefaultProbingStrategy,
         typename Hash = DefaultHash<Key>,
         typename IsEqual = DefaultEquality,
         typename Slot = typename DefaultSetSlot<Key>::type>
using RawSet = Set<Key, InlineBufferCapacity, ProbingStrategy, Hash, IsEqual, Slot, RawAllocator>;

}  // namespace blender