text: // Copyright 2018 The Abseil Authors.
  //
  // 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
  //
  //      https://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.
  //
  // -----------------------------------------------------------------------------
  // File: flat_hash_set.h
  // -----------------------------------------------------------------------------
  //
  // An `absl::flat_hash_set<T>` is an unordered associative container designed to
  // be a more efficient replacement for `std::unordered_set`. Like
  // `unordered_set`, search, insertion, and deletion of set elements can be done
  // as an `O(1)` operation. However, `flat_hash_set` (and other unordered
  // associative containers known as the collection of Abseil "Swiss tables")
  // contain other optimizations that result in both memory and computation
  // advantages.
  //
  // In most cases, your default choice for a hash set should be a set of type
  // `flat_hash_set`.
  #ifndef ABSL_CONTAINER_FLAT_HASH_SET_H_
  #define ABSL_CONTAINER_FLAT_HASH_SET_H_
  
  #include <type_traits>
  #include <utility>
  
  #include "absl/algorithm/container.h"
  #include "absl/base/macros.h"
  #include "absl/container/internal/container_memory.h"
  #include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
  #include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
  #include "absl/memory/memory.h"
  
  namespace absl {
  ABSL_NAMESPACE_BEGIN
  namespace container_internal {
  template <typename T>
  struct FlatHashSetPolicy;
  }  // namespace container_internal
  
  // -----------------------------------------------------------------------------
  // absl::flat_hash_set
  // -----------------------------------------------------------------------------
  //
  // An `absl::flat_hash_set<T>` is an unordered associative container which has
  // been optimized for both speed and memory footprint in most common use cases.
  // Its interface is similar to that of `std::unordered_set<T>` with the
  // following notable differences:
  //
  // * Requires keys that are CopyConstructible
  // * Supports heterogeneous lookup, through `find()` and `insert()`, provided
  //   that the set is provided a compatible heterogeneous hashing function and
  //   equality operator.
  // * Invalidates any references and pointers to elements within the table after
  //   `rehash()`.
  // * Contains a `capacity()` member function indicating the number of element
  //   slots (open, deleted, and empty) within the hash set.
  // * Returns `void` from the `erase(iterator)` overload.
  //
  // By default, `flat_hash_set` uses the `absl::Hash` hashing framework. All
  // fundamental and Abseil types that support the `absl::Hash` framework have a
  // compatible equality operator for comparing insertions into `flat_hash_map`.
  // If your type is not yet supported by the `absl::Hash` framework, see
  // absl/hash/hash.h for information on extending Abseil hashing to user-defined
  // types.
  //
  // NOTE: A `flat_hash_set` stores its keys directly inside its implementation
  // array to avoid memory indirection. Because a `flat_hash_set` is designed to
  // move data when rehashed, set keys will not retain pointer stability. If you
  // require pointer stability, consider using
  // `absl::flat_hash_set<std::unique_ptr<T>>`. If your type is not moveable and
  // you require pointer stability, consider `absl::node_hash_set` instead.
  //
  // Example:
  //
  //   // Create a flat hash set of three strings
  //   absl::flat_hash_set<std::string> ducks =
  //     {"huey", "dewey", "louie"};
  //
  //  // Insert a new element into the flat hash set
  //  ducks.insert("donald");
  //
  //  // Force a rehash of the flat hash set
  //  ducks.rehash(0);
  //
  //  // See if "dewey" is present
  //  if (ducks.contains("dewey")) {
  //    std::cout << "We found dewey!" << std::endl;
  //  }
  template <class T, class Hash = absl::container_internal::hash_default_hash<T>,
            class Eq = absl::container_internal::hash_default_eq<T>,
            class Allocator = std::allocator<T>>
  class flat_hash_set
      : public absl::container_internal::raw_hash_set<
            absl::container_internal::FlatHashSetPolicy<T>, Hash, Eq, Allocator> {
    using Base = typename flat_hash_set::raw_hash_set;
  
   public:
    // Constructors and Assignment Operators
    //
    // A flat_hash_set supports the same overload set as `std::unordered_map`
    // for construction and assignment:
    //
    // *  Default constructor
    //
    //    // No allocation for the table's elements is made.
    //    absl::flat_hash_set<std::string> set1;
    //
    // * Initializer List constructor
    //
    //   absl::flat_hash_set<std::string> set2 =
    //       {{"huey"}, {"dewey"}, {"louie"},};
    //
    // * Copy constructor
    //
    //   absl::flat_hash_set<std::string> set3(set2);
    //
    // * Copy assignment operator
    //
    //  // Hash functor and Comparator are copied as well
    //  absl::flat_hash_set<std::string> set4;
    //  set4 = set3;
    //
    // * Move constructor
    //
    //   // Move is guaranteed efficient
    //   absl::flat_hash_set<std::string> set5(std::move(set4));
    //
    // * Move assignment operator
    //
    //   // May be efficient if allocators are compatible
    //   absl::flat_hash_set<std::string> set6;
    //   set6 = std::move(set5);
    //
    // * Range constructor
    //
    //   std::vector<std::string> v = {"a", "b"};
    //   absl::flat_hash_set<std::string> set7(v.begin(), v.end());
    flat_hash_set() {}
    using Base::Base;
  
    // flat_hash_set::begin()
    //
    // Returns an iterator to the beginning of the `flat_hash_set`.
    using Base::begin;
  
    // flat_hash_set::cbegin()
    //
    // Returns a const iterator to the beginning of the `flat_hash_set`.
    using Base::cbegin;
  
    // flat_hash_set::cend()
    //
    // Returns a const iterator to the end of the `flat_hash_set`.
    using Base::cend;
  
    // flat_hash_set::end()
    //
    // Returns an iterator to the end of the `flat_hash_set`.
    using Base::end;
  
    // flat_hash_set::capacity()
    //
    // Returns the number of element slots (assigned, deleted, and empty)
    // available within the `flat_hash_set`.
    //
    // NOTE: this member function is particular to `absl::flat_hash_set` and is
    // not provided in the `std::unordered_map` API.
    using Base::capacity;
  
    // flat_hash_set::empty()
    //
    // Returns whether or not the `flat_hash_set` is empty.
    using Base::empty;
  
    // flat_hash_set::max_size()
    //
    // Returns the largest theoretical possible number of elements within a
    // `flat_hash_set` under current memory constraints. This value can be thought
    // of the largest value of `std::distance(begin(), end())` for a
    // `flat_hash_set<T>`.
    using Base::max_size;
  
    // flat_hash_set::size()
    //
    // Returns the number of elements currently within the `flat_hash_set`.
    using Base::size;
  
    // flat_hash_set::clear()
    //
    // Removes all elements from the `flat_hash_set`. Invalidates any references,
    // pointers, or iterators referring to contained elements.
    //
    // NOTE: this operation may shrink the underlying buffer. To avoid shrinking
    // the underlying buffer call `erase(begin(), end())`.
    using Base::clear;
  
    // flat_hash_set::erase()
    //
    // Erases elements within the `flat_hash_set`. Erasing does not trigger a
    // rehash. Overloads are listed below.
    //
    // void erase(const_iterator pos):
    //
    //   Erases the element at `position` of the `flat_hash_set`, returning
    //   `void`.
    //
    //   NOTE: returning `void` in this case is different than that of STL
    //   containers in general and `std::unordered_set` in particular (which
    //   return an iterator to the element following the erased element). If that
    //   iterator is needed, simply post increment the iterator:
    //
    //     set.erase(it++);
    //
    // iterator erase(const_iterator first, const_iterator last):
    //
    //   Erases the elements in the open interval [`first`, `last`), returning an
    //   iterator pointing to `last`.
    //
    // size_type erase(const key_type& key):
    //
    //   Erases the element with the matching key, if it exists, returning the
    //   number of elements erased (0 or 1).
    using Base::erase;
  
    // flat_hash_set::insert()
    //
    // Inserts an element of the specified value into the `flat_hash_set`,
    // returning an iterator pointing to the newly inserted element, provided that
    // an element with the given key does not already exist. If rehashing occurs
    // due to the insertion, all iterators are invalidated. Overloads are listed
    // below.
    //
    // std::pair<iterator,bool> insert(const T& value):
    //
    //   Inserts a value into the `flat_hash_set`. Returns a pair consisting of an
    //   iterator to the inserted element (or to the element that prevented the
    //   insertion) and a bool denoting whether the insertion took place.
    //
    // std::pair<iterator,bool> insert(T&& value):
    //
    //   Inserts a moveable value into the `flat_hash_set`. Returns a pair
    //   consisting of an iterator to the inserted element (or to the element that
    //   prevented the insertion) and a bool denoting whether the insertion took
    //   place.
    //
    // iterator insert(const_iterator hint, const T& value):
    // iterator insert(const_iterator hint, T&& value):
    //
    //   Inserts a value, using the position of `hint` as a non-binding suggestion
    //   for where to begin the insertion search. Returns an iterator to the
    //   inserted element, or to the existing element that prevented the
    //   insertion.
    //
    // void insert(InputIterator first, InputIterator last):
    //
    //   Inserts a range of values [`first`, `last`).
    //
    //   NOTE: Although the STL does not specify which element may be inserted if
    //   multiple keys compare equivalently, for `flat_hash_set` we guarantee the
    //   first match is inserted.
    //
    // void insert(std::initializer_list<T> ilist):
    //
    //   Inserts the elements within the initializer list `ilist`.
    //
    //   NOTE: Although the STL does not specify which element may be inserted if
    //   multiple keys compare equivalently within the initializer list, for
    //   `flat_hash_set` we guarantee the first match is inserted.
    using Base::insert;
  
    // flat_hash_set::emplace()
    //
    // Inserts an element of the specified value by constructing it in-place
    // within the `flat_hash_set`, provided that no element with the given key
    // already exists.
    //
    // The element may be constructed even if there already is an element with the
    // key in the container, in which case the newly constructed element will be
    // destroyed immediately.
    //
    // If rehashing occurs due to the insertion, all iterators are invalidated.
    using Base::emplace;
  
    // flat_hash_set::emplace_hint()
    //
    // Inserts an element of the specified value by constructing it in-place
    // within the `flat_hash_set`, using the position of `hint` as a non-binding
    // suggestion for where to begin the insertion search, and only inserts
    // provided that no element with the given key already exists.
    //
    // The element may be constructed even if there already is an element with the
    // key in the container, in which case the newly constructed element will be
    // destroyed immediately.
    //
    // If rehashing occurs due to the insertion, all iterators are invalidated.
    using Base::emplace_hint;
  
    // flat_hash_set::extract()
    //
    // Extracts the indicated element, erasing it in the process, and returns it
    // as a C++17-compatible node handle. Overloads are listed below.
    //
    // node_type extract(const_iterator position):
    //
    //   Extracts the element at the indicated position and returns a node handle
    //   owning that extracted data.
    //
    // node_type extract(const key_type& x):
    //
    //   Extracts the element with the key matching the passed key value and
    //   returns a node handle owning that extracted data. If the `flat_hash_set`
    //   does not contain an element with a matching key, this function returns an
    //   empty node handle.
    using Base::extract;
  
    // flat_hash_set::merge()
    //
    // Extracts elements from a given `source` flat hash set into this
    // `flat_hash_set`. If the destination `flat_hash_set` already contains an
    // element with an equivalent key, that element is not extracted.
    using Base::merge;
  
    // flat_hash_set::swap(flat_hash_set& other)
    //
    // Exchanges the contents of this `flat_hash_set` with those of the `other`
    // flat hash map, avoiding invocation of any move, copy, or swap operations on
    // individual elements.
    //
    // All iterators and references on the `flat_hash_set` remain valid, excepting
    // for the past-the-end iterator, which is invalidated.
    //
    // `swap()` requires that the flat hash set's hashing and key equivalence
    // functions be Swappable, and are exchaged using unqualified calls to
    // non-member `swap()`. If the map's allocator has
    // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
    // set to `true`, the allocators are also exchanged using an unqualified call
    // to non-member `swap()`; otherwise, the allocators are not swapped.
    using Base::swap;
  
    // flat_hash_set::rehash(count)
    //
    // Rehashes the `flat_hash_set`, setting the number of slots to be at least
    // the passed value. If the new number of slots increases the load factor more
    // than the current maximum load factor
    // (`count` < `size()` / `max_load_factor()`), then the new number of slots
    // will be at least `size()` / `max_load_factor()`.
    //
    // To force a rehash, pass rehash(0).
    //
    // NOTE: unlike behavior in `std::unordered_set`, references are also
    // invalidated upon a `rehash()`.
    using Base::rehash;
  
    // flat_hash_set::reserve(count)
    //
    // Sets the number of slots in the `flat_hash_set` to the number needed to
    // accommodate at least `count` total elements without exceeding the current
    // maximum load factor, and may rehash the container if needed.
    using Base::reserve;
  
    // flat_hash_set::contains()
    //
    // Determines whether an element comparing equal to the given `key` exists
    // within the `flat_hash_set`, returning `true` if so or `false` otherwise.
    using Base::contains;
  
    // flat_hash_set::count(const Key& key) const
    //
    // Returns the number of elements comparing equal to the given `key` within
    // the `flat_hash_set`. note that this function will return either `1` or `0`
    // since duplicate elements are not allowed within a `flat_hash_set`.
    using Base::count;
  
    // flat_hash_set::equal_range()
    //
    // Returns a closed range [first, last], defined by a `std::pair` of two
    // iterators, containing all elements with the passed key in the
    // `flat_hash_set`.
    using Base::equal_range;
  
    // flat_hash_set::find()
    //
    // Finds an element with the passed `key` within the `flat_hash_set`.
    using Base::find;
  
    // flat_hash_set::bucket_count()
    //
    // Returns the number of "buckets" within the `flat_hash_set`. Note that
    // because a flat hash map contains all elements within its internal storage,
    // this value simply equals the current capacity of the `flat_hash_set`.
    using Base::bucket_count;
  
    // flat_hash_set::load_factor()
    //
    // Returns the current load factor of the `flat_hash_set` (the average number
    // of slots occupied with a value within the hash map).
    using Base::load_factor;
  
    // flat_hash_set::max_load_factor()
    //
    // Manages the maximum load factor of the `flat_hash_set`. Overloads are
    // listed below.
    //
    // float flat_hash_set::max_load_factor()
    //
    //   Returns the current maximum load factor of the `flat_hash_set`.
    //
    // void flat_hash_set::max_load_factor(float ml)
    //
    //   Sets the maximum load factor of the `flat_hash_set` to the passed value.
    //
    //   NOTE: This overload is provided only for API compatibility with the STL;
    //   `flat_hash_set` will ignore any set load factor and manage its rehashing
    //   internally as an implementation detail.
    using Base::max_load_factor;
  
    // flat_hash_set::get_allocator()
    //
    // Returns the allocator function associated with this `flat_hash_set`.
    using Base::get_allocator;
  
    // flat_hash_set::hash_function()
    //
    // Returns the hashing function used to hash the keys within this
    // `flat_hash_set`.
    using Base::hash_function;
  
    // flat_hash_set::key_eq()
    //
    // Returns the function used for comparing keys equality.
    using Base::key_eq;
  };
  
  // erase_if(flat_hash_set<>, Pred)
  //
  // Erases all elements that satisfy the predicate `pred` from the container `c`.
  template <typename T, typename H, typename E, typename A, typename Predicate>
  void erase_if(flat_hash_set<T, H, E, A>& c, Predicate pred) {
    container_internal::EraseIf(pred, &c);
  }
  
  namespace container_internal {
  
  template <class T>
  struct FlatHashSetPolicy {
    using slot_type = T;
    using key_type = T;
    using init_type = T;
    using constant_iterators = std::true_type;
  
    template <class Allocator, class... Args>
    static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
      absl::allocator_traits<Allocator>::construct(*alloc, slot,
                                                   std::forward<Args>(args)...);
    }
  
    template <class Allocator>
    static void destroy(Allocator* alloc, slot_type* slot) {
      absl::allocator_traits<Allocator>::destroy(*alloc, slot);
    }
  
    template <class Allocator>
    static void transfer(Allocator* alloc, slot_type* new_slot,
                         slot_type* old_slot) {
      construct(alloc, new_slot, std::move(*old_slot));
      destroy(alloc, old_slot);
    }
  
    static T& element(slot_type* slot) { return *slot; }
  
    template <class F, class... Args>
    static decltype(absl::container_internal::DecomposeValue(
        std::declval<F>(), std::declval<Args>()...))
    apply(F&& f, Args&&... args) {
      return absl::container_internal::DecomposeValue(
          std::forward<F>(f), std::forward<Args>(args)...);
    }
  
    static size_t space_used(const T*) { return 0; }
  };
  }  // namespace container_internal
  
  namespace container_algorithm_internal {
  
  // Specialization of trait in absl/algorithm/container.h
  template <class Key, class Hash, class KeyEqual, class Allocator>
  struct IsUnorderedContainer<absl::flat_hash_set<Key, Hash, KeyEqual, Allocator>>
      : std::true_type {};
  
  }  // namespace container_algorithm_internal
  
  ABSL_NAMESPACE_END
  }  // namespace absl
  
  #endif  // ABSL_CONTAINER_FLAT_HASH_SET_H_
  