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flat_hash_set.h (22079B)

---

// 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`.
//
// `flat_hash_set` is not exception-safe.

#ifndef ABSL_CONTAINER_FLAT_HASH_SET_H_
#define ABSL_CONTAINER_FLAT_HASH_SET_H_

#include <cstddef>
#include <memory>
#include <type_traits>
#include <utility>

#include "absl/algorithm/container.h"
#include "absl/base/attributes.h"
#include "absl/base/macros.h"
#include "absl/container/hash_container_defaults.h"
#include "absl/container/internal/container_memory.h"
#include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
#include "absl/memory/memory.h"
#include "absl/meta/type_traits.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 MoveConstructible
// * Supports heterogeneous lookup, through `find()` and `insert()`, provided
//   that the set is provided a compatible heterogeneous hashing function and
//   equality operator. See below for details.
// * Invalidates any references and pointers to elements within the table after
//   `rehash()` and when the table is moved.
// * 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_set`.
// 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.
//
// Using `absl::flat_hash_set` at interface boundaries in dynamically loaded
// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
// be randomized across dynamically loaded libraries.
//
// To achieve heterogeneous lookup for custom types either `Hash` and `Eq` type
// parameters can be used or `T` should have public inner types
// `absl_container_hash` and (optionally) `absl_container_eq`. In either case,
// `typename Hash::is_transparent` and `typename Eq::is_transparent` should be
// well-formed. Both types are basically functors:
// * `Hash` should support `size_t operator()(U val) const` that returns a hash
// for the given `val`.
// * `Eq` should support `bool operator()(U lhs, V rhs) const` that returns true
// if `lhs` is equal to `rhs`.
//
// In most cases `T` needs only to provide the `absl_container_hash`. In this
// case `std::equal_to<void>` will be used instead of `eq` part.
//
// 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.
//
// PERFORMANCE WARNING: Erasure & sparsity can negatively affect performance:
//  * Iteration takes O(capacity) time, not O(size).
//  * erase() slows down begin() and ++iterator.
//  * Capacity only shrinks on rehash() or clear() -- not on erase().
//
// 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 = DefaultHashContainerHash<T>,
         class Eq = DefaultHashContainerEq<T>,
         class Allocator = std::allocator<T>>
class ABSL_ATTRIBUTE_OWNER 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_set`
 // 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_set` 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`. The special case of calling
 //   `erase(begin(), end())` resets the reserved growth such that if
 //   `reserve(N)` has previously been called and there has been no intervening
 //   call to `clear()`, then after calling `erase(begin(), end())`, it is safe
 //   to assume that inserting N elements will not cause a rehash.
 //
 // 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 set.
 //
 // 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 exchanged using unqualified calls to
 // non-member `swap()`. If the set'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 set 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 set).
 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`.
// Returns the number of erased elements.
template <typename T, typename H, typename E, typename A, typename Predicate>
typename flat_hash_set<T, H, E, A>::size_type erase_if(
   flat_hash_set<T, H, E, A>& c, Predicate pred) {
 return container_internal::EraseIf(pred, &c);
}

// swap(flat_hash_set<>, flat_hash_set<>)
//
// Swaps the contents of two `flat_hash_set` containers.
//
// NOTE: we need to define this function template in order for
// `flat_hash_set::swap` to be called instead of `std::swap`. Even though we
// have `swap(raw_hash_set&, raw_hash_set&)` defined, that function requires a
// derived-to-base conversion, whereas `std::swap` is a function template so
// `std::swap` will be preferred by compiler.
template <typename T, typename H, typename E, typename A>
void swap(flat_hash_set<T, H, E, A>& x,
         flat_hash_set<T, H, E, A>& y) noexcept(noexcept(x.swap(y))) {
 return x.swap(y);
}

namespace container_internal {

// c_for_each_fast(flat_hash_set<>, Function)
//
// Container-based version of the <algorithm> `std::for_each()` function to
// apply a function to a container's elements.
// There is no guarantees on the order of the function calls.
// Erasure and/or insertion of elements in the function is not allowed.
template <typename T, typename H, typename E, typename A, typename Function>
decay_t<Function> c_for_each_fast(const flat_hash_set<T, H, E, A>& c,
                                 Function&& f) {
 container_internal::ForEach(f, &c);
 return f;
}
template <typename T, typename H, typename E, typename A, typename Function>
decay_t<Function> c_for_each_fast(flat_hash_set<T, H, E, A>& c, Function&& f) {
 container_internal::ForEach(f, &c);
 return f;
}
template <typename T, typename H, typename E, typename A, typename Function>
decay_t<Function> c_for_each_fast(flat_hash_set<T, H, E, A>&& c, Function&& f) {
 container_internal::ForEach(f, &c);
 return f;
}

}  // namespace container_internal

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)...);
 }

 // Return std::true_type in case destroy is trivial.
 template <class Allocator>
 static auto destroy(Allocator* alloc, slot_type* slot) {
   absl::allocator_traits<Allocator>::destroy(*alloc, slot);
   return IsDestructionTrivial<Allocator, slot_type>();
 }

 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; }

 template <class Hash>
 static constexpr HashSlotFn get_hash_slot_fn() {
   return &TypeErasedApplyToSlotFn<Hash, T>;
 }
};
}  // 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_