flat_hash_set.h
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1 // Copyright 2018 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 //
15 // -----------------------------------------------------------------------------
16 // File: flat_hash_set.h
17 // -----------------------------------------------------------------------------
18 //
19 // An `absl::flat_hash_set<T>` is an unordered associative container designed to
20 // be a more efficient replacement for `std::unordered_set`. Like
21 // `unordered_set`, search, insertion, and deletion of set elements can be done
22 // as an `O(1)` operation. However, `flat_hash_set` (and other unordered
23 // associative containers known as the collection of Abseil "Swiss tables")
24 // contain other optimizations that result in both memory and computation
25 // advantages.
26 //
27 // In most cases, your default choice for a hash set should be a set of type
28 // `flat_hash_set`.
29 #ifndef ABSL_CONTAINER_FLAT_HASH_SET_H_
30 #define ABSL_CONTAINER_FLAT_HASH_SET_H_
31 
32 #include <type_traits>
33 #include <utility>
34 
36 #include "absl/base/macros.h"
38 #include "absl/container/internal/hash_function_defaults.h" // IWYU pragma: export
39 #include "absl/container/internal/raw_hash_set.h" // IWYU pragma: export
40 #include "absl/memory/memory.h"
41 
42 namespace absl {
43 namespace container_internal {
44 template <typename T>
46 } // namespace container_internal
47 
48 // -----------------------------------------------------------------------------
49 // absl::flat_hash_set
50 // -----------------------------------------------------------------------------
51 //
52 // An `absl::flat_hash_set<T>` is an unordered associative container which has
53 // been optimized for both speed and memory footprint in most common use cases.
54 // Its interface is similar to that of `std::unordered_set<T>` with the
55 // following notable differences:
56 //
57 // * Requires keys that are CopyConstructible
58 // * Supports heterogeneous lookup, through `find()`, `operator[]()` and
59 // `insert()`, provided that the set is provided a compatible heterogeneous
60 // hashing function and equality operator.
61 // * Invalidates any references and pointers to elements within the table after
62 // `rehash()`.
63 // * Contains a `capacity()` member function indicating the number of element
64 // slots (open, deleted, and empty) within the hash set.
65 // * Returns `void` from the `erase(iterator)` overload.
66 //
67 // By default, `flat_hash_set` uses the `absl::Hash` hashing framework. All
68 // fundamental and Abseil types that support the `absl::Hash` framework have a
69 // compatible equality operator for comparing insertions into `flat_hash_map`.
70 // If your type is not yet supported by the `absl::Hash` framework, see
71 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
72 // types.
73 //
74 // NOTE: A `flat_hash_set` stores its keys directly inside its implementation
75 // array to avoid memory indirection. Because a `flat_hash_set` is designed to
76 // move data when rehashed, set keys will not retain pointer stability. If you
77 // require pointer stability, consider using
78 // `absl::flat_hash_set<std::unique_ptr<T>>`. If your type is not moveable and
79 // you require pointer stability, consider `absl::node_hash_set` instead.
80 //
81 // Example:
82 //
83 // // Create a flat hash set of three strings
84 // absl::flat_hash_set<std::string> ducks =
85 // {"huey", "dewey", "louie"};
86 //
87 // // Insert a new element into the flat hash set
88 // ducks.insert("donald");
89 //
90 // // Force a rehash of the flat hash set
91 // ducks.rehash(0);
92 //
93 // // See if "dewey" is present
94 // if (ducks.contains("dewey")) {
95 // std::cout << "We found dewey!" << std::endl;
96 // }
97 template <class T, class Hash = absl::container_internal::hash_default_hash<T>,
98  class Eq = absl::container_internal::hash_default_eq<T>,
99  class Allocator = std::allocator<T>>
102  absl::container_internal::FlatHashSetPolicy<T>, Hash, Eq, Allocator> {
104 
105  public:
106  // Constructors and Assignment Operators
107  //
108  // A flat_hash_set supports the same overload set as `std::unordered_map`
109  // for construction and assignment:
110  //
111  // * Default constructor
112  //
113  // // No allocation for the table's elements is made.
114  // absl::flat_hash_set<std::string> set1;
115  //
116  // * Initializer List constructor
117  //
118  // absl::flat_hash_set<std::string> set2 =
119  // {{"huey"}, {"dewey"}, {"louie"},};
120  //
121  // * Copy constructor
122  //
123  // absl::flat_hash_set<std::string> set3(set2);
124  //
125  // * Copy assignment operator
126  //
127  // // Hash functor and Comparator are copied as well
128  // absl::flat_hash_set<std::string> set4;
129  // set4 = set3;
130  //
131  // * Move constructor
132  //
133  // // Move is guaranteed efficient
134  // absl::flat_hash_set<std::string> set5(std::move(set4));
135  //
136  // * Move assignment operator
137  //
138  // // May be efficient if allocators are compatible
139  // absl::flat_hash_set<std::string> set6;
140  // set6 = std::move(set5);
141  //
142  // * Range constructor
143  //
144  // std::vector<std::string> v = {"a", "b"};
145  // absl::flat_hash_set<std::string> set7(v.begin(), v.end());
147  using Base::Base;
148 
149  // flat_hash_set::begin()
150  //
151  // Returns an iterator to the beginning of the `flat_hash_set`.
152  using Base::begin;
153 
154  // flat_hash_set::cbegin()
155  //
156  // Returns a const iterator to the beginning of the `flat_hash_set`.
157  using Base::cbegin;
158 
159  // flat_hash_set::cend()
160  //
161  // Returns a const iterator to the end of the `flat_hash_set`.
162  using Base::cend;
163 
164  // flat_hash_set::end()
165  //
166  // Returns an iterator to the end of the `flat_hash_set`.
167  using Base::end;
168 
169  // flat_hash_set::capacity()
170  //
171  // Returns the number of element slots (assigned, deleted, and empty)
172  // available within the `flat_hash_set`.
173  //
174  // NOTE: this member function is particular to `absl::flat_hash_set` and is
175  // not provided in the `std::unordered_map` API.
176  using Base::capacity;
177 
178  // flat_hash_set::empty()
179  //
180  // Returns whether or not the `flat_hash_set` is empty.
181  using Base::empty;
182 
183  // flat_hash_set::max_size()
184  //
185  // Returns the largest theoretical possible number of elements within a
186  // `flat_hash_set` under current memory constraints. This value can be thought
187  // of the largest value of `std::distance(begin(), end())` for a
188  // `flat_hash_set<T>`.
189  using Base::max_size;
190 
191  // flat_hash_set::size()
192  //
193  // Returns the number of elements currently within the `flat_hash_set`.
194  using Base::size;
195 
196  // flat_hash_set::clear()
197  //
198  // Removes all elements from the `flat_hash_set`. Invalidates any references,
199  // pointers, or iterators referring to contained elements.
200  //
201  // NOTE: this operation may shrink the underlying buffer. To avoid shrinking
202  // the underlying buffer call `erase(begin(), end())`.
203  using Base::clear;
204 
205  // flat_hash_set::erase()
206  //
207  // Erases elements within the `flat_hash_set`. Erasing does not trigger a
208  // rehash. Overloads are listed below.
209  //
210  // void erase(const_iterator pos):
211  //
212  // Erases the element at `position` of the `flat_hash_set`, returning
213  // `void`.
214  //
215  // NOTE: returning `void` in this case is different than that of STL
216  // containers in general and `std::unordered_set` in particular (which
217  // return an iterator to the element following the erased element). If that
218  // iterator is needed, simply post increment the iterator:
219  //
220  // set.erase(it++);
221  //
222  // iterator erase(const_iterator first, const_iterator last):
223  //
224  // Erases the elements in the open interval [`first`, `last`), returning an
225  // iterator pointing to `last`.
226  //
227  // size_type erase(const key_type& key):
228  //
229  // Erases the element with the matching key, if it exists.
230  using Base::erase;
231 
232  // flat_hash_set::insert()
233  //
234  // Inserts an element of the specified value into the `flat_hash_set`,
235  // returning an iterator pointing to the newly inserted element, provided that
236  // an element with the given key does not already exist. If rehashing occurs
237  // due to the insertion, all iterators are invalidated. Overloads are listed
238  // below.
239  //
240  // std::pair<iterator,bool> insert(const T& value):
241  //
242  // Inserts a value into the `flat_hash_set`. Returns a pair consisting of an
243  // iterator to the inserted element (or to the element that prevented the
244  // insertion) and a bool denoting whether the insertion took place.
245  //
246  // std::pair<iterator,bool> insert(T&& value):
247  //
248  // Inserts a moveable value into the `flat_hash_set`. Returns a pair
249  // consisting of an iterator to the inserted element (or to the element that
250  // prevented the insertion) and a bool denoting whether the insertion took
251  // place.
252  //
253  // iterator insert(const_iterator hint, const T& value):
254  // iterator insert(const_iterator hint, T&& value):
255  //
256  // Inserts a value, using the position of `hint` as a non-binding suggestion
257  // for where to begin the insertion search. Returns an iterator to the
258  // inserted element, or to the existing element that prevented the
259  // insertion.
260  //
261  // void insert(InputIterator first, InputIterator last):
262  //
263  // Inserts a range of values [`first`, `last`).
264  //
265  // NOTE: Although the STL does not specify which element may be inserted if
266  // multiple keys compare equivalently, for `flat_hash_set` we guarantee the
267  // first match is inserted.
268  //
269  // void insert(std::initializer_list<T> ilist):
270  //
271  // Inserts the elements within the initializer list `ilist`.
272  //
273  // NOTE: Although the STL does not specify which element may be inserted if
274  // multiple keys compare equivalently within the initializer list, for
275  // `flat_hash_set` we guarantee the first match is inserted.
276  using Base::insert;
277 
278  // flat_hash_set::emplace()
279  //
280  // Inserts an element of the specified value by constructing it in-place
281  // within the `flat_hash_set`, provided that no element with the given key
282  // already exists.
283  //
284  // The element may be constructed even if there already is an element with the
285  // key in the container, in which case the newly constructed element will be
286  // destroyed immediately.
287  //
288  // If rehashing occurs due to the insertion, all iterators are invalidated.
289  using Base::emplace;
290 
291  // flat_hash_set::emplace_hint()
292  //
293  // Inserts an element of the specified value by constructing it in-place
294  // within the `flat_hash_set`, using the position of `hint` as a non-binding
295  // suggestion for where to begin the insertion search, and only inserts
296  // provided that no element with the given key already exists.
297  //
298  // The element may be constructed even if there already is an element with the
299  // key in the container, in which case the newly constructed element will be
300  // destroyed immediately.
301  //
302  // If rehashing occurs due to the insertion, all iterators are invalidated.
303  using Base::emplace_hint;
304 
305  // flat_hash_set::extract()
306  //
307  // Extracts the indicated element, erasing it in the process, and returns it
308  // as a C++17-compatible node handle. Overloads are listed below.
309  //
310  // node_type extract(const_iterator position):
311  //
312  // Extracts the element at the indicated position and returns a node handle
313  // owning that extracted data.
314  //
315  // node_type extract(const key_type& x):
316  //
317  // Extracts the element with the key matching the passed key value and
318  // returns a node handle owning that extracted data. If the `flat_hash_set`
319  // does not contain an element with a matching key, this function returns an
320  // empty node handle.
321  using Base::extract;
322 
323  // flat_hash_set::merge()
324  //
325  // Extracts elements from a given `source` flat hash map into this
326  // `flat_hash_set`. If the destination `flat_hash_set` already contains an
327  // element with an equivalent key, that element is not extracted.
328  using Base::merge;
329 
330  // flat_hash_set::swap(flat_hash_set& other)
331  //
332  // Exchanges the contents of this `flat_hash_set` with those of the `other`
333  // flat hash map, avoiding invocation of any move, copy, or swap operations on
334  // individual elements.
335  //
336  // All iterators and references on the `flat_hash_set` remain valid, excepting
337  // for the past-the-end iterator, which is invalidated.
338  //
339  // `swap()` requires that the flat hash set's hashing and key equivalence
340  // functions be Swappable, and are exchaged using unqualified calls to
341  // non-member `swap()`. If the map's allocator has
342  // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
343  // set to `true`, the allocators are also exchanged using an unqualified call
344  // to non-member `swap()`; otherwise, the allocators are not swapped.
345  using Base::swap;
346 
347  // flat_hash_set::rehash(count)
348  //
349  // Rehashes the `flat_hash_set`, setting the number of slots to be at least
350  // the passed value. If the new number of slots increases the load factor more
351  // than the current maximum load factor
352  // (`count` < `size()` / `max_load_factor()`), then the new number of slots
353  // will be at least `size()` / `max_load_factor()`.
354  //
355  // To force a rehash, pass rehash(0).
356  //
357  // NOTE: unlike behavior in `std::unordered_set`, references are also
358  // invalidated upon a `rehash()`.
359  using Base::rehash;
360 
361  // flat_hash_set::reserve(count)
362  //
363  // Sets the number of slots in the `flat_hash_set` to the number needed to
364  // accommodate at least `count` total elements without exceeding the current
365  // maximum load factor, and may rehash the container if needed.
366  using Base::reserve;
367 
368  // flat_hash_set::contains()
369  //
370  // Determines whether an element comparing equal to the given `key` exists
371  // within the `flat_hash_set`, returning `true` if so or `false` otherwise.
372  using Base::contains;
373 
374  // flat_hash_set::count(const Key& key) const
375  //
376  // Returns the number of elements comparing equal to the given `key` within
377  // the `flat_hash_set`. note that this function will return either `1` or `0`
378  // since duplicate elements are not allowed within a `flat_hash_set`.
379  using Base::count;
380 
381  // flat_hash_set::equal_range()
382  //
383  // Returns a closed range [first, last], defined by a `std::pair` of two
384  // iterators, containing all elements with the passed key in the
385  // `flat_hash_set`.
386  using Base::equal_range;
387 
388  // flat_hash_set::find()
389  //
390  // Finds an element with the passed `key` within the `flat_hash_set`.
391  using Base::find;
392 
393  // flat_hash_set::bucket_count()
394  //
395  // Returns the number of "buckets" within the `flat_hash_set`. Note that
396  // because a flat hash map contains all elements within its internal storage,
397  // this value simply equals the current capacity of the `flat_hash_set`.
398  using Base::bucket_count;
399 
400  // flat_hash_set::load_factor()
401  //
402  // Returns the current load factor of the `flat_hash_set` (the average number
403  // of slots occupied with a value within the hash map).
404  using Base::load_factor;
405 
406  // flat_hash_set::max_load_factor()
407  //
408  // Manages the maximum load factor of the `flat_hash_set`. Overloads are
409  // listed below.
410  //
411  // float flat_hash_set::max_load_factor()
412  //
413  // Returns the current maximum load factor of the `flat_hash_set`.
414  //
415  // void flat_hash_set::max_load_factor(float ml)
416  //
417  // Sets the maximum load factor of the `flat_hash_set` to the passed value.
418  //
419  // NOTE: This overload is provided only for API compatibility with the STL;
420  // `flat_hash_set` will ignore any set load factor and manage its rehashing
421  // internally as an implementation detail.
422  using Base::max_load_factor;
423 
424  // flat_hash_set::get_allocator()
425  //
426  // Returns the allocator function associated with this `flat_hash_set`.
427  using Base::get_allocator;
428 
429  // flat_hash_set::hash_function()
430  //
431  // Returns the hashing function used to hash the keys within this
432  // `flat_hash_set`.
433  using Base::hash_function;
434 
435  // flat_hash_set::key_eq()
436  //
437  // Returns the function used for comparing keys equality.
438  using Base::key_eq;
439 };
440 
441 namespace container_internal {
442 
443 template <class T>
444 struct FlatHashSetPolicy {
445  using slot_type = T;
446  using key_type = T;
447  using init_type = T;
448  using constant_iterators = std::true_type;
449 
450  template <class Allocator, class... Args>
451  static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
453  std::forward<Args>(args)...);
454  }
455 
456  template <class Allocator>
457  static void destroy(Allocator* alloc, slot_type* slot) {
459  }
460 
461  template <class Allocator>
462  static void transfer(Allocator* alloc, slot_type* new_slot,
463  slot_type* old_slot) {
464  construct(alloc, new_slot, std::move(*old_slot));
465  destroy(alloc, old_slot);
466  }
467 
468  static T& element(slot_type* slot) { return *slot; }
469 
470  template <class F, class... Args>
472  std::declval<F>(), std::declval<Args>()...))
473  apply(F&& f, Args&&... args) {
475  std::forward<F>(f), std::forward<Args>(args)...);
476  }
477 
478  static size_t space_used(const T*) { return 0; }
479 };
480 } // namespace container_internal
481 
482 namespace container_algorithm_internal {
483 
484 // Specialization of trait in absl/algorithm/container.h
485 template <class Key, class Hash, class KeyEqual, class Allocator>
486 struct IsUnorderedContainer<absl::flat_hash_set<Key, Hash, KeyEqual, Allocator>>
487  : std::true_type {};
488 
489 } // namespace container_algorithm_internal
490 
491 } // namespace absl
492 
493 #endif // ABSL_CONTAINER_FLAT_HASH_SET_H_
hash_default_eq< T > key_eq
char * begin
raw_hash_set() noexcept(std::is_nothrow_default_constructible< hasher >::value &&std::is_nothrow_default_constructible< key_equal >::value &&std::is_nothrow_default_constructible< allocator_type >::value)
Definition: raw_hash_set.h:707
static void destroy(Allocator *alloc, slot_type *slot)
decltype(std::declval< F >()(std::declval< const Arg & >(), std::declval< Arg >())) DecomposeValue(F &&f, Arg &&arg)
static void destroy(Alloc &a, T *p)
Definition: memory.h:542
char * end
typename flat_hash_set::raw_hash_set Base
Definition: algorithm.h:29
static std::function< void(void *, Slot *)> destroy
static void construct(Allocator *alloc, slot_type *slot, Args &&... args)
void swap(absl::InlinedVector< T, N, A > &a, absl::InlinedVector< T, N, A > &b) noexcept(noexcept(a.swap(b)))
static std::function< void(void *, Slot *, Slot)> construct
auto apply(Functor &&functor, Tuple &&t) -> decltype(utility_internal::apply_helper(absl::forward< Functor >(functor), absl::forward< Tuple >(t), absl::make_index_sequence< std::tuple_size< typename std::remove_reference< Tuple >::type >::value >
Definition: utility.h:287
uintptr_t size
static void transfer(Allocator *alloc, slot_type *new_slot, slot_type *old_slot)
std::allocator< int > alloc
constexpr absl::remove_reference_t< T > && move(T &&t) noexcept
Definition: utility.h:219
static void construct(Alloc &a, T *p, Args &&... args)
Definition: memory.h:534


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