about summary refs log blame commit diff
path: root/absl/container/internal/btree.h
blob: 707e9f0e48bfb3dcdce0764b0cde499df9a27ea7 (plain) (tree)
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013







































































                                                                                
                    










































































































































































































































































































































































































































































































































































































































































































































































































































                                                                                    

                                                               
 
                                


                 
                                


































































                                                                                
                                                         

















                                                            
                                                                              
 
                                                 



































































                                                                                

                                                                        



                                                                      
                                                               


                                         
                                                               






















































































                                                                                
                                                                           

































































                                                                                
                   













                                                                             
                                                                            











                                                             
                                                                      
            
                                                                       















                                                                               
                               
                           
                            




                                                                               
                               
                           
                            




                                                         
                                                              




























                                                                               
                                                   


                                                       
                                               



































































































                                                                                

                                                                






















































































































































































































































































































































































































































































































































































































                                                                                


















































                                                                               
                                          













                                                                             
                                                        


































































































                                                                                
                                       



















































































































































































                                                                               
                                                               




































































































































































































                                                                               

                                                                        
















                                                             


                                                                            





                                  
                  


                                           
// 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.

// A btree implementation of the STL set and map interfaces. A btree is smaller
// and generally also faster than STL set/map (refer to the benchmarks below).
// The red-black tree implementation of STL set/map has an overhead of 3
// pointers (left, right and parent) plus the node color information for each
// stored value. So a set<int32_t> consumes 40 bytes for each value stored in
// 64-bit mode. This btree implementation stores multiple values on fixed
// size nodes (usually 256 bytes) and doesn't store child pointers for leaf
// nodes. The result is that a btree_set<int32_t> may use much less memory per
// stored value. For the random insertion benchmark in btree_bench.cc, a
// btree_set<int32_t> with node-size of 256 uses 5.1 bytes per stored value.
//
// The packing of multiple values on to each node of a btree has another effect
// besides better space utilization: better cache locality due to fewer cache
// lines being accessed. Better cache locality translates into faster
// operations.
//
// CAVEATS
//
// Insertions and deletions on a btree can cause splitting, merging or
// rebalancing of btree nodes. And even without these operations, insertions
// and deletions on a btree will move values around within a node. In both
// cases, the result is that insertions and deletions can invalidate iterators
// pointing to values other than the one being inserted/deleted. Therefore, this
// container does not provide pointer stability. This is notably different from
// STL set/map which takes care to not invalidate iterators on insert/erase
// except, of course, for iterators pointing to the value being erased.  A
// partial workaround when erasing is available: erase() returns an iterator
// pointing to the item just after the one that was erased (or end() if none
// exists).

#ifndef ABSL_CONTAINER_INTERNAL_BTREE_H_
#define ABSL_CONTAINER_INTERNAL_BTREE_H_

#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <functional>
#include <iterator>
#include <limits>
#include <new>
#include <string>
#include <type_traits>
#include <utility>

#include "absl/base/macros.h"
#include "absl/container/internal/common.h"
#include "absl/container/internal/compressed_tuple.h"
#include "absl/container/internal/container_memory.h"
#include "absl/container/internal/layout.h"
#include "absl/memory/memory.h"
#include "absl/meta/type_traits.h"
#include "absl/strings/string_view.h"
#include "absl/types/compare.h"
#include "absl/utility/utility.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace container_internal {

// A helper class that indicates if the Compare parameter is a key-compare-to
// comparator.
template <typename Compare, typename T>
using btree_is_key_compare_to =
    std::is_convertible<absl::result_of_t<Compare(const T &, const T &)>,
                        absl::weak_ordering>;

struct StringBtreeDefaultLess {
  using is_transparent = void;

  StringBtreeDefaultLess() = default;

  // Compatibility constructor.
  StringBtreeDefaultLess(std::less<std::string>) {}  // NOLINT
  StringBtreeDefaultLess(std::less<string_view>) {}  // NOLINT

  absl::weak_ordering operator()(absl::string_view lhs,
                                 absl::string_view rhs) const {
    return compare_internal::compare_result_as_ordering(lhs.compare(rhs));
  }
};

struct StringBtreeDefaultGreater {
  using is_transparent = void;

  StringBtreeDefaultGreater() = default;

  StringBtreeDefaultGreater(std::greater<std::string>) {}  // NOLINT
  StringBtreeDefaultGreater(std::greater<string_view>) {}  // NOLINT

  absl::weak_ordering operator()(absl::string_view lhs,
                                 absl::string_view rhs) const {
    return compare_internal::compare_result_as_ordering(rhs.compare(lhs));
  }
};

// A helper class to convert a boolean comparison into a three-way "compare-to"
// comparison that returns a negative value to indicate less-than, zero to
// indicate equality and a positive value to indicate greater-than. This helper
// class is specialized for less<std::string>, greater<std::string>,
// less<string_view>, and greater<string_view>.
//
// key_compare_to_adapter is provided so that btree users
// automatically get the more efficient compare-to code when using common
// google string types with common comparison functors.
// These string-like specializations also turn on heterogeneous lookup by
// default.
template <typename Compare>
struct key_compare_to_adapter {
  using type = Compare;
};

template <>
struct key_compare_to_adapter<std::less<std::string>> {
  using type = StringBtreeDefaultLess;
};

template <>
struct key_compare_to_adapter<std::greater<std::string>> {
  using type = StringBtreeDefaultGreater;
};

template <>
struct key_compare_to_adapter<std::less<absl::string_view>> {
  using type = StringBtreeDefaultLess;
};

template <>
struct key_compare_to_adapter<std::greater<absl::string_view>> {
  using type = StringBtreeDefaultGreater;
};

template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
          bool Multi, typename SlotPolicy>
struct common_params {
  // If Compare is a common comparator for a std::string-like type, then we adapt it
  // to use heterogeneous lookup and to be a key-compare-to comparator.
  using key_compare = typename key_compare_to_adapter<Compare>::type;
  // A type which indicates if we have a key-compare-to functor or a plain old
  // key-compare functor.
  using is_key_compare_to = btree_is_key_compare_to<key_compare, Key>;

  using allocator_type = Alloc;
  using key_type = Key;
  using size_type = std::make_signed<size_t>::type;
  using difference_type = ptrdiff_t;

  // True if this is a multiset or multimap.
  using is_multi_container = std::integral_constant<bool, Multi>;

  using slot_policy = SlotPolicy;
  using slot_type = typename slot_policy::slot_type;
  using value_type = typename slot_policy::value_type;
  using init_type = typename slot_policy::mutable_value_type;
  using pointer = value_type *;
  using const_pointer = const value_type *;
  using reference = value_type &;
  using const_reference = const value_type &;

  enum {
    kTargetNodeSize = TargetNodeSize,

    // Upper bound for the available space for values. This is largest for leaf
    // nodes, which have overhead of at least a pointer + 4 bytes (for storing
    // 3 field_types and an enum).
    kNodeValueSpace =
        TargetNodeSize - /*minimum overhead=*/(sizeof(void *) + 4),
  };

  // This is an integral type large enough to hold as many
  // ValueSize-values as will fit a node of TargetNodeSize bytes.
  using node_count_type =
      absl::conditional_t<(kNodeValueSpace / sizeof(value_type) >
                           (std::numeric_limits<uint8_t>::max)()),
                          uint16_t, uint8_t>;  // NOLINT

  // The following methods are necessary for passing this struct as PolicyTraits
  // for node_handle and/or are used within btree.
  static value_type &element(slot_type *slot) {
    return slot_policy::element(slot);
  }
  static const value_type &element(const slot_type *slot) {
    return slot_policy::element(slot);
  }
  template <class... Args>
  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
    slot_policy::construct(alloc, slot, std::forward<Args>(args)...);
  }
  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
    slot_policy::construct(alloc, slot, other);
  }
  static void destroy(Alloc *alloc, slot_type *slot) {
    slot_policy::destroy(alloc, slot);
  }
  static void transfer(Alloc *alloc, slot_type *new_slot, slot_type *old_slot) {
    construct(alloc, new_slot, old_slot);
    destroy(alloc, old_slot);
  }
  static void swap(Alloc *alloc, slot_type *a, slot_type *b) {
    slot_policy::swap(alloc, a, b);
  }
  static void move(Alloc *alloc, slot_type *src, slot_type *dest) {
    slot_policy::move(alloc, src, dest);
  }
  static void move(Alloc *alloc, slot_type *first, slot_type *last,
                   slot_type *result) {
    slot_policy::move(alloc, first, last, result);
  }
};

// A parameters structure for holding the type parameters for a btree_map.
// Compare and Alloc should be nothrow copy-constructible.
template <typename Key, typename Data, typename Compare, typename Alloc,
          int TargetNodeSize, bool Multi>
struct map_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
                                  map_slot_policy<Key, Data>> {
  using super_type = typename map_params::common_params;
  using mapped_type = Data;
  // This type allows us to move keys when it is safe to do so. It is safe
  // for maps in which value_type and mutable_value_type are layout compatible.
  using slot_policy = typename super_type::slot_policy;
  using slot_type = typename super_type::slot_type;
  using value_type = typename super_type::value_type;
  using init_type = typename super_type::init_type;

  using key_compare = typename super_type::key_compare;
  // Inherit from key_compare for empty base class optimization.
  struct value_compare : private key_compare {
    value_compare() = default;
    explicit value_compare(const key_compare &cmp) : key_compare(cmp) {}

    template <typename T, typename U>
    auto operator()(const T &left, const U &right) const
        -> decltype(std::declval<key_compare>()(left.first, right.first)) {
      return key_compare::operator()(left.first, right.first);
    }
  };
  using is_map_container = std::true_type;

  static const Key &key(const value_type &x) { return x.first; }
  static const Key &key(const init_type &x) { return x.first; }
  static const Key &key(const slot_type *x) { return slot_policy::key(x); }
  static mapped_type &value(value_type *value) { return value->second; }
};

// This type implements the necessary functions from the
// absl::container_internal::slot_type interface.
template <typename Key>
struct set_slot_policy {
  using slot_type = Key;
  using value_type = Key;
  using mutable_value_type = Key;

  static value_type &element(slot_type *slot) { return *slot; }
  static const value_type &element(const slot_type *slot) { return *slot; }

  template <typename Alloc, class... Args>
  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
    absl::allocator_traits<Alloc>::construct(*alloc, slot,
                                             std::forward<Args>(args)...);
  }

  template <typename Alloc>
  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
    absl::allocator_traits<Alloc>::construct(*alloc, slot, std::move(*other));
  }

  template <typename Alloc>
  static void destroy(Alloc *alloc, slot_type *slot) {
    absl::allocator_traits<Alloc>::destroy(*alloc, slot);
  }

  template <typename Alloc>
  static void swap(Alloc * /*alloc*/, slot_type *a, slot_type *b) {
    using std::swap;
    swap(*a, *b);
  }

  template <typename Alloc>
  static void move(Alloc * /*alloc*/, slot_type *src, slot_type *dest) {
    *dest = std::move(*src);
  }

  template <typename Alloc>
  static void move(Alloc *alloc, slot_type *first, slot_type *last,
                   slot_type *result) {
    for (slot_type *src = first, *dest = result; src != last; ++src, ++dest)
      move(alloc, src, dest);
  }
};

// A parameters structure for holding the type parameters for a btree_set.
// Compare and Alloc should be nothrow copy-constructible.
template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
          bool Multi>
struct set_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
                                  set_slot_policy<Key>> {
  using value_type = Key;
  using slot_type = typename set_params::common_params::slot_type;
  using value_compare = typename set_params::common_params::key_compare;
  using is_map_container = std::false_type;

  static const Key &key(const value_type &x) { return x; }
  static const Key &key(const slot_type *x) { return *x; }
};

// An adapter class that converts a lower-bound compare into an upper-bound
// compare. Note: there is no need to make a version of this adapter specialized
// for key-compare-to functors because the upper-bound (the first value greater
// than the input) is never an exact match.
template <typename Compare>
struct upper_bound_adapter {
  explicit upper_bound_adapter(const Compare &c) : comp(c) {}
  template <typename K, typename LK>
  bool operator()(const K &a, const LK &b) const {
    // Returns true when a is not greater than b.
    return !compare_internal::compare_result_as_less_than(comp(b, a));
  }

 private:
  Compare comp;
};

enum class MatchKind : uint8_t { kEq, kNe };

template <typename V, bool IsCompareTo>
struct SearchResult {
  V value;
  MatchKind match;

  static constexpr bool HasMatch() { return true; }
  bool IsEq() const { return match == MatchKind::kEq; }
};

// When we don't use CompareTo, `match` is not present.
// This ensures that callers can't use it accidentally when it provides no
// useful information.
template <typename V>
struct SearchResult<V, false> {
  V value;

  static constexpr bool HasMatch() { return false; }
  static constexpr bool IsEq() { return false; }
};

// A node in the btree holding. The same node type is used for both internal
// and leaf nodes in the btree, though the nodes are allocated in such a way
// that the children array is only valid in internal nodes.
template <typename Params>
class btree_node {
  using is_key_compare_to = typename Params::is_key_compare_to;
  using is_multi_container = typename Params::is_multi_container;
  using field_type = typename Params::node_count_type;
  using allocator_type = typename Params::allocator_type;
  using slot_type = typename Params::slot_type;

 public:
  using params_type = Params;
  using key_type = typename Params::key_type;
  using value_type = typename Params::value_type;
  using pointer = typename Params::pointer;
  using const_pointer = typename Params::const_pointer;
  using reference = typename Params::reference;
  using const_reference = typename Params::const_reference;
  using key_compare = typename Params::key_compare;
  using size_type = typename Params::size_type;
  using difference_type = typename Params::difference_type;

  // Btree decides whether to use linear node search as follows:
  //   - If the key is arithmetic and the comparator is std::less or
  //     std::greater, choose linear.
  //   - Otherwise, choose binary.
  // TODO(ezb): Might make sense to add condition(s) based on node-size.
  using use_linear_search = std::integral_constant<
      bool,
                std::is_arithmetic<key_type>::value &&
                    (std::is_same<std::less<key_type>, key_compare>::value ||
                     std::is_same<std::greater<key_type>, key_compare>::value)>;

  // This class is organized by gtl::Layout as if it had the following
  // structure:
  //   // A pointer to the node's parent.
  //   btree_node *parent;
  //
  //   // The position of the node in the node's parent.
  //   field_type position;
  //   // The index of the first populated value in `values`.
  //   // TODO(ezb): right now, `start` is always 0. Update insertion/merge
  //   // logic to allow for floating storage within nodes.
  //   field_type start;
  //   // The count of the number of populated values in the node.
  //   field_type count;
  //   // The maximum number of values the node can hold. This is an integer in
  //   // [1, kNodeValues] for root leaf nodes, kNodeValues for non-root leaf
  //   // nodes, and kInternalNodeMaxCount (as a sentinel value) for internal
  //   // nodes (even though there are still kNodeValues values in the node).
  //   // TODO(ezb): make max_count use only 4 bits and record log2(capacity)
  //   // to free extra bits for is_root, etc.
  //   field_type max_count;
  //
  //   // The array of values. The capacity is `max_count` for leaf nodes and
  //   // kNodeValues for internal nodes. Only the values in
  //   // [start, start + count) have been initialized and are valid.
  //   slot_type values[max_count];
  //
  //   // The array of child pointers. The keys in children[i] are all less
  //   // than key(i). The keys in children[i + 1] are all greater than key(i).
  //   // There are 0 children for leaf nodes and kNodeValues + 1 children for
  //   // internal nodes.
  //   btree_node *children[kNodeValues + 1];
  //
  // This class is only constructed by EmptyNodeType. Normally, pointers to the
  // layout above are allocated, cast to btree_node*, and de-allocated within
  // the btree implementation.
  ~btree_node() = default;
  btree_node(btree_node const &) = delete;
  btree_node &operator=(btree_node const &) = delete;

  // Public for EmptyNodeType.
  constexpr static size_type Alignment() {
    static_assert(LeafLayout(1).Alignment() == InternalLayout().Alignment(),
                  "Alignment of all nodes must be equal.");
    return InternalLayout().Alignment();
  }

 protected:
  btree_node() = default;

 private:
  using layout_type = absl::container_internal::Layout<btree_node *, field_type,
                                                       slot_type, btree_node *>;
  constexpr static size_type SizeWithNValues(size_type n) {
    return layout_type(/*parent*/ 1,
                       /*position, start, count, max_count*/ 4,
                       /*values*/ n,
                       /*children*/ 0)
        .AllocSize();
  }
  // A lower bound for the overhead of fields other than values in a leaf node.
  constexpr static size_type MinimumOverhead() {
    return SizeWithNValues(1) - sizeof(value_type);
  }

  // Compute how many values we can fit onto a leaf node taking into account
  // padding.
  constexpr static size_type NodeTargetValues(const int begin, const int end) {
    return begin == end ? begin
                        : SizeWithNValues((begin + end) / 2 + 1) >
                                  params_type::kTargetNodeSize
                              ? NodeTargetValues(begin, (begin + end) / 2)
                              : NodeTargetValues((begin + end) / 2 + 1, end);
  }

  enum {
    kTargetNodeSize = params_type::kTargetNodeSize,
    kNodeTargetValues = NodeTargetValues(0, params_type::kTargetNodeSize),

    // We need a minimum of 3 values per internal node in order to perform
    // splitting (1 value for the two nodes involved in the split and 1 value
    // propagated to the parent as the delimiter for the split).
    kNodeValues = kNodeTargetValues >= 3 ? kNodeTargetValues : 3,

    // The node is internal (i.e. is not a leaf node) if and only if `max_count`
    // has this value.
    kInternalNodeMaxCount = 0,
  };

  // Leaves can have less than kNodeValues values.
  constexpr static layout_type LeafLayout(const int max_values = kNodeValues) {
    return layout_type(/*parent*/ 1,
                       /*position, start, count, max_count*/ 4,
                       /*values*/ max_values,
                       /*children*/ 0);
  }
  constexpr static layout_type InternalLayout() {
    return layout_type(/*parent*/ 1,
                       /*position, start, count, max_count*/ 4,
                       /*values*/ kNodeValues,
                       /*children*/ kNodeValues + 1);
  }
  constexpr static size_type LeafSize(const int max_values = kNodeValues) {
    return LeafLayout(max_values).AllocSize();
  }
  constexpr static size_type InternalSize() {
    return InternalLayout().AllocSize();
  }

  // N is the index of the type in the Layout definition.
  // ElementType<N> is the Nth type in the Layout definition.
  template <size_type N>
  inline typename layout_type::template ElementType<N> *GetField() {
    // We assert that we don't read from values that aren't there.
    assert(N < 3 || !leaf());
    return InternalLayout().template Pointer<N>(reinterpret_cast<char *>(this));
  }
  template <size_type N>
  inline const typename layout_type::template ElementType<N> *GetField() const {
    assert(N < 3 || !leaf());
    return InternalLayout().template Pointer<N>(
        reinterpret_cast<const char *>(this));
  }
  void set_parent(btree_node *p) { *GetField<0>() = p; }
  field_type &mutable_count() { return GetField<1>()[2]; }
  slot_type *slot(int i) { return &GetField<2>()[i]; }
  const slot_type *slot(int i) const { return &GetField<2>()[i]; }
  void set_position(field_type v) { GetField<1>()[0] = v; }
  void set_start(field_type v) { GetField<1>()[1] = v; }
  void set_count(field_type v) { GetField<1>()[2] = v; }
  // This method is only called by the node init methods.
  void set_max_count(field_type v) { GetField<1>()[3] = v; }

 public:
  // Whether this is a leaf node or not. This value doesn't change after the
  // node is created.
  bool leaf() const { return GetField<1>()[3] != kInternalNodeMaxCount; }

  // Getter for the position of this node in its parent.
  field_type position() const { return GetField<1>()[0]; }

  // Getter for the offset of the first value in the `values` array.
  field_type start() const { return GetField<1>()[1]; }

  // Getters for the number of values stored in this node.
  field_type count() const { return GetField<1>()[2]; }
  field_type max_count() const {
    // Internal nodes have max_count==kInternalNodeMaxCount.
    // Leaf nodes have max_count in [1, kNodeValues].
    const field_type max_count = GetField<1>()[3];
    return max_count == field_type{kInternalNodeMaxCount}
               ? field_type{kNodeValues}
               : max_count;
  }

  // Getter for the parent of this node.
  btree_node *parent() const { return *GetField<0>(); }
  // Getter for whether the node is the root of the tree. The parent of the
  // root of the tree is the leftmost node in the tree which is guaranteed to
  // be a leaf.
  bool is_root() const { return parent()->leaf(); }
  void make_root() {
    assert(parent()->is_root());
    set_parent(parent()->parent());
  }

  // Getters for the key/value at position i in the node.
  const key_type &key(int i) const { return params_type::key(slot(i)); }
  reference value(int i) { return params_type::element(slot(i)); }
  const_reference value(int i) const { return params_type::element(slot(i)); }

  // Getters/setter for the child at position i in the node.
  btree_node *child(int i) const { return GetField<3>()[i]; }
  btree_node *&mutable_child(int i) { return GetField<3>()[i]; }
  void clear_child(int i) {
    absl::container_internal::SanitizerPoisonObject(&mutable_child(i));
  }
  void set_child(int i, btree_node *c) {
    absl::container_internal::SanitizerUnpoisonObject(&mutable_child(i));
    mutable_child(i) = c;
    c->set_position(i);
  }
  void init_child(int i, btree_node *c) {
    set_child(i, c);
    c->set_parent(this);
  }

  // Returns the position of the first value whose key is not less than k.
  template <typename K>
  SearchResult<int, is_key_compare_to::value> lower_bound(
      const K &k, const key_compare &comp) const {
    return use_linear_search::value ? linear_search(k, comp)
                                    : binary_search(k, comp);
  }
  // Returns the position of the first value whose key is greater than k.
  template <typename K>
  int upper_bound(const K &k, const key_compare &comp) const {
    auto upper_compare = upper_bound_adapter<key_compare>(comp);
    return use_linear_search::value ? linear_search(k, upper_compare).value
                                    : binary_search(k, upper_compare).value;
  }

  template <typename K, typename Compare>
  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
  linear_search(const K &k, const Compare &comp) const {
    return linear_search_impl(k, 0, count(), comp,
                              btree_is_key_compare_to<Compare, key_type>());
  }

  template <typename K, typename Compare>
  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
  binary_search(const K &k, const Compare &comp) const {
    return binary_search_impl(k, 0, count(), comp,
                              btree_is_key_compare_to<Compare, key_type>());
  }

  // Returns the position of the first value whose key is not less than k using
  // linear search performed using plain compare.
  template <typename K, typename Compare>
  SearchResult<int, false> linear_search_impl(
      const K &k, int s, const int e, const Compare &comp,
      std::false_type /* IsCompareTo */) const {
    while (s < e) {
      if (!comp(key(s), k)) {
        break;
      }
      ++s;
    }
    return {s};
  }

  // Returns the position of the first value whose key is not less than k using
  // linear search performed using compare-to.
  template <typename K, typename Compare>
  SearchResult<int, true> linear_search_impl(
      const K &k, int s, const int e, const Compare &comp,
      std::true_type /* IsCompareTo */) const {
    while (s < e) {
      const absl::weak_ordering c = comp(key(s), k);
      if (c == 0) {
        return {s, MatchKind::kEq};
      } else if (c > 0) {
        break;
      }
      ++s;
    }
    return {s, MatchKind::kNe};
  }

  // Returns the position of the first value whose key is not less than k using
  // binary search performed using plain compare.
  template <typename K, typename Compare>
  SearchResult<int, false> binary_search_impl(
      const K &k, int s, int e, const Compare &comp,
      std::false_type /* IsCompareTo */) const {
    while (s != e) {
      const int mid = (s + e) >> 1;
      if (comp(key(mid), k)) {
        s = mid + 1;
      } else {
        e = mid;
      }
    }
    return {s};
  }

  // Returns the position of the first value whose key is not less than k using
  // binary search performed using compare-to.
  template <typename K, typename CompareTo>
  SearchResult<int, true> binary_search_impl(
      const K &k, int s, int e, const CompareTo &comp,
      std::true_type /* IsCompareTo */) const {
    if (is_multi_container::value) {
      MatchKind exact_match = MatchKind::kNe;
      while (s != e) {
        const int mid = (s + e) >> 1;
        const absl::weak_ordering c = comp(key(mid), k);
        if (c < 0) {
          s = mid + 1;
        } else {
          e = mid;
          if (c == 0) {
            // Need to return the first value whose key is not less than k,
            // which requires continuing the binary search if this is a
            // multi-container.
            exact_match = MatchKind::kEq;
          }
        }
      }
      return {s, exact_match};
    } else {  // Not a multi-container.
      while (s != e) {
        const int mid = (s + e) >> 1;
        const absl::weak_ordering c = comp(key(mid), k);
        if (c < 0) {
          s = mid + 1;
        } else if (c > 0) {
          e = mid;
        } else {
          return {mid, MatchKind::kEq};
        }
      }
      return {s, MatchKind::kNe};
    }
  }

  // Emplaces a value at position i, shifting all existing values and
  // children at positions >= i to the right by 1.
  template <typename... Args>
  void emplace_value(size_type i, allocator_type *alloc, Args &&... args);

  // Removes the value at position i, shifting all existing values and children
  // at positions > i to the left by 1.
  void remove_value(int i, allocator_type *alloc);

  // Removes the values at positions [i, i + to_erase), shifting all values
  // after that range to the left by to_erase. Does not change children at all.
  void remove_values_ignore_children(int i, int to_erase,
                                     allocator_type *alloc);

  // Rebalances a node with its right sibling.
  void rebalance_right_to_left(int to_move, btree_node *right,
                               allocator_type *alloc);
  void rebalance_left_to_right(int to_move, btree_node *right,
                               allocator_type *alloc);

  // Splits a node, moving a portion of the node's values to its right sibling.
  void split(int insert_position, btree_node *dest, allocator_type *alloc);

  // Merges a node with its right sibling, moving all of the values and the
  // delimiting key in the parent node onto itself.
  void merge(btree_node *sibling, allocator_type *alloc);

  // Swap the contents of "this" and "src".
  void swap(btree_node *src, allocator_type *alloc);

  // Node allocation/deletion routines.
  static btree_node *init_leaf(btree_node *n, btree_node *parent,
                               int max_count) {
    n->set_parent(parent);
    n->set_position(0);
    n->set_start(0);
    n->set_count(0);
    n->set_max_count(max_count);
    absl::container_internal::SanitizerPoisonMemoryRegion(
        n->slot(0), max_count * sizeof(slot_type));
    return n;
  }
  static btree_node *init_internal(btree_node *n, btree_node *parent) {
    init_leaf(n, parent, kNodeValues);
    // Set `max_count` to a sentinel value to indicate that this node is
    // internal.
    n->set_max_count(kInternalNodeMaxCount);
    absl::container_internal::SanitizerPoisonMemoryRegion(
        &n->mutable_child(0), (kNodeValues + 1) * sizeof(btree_node *));
    return n;
  }
  void destroy(allocator_type *alloc) {
    for (int i = 0; i < count(); ++i) {
      value_destroy(i, alloc);
    }
  }

 public:
  // Exposed only for tests.
  static bool testonly_uses_linear_node_search() {
    return use_linear_search::value;
  }

 private:
  template <typename... Args>
  void value_init(const size_type i, allocator_type *alloc, Args &&... args) {
    absl::container_internal::SanitizerUnpoisonObject(slot(i));
    params_type::construct(alloc, slot(i), std::forward<Args>(args)...);
  }
  void value_destroy(const size_type i, allocator_type *alloc) {
    params_type::destroy(alloc, slot(i));
    absl::container_internal::SanitizerPoisonObject(slot(i));
  }

  // Move n values starting at value i in this node into the values starting at
  // value j in node x.
  void uninitialized_move_n(const size_type n, const size_type i,
                            const size_type j, btree_node *x,
                            allocator_type *alloc) {
    absl::container_internal::SanitizerUnpoisonMemoryRegion(
        x->slot(j), n * sizeof(slot_type));
    for (slot_type *src = slot(i), *end = src + n, *dest = x->slot(j);
         src != end; ++src, ++dest) {
      params_type::construct(alloc, dest, src);
    }
  }

  // Destroys a range of n values, starting at index i.
  void value_destroy_n(const size_type i, const size_type n,
                       allocator_type *alloc) {
    for (int j = 0; j < n; ++j) {
      value_destroy(i + j, alloc);
    }
  }

  template <typename P>
  friend class btree;
  template <typename N, typename R, typename P>
  friend struct btree_iterator;
  friend class BtreeNodePeer;
};

template <typename Node, typename Reference, typename Pointer>
struct btree_iterator {
 private:
  using key_type = typename Node::key_type;
  using size_type = typename Node::size_type;
  using params_type = typename Node::params_type;

  using node_type = Node;
  using normal_node = typename std::remove_const<Node>::type;
  using const_node = const Node;
  using normal_pointer = typename params_type::pointer;
  using normal_reference = typename params_type::reference;
  using const_pointer = typename params_type::const_pointer;
  using const_reference = typename params_type::const_reference;
  using slot_type = typename params_type::slot_type;

  using iterator =
      btree_iterator<normal_node, normal_reference, normal_pointer>;
  using const_iterator =
      btree_iterator<const_node, const_reference, const_pointer>;

 public:
  // These aliases are public for std::iterator_traits.
  using difference_type = typename Node::difference_type;
  using value_type = typename params_type::value_type;
  using pointer = Pointer;
  using reference = Reference;
  using iterator_category = std::bidirectional_iterator_tag;

  btree_iterator() : node(nullptr), position(-1) {}
  btree_iterator(Node *n, int p) : node(n), position(p) {}

  // NOTE: this SFINAE allows for implicit conversions from iterator to
  // const_iterator, but it specifically avoids defining copy constructors so
  // that btree_iterator can be trivially copyable. This is for performance and
  // binary size reasons.
  template <typename N, typename R, typename P,
            absl::enable_if_t<
                std::is_same<btree_iterator<N, R, P>, iterator>::value &&
                    std::is_same<btree_iterator, const_iterator>::value,
                int> = 0>
  btree_iterator(const btree_iterator<N, R, P> &x)  // NOLINT
      : node(x.node), position(x.position) {}

 private:
  // This SFINAE allows explicit conversions from const_iterator to
  // iterator, but also avoids defining a copy constructor.
  // NOTE: the const_cast is safe because this constructor is only called by
  // non-const methods and the container owns the nodes.
  template <typename N, typename R, typename P,
            absl::enable_if_t<
                std::is_same<btree_iterator<N, R, P>, const_iterator>::value &&
                    std::is_same<btree_iterator, iterator>::value,
                int> = 0>
  explicit btree_iterator(const btree_iterator<N, R, P> &x)
      : node(const_cast<node_type *>(x.node)), position(x.position) {}

  // Increment/decrement the iterator.
  void increment() {
    if (node->leaf() && ++position < node->count()) {
      return;
    }
    increment_slow();
  }
  void increment_slow();

  void decrement() {
    if (node->leaf() && --position >= 0) {
      return;
    }
    decrement_slow();
  }
  void decrement_slow();

 public:
  bool operator==(const const_iterator &x) const {
    return node == x.node && position == x.position;
  }
  bool operator!=(const const_iterator &x) const {
    return node != x.node || position != x.position;
  }

  // Accessors for the key/value the iterator is pointing at.
  reference operator*() const { return node->value(position); }
  pointer operator->() const { return &node->value(position); }

  btree_iterator &operator++() {
    increment();
    return *this;
  }
  btree_iterator &operator--() {
    decrement();
    return *this;
  }
  btree_iterator operator++(int) {
    btree_iterator tmp = *this;
    ++*this;
    return tmp;
  }
  btree_iterator operator--(int) {
    btree_iterator tmp = *this;
    --*this;
    return tmp;
  }

 private:
  template <typename Params>
  friend class btree;
  template <typename Tree>
  friend class btree_container;
  template <typename Tree>
  friend class btree_set_container;
  template <typename Tree>
  friend class btree_map_container;
  template <typename Tree>
  friend class btree_multiset_container;
  template <typename N, typename R, typename P>
  friend struct btree_iterator;
  template <typename TreeType, typename CheckerType>
  friend class base_checker;

  const key_type &key() const { return node->key(position); }
  slot_type *slot() { return node->slot(position); }

  // The node in the tree the iterator is pointing at.
  Node *node;
  // The position within the node of the tree the iterator is pointing at.
  // TODO(ezb): make this a field_type
  int position;
};

template <typename Params>
class btree {
  using node_type = btree_node<Params>;
  using is_key_compare_to = typename Params::is_key_compare_to;

  // We use a static empty node for the root/leftmost/rightmost of empty btrees
  // in order to avoid branching in begin()/end().
  struct alignas(node_type::Alignment()) EmptyNodeType : node_type {
    using field_type = typename node_type::field_type;
    node_type *parent;
    field_type position = 0;
    field_type start = 0;
    field_type count = 0;
    // max_count must be != kInternalNodeMaxCount (so that this node is regarded
    // as a leaf node). max_count() is never called when the tree is empty.
    field_type max_count = node_type::kInternalNodeMaxCount + 1;

#ifdef _MSC_VER
    // MSVC has constexpr code generations bugs here.
    EmptyNodeType() : parent(this) {}
#else
    constexpr EmptyNodeType(node_type *p) : parent(p) {}
#endif
  };

  static node_type *EmptyNode() {
#ifdef _MSC_VER
    static EmptyNodeType *empty_node = new EmptyNodeType;
    // This assert fails on some other construction methods.
    assert(empty_node->parent == empty_node);
    return empty_node;
#else
    static constexpr EmptyNodeType empty_node(
        const_cast<EmptyNodeType *>(&empty_node));
    return const_cast<EmptyNodeType *>(&empty_node);
#endif
  }

  enum {
    kNodeValues = node_type::kNodeValues,
    kMinNodeValues = kNodeValues / 2,
  };

  struct node_stats {
    using size_type = typename Params::size_type;

    node_stats(size_type l, size_type i) : leaf_nodes(l), internal_nodes(i) {}

    node_stats &operator+=(const node_stats &x) {
      leaf_nodes += x.leaf_nodes;
      internal_nodes += x.internal_nodes;
      return *this;
    }

    size_type leaf_nodes;
    size_type internal_nodes;
  };

 public:
  using key_type = typename Params::key_type;
  using value_type = typename Params::value_type;
  using size_type = typename Params::size_type;
  using difference_type = typename Params::difference_type;
  using key_compare = typename Params::key_compare;
  using value_compare = typename Params::value_compare;
  using allocator_type = typename Params::allocator_type;
  using reference = typename Params::reference;
  using const_reference = typename Params::const_reference;
  using pointer = typename Params::pointer;
  using const_pointer = typename Params::const_pointer;
  using iterator = btree_iterator<node_type, reference, pointer>;
  using const_iterator = typename iterator::const_iterator;
  using reverse_iterator = std::reverse_iterator<iterator>;
  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
  using node_handle_type = node_handle<Params, Params, allocator_type>;

  // Internal types made public for use by btree_container types.
  using params_type = Params;
  using slot_type = typename Params::slot_type;

 private:
  // For use in copy_or_move_values_in_order.
  const value_type &maybe_move_from_iterator(const_iterator x) { return *x; }
  value_type &&maybe_move_from_iterator(iterator x) { return std::move(*x); }

  // Copies or moves (depending on the template parameter) the values in
  // x into this btree in their order in x. This btree must be empty before this
  // method is called. This method is used in copy construction, copy
  // assignment, and move assignment.
  template <typename Btree>
  void copy_or_move_values_in_order(Btree *x);

  // Validates that various assumptions/requirements are true at compile time.
  constexpr static bool static_assert_validation();

 public:
  btree(const key_compare &comp, const allocator_type &alloc);

  btree(const btree &x);
  btree(btree &&x) noexcept
      : root_(std::move(x.root_)),
        rightmost_(absl::exchange(x.rightmost_, EmptyNode())),
        size_(absl::exchange(x.size_, 0)) {
    x.mutable_root() = EmptyNode();
  }

  ~btree() {
    // Put static_asserts in destructor to avoid triggering them before the type
    // is complete.
    static_assert(static_assert_validation(), "This call must be elided.");
    clear();
  }

  // Assign the contents of x to *this.
  btree &operator=(const btree &x);
  btree &operator=(btree &&x) noexcept;

  iterator begin() { return iterator(leftmost(), 0); }
  const_iterator begin() const { return const_iterator(leftmost(), 0); }
  iterator end() { return iterator(rightmost_, rightmost_->count()); }
  const_iterator end() const {
    return const_iterator(rightmost_, rightmost_->count());
  }
  reverse_iterator rbegin() { return reverse_iterator(end()); }
  const_reverse_iterator rbegin() const {
    return const_reverse_iterator(end());
  }
  reverse_iterator rend() { return reverse_iterator(begin()); }
  const_reverse_iterator rend() const {
    return const_reverse_iterator(begin());
  }

  // Finds the first element whose key is not less than key.
  template <typename K>
  iterator lower_bound(const K &key) {
    return internal_end(internal_lower_bound(key));
  }
  template <typename K>
  const_iterator lower_bound(const K &key) const {
    return internal_end(internal_lower_bound(key));
  }

  // Finds the first element whose key is greater than key.
  template <typename K>
  iterator upper_bound(const K &key) {
    return internal_end(internal_upper_bound(key));
  }
  template <typename K>
  const_iterator upper_bound(const K &key) const {
    return internal_end(internal_upper_bound(key));
  }

  // Finds the range of values which compare equal to key. The first member of
  // the returned pair is equal to lower_bound(key). The second member pair of
  // the pair is equal to upper_bound(key).
  template <typename K>
  std::pair<iterator, iterator> equal_range(const K &key) {
    return {lower_bound(key), upper_bound(key)};
  }
  template <typename K>
  std::pair<const_iterator, const_iterator> equal_range(const K &key) const {
    return {lower_bound(key), upper_bound(key)};
  }

  // Inserts a value into the btree only if it does not already exist. The
  // boolean return value indicates whether insertion succeeded or failed.
  // Requirement: if `key` already exists in the btree, does not consume `args`.
  // Requirement: `key` is never referenced after consuming `args`.
  template <typename... Args>
  std::pair<iterator, bool> insert_unique(const key_type &key, Args &&... args);

  // Inserts with hint. Checks to see if the value should be placed immediately
  // before `position` in the tree. If so, then the insertion will take
  // amortized constant time. If not, the insertion will take amortized
  // logarithmic time as if a call to insert_unique() were made.
  // Requirement: if `key` already exists in the btree, does not consume `args`.
  // Requirement: `key` is never referenced after consuming `args`.
  template <typename... Args>
  std::pair<iterator, bool> insert_hint_unique(iterator position,
                                               const key_type &key,
                                               Args &&... args);

  // Insert a range of values into the btree.
  template <typename InputIterator>
  void insert_iterator_unique(InputIterator b, InputIterator e);

  // Inserts a value into the btree.
  template <typename ValueType>
  iterator insert_multi(const key_type &key, ValueType &&v);

  // Inserts a value into the btree.
  template <typename ValueType>
  iterator insert_multi(ValueType &&v) {
    return insert_multi(params_type::key(v), std::forward<ValueType>(v));
  }

  // Insert with hint. Check to see if the value should be placed immediately
  // before position in the tree. If it does, then the insertion will take
  // amortized constant time. If not, the insertion will take amortized
  // logarithmic time as if a call to insert_multi(v) were made.
  template <typename ValueType>
  iterator insert_hint_multi(iterator position, ValueType &&v);

  // Insert a range of values into the btree.
  template <typename InputIterator>
  void insert_iterator_multi(InputIterator b, InputIterator e);

  // Erase the specified iterator from the btree. The iterator must be valid
  // (i.e. not equal to end()).  Return an iterator pointing to the node after
  // the one that was erased (or end() if none exists).
  // Requirement: does not read the value at `*iter`.
  iterator erase(iterator iter);

  // Erases range. Returns the number of keys erased and an iterator pointing
  // to the element after the last erased element.
  std::pair<size_type, iterator> erase_range(iterator begin, iterator end);

  // Erases the specified key from the btree. Returns 1 if an element was
  // erased and 0 otherwise.
  template <typename K>
  size_type erase_unique(const K &key);

  // Erases all of the entries matching the specified key from the
  // btree. Returns the number of elements erased.
  template <typename K>
  size_type erase_multi(const K &key);

  // Finds the iterator corresponding to a key or returns end() if the key is
  // not present.
  template <typename K>
  iterator find(const K &key) {
    return internal_end(internal_find(key));
  }
  template <typename K>
  const_iterator find(const K &key) const {
    return internal_end(internal_find(key));
  }

  // Returns a count of the number of times the key appears in the btree.
  template <typename K>
  size_type count_unique(const K &key) const {
    const iterator begin = internal_find(key);
    if (begin.node == nullptr) {
      // The key doesn't exist in the tree.
      return 0;
    }
    return 1;
  }
  // Returns a count of the number of times the key appears in the btree.
  template <typename K>
  size_type count_multi(const K &key) const {
    const auto range = equal_range(key);
    return std::distance(range.first, range.second);
  }

  // Clear the btree, deleting all of the values it contains.
  void clear();

  // Swap the contents of *this and x.
  void swap(btree &x);

  const key_compare &key_comp() const noexcept {
    return root_.template get<0>();
  }
  template <typename K, typename LK>
  bool compare_keys(const K &x, const LK &y) const {
    return compare_internal::compare_result_as_less_than(key_comp()(x, y));
  }

  value_compare value_comp() const { return value_compare(key_comp()); }

  // Verifies the structure of the btree.
  void verify() const;

  // Size routines.
  size_type size() const { return size_; }
  size_type max_size() const { return (std::numeric_limits<size_type>::max)(); }
  bool empty() const { return size_ == 0; }

  // The height of the btree. An empty tree will have height 0.
  size_type height() const {
    size_type h = 0;
    if (!empty()) {
      // Count the length of the chain from the leftmost node up to the
      // root. We actually count from the root back around to the level below
      // the root, but the calculation is the same because of the circularity
      // of that traversal.
      const node_type *n = root();
      do {
        ++h;
        n = n->parent();
      } while (n != root());
    }
    return h;
  }

  // The number of internal, leaf and total nodes used by the btree.
  size_type leaf_nodes() const { return internal_stats(root()).leaf_nodes; }
  size_type internal_nodes() const {
    return internal_stats(root()).internal_nodes;
  }
  size_type nodes() const {
    node_stats stats = internal_stats(root());
    return stats.leaf_nodes + stats.internal_nodes;
  }

  // The total number of bytes used by the btree.
  size_type bytes_used() const {
    node_stats stats = internal_stats(root());
    if (stats.leaf_nodes == 1 && stats.internal_nodes == 0) {
      return sizeof(*this) + node_type::LeafSize(root()->max_count());
    } else {
      return sizeof(*this) + stats.leaf_nodes * node_type::LeafSize() +
             stats.internal_nodes * node_type::InternalSize();
    }
  }

  // The average number of bytes used per value stored in the btree.
  static double average_bytes_per_value() {
    // Returns the number of bytes per value on a leaf node that is 75%
    // full. Experimentally, this matches up nicely with the computed number of
    // bytes per value in trees that had their values inserted in random order.
    return node_type::LeafSize() / (kNodeValues * 0.75);
  }

  // The fullness of the btree. Computed as the number of elements in the btree
  // divided by the maximum number of elements a tree with the current number
  // of nodes could hold. A value of 1 indicates perfect space
  // utilization. Smaller values indicate space wastage.
  // Returns 0 for empty trees.
  double fullness() const {
    if (empty()) return 0.0;
    return static_cast<double>(size()) / (nodes() * kNodeValues);
  }
  // The overhead of the btree structure in bytes per node. Computed as the
  // total number of bytes used by the btree minus the number of bytes used for
  // storing elements divided by the number of elements.
  // Returns 0 for empty trees.
  double overhead() const {
    if (empty()) return 0.0;
    return (bytes_used() - size() * sizeof(value_type)) /
           static_cast<double>(size());
  }

  // The allocator used by the btree.
  allocator_type get_allocator() const { return allocator(); }

 private:
  // Internal accessor routines.
  node_type *root() { return root_.template get<2>(); }
  const node_type *root() const { return root_.template get<2>(); }
  node_type *&mutable_root() noexcept { return root_.template get<2>(); }
  key_compare *mutable_key_comp() noexcept { return &root_.template get<0>(); }

  // The leftmost node is stored as the parent of the root node.
  node_type *leftmost() { return root()->parent(); }
  const node_type *leftmost() const { return root()->parent(); }

  // Allocator routines.
  allocator_type *mutable_allocator() noexcept {
    return &root_.template get<1>();
  }
  const allocator_type &allocator() const noexcept {
    return root_.template get<1>();
  }

  // Allocates a correctly aligned node of at least size bytes using the
  // allocator.
  node_type *allocate(const size_type size) {
    return reinterpret_cast<node_type *>(
        absl::container_internal::Allocate<node_type::Alignment()>(
            mutable_allocator(), size));
  }

  // Node creation/deletion routines.
  node_type *new_internal_node(node_type *parent) {
    node_type *p = allocate(node_type::InternalSize());
    return node_type::init_internal(p, parent);
  }
  node_type *new_leaf_node(node_type *parent) {
    node_type *p = allocate(node_type::LeafSize());
    return node_type::init_leaf(p, parent, kNodeValues);
  }
  node_type *new_leaf_root_node(const int max_count) {
    node_type *p = allocate(node_type::LeafSize(max_count));
    return node_type::init_leaf(p, p, max_count);
  }

  // Deletion helper routines.
  void erase_same_node(iterator begin, iterator end);
  iterator erase_from_leaf_node(iterator begin, size_type to_erase);
  iterator rebalance_after_delete(iterator iter);

  // Deallocates a node of a certain size in bytes using the allocator.
  void deallocate(const size_type size, node_type *node) {
    absl::container_internal::Deallocate<node_type::Alignment()>(
        mutable_allocator(), node, size);
  }

  void delete_internal_node(node_type *node) {
    node->destroy(mutable_allocator());
    deallocate(node_type::InternalSize(), node);
  }
  void delete_leaf_node(node_type *node) {
    node->destroy(mutable_allocator());
    deallocate(node_type::LeafSize(node->max_count()), node);
  }

  // Rebalances or splits the node iter points to.
  void rebalance_or_split(iterator *iter);

  // Merges the values of left, right and the delimiting key on their parent
  // onto left, removing the delimiting key and deleting right.
  void merge_nodes(node_type *left, node_type *right);

  // Tries to merge node with its left or right sibling, and failing that,
  // rebalance with its left or right sibling. Returns true if a merge
  // occurred, at which point it is no longer valid to access node. Returns
  // false if no merging took place.
  bool try_merge_or_rebalance(iterator *iter);

  // Tries to shrink the height of the tree by 1.
  void try_shrink();

  iterator internal_end(iterator iter) {
    return iter.node != nullptr ? iter : end();
  }
  const_iterator internal_end(const_iterator iter) const {
    return iter.node != nullptr ? iter : end();
  }

  // Emplaces a value into the btree immediately before iter. Requires that
  // key(v) <= iter.key() and (--iter).key() <= key(v).
  template <typename... Args>
  iterator internal_emplace(iterator iter, Args &&... args);

  // Returns an iterator pointing to the first value >= the value "iter" is
  // pointing at. Note that "iter" might be pointing to an invalid location as
  // iter.position == iter.node->count(). This routine simply moves iter up in
  // the tree to a valid location.
  // Requires: iter.node is non-null.
  template <typename IterType>
  static IterType internal_last(IterType iter);

  // Returns an iterator pointing to the leaf position at which key would
  // reside in the tree. We provide 2 versions of internal_locate. The first
  // version uses a less-than comparator and is incapable of distinguishing when
  // there is an exact match. The second version is for the key-compare-to
  // specialization and distinguishes exact matches. The key-compare-to
  // specialization allows the caller to avoid a subsequent comparison to
  // determine if an exact match was made, which is important for keys with
  // expensive comparison, such as strings.
  template <typename K>
  SearchResult<iterator, is_key_compare_to::value> internal_locate(
      const K &key) const;

  template <typename K>
  SearchResult<iterator, false> internal_locate_impl(
      const K &key, std::false_type /* IsCompareTo */) const;

  template <typename K>
  SearchResult<iterator, true> internal_locate_impl(
      const K &key, std::true_type /* IsCompareTo */) const;

  // Internal routine which implements lower_bound().
  template <typename K>
  iterator internal_lower_bound(const K &key) const;

  // Internal routine which implements upper_bound().
  template <typename K>
  iterator internal_upper_bound(const K &key) const;

  // Internal routine which implements find().
  template <typename K>
  iterator internal_find(const K &key) const;

  // Deletes a node and all of its children.
  void internal_clear(node_type *node);

  // Verifies the tree structure of node.
  int internal_verify(const node_type *node, const key_type *lo,
                      const key_type *hi) const;

  node_stats internal_stats(const node_type *node) const {
    // The root can be a static empty node.
    if (node == nullptr || (node == root() && empty())) {
      return node_stats(0, 0);
    }
    if (node->leaf()) {
      return node_stats(1, 0);
    }
    node_stats res(0, 1);
    for (int i = 0; i <= node->count(); ++i) {
      res += internal_stats(node->child(i));
    }
    return res;
  }

 public:
  // Exposed only for tests.
  static bool testonly_uses_linear_node_search() {
    return node_type::testonly_uses_linear_node_search();
  }

 private:
  // We use compressed tuple in order to save space because key_compare and
  // allocator_type are usually empty.
  absl::container_internal::CompressedTuple<key_compare, allocator_type,
                                            node_type *>
      root_;

  // A pointer to the rightmost node. Note that the leftmost node is stored as
  // the root's parent.
  node_type *rightmost_;

  // Number of values.
  size_type size_;
};

////
// btree_node methods
template <typename P>
template <typename... Args>
inline void btree_node<P>::emplace_value(const size_type i,
                                         allocator_type *alloc,
                                         Args &&... args) {
  assert(i <= count());
  // Shift old values to create space for new value and then construct it in
  // place.
  if (i < count()) {
    value_init(count(), alloc, slot(count() - 1));
    for (size_type j = count() - 1; j > i; --j)
      params_type::move(alloc, slot(j - 1), slot(j));
    value_destroy(i, alloc);
  }
  value_init(i, alloc, std::forward<Args>(args)...);
  set_count(count() + 1);

  if (!leaf() && count() > i + 1) {
    for (int j = count(); j > i + 1; --j) {
      set_child(j, child(j - 1));
    }
    clear_child(i + 1);
  }
}

template <typename P>
inline void btree_node<P>::remove_value(const int i, allocator_type *alloc) {
  if (!leaf() && count() > i + 1) {
    assert(child(i + 1)->count() == 0);
    for (size_type j = i + 1; j < count(); ++j) {
      set_child(j, child(j + 1));
    }
    clear_child(count());
  }

  remove_values_ignore_children(i, /*to_erase=*/1, alloc);
}

template <typename P>
inline void btree_node<P>::remove_values_ignore_children(
    const int i, const int to_erase, allocator_type *alloc) {
  params_type::move(alloc, slot(i + to_erase), slot(count()), slot(i));
  value_destroy_n(count() - to_erase, to_erase, alloc);
  set_count(count() - to_erase);
}

template <typename P>
void btree_node<P>::rebalance_right_to_left(const int to_move,
                                            btree_node *right,
                                            allocator_type *alloc) {
  assert(parent() == right->parent());
  assert(position() + 1 == right->position());
  assert(right->count() >= count());
  assert(to_move >= 1);
  assert(to_move <= right->count());

  // 1) Move the delimiting value in the parent to the left node.
  value_init(count(), alloc, parent()->slot(position()));

  // 2) Move the (to_move - 1) values from the right node to the left node.
  right->uninitialized_move_n(to_move - 1, 0, count() + 1, this, alloc);

  // 3) Move the new delimiting value to the parent from the right node.
  params_type::move(alloc, right->slot(to_move - 1),
                    parent()->slot(position()));

  // 4) Shift the values in the right node to their correct position.
  params_type::move(alloc, right->slot(to_move), right->slot(right->count()),
                    right->slot(0));

  // 5) Destroy the now-empty to_move entries in the right node.
  right->value_destroy_n(right->count() - to_move, to_move, alloc);

  if (!leaf()) {
    // Move the child pointers from the right to the left node.
    for (int i = 0; i < to_move; ++i) {
      init_child(count() + i + 1, right->child(i));
    }
    for (int i = 0; i <= right->count() - to_move; ++i) {
      assert(i + to_move <= right->max_count());
      right->init_child(i, right->child(i + to_move));
      right->clear_child(i + to_move);
    }
  }

  // Fixup the counts on the left and right nodes.
  set_count(count() + to_move);
  right->set_count(right->count() - to_move);
}

template <typename P>
void btree_node<P>::rebalance_left_to_right(const int to_move,
                                            btree_node *right,
                                            allocator_type *alloc) {
  assert(parent() == right->parent());
  assert(position() + 1 == right->position());
  assert(count() >= right->count());
  assert(to_move >= 1);
  assert(to_move <= count());

  // Values in the right node are shifted to the right to make room for the
  // new to_move values. Then, the delimiting value in the parent and the
  // other (to_move - 1) values in the left node are moved into the right node.
  // Lastly, a new delimiting value is moved from the left node into the
  // parent, and the remaining empty left node entries are destroyed.

  if (right->count() >= to_move) {
    // The original location of the right->count() values are sufficient to hold
    // the new to_move entries from the parent and left node.

    // 1) Shift existing values in the right node to their correct positions.
    right->uninitialized_move_n(to_move, right->count() - to_move,
                                right->count(), right, alloc);
    for (slot_type *src = right->slot(right->count() - to_move - 1),
                   *dest = right->slot(right->count() - 1),
                   *end = right->slot(0);
         src >= end; --src, --dest) {
      params_type::move(alloc, src, dest);
    }

    // 2) Move the delimiting value in the parent to the right node.
    params_type::move(alloc, parent()->slot(position()),
                      right->slot(to_move - 1));

    // 3) Move the (to_move - 1) values from the left node to the right node.
    params_type::move(alloc, slot(count() - (to_move - 1)), slot(count()),
                      right->slot(0));
  } else {
    // The right node does not have enough initialized space to hold the new
    // to_move entries, so part of them will move to uninitialized space.

    // 1) Shift existing values in the right node to their correct positions.
    right->uninitialized_move_n(right->count(), 0, to_move, right, alloc);

    // 2) Move the delimiting value in the parent to the right node.
    right->value_init(to_move - 1, alloc, parent()->slot(position()));

    // 3) Move the (to_move - 1) values from the left node to the right node.
    const size_type uninitialized_remaining = to_move - right->count() - 1;
    uninitialized_move_n(uninitialized_remaining,
                         count() - uninitialized_remaining, right->count(),
                         right, alloc);
    params_type::move(alloc, slot(count() - (to_move - 1)),
                      slot(count() - uninitialized_remaining), right->slot(0));
  }

  // 4) Move the new delimiting value to the parent from the left node.
  params_type::move(alloc, slot(count() - to_move), parent()->slot(position()));

  // 5) Destroy the now-empty to_move entries in the left node.
  value_destroy_n(count() - to_move, to_move, alloc);

  if (!leaf()) {
    // Move the child pointers from the left to the right node.
    for (int i = right->count(); i >= 0; --i) {
      right->init_child(i + to_move, right->child(i));
      right->clear_child(i);
    }
    for (int i = 1; i <= to_move; ++i) {
      right->init_child(i - 1, child(count() - to_move + i));
      clear_child(count() - to_move + i);
    }
  }

  // Fixup the counts on the left and right nodes.
  set_count(count() - to_move);
  right->set_count(right->count() + to_move);
}

template <typename P>
void btree_node<P>::split(const int insert_position, btree_node *dest,
                          allocator_type *alloc) {
  assert(dest->count() == 0);
  assert(max_count() == kNodeValues);

  // We bias the split based on the position being inserted. If we're
  // inserting at the beginning of the left node then bias the split to put
  // more values on the right node. If we're inserting at the end of the
  // right node then bias the split to put more values on the left node.
  if (insert_position == 0) {
    dest->set_count(count() - 1);
  } else if (insert_position == kNodeValues) {
    dest->set_count(0);
  } else {
    dest->set_count(count() / 2);
  }
  set_count(count() - dest->count());
  assert(count() >= 1);

  // Move values from the left sibling to the right sibling.
  uninitialized_move_n(dest->count(), count(), 0, dest, alloc);

  // Destroy the now-empty entries in the left node.
  value_destroy_n(count(), dest->count(), alloc);

  // The split key is the largest value in the left sibling.
  set_count(count() - 1);
  parent()->emplace_value(position(), alloc, slot(count()));
  value_destroy(count(), alloc);
  parent()->init_child(position() + 1, dest);

  if (!leaf()) {
    for (int i = 0; i <= dest->count(); ++i) {
      assert(child(count() + i + 1) != nullptr);
      dest->init_child(i, child(count() + i + 1));
      clear_child(count() + i + 1);
    }
  }
}

template <typename P>
void btree_node<P>::merge(btree_node *src, allocator_type *alloc) {
  assert(parent() == src->parent());
  assert(position() + 1 == src->position());

  // Move the delimiting value to the left node.
  value_init(count(), alloc, parent()->slot(position()));

  // Move the values from the right to the left node.
  src->uninitialized_move_n(src->count(), 0, count() + 1, this, alloc);

  // Destroy the now-empty entries in the right node.
  src->value_destroy_n(0, src->count(), alloc);

  if (!leaf()) {
    // Move the child pointers from the right to the left node.
    for (int i = 0; i <= src->count(); ++i) {
      init_child(count() + i + 1, src->child(i));
      src->clear_child(i);
    }
  }

  // Fixup the counts on the src and dest nodes.
  set_count(1 + count() + src->count());
  src->set_count(0);

  // Remove the value on the parent node.
  parent()->remove_value(position(), alloc);
}

template <typename P>
void btree_node<P>::swap(btree_node *x, allocator_type *alloc) {
  using std::swap;
  assert(leaf() == x->leaf());

  // Determine which is the smaller/larger node.
  btree_node *smaller = this, *larger = x;
  if (smaller->count() > larger->count()) {
    swap(smaller, larger);
  }

  // Swap the values.
  for (slot_type *a = smaller->slot(0), *b = larger->slot(0),
                 *end = a + smaller->count();
       a != end; ++a, ++b) {
    params_type::swap(alloc, a, b);
  }

  // Move values that can't be swapped.
  const size_type to_move = larger->count() - smaller->count();
  larger->uninitialized_move_n(to_move, smaller->count(), smaller->count(),
                               smaller, alloc);
  larger->value_destroy_n(smaller->count(), to_move, alloc);

  if (!leaf()) {
    // Swap the child pointers.
    std::swap_ranges(&smaller->mutable_child(0),
                     &smaller->mutable_child(smaller->count() + 1),
                     &larger->mutable_child(0));
    // Update swapped children's parent pointers.
    int i = 0;
    for (; i <= smaller->count(); ++i) {
      smaller->child(i)->set_parent(smaller);
      larger->child(i)->set_parent(larger);
    }
    // Move the child pointers that couldn't be swapped.
    for (; i <= larger->count(); ++i) {
      smaller->init_child(i, larger->child(i));
      larger->clear_child(i);
    }
  }

  // Swap the counts.
  swap(mutable_count(), x->mutable_count());
}

////
// btree_iterator methods
template <typename N, typename R, typename P>
void btree_iterator<N, R, P>::increment_slow() {
  if (node->leaf()) {
    assert(position >= node->count());
    btree_iterator save(*this);
    while (position == node->count() && !node->is_root()) {
      assert(node->parent()->child(node->position()) == node);
      position = node->position();
      node = node->parent();
    }
    if (position == node->count()) {
      *this = save;
    }
  } else {
    assert(position < node->count());
    node = node->child(position + 1);
    while (!node->leaf()) {
      node = node->child(0);
    }
    position = 0;
  }
}

template <typename N, typename R, typename P>
void btree_iterator<N, R, P>::decrement_slow() {
  if (node->leaf()) {
    assert(position <= -1);
    btree_iterator save(*this);
    while (position < 0 && !node->is_root()) {
      assert(node->parent()->child(node->position()) == node);
      position = node->position() - 1;
      node = node->parent();
    }
    if (position < 0) {
      *this = save;
    }
  } else {
    assert(position >= 0);
    node = node->child(position);
    while (!node->leaf()) {
      node = node->child(node->count());
    }
    position = node->count() - 1;
  }
}

////
// btree methods
template <typename P>
template <typename Btree>
void btree<P>::copy_or_move_values_in_order(Btree *x) {
  static_assert(std::is_same<btree, Btree>::value ||
                    std::is_same<const btree, Btree>::value,
                "Btree type must be same or const.");
  assert(empty());

  // We can avoid key comparisons because we know the order of the
  // values is the same order we'll store them in.
  auto iter = x->begin();
  if (iter == x->end()) return;
  insert_multi(maybe_move_from_iterator(iter));
  ++iter;
  for (; iter != x->end(); ++iter) {
    // If the btree is not empty, we can just insert the new value at the end
    // of the tree.
    internal_emplace(end(), maybe_move_from_iterator(iter));
  }
}

template <typename P>
constexpr bool btree<P>::static_assert_validation() {
  static_assert(std::is_nothrow_copy_constructible<key_compare>::value,
                "Key comparison must be nothrow copy constructible");
  static_assert(std::is_nothrow_copy_constructible<allocator_type>::value,
                "Allocator must be nothrow copy constructible");
  static_assert(type_traits_internal::is_trivially_copyable<iterator>::value,
                "iterator not trivially copyable.");

  // Note: We assert that kTargetValues, which is computed from
  // Params::kTargetNodeSize, must fit the node_type::field_type.
  static_assert(
      kNodeValues < (1 << (8 * sizeof(typename node_type::field_type))),
      "target node size too large");

  // Verify that key_compare returns an absl::{weak,strong}_ordering or bool.
  using compare_result_type =
      absl::result_of_t<key_compare(key_type, key_type)>;
  static_assert(
      std::is_same<compare_result_type, bool>::value ||
          std::is_convertible<compare_result_type, absl::weak_ordering>::value,
      "key comparison function must return absl::{weak,strong}_ordering or "
      "bool.");

  // Test the assumption made in setting kNodeValueSpace.
  static_assert(node_type::MinimumOverhead() >= sizeof(void *) + 4,
                "node space assumption incorrect");

  return true;
}

template <typename P>
btree<P>::btree(const key_compare &comp, const allocator_type &alloc)
    : root_(comp, alloc, EmptyNode()), rightmost_(EmptyNode()), size_(0) {}

template <typename P>
btree<P>::btree(const btree &x) : btree(x.key_comp(), x.allocator()) {
  copy_or_move_values_in_order(&x);
}

template <typename P>
template <typename... Args>
auto btree<P>::insert_unique(const key_type &key, Args &&... args)
    -> std::pair<iterator, bool> {
  if (empty()) {
    mutable_root() = rightmost_ = new_leaf_root_node(1);
  }

  auto res = internal_locate(key);
  iterator &iter = res.value;

  if (res.HasMatch()) {
    if (res.IsEq()) {
      // The key already exists in the tree, do nothing.
      return {iter, false};
    }
  } else {
    iterator last = internal_last(iter);
    if (last.node && !compare_keys(key, last.key())) {
      // The key already exists in the tree, do nothing.
      return {last, false};
    }
  }
  return {internal_emplace(iter, std::forward<Args>(args)...), true};
}

template <typename P>
template <typename... Args>
inline auto btree<P>::insert_hint_unique(iterator position, const key_type &key,
                                         Args &&... args)
    -> std::pair<iterator, bool> {
  if (!empty()) {
    if (position == end() || compare_keys(key, position.key())) {
      iterator prev = position;
      if (position == begin() || compare_keys((--prev).key(), key)) {
        // prev.key() < key < position.key()
        return {internal_emplace(position, std::forward<Args>(args)...), true};
      }
    } else if (compare_keys(position.key(), key)) {
      ++position;
      if (position == end() || compare_keys(key, position.key())) {
        // {original `position`}.key() < key < {current `position`}.key()
        return {internal_emplace(position, std::forward<Args>(args)...), true};
      }
    } else {
      // position.key() == key
      return {position, false};
    }
  }
  return insert_unique(key, std::forward<Args>(args)...);
}

template <typename P>
template <typename InputIterator>
void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e) {
  for (; b != e; ++b) {
    insert_hint_unique(end(), params_type::key(*b), *b);
  }
}

template <typename P>
template <typename ValueType>
auto btree<P>::insert_multi(const key_type &key, ValueType &&v) -> iterator {
  if (empty()) {
    mutable_root() = rightmost_ = new_leaf_root_node(1);
  }

  iterator iter = internal_upper_bound(key);
  if (iter.node == nullptr) {
    iter = end();
  }
  return internal_emplace(iter, std::forward<ValueType>(v));
}

template <typename P>
template <typename ValueType>
auto btree<P>::insert_hint_multi(iterator position, ValueType &&v) -> iterator {
  if (!empty()) {
    const key_type &key = params_type::key(v);
    if (position == end() || !compare_keys(position.key(), key)) {
      iterator prev = position;
      if (position == begin() || !compare_keys(key, (--prev).key())) {
        // prev.key() <= key <= position.key()
        return internal_emplace(position, std::forward<ValueType>(v));
      }
    } else {
      iterator next = position;
      ++next;
      if (next == end() || !compare_keys(next.key(), key)) {
        // position.key() < key <= next.key()
        return internal_emplace(next, std::forward<ValueType>(v));
      }
    }
  }
  return insert_multi(std::forward<ValueType>(v));
}

template <typename P>
template <typename InputIterator>
void btree<P>::insert_iterator_multi(InputIterator b, InputIterator e) {
  for (; b != e; ++b) {
    insert_hint_multi(end(), *b);
  }
}

template <typename P>
auto btree<P>::operator=(const btree &x) -> btree & {
  if (this != &x) {
    clear();

    *mutable_key_comp() = x.key_comp();
    if (absl::allocator_traits<
            allocator_type>::propagate_on_container_copy_assignment::value) {
      *mutable_allocator() = x.allocator();
    }

    copy_or_move_values_in_order(&x);
  }
  return *this;
}

template <typename P>
auto btree<P>::operator=(btree &&x) noexcept -> btree & {
  if (this != &x) {
    clear();

    using std::swap;
    if (absl::allocator_traits<
            allocator_type>::propagate_on_container_copy_assignment::value) {
      // Note: `root_` also contains the allocator and the key comparator.
      swap(root_, x.root_);
      swap(rightmost_, x.rightmost_);
      swap(size_, x.size_);
    } else {
      if (allocator() == x.allocator()) {
        swap(mutable_root(), x.mutable_root());
        swap(*mutable_key_comp(), *x.mutable_key_comp());
        swap(rightmost_, x.rightmost_);
        swap(size_, x.size_);
      } else {
        // We aren't allowed to propagate the allocator and the allocator is
        // different so we can't take over its memory. We must move each element
        // individually. We need both `x` and `this` to have `x`s key comparator
        // while moving the values so we can't swap the key comparators.
        *mutable_key_comp() = x.key_comp();
        copy_or_move_values_in_order(&x);
      }
    }
  }
  return *this;
}

template <typename P>
auto btree<P>::erase(iterator iter) -> iterator {
  bool internal_delete = false;
  if (!iter.node->leaf()) {
    // Deletion of a value on an internal node. First, move the largest value
    // from our left child here, then delete that position (in remove_value()
    // below). We can get to the largest value from our left child by
    // decrementing iter.
    iterator internal_iter(iter);
    --iter;
    assert(iter.node->leaf());
    params_type::move(mutable_allocator(), iter.node->slot(iter.position),
                      internal_iter.node->slot(internal_iter.position));
    internal_delete = true;
  }

  // Delete the key from the leaf.
  iter.node->remove_value(iter.position, mutable_allocator());
  --size_;

  // We want to return the next value after the one we just erased. If we
  // erased from an internal node (internal_delete == true), then the next
  // value is ++(++iter). If we erased from a leaf node (internal_delete ==
  // false) then the next value is ++iter. Note that ++iter may point to an
  // internal node and the value in the internal node may move to a leaf node
  // (iter.node) when rebalancing is performed at the leaf level.

  iterator res = rebalance_after_delete(iter);

  // If we erased from an internal node, advance the iterator.
  if (internal_delete) {
    ++res;
  }
  return res;
}

template <typename P>
auto btree<P>::rebalance_after_delete(iterator iter) -> iterator {
  // Merge/rebalance as we walk back up the tree.
  iterator res(iter);
  bool first_iteration = true;
  for (;;) {
    if (iter.node == root()) {
      try_shrink();
      if (empty()) {
        return end();
      }
      break;
    }
    if (iter.node->count() >= kMinNodeValues) {
      break;
    }
    bool merged = try_merge_or_rebalance(&iter);
    // On the first iteration, we should update `res` with `iter` because `res`
    // may have been invalidated.
    if (first_iteration) {
      res = iter;
      first_iteration = false;
    }
    if (!merged) {
      break;
    }
    iter.position = iter.node->position();
    iter.node = iter.node->parent();
  }

  // Adjust our return value. If we're pointing at the end of a node, advance
  // the iterator.
  if (res.position == res.node->count()) {
    res.position = res.node->count() - 1;
    ++res;
  }

  return res;
}

template <typename P>
auto btree<P>::erase_range(iterator begin, iterator end)
    -> std::pair<size_type, iterator> {
  difference_type count = std::distance(begin, end);
  assert(count >= 0);

  if (count == 0) {
    return {0, begin};
  }

  if (count == size_) {
    clear();
    return {count, this->end()};
  }

  if (begin.node == end.node) {
    erase_same_node(begin, end);
    size_ -= count;
    return {count, rebalance_after_delete(begin)};
  }

  const size_type target_size = size_ - count;
  while (size_ > target_size) {
    if (begin.node->leaf()) {
      const size_type remaining_to_erase = size_ - target_size;
      const size_type remaining_in_node = begin.node->count() - begin.position;
      begin = erase_from_leaf_node(
          begin, (std::min)(remaining_to_erase, remaining_in_node));
    } else {
      begin = erase(begin);
    }
  }
  return {count, begin};
}

template <typename P>
void btree<P>::erase_same_node(iterator begin, iterator end) {
  assert(begin.node == end.node);
  assert(end.position > begin.position);

  node_type *node = begin.node;
  size_type to_erase = end.position - begin.position;
  if (!node->leaf()) {
    // Delete all children between begin and end.
    for (size_type i = 0; i < to_erase; ++i) {
      internal_clear(node->child(begin.position + i + 1));
    }
    // Rotate children after end into new positions.
    for (size_type i = begin.position + to_erase + 1; i <= node->count(); ++i) {
      node->set_child(i - to_erase, node->child(i));
      node->clear_child(i);
    }
  }
  node->remove_values_ignore_children(begin.position, to_erase,
                                      mutable_allocator());

  // Do not need to update rightmost_, because
  // * either end == this->end(), and therefore node == rightmost_, and still
  //   exists
  // * or end != this->end(), and therefore rightmost_ hasn't been erased, since
  //   it wasn't covered in [begin, end)
}

template <typename P>
auto btree<P>::erase_from_leaf_node(iterator begin, size_type to_erase)
    -> iterator {
  node_type *node = begin.node;
  assert(node->leaf());
  assert(node->count() > begin.position);
  assert(begin.position + to_erase <= node->count());

  node->remove_values_ignore_children(begin.position, to_erase,
                                      mutable_allocator());

  size_ -= to_erase;

  return rebalance_after_delete(begin);
}

template <typename P>
template <typename K>
auto btree<P>::erase_unique(const K &key) -> size_type {
  const iterator iter = internal_find(key);
  if (iter.node == nullptr) {
    // The key doesn't exist in the tree, return nothing done.
    return 0;
  }
  erase(iter);
  return 1;
}

template <typename P>
template <typename K>
auto btree<P>::erase_multi(const K &key) -> size_type {
  const iterator begin = internal_lower_bound(key);
  if (begin.node == nullptr) {
    // The key doesn't exist in the tree, return nothing done.
    return 0;
  }
  // Delete all of the keys between begin and upper_bound(key).
  const iterator end = internal_end(internal_upper_bound(key));
  return erase_range(begin, end).first;
}

template <typename P>
void btree<P>::clear() {
  if (!empty()) {
    internal_clear(root());
  }
  mutable_root() = EmptyNode();
  rightmost_ = EmptyNode();
  size_ = 0;
}

template <typename P>
void btree<P>::swap(btree &x) {
  using std::swap;
  if (absl::allocator_traits<
          allocator_type>::propagate_on_container_swap::value) {
    // Note: `root_` also contains the allocator and the key comparator.
    swap(root_, x.root_);
  } else {
    // It's undefined behavior if the allocators are unequal here.
    assert(allocator() == x.allocator());
    swap(mutable_root(), x.mutable_root());
    swap(*mutable_key_comp(), *x.mutable_key_comp());
  }
  swap(rightmost_, x.rightmost_);
  swap(size_, x.size_);
}

template <typename P>
void btree<P>::verify() const {
  assert(root() != nullptr);
  assert(leftmost() != nullptr);
  assert(rightmost_ != nullptr);
  assert(empty() || size() == internal_verify(root(), nullptr, nullptr));
  assert(leftmost() == (++const_iterator(root(), -1)).node);
  assert(rightmost_ == (--const_iterator(root(), root()->count())).node);
  assert(leftmost()->leaf());
  assert(rightmost_->leaf());
}

template <typename P>
void btree<P>::rebalance_or_split(iterator *iter) {
  node_type *&node = iter->node;
  int &insert_position = iter->position;
  assert(node->count() == node->max_count());
  assert(kNodeValues == node->max_count());

  // First try to make room on the node by rebalancing.
  node_type *parent = node->parent();
  if (node != root()) {
    if (node->position() > 0) {
      // Try rebalancing with our left sibling.
      node_type *left = parent->child(node->position() - 1);
      assert(left->max_count() == kNodeValues);
      if (left->count() < kNodeValues) {
        // We bias rebalancing based on the position being inserted. If we're
        // inserting at the end of the right node then we bias rebalancing to
        // fill up the left node.
        int to_move = (kNodeValues - left->count()) /
                      (1 + (insert_position < kNodeValues));
        to_move = (std::max)(1, to_move);

        if (((insert_position - to_move) >= 0) ||
            ((left->count() + to_move) < kNodeValues)) {
          left->rebalance_right_to_left(to_move, node, mutable_allocator());

          assert(node->max_count() - node->count() == to_move);
          insert_position = insert_position - to_move;
          if (insert_position < 0) {
            insert_position = insert_position + left->count() + 1;
            node = left;
          }

          assert(node->count() < node->max_count());
          return;
        }
      }
    }

    if (node->position() < parent->count()) {
      // Try rebalancing with our right sibling.
      node_type *right = parent->child(node->position() + 1);
      assert(right->max_count() == kNodeValues);
      if (right->count() < kNodeValues) {
        // We bias rebalancing based on the position being inserted. If we're
        // inserting at the beginning of the left node then we bias rebalancing
        // to fill up the right node.
        int to_move =
            (kNodeValues - right->count()) / (1 + (insert_position > 0));
        to_move = (std::max)(1, to_move);

        if ((insert_position <= (node->count() - to_move)) ||
            ((right->count() + to_move) < kNodeValues)) {
          node->rebalance_left_to_right(to_move, right, mutable_allocator());

          if (insert_position > node->count()) {
            insert_position = insert_position - node->count() - 1;
            node = right;
          }

          assert(node->count() < node->max_count());
          return;
        }
      }
    }

    // Rebalancing failed, make sure there is room on the parent node for a new
    // value.
    assert(parent->max_count() == kNodeValues);
    if (parent->count() == kNodeValues) {
      iterator parent_iter(node->parent(), node->position());
      rebalance_or_split(&parent_iter);
    }
  } else {
    // Rebalancing not possible because this is the root node.
    // Create a new root node and set the current root node as the child of the
    // new root.
    parent = new_internal_node(parent);
    parent->init_child(0, root());
    mutable_root() = parent;
    // If the former root was a leaf node, then it's now the rightmost node.
    assert(!parent->child(0)->leaf() || parent->child(0) == rightmost_);
  }

  // Split the node.
  node_type *split_node;
  if (node->leaf()) {
    split_node = new_leaf_node(parent);
    node->split(insert_position, split_node, mutable_allocator());
    if (rightmost_ == node) rightmost_ = split_node;
  } else {
    split_node = new_internal_node(parent);
    node->split(insert_position, split_node, mutable_allocator());
  }

  if (insert_position > node->count()) {
    insert_position = insert_position - node->count() - 1;
    node = split_node;
  }
}

template <typename P>
void btree<P>::merge_nodes(node_type *left, node_type *right) {
  left->merge(right, mutable_allocator());
  if (right->leaf()) {
    if (rightmost_ == right) rightmost_ = left;
    delete_leaf_node(right);
  } else {
    delete_internal_node(right);
  }
}

template <typename P>
bool btree<P>::try_merge_or_rebalance(iterator *iter) {
  node_type *parent = iter->node->parent();
  if (iter->node->position() > 0) {
    // Try merging with our left sibling.
    node_type *left = parent->child(iter->node->position() - 1);
    assert(left->max_count() == kNodeValues);
    if ((1 + left->count() + iter->node->count()) <= kNodeValues) {
      iter->position += 1 + left->count();
      merge_nodes(left, iter->node);
      iter->node = left;
      return true;
    }
  }
  if (iter->node->position() < parent->count()) {
    // Try merging with our right sibling.
    node_type *right = parent->child(iter->node->position() + 1);
    assert(right->max_count() == kNodeValues);
    if ((1 + iter->node->count() + right->count()) <= kNodeValues) {
      merge_nodes(iter->node, right);
      return true;
    }
    // Try rebalancing with our right sibling. We don't perform rebalancing if
    // we deleted the first element from iter->node and the node is not
    // empty. This is a small optimization for the common pattern of deleting
    // from the front of the tree.
    if ((right->count() > kMinNodeValues) &&
        ((iter->node->count() == 0) || (iter->position > 0))) {
      int to_move = (right->count() - iter->node->count()) / 2;
      to_move = (std::min)(to_move, right->count() - 1);
      iter->node->rebalance_right_to_left(to_move, right, mutable_allocator());
      return false;
    }
  }
  if (iter->node->position() > 0) {
    // Try rebalancing with our left sibling. We don't perform rebalancing if
    // we deleted the last element from iter->node and the node is not
    // empty. This is a small optimization for the common pattern of deleting
    // from the back of the tree.
    node_type *left = parent->child(iter->node->position() - 1);
    if ((left->count() > kMinNodeValues) &&
        ((iter->node->count() == 0) ||
         (iter->position < iter->node->count()))) {
      int to_move = (left->count() - iter->node->count()) / 2;
      to_move = (std::min)(to_move, left->count() - 1);
      left->rebalance_left_to_right(to_move, iter->node, mutable_allocator());
      iter->position += to_move;
      return false;
    }
  }
  return false;
}

template <typename P>
void btree<P>::try_shrink() {
  if (root()->count() > 0) {
    return;
  }
  // Deleted the last item on the root node, shrink the height of the tree.
  if (root()->leaf()) {
    assert(size() == 0);
    delete_leaf_node(root());
    mutable_root() = EmptyNode();
    rightmost_ = EmptyNode();
  } else {
    node_type *child = root()->child(0);
    child->make_root();
    delete_internal_node(root());
    mutable_root() = child;
  }
}

template <typename P>
template <typename IterType>
inline IterType btree<P>::internal_last(IterType iter) {
  assert(iter.node != nullptr);
  while (iter.position == iter.node->count()) {
    iter.position = iter.node->position();
    iter.node = iter.node->parent();
    if (iter.node->leaf()) {
      iter.node = nullptr;
      break;
    }
  }
  return iter;
}

template <typename P>
template <typename... Args>
inline auto btree<P>::internal_emplace(iterator iter, Args &&... args)
    -> iterator {
  if (!iter.node->leaf()) {
    // We can't insert on an internal node. Instead, we'll insert after the
    // previous value which is guaranteed to be on a leaf node.
    --iter;
    ++iter.position;
  }
  const int max_count = iter.node->max_count();
  if (iter.node->count() == max_count) {
    // Make room in the leaf for the new item.
    if (max_count < kNodeValues) {
      // Insertion into the root where the root is smaller than the full node
      // size. Simply grow the size of the root node.
      assert(iter.node == root());
      iter.node =
          new_leaf_root_node((std::min<int>)(kNodeValues, 2 * max_count));
      iter.node->swap(root(), mutable_allocator());
      delete_leaf_node(root());
      mutable_root() = iter.node;
      rightmost_ = iter.node;
    } else {
      rebalance_or_split(&iter);
    }
  }
  iter.node->emplace_value(iter.position, mutable_allocator(),
                           std::forward<Args>(args)...);
  ++size_;
  return iter;
}

template <typename P>
template <typename K>
inline auto btree<P>::internal_locate(const K &key) const
    -> SearchResult<iterator, is_key_compare_to::value> {
  return internal_locate_impl(key, is_key_compare_to());
}

template <typename P>
template <typename K>
inline auto btree<P>::internal_locate_impl(
    const K &key, std::false_type /* IsCompareTo */) const
    -> SearchResult<iterator, false> {
  iterator iter(const_cast<node_type *>(root()), 0);
  for (;;) {
    iter.position = iter.node->lower_bound(key, key_comp()).value;
    // NOTE: we don't need to walk all the way down the tree if the keys are
    // equal, but determining equality would require doing an extra comparison
    // on each node on the way down, and we will need to go all the way to the
    // leaf node in the expected case.
    if (iter.node->leaf()) {
      break;
    }
    iter.node = iter.node->child(iter.position);
  }
  return {iter};
}

template <typename P>
template <typename K>
inline auto btree<P>::internal_locate_impl(
    const K &key, std::true_type /* IsCompareTo */) const
    -> SearchResult<iterator, true> {
  iterator iter(const_cast<node_type *>(root()), 0);
  for (;;) {
    SearchResult<int, true> res = iter.node->lower_bound(key, key_comp());
    iter.position = res.value;
    if (res.match == MatchKind::kEq) {
      return {iter, MatchKind::kEq};
    }
    if (iter.node->leaf()) {
      break;
    }
    iter.node = iter.node->child(iter.position);
  }
  return {iter, MatchKind::kNe};
}

template <typename P>
template <typename K>
auto btree<P>::internal_lower_bound(const K &key) const -> iterator {
  iterator iter(const_cast<node_type *>(root()), 0);
  for (;;) {
    iter.position = iter.node->lower_bound(key, key_comp()).value;
    if (iter.node->leaf()) {
      break;
    }
    iter.node = iter.node->child(iter.position);
  }
  return internal_last(iter);
}

template <typename P>
template <typename K>
auto btree<P>::internal_upper_bound(const K &key) const -> iterator {
  iterator iter(const_cast<node_type *>(root()), 0);
  for (;;) {
    iter.position = iter.node->upper_bound(key, key_comp());
    if (iter.node->leaf()) {
      break;
    }
    iter.node = iter.node->child(iter.position);
  }
  return internal_last(iter);
}

template <typename P>
template <typename K>
auto btree<P>::internal_find(const K &key) const -> iterator {
  auto res = internal_locate(key);
  if (res.HasMatch()) {
    if (res.IsEq()) {
      return res.value;
    }
  } else {
    const iterator iter = internal_last(res.value);
    if (iter.node != nullptr && !compare_keys(key, iter.key())) {
      return iter;
    }
  }
  return {nullptr, 0};
}

template <typename P>
void btree<P>::internal_clear(node_type *node) {
  if (!node->leaf()) {
    for (int i = 0; i <= node->count(); ++i) {
      internal_clear(node->child(i));
    }
    delete_internal_node(node);
  } else {
    delete_leaf_node(node);
  }
}

template <typename P>
int btree<P>::internal_verify(const node_type *node, const key_type *lo,
                              const key_type *hi) const {
  assert(node->count() > 0);
  assert(node->count() <= node->max_count());
  if (lo) {
    assert(!compare_keys(node->key(0), *lo));
  }
  if (hi) {
    assert(!compare_keys(*hi, node->key(node->count() - 1)));
  }
  for (int i = 1; i < node->count(); ++i) {
    assert(!compare_keys(node->key(i), node->key(i - 1)));
  }
  int count = node->count();
  if (!node->leaf()) {
    for (int i = 0; i <= node->count(); ++i) {
      assert(node->child(i) != nullptr);
      assert(node->child(i)->parent() == node);
      assert(node->child(i)->position() == i);
      count +=
          internal_verify(node->child(i), (i == 0) ? lo : &node->key(i - 1),
                          (i == node->count()) ? hi : &node->key(i));
    }
  }
  return count;
}

}  // namespace container_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_CONTAINER_INTERNAL_BTREE_H_