1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
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
/*
 * Copyright (C) 2021-2022 taylor.fish <contact@taylor.fish>
 *
 * This file is part of btree-vec.
 *
 * btree-vec is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * btree-vec is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with btree-vec. If not, see <https://www.gnu.org/licenses/>.
 */

#![cfg_attr(not(all(test, btree_vec_debug)), no_std)]
#![cfg_attr(
    any(feature = "allocator_api", has_allocator_api),
    feature(allocator_api)
)]
#![cfg_attr(feature = "dropck_eyepatch", feature(dropck_eyepatch))]
#![deny(unsafe_op_in_unsafe_fn)]

//! This crate provides a growable array (vector) implemented using a B-tree
//! (more specifically, a B+ tree). It provides non-amortized O(log n) random
//! accesses, insertions, and removals, as well as O(n) iteration. The
//! branching factor is also customizable.
//!
//! The design is similar to [unsorted counted B-trees][cb] as described by
//! Simon Tatham.
//!
//! [cb]: https://www.chiark.greenend.org.uk/~sgtatham/algorithms/cbtree.html
//!
//! For now, the vector supports insertions and removals only of single
//! elements, but bulk operations, including implementations of [`Extend`]
//! and [`FromIterator`], may be added in the future.
//!
//! Example
//! -------
//!
//! ```rust
//! # use btree_vec::BTreeVec;
//! let mut vec = BTreeVec::new();
//! for i in 0..20 {
//!     vec.push(i);
//! }
//! for i in 0..10 {
//!     assert!(vec.remove(i) == i * 2);
//! }
//! for i in 0..10 {
//!     assert!(vec[i] == i * 2 + 1);
//! }
//! for i in 0..10 {
//!     vec.insert(i * 2, i * 2);
//! }
//! assert!(vec.len() == 20);
//! for (i, n) in vec.iter().copied().enumerate() {
//!     assert!(i == n);
//! }
//! ```
//!
//! Crate features
//! --------------
//!
//! If the crate feature `dropck_eyepatch` is enabled, items in a [`BTreeVec`]
//! can contain references with the same life as the vector itself. This
//! requires Rust nightly, as the unstable language feature [`dropck_eyepatch`]
//! must be used.
//!
//! If the crate feature `allocator_api` is enabled, you can configure
//! [`BTreeVec`] with the unstable [`Allocator`] trait. Alternatively, if the
//! feature `allocator-fallback` is enabled, this crate will use the allocator
//! API provided by [allocator-fallback] instead of the standard library’s.
//!
//! [`dropck_eyepatch`]: https://github.com/rust-lang/rust/issues/34761
//! [allocator-fallback]: https://docs.rs/allocator-fallback
//!
//! [`Extend`]: core::iter::Extend
//! [`FromIterator`]: core::iter::FromIterator
//! [`Allocator`]: alloc::alloc::Allocator

extern crate alloc;

#[cfg(feature = "allocator_api")]
use alloc::alloc as allocator;

#[cfg(not(feature = "allocator_api"))]
#[cfg(feature = "allocator-fallback")]
use allocator_fallback as allocator;

#[cfg(not(any_allocator_api))]
#[path = "alloc_fallback.rs"]
mod allocator;

use alloc::boxed::Box;
use allocator::{Allocator, Global};
use core::fmt::{self, Debug, Formatter};
use core::iter::{ExactSizeIterator, FusedIterator};
use core::marker::PhantomData;
use core::ops::{Index, IndexMut};
use core::ptr::NonNull;

#[cfg(btree_vec_debug)]
#[allow(dead_code)]
pub mod debug;
mod insert;
mod node;
mod remove;
#[cfg(test)]
mod tests;
mod verified_alloc;

use insert::{insert, ItemInsertion};
use node::{LeafRef, Mutable, Node, NodeRef};
use node::{PrefixCast, PrefixPtr, PrefixRef};
use remove::remove;
use verified_alloc::VerifiedAlloc;

/// A growable array (vector) implemented as a B+ tree.
///
/// Provides non-amortized O(log n) random accesses, insertions, and removals,
/// and O(n) iteration.
///
/// `B` is the branching factor. It must be at least 3. The standard library
/// uses a value of 6 for its B-tree structures. Larger values are better when
/// `T` is smaller.
pub struct BTreeVec<T, const B: usize = 12, A: Allocator = Global> {
    root: Option<PrefixPtr<T, B>>,
    size: usize,
    alloc: VerifiedAlloc<A>,
    /// Lets dropck know that `T` may be dropped.
    phantom: PhantomData<Box<T>>,
}

// SAFETY: `BTreeVec` owns its data, so it can be sent to another thread.
unsafe impl<T, const B: usize, A> Send for BTreeVec<T, B, A>
where
    T: Send,
    A: Allocator,
{
}

// SAFETY: `BTreeVec` owns its data and provides access to it only through
// standard borrows.
unsafe impl<T, const B: usize, A> Sync for BTreeVec<T, B, A>
where
    T: Sync,
    A: Allocator,
{
}

fn leaf_for<T, const B: usize, R>(
    mut root: PrefixRef<T, B, R>,
    mut index: usize,
) -> (LeafRef<T, B, R>, usize) {
    loop {
        let node = match root.cast() {
            PrefixCast::Leaf(node) => return (node, index),
            PrefixCast::Internal(node) => node,
        };
        let last = node.length() - 1;
        let mut sizes = node.sizes.iter().copied().take(last);
        let index = sizes
            .position(|size| {
                if let Some(n) = index.checked_sub(size) {
                    index = n;
                    false
                } else {
                    true
                }
            })
            .unwrap_or(last);
        root = node.into_child(index);
    }
}

impl<T> BTreeVec<T> {
    /// Creates a new [`BTreeVec`]. Note that this function is implemented
    /// only for the default value of `B`; see [`Self::create`] for an
    /// equivalent that works with all values of `B`.
    pub fn new() -> Self {
        Self::create()
    }
}

impl<T, A: Allocator> BTreeVec<T, 12, A> {
    #[cfg_attr(
        not(any(feature = "allocator_api", feature = "allocator-fallback")),
        doc(hidden)
    )]
    /// Creates a new [`BTreeVec`] with the given allocator. Note that this
    /// function is implemented only for the default value of `B`; see
    /// [`Self::create_in`] for an equivalent that works with all values of
    /// `B`.
    pub fn new_in(alloc: A) -> Self {
        Self::create_in(alloc)
    }
}

impl<T, const B: usize> BTreeVec<T, B> {
    /// Creates a new [`BTreeVec`]. This function exists because
    /// [`BTreeVec::new`] is implemented only for the default value of `B`.
    pub fn create() -> Self {
        Self::create_in(Global)
    }
}

impl<T, const B: usize, A: Allocator> BTreeVec<T, B, A> {
    #[cfg_attr(
        not(any(feature = "allocator_api", feature = "allocator-fallback")),
        doc(hidden)
    )]
    /// Creates a new [`BTreeVec`] with the given allocator. This function
    /// exists because [`BTreeVec::new_in`] is implemented only for the default
    /// value of `B`.
    pub fn create_in(alloc: A) -> Self {
        assert!(B >= 3);
        // SAFETY:
        //
        // * All nodes are allocated by `alloc`, either via the calls to
        //  `insert` and `LeafRef::alloc` in `Self::insert`. Nodes are
        //  deallocated in two places: via the call to `remove` in
        //  `Self::remove`, and via the call to `NodeRef::destroy` in
        //  `Self::drop`. In both of these cases, `alloc` is provided as the
        //  allocator with which to deallocate the nodes.
        //
        // * When `alloc` (`Self.alloc`) is dropped, `Self::drop` will have
        //   run, which destroys all nodes. If `alloc`'s memory is reused
        //   (e.g., via `mem::forget`), the only way this can happen is if the
        //   operation that made its memory able to be reused applied to the
        //   entire `BTreeVec`. Thus, all allocated nodes will become
        //   inaccessible as they are not exposed via any public APIs,
        //   guaranteeing that they will never be accessed.
        let alloc = unsafe { VerifiedAlloc::new(alloc) };
        Self {
            root: None,
            size: 0,
            alloc,
            phantom: PhantomData,
        }
    }

    /// # Safety
    ///
    /// * There must not be any mutable references, including other
    ///   [`NodeRef`]s where `R` is [`Mutable`], to any data accessible via the
    ///   returned [`NodeRef`].
    ///
    /// [`Mutable`]: node::Mutable
    unsafe fn leaf_for(&self, index: usize) -> (LeafRef<T, B>, usize) {
        // SAFETY: Caller guarantees safety.
        leaf_for(unsafe { NodeRef::new(self.root.unwrap()) }, index)
    }

    /// # Safety
    ///
    /// There must be no other references, including [`NodeRef`]s, to any data
    /// accessible via the returned [`NodeRef`].
    unsafe fn leaf_for_mut(
        &mut self,
        index: usize,
    ) -> (LeafRef<T, B, Mutable>, usize) {
        // SAFETY: Caller guarantees safety.
        leaf_for(unsafe { NodeRef::new_mutable(self.root.unwrap()) }, index)
    }

    /// Gets the length of the vector.
    pub fn len(&self) -> usize {
        self.size
    }

    /// Checks whether the vector is empty.
    pub fn is_empty(&self) -> bool {
        self.size == 0
    }

    /// Gets the item at `index`, or [`None`] if no such item exists.
    pub fn get(&self, index: usize) -> Option<&T> {
        (index < self.size).then(|| {
            // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with
            // standard borrowing rules, so there are no existing mutable
            // references.
            let (leaf, index) = unsafe { self.leaf_for(index) };
            leaf.into_child(index)
        })
    }

    /// Gets a mutable reference to the item at `index`, or [`None`] if no such
    /// item exists.
    pub fn get_mut(&mut self, index: usize) -> Option<&mut T> {
        (index < self.size).then(|| {
            // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with
            // standard borrowing rules, so there are no existing references.
            let (leaf, index) = unsafe { self.leaf_for_mut(index) };
            leaf.into_child_mut(index)
        })
    }

    /// Gets the first item in the vector, or [`None`] if the vector is empty.
    pub fn first(&self) -> Option<&T> {
        self.get(0)
    }

    /// Gets a mutable reference to the first item in the vector, or [`None`]
    /// if the vector is empty.
    pub fn first_mut(&mut self) -> Option<&mut T> {
        self.get_mut(0)
    }

    /// Gets the last item in the vector, or [`None`] if the vector is empty.
    pub fn last(&self) -> Option<&T> {
        self.size.checked_sub(1).and_then(|s| self.get(s))
    }

    /// Gets a mutable reference to the last item in the vector, or [`None`] if
    /// the vector is empty.
    pub fn last_mut(&mut self) -> Option<&mut T> {
        self.size.checked_sub(1).and_then(move |s| self.get_mut(s))
    }

    /// Inserts `item` at `index`.
    ///
    /// # Panics
    ///
    /// Panics if `index` is greater than [`self.len()`](Self::len).
    pub fn insert(&mut self, index: usize, item: T) {
        assert!(index <= self.size);
        self.root.get_or_insert_with(|| {
            LeafRef::alloc(&self.alloc).into_prefix().as_ptr()
        });
        // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with standard
        // borrowing rules, so there are no existing references.
        let (leaf, index) = unsafe { self.leaf_for_mut(index) };
        let root = insert(
            ItemInsertion {
                node: leaf,
                index,
                item,
                root_size: self.size,
            },
            &self.alloc,
        );
        self.root = Some(root.as_ptr());
        self.size += 1;
    }

    /// Inserts `item` at the end of the vector.
    pub fn push(&mut self, item: T) {
        self.insert(self.size, item);
    }

    /// Removes and returns the item at `index`.
    ///
    /// # Panics
    ///
    /// Panics if `index` is not less than [`self.len()`](Self::len).
    pub fn remove(&mut self, index: usize) -> T {
        assert!(index < self.size);
        // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with
        // standard borrowing rules, so there are no existing references.
        let (leaf, index) = unsafe { self.leaf_for_mut(index) };
        let (root, item) = remove(leaf, index, &self.alloc);
        self.root = Some(root.as_ptr());
        self.size -= 1;
        item
    }

    /// Removes and returns the last item in the vector, or [`None`] if the
    /// vector is empty.
    pub fn pop(&mut self) -> Option<T> {
        self.size.checked_sub(1).map(|s| self.remove(s))
    }

    /// Gets an iterator that returns references to each item in the vector.
    pub fn iter(&self) -> Iter<'_, T, B> {
        // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with standard
        // borrowing rules, so there are no existing mutable references.
        Iter {
            leaf: self.root.map(|_| unsafe { self.leaf_for(0) }.0),
            index: 0,
            remaining: self.len(),
            phantom: PhantomData,
        }
    }

    /// Gets an iterator that returns mutable references to each item in the
    /// vector.
    pub fn iter_mut(&mut self) -> IterMut<'_, T, B> {
        // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with standard
        // borrowing rules, so there are no existing references.
        IterMut {
            leaf: self.root.map(|_| unsafe { self.leaf_for_mut(0) }.0),
            index: 0,
            remaining: self.len(),
            phantom: PhantomData,
        }
    }
}

impl<T, const B: usize, A> Default for BTreeVec<T, B, A>
where
    A: Allocator + Default,
{
    fn default() -> Self {
        Self::create_in(A::default())
    }
}

impl<T, const B: usize, A: Allocator> Index<usize> for BTreeVec<T, B, A> {
    type Output = T;

    fn index(&self, index: usize) -> &T {
        self.get(index).unwrap()
    }
}

impl<T, const B: usize, A: Allocator> IndexMut<usize> for BTreeVec<T, B, A> {
    fn index_mut(&mut self, index: usize) -> &mut T {
        self.get_mut(index).unwrap()
    }
}

impl<T: Debug, const B: usize, A: Allocator> Debug for BTreeVec<T, B, A> {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.iter()).finish()
    }
}

// SAFETY: This `Drop` impl does not directly or indirectly access any data in
// any `T`, except for calling its destructor (see [1]), and `Self` contains a
// `PhantomData<Box<T>>` so dropck knows that `T` may be dropped (see [2]).
//
// [1]: https://doc.rust-lang.org/nomicon/dropck.html
// [2]: https://forge.rust-lang.org/libs/maintaining-std.html
//      #is-there-a-manual-drop-implementation
#[cfg_attr(feature = "dropck_eyepatch", add_syntax::prepend(unsafe))]
impl<#[cfg_attr(feature = "dropck_eyepatch", may_dangle)] T, const B: usize, A>
    Drop for BTreeVec<T, B, A>
where
    A: Allocator,
{
    fn drop(&mut self) {
        if let Some(root) = self.root {
            // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with
            // standard borrowing rules, so there are no existing
            // references.
            unsafe { NodeRef::new_mutable(root) }.destroy(&self.alloc);
        }
    }
}

fn nth<T, const B: usize, R>(
    leaf: LeafRef<T, B, R>,
    index: usize,
    mut n: usize,
) -> Option<(LeafRef<T, B, R>, usize)> {
    if let Some(new) = n.checked_sub(leaf.length() - index) {
        n = new;
    } else {
        return Some((leaf, index + n));
    };
    let mut child_index = leaf.index();
    let mut parent = leaf.into_parent().ok()?;
    loop {
        let sizes = parent.sizes[..parent.length()].iter().copied();
        for (i, size) in sizes.enumerate().skip(child_index + 1) {
            if let Some(new) = n.checked_sub(size) {
                n = new;
            } else {
                return Some(leaf_for(parent.into_child(i), n));
            }
        }
        child_index = parent.index();
        parent = parent.into_parent().ok()?;
    }
}

/// An iterator over the items in a [`BTreeVec`].
pub struct Iter<'a, T, const B: usize> {
    leaf: Option<LeafRef<T, B>>,
    index: usize,
    remaining: usize,
    phantom: PhantomData<&'a T>,
}

impl<'a, T, const B: usize> Iterator for Iter<'a, T, B> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        let mut leaf = self.leaf?;
        if self.index == leaf.length() {
            self.leaf = self.leaf.take().unwrap().into_next().ok();
            leaf = self.leaf?;
            self.index = 0;
        }
        let index = self.index;
        self.index += 1;
        Some(leaf.into_child(index))
    }

    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let (leaf, i) = nth(self.leaf.take()?, self.index, n)?;
        self.index = i + 1;
        Some(self.leaf.insert(leaf).into_child(i))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.remaining, Some(self.remaining))
    }
}

impl<T, const B: usize> FusedIterator for Iter<'_, T, B> {}

impl<T, const B: usize> ExactSizeIterator for Iter<'_, T, B> {
    fn len(&self) -> usize {
        let (lower, upper) = self.size_hint();
        debug_assert_eq!(Some(lower), upper);
        lower
    }
}

impl<T, const B: usize> Clone for Iter<'_, T, B> {
    fn clone(&self) -> Self {
        Self {
            leaf: self.leaf,
            index: self.index,
            remaining: self.remaining,
            phantom: self.phantom,
        }
    }
}

// SAFETY: This type yields immutable references to items in the vector, so it
// can be `Send` as long as `T` is `Sync` (which means `&T` is `Send`).
unsafe impl<T: Sync, const B: usize> Send for Iter<'_, T, B> {}

// SAFETY: This type has no `&self` methods that access shared data or fields
// with non-`Sync` interior mutability, but `T` must be `Sync` to match the
// `Send` impl, since this type implements `Clone`, effectively allowing it to
// be sent.
unsafe impl<T: Sync, const B: usize> Sync for Iter<'_, T, B> {}

impl<'a, T, const B: usize, A> IntoIterator for &'a BTreeVec<T, B, A>
where
    A: Allocator,
{
    type Item = &'a T;
    type IntoIter = Iter<'a, T, B>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

/// A mutable iterator over the items in a [`BTreeVec`].
pub struct IterMut<'a, T, const B: usize> {
    leaf: Option<LeafRef<T, B, Mutable>>,
    index: usize,
    remaining: usize,
    phantom: PhantomData<&'a mut T>,
}

impl<'a, T, const B: usize> Iterator for IterMut<'a, T, B> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        let mut leaf = self.leaf.as_mut()?;
        if self.index == leaf.length() {
            self.leaf = self.leaf.take().unwrap().into_next().ok();
            leaf = self.leaf.as_mut()?;
            self.index = 0;
        }
        let index = self.index;
        self.index += 1;
        // SAFETY: Extending the lifetime to `'a` is okay because `'a` doesn't
        // outlive the `BTreeVec` and we won't access this index again for the
        // life of the iterator.
        Some(unsafe { NonNull::from(leaf.child_mut(index)).as_mut() })
    }

    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let (leaf, i) = nth(self.leaf.take()?, self.index, n)?;
        self.index = i + 1;
        // SAFETY: Extending the lifetime to `'a` is okay because `'a` doesn't
        // outlive the `BTreeVec` and we won't access this index again for the
        // life of the iterator.
        Some(unsafe {
            NonNull::from(self.leaf.insert(leaf).child_mut(i)).as_mut()
        })
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.remaining, Some(self.remaining))
    }
}

impl<T, const B: usize> FusedIterator for IterMut<'_, T, B> {}

impl<T, const B: usize> ExactSizeIterator for IterMut<'_, T, B> {
    fn len(&self) -> usize {
        let (lower, upper) = self.size_hint();
        debug_assert_eq!(Some(lower), upper);
        lower
    }
}

// SAFETY: This type yields mutable references to items in the vector, so it
// can be `Send` as long as `T` is `Send`. `T` doesn't need to be `Sync`
// because no other iterator that yields items from the vector can exist at the
// same time as this iterator.
unsafe impl<T: Send, const B: usize> Send for IterMut<'_, T, B> {}

// SAFETY: This type has no `&self` methods that access any fields.
unsafe impl<T, const B: usize> Sync for IterMut<'_, T, B> {}

impl<'a, T, const B: usize, A> IntoIterator for &'a mut BTreeVec<T, B, A>
where
    A: Allocator,
{
    type Item = &'a mut T;
    type IntoIter = IterMut<'a, T, B>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter_mut()
    }
}

/// An owning iterator over the items in a [`BTreeVec`].
pub struct IntoIter<T, const B: usize, A: Allocator = Global> {
    leaf: Option<LeafRef<T, B, Mutable>>,
    length: usize,
    index: usize,
    remaining: usize,
    _tree: BTreeVec<T, B, A>,
}

impl<T, const B: usize, A: Allocator> Iterator for IntoIter<T, B, A> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        let mut leaf = self.leaf.as_mut()?;
        if self.index == self.length {
            self.leaf = self.leaf.take().unwrap().into_next().ok();
            leaf = self.leaf.as_mut()?;
            self.index = 0;
            self.length = leaf.length();
            leaf.set_zero_length();
        }
        let index = self.index;
        self.index += 1;
        self.remaining -= 1;
        // SAFETY: We haven't taken the item at `index` yet.
        Some(unsafe { leaf.take_raw_child(index).assume_init() })
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.remaining, Some(self.remaining))
    }
}

impl<T, const B: usize, A: Allocator> FusedIterator for IntoIter<T, B, A> {}

impl<T, const B: usize> ExactSizeIterator for IntoIter<T, B> {
    fn len(&self) -> usize {
        let (lower, upper) = self.size_hint();
        debug_assert_eq!(Some(lower), upper);
        lower
    }
}

// SAFETY: This type owns the items in the vector, so it can be `Send` as long
// as `T` is `Send`.
unsafe impl<T, const B: usize, A> Send for IntoIter<T, B, A>
where
    T: Send,
    A: Allocator,
{
}

// SAFETY: This type has no `&self` methods that access any fields.
unsafe impl<T, const B: usize, A: Allocator> Sync for IntoIter<T, B, A> {}

impl<T, const B: usize, A: Allocator> Drop for IntoIter<T, B, A> {
    fn drop(&mut self) {
        let mut leaf = if let Some(leaf) = self.leaf.take() {
            leaf
        } else {
            return;
        };
        for i in self.index..self.length {
            // SAFETY: We haven't taken the item at `index` yet.
            unsafe {
                leaf.take_raw_child(i).assume_init();
            }
        }
    }
}

impl<T, const B: usize, A: Allocator> IntoIterator for BTreeVec<T, B, A> {
    type Item = T;
    type IntoIter = IntoIter<T, B, A>;

    fn into_iter(mut self) -> Self::IntoIter {
        // SAFETY: `BTreeVec` uses `NodeRef`s in accordance with standard
        // borrowing rules, so because we own the `BTreeVec`, there are no
        // existing references.
        let leaf = self.root.map(|_| unsafe { self.leaf_for_mut(0) }.0);
        IntoIter {
            index: 0,
            length: leaf.as_ref().map_or(0, |leaf| leaf.length()),
            leaf,
            remaining: self.len(),
            _tree: self,
        }
    }
}