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
 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
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

//! A fixed capacity smart array.
//!
//! See [`Chunk`](struct.Chunk.html)

use std::borrow::{Borrow, BorrowMut};
use std::cmp::Ordering;
use std::fmt::{Debug, Error, Formatter};
use std::hash::{Hash, Hasher};
use std::io;
use std::iter::{FromIterator, FusedIterator};
use std::mem::{self, replace, ManuallyDrop};
use std::ops::{Deref, DerefMut, Index, IndexMut};
use std::ptr;
use std::slice::{
    from_raw_parts, from_raw_parts_mut, Iter as SliceIter, IterMut as SliceIterMut, SliceIndex,
};

use typenum::U64;

use crate::types::ChunkLength;

/// A fixed capacity smart array.
///
/// An inline array of items with a variable length but a fixed, preallocated
/// capacity given by the `N` type, which must be an [`Unsigned`][Unsigned] type
/// level numeral.
///
/// It's 'smart' because it's able to reorganise its contents based on expected
/// behaviour. If you construct one using `push_back`, it will be laid out like
/// a `Vec` with space at the end. If you `push_front` it will start filling in
/// values from the back instead of the front, so that you still get linear time
/// push as long as you don't reverse direction. If you do, and there's no room
/// at the end you're pushing to, it'll shift its contents over to the other
/// side, creating more space to push into. This technique is tuned for
/// `Chunk`'s expected use case in [im::Vector]: usually, chunks always see
/// either `push_front` or `push_back`, but not both unless they move around
/// inside the tree, in which case they're able to reorganise themselves with
/// reasonable efficiency to suit their new usage patterns.
///
/// It maintains a `left` index and a `right` index instead of a simple length
/// counter in order to accomplish this, much like a ring buffer would, except
/// that the `Chunk` keeps all its items sequentially in memory so that you can
/// always get a `&[A]` slice for them, at the price of the occasional
/// reordering operation. The allocated size of a `Chunk` is thus `usize` * 2 +
/// `A` * `N`.
///
/// This technique also lets us choose to shift the shortest side to account for
/// the inserted or removed element when performing insert and remove
/// operations, unlike `Vec` where you always need to shift the right hand side.
///
/// Unlike a `Vec`, the `Chunk` has a fixed capacity and cannot grow beyond it.
/// Being intended for low level use, it expects you to know or test whether
/// you're pushing to a full array, and has an API more geared towards panics
/// than returning `Option`s, on the assumption that you know what you're doing.
/// Of course, if you don't, you can expect it to panic immediately rather than
/// do something undefined and usually bad.
///
/// ## Isn't this just a less efficient ring buffer?
///
/// You might be wondering why you would want to use this data structure rather
/// than a [ring buffer], which is similar but doesn't need to shift its content
/// around when it hits the sides of the allocated buffer. The answer is that
/// `Chunk` can be dereferenced into a slice, while a ring buffer can not. If
/// you don't need to be able to do that, a ring buffer will generally be the
/// more efficient choice.
///
/// # Examples
///
/// ```rust
/// # #[macro_use] extern crate sized_chunks;
/// # extern crate typenum;
/// # use sized_chunks::Chunk;
/// # use typenum::U64;
/// # fn main() {
/// // Construct a chunk with a 64 item capacity
/// let mut chunk = Chunk::<i32, U64>::new();
/// // Fill it with descending numbers
/// chunk.extend((0..64).rev());
/// // It derefs to a slice so we can use standard slice methods
/// chunk.sort();
/// // It's got all the amenities like `FromIterator` and `Eq`
/// let expected: Chunk<i32, U64> = (0..64).collect();
/// assert_eq!(expected, chunk);
/// # }
/// ```
///
/// [Unsigned]: https://docs.rs/typenum/1.10.0/typenum/marker_traits/trait.Unsigned.html
/// [im::Vector]: https://docs.rs/im/latest/im/vector/enum.Vector.html
/// [ring buffer]: https://en.wikipedia.org/wiki/Circular_buffer
pub struct Chunk<A, N = U64>
where
    N: ChunkLength<A>,
{
    left: usize,
    right: usize,
    data: ManuallyDrop<N::SizedType>,
}

impl<A, N> Drop for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn drop(&mut self) {
        if mem::needs_drop::<A>() {
            for i in self.left..self.right {
                unsafe { Chunk::force_drop(i, self) }
            }
        }
    }
}

impl<A, N> Clone for Chunk<A, N>
where
    A: Clone,
    N: ChunkLength<A>,
{
    fn clone(&self) -> Self {
        let mut out = Self::new();
        out.left = self.left;
        out.right = self.right;
        for index in self.left..self.right {
            unsafe { Chunk::force_write(index, self.values()[index].clone(), &mut out) }
        }
        out
    }
}

impl<A, N> Chunk<A, N>
where
    N: ChunkLength<A>,
{
    /// Construct a new empty chunk.
    pub fn new() -> Self {
        let mut chunk: Self;
        unsafe {
            chunk = mem::zeroed();
            ptr::write(&mut chunk.left, 0);
            ptr::write(&mut chunk.right, 0);
        }
        chunk
    }

    /// Construct a new chunk with one item.
    pub fn unit(value: A) -> Self {
        let mut chunk: Self;
        unsafe {
            chunk = mem::zeroed();
            ptr::write(&mut chunk.left, 0);
            ptr::write(&mut chunk.right, 1);
            Chunk::force_write(0, value, &mut chunk);
        }
        chunk
    }

    /// Construct a new chunk with two items.
    pub fn pair(left: A, right: A) -> Self {
        let mut chunk: Self;
        unsafe {
            chunk = mem::zeroed();
            ptr::write(&mut chunk.left, 0);
            ptr::write(&mut chunk.right, 2);
            Chunk::force_write(0, left, &mut chunk);
            Chunk::force_write(1, right, &mut chunk);
        }
        chunk
    }

    /// Construct a new chunk and move every item from `other` into the new
    /// chunk.
    ///
    /// Time: O(n)
    pub fn drain_from(other: &mut Self) -> Self {
        let other_len = other.len();
        Self::from_front(other, other_len)
    }

    /// Construct a new chunk and populate it by taking `count` items from the
    /// iterator `iter`.
    ///
    /// Panics if the iterator contains less than `count` items.
    ///
    /// Time: O(n)
    pub fn collect_from<I>(iter: &mut I, mut count: usize) -> Self
    where
        I: Iterator<Item = A>,
    {
        let mut chunk = Self::new();
        while count > 0 {
            count -= 1;
            chunk.push_back(
                iter.next()
                    .expect("Chunk::collect_from: underfull iterator"),
            );
        }
        chunk
    }

    /// Construct a new chunk and populate it by taking `count` items from the
    /// front of `other`.
    ///
    /// Time: O(n) for the number of items moved
    pub fn from_front(other: &mut Self, count: usize) -> Self {
        let other_len = other.len();
        debug_assert!(count <= other_len);
        let mut chunk = Self::new();
        unsafe { Chunk::force_copy_to(other.left, 0, count, other, &mut chunk) };
        chunk.right = count;
        other.left += count;
        chunk
    }

    /// Construct a new chunk and populate it by taking `count` items from the
    /// back of `other`.
    ///
    /// Time: O(n) for the number of items moved
    pub fn from_back(other: &mut Self, count: usize) -> Self {
        let other_len = other.len();
        debug_assert!(count <= other_len);
        let mut chunk = Self::new();
        unsafe { Chunk::force_copy_to(other.right - count, 0, count, other, &mut chunk) };
        chunk.right = count;
        other.right -= count;
        chunk
    }

    /// Get the length of the chunk.
    #[inline]
    pub fn len(&self) -> usize {
        self.right - self.left
    }

    /// Get the capacity of a chunk of this type.
    #[inline]
    pub fn capacity() -> usize {
        N::USIZE
    }

    /// Test if the chunk is empty.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.left == self.right
    }

    /// Test if the chunk is at capacity.
    #[inline]
    pub fn is_full(&self) -> bool {
        self.left == 0 && self.right == N::USIZE
    }

    #[inline]
    fn values(&self) -> &[A] {
        unsafe {
            from_raw_parts(
                &self.data as *const ManuallyDrop<N::SizedType> as *const A,
                N::USIZE,
            )
        }
    }

    #[inline]
    fn values_mut(&mut self) -> &mut [A] {
        unsafe {
            from_raw_parts_mut(
                &mut self.data as *mut ManuallyDrop<N::SizedType> as *mut A,
                N::USIZE,
            )
        }
    }

    /// Copy the value at an index, discarding ownership of the copied value
    #[inline]
    unsafe fn force_read(index: usize, chunk: &mut Self) -> A {
        ptr::read(&chunk.values()[index])
    }

    /// Write a value at an index without trying to drop what's already there
    #[inline]
    unsafe fn force_write(index: usize, value: A, chunk: &mut Self) {
        ptr::write(&mut chunk.values_mut()[index], value)
    }

    /// Drop the value at an index
    #[inline]
    unsafe fn force_drop(index: usize, chunk: &mut Self) {
        ptr::drop_in_place(&mut chunk.values_mut()[index])
    }

    /// Copy a range within a chunk
    #[inline]
    unsafe fn force_copy(from: usize, to: usize, count: usize, chunk: &mut Self) {
        if count > 0 {
            ptr::copy(&chunk.values()[from], &mut chunk.values_mut()[to], count)
        }
    }

    /// Copy a range between chunks
    #[inline]
    unsafe fn force_copy_to(
        from: usize,
        to: usize,
        count: usize,
        chunk: &mut Self,
        other: &mut Self,
    ) {
        if count > 0 {
            ptr::copy_nonoverlapping(&chunk.values()[from], &mut other.values_mut()[to], count)
        }
    }

    /// Push an item to the front of the chunk.
    ///
    /// Panics if the capacity of the chunk is exceeded.
    ///
    /// Time: O(1) if there's room at the front, O(n) otherwise
    pub fn push_front(&mut self, value: A) {
        if self.is_full() {
            panic!("Chunk::push_front: can't push to full chunk");
        }
        if self.is_empty() {
            self.left = N::USIZE;
            self.right = N::USIZE;
        } else if self.left == 0 {
            self.left = N::USIZE - self.right;
            unsafe { Chunk::force_copy(0, self.left, self.right, self) };
            self.right = N::USIZE;
        }
        self.left -= 1;
        unsafe { Chunk::force_write(self.left, value, self) }
    }

    /// Push an item to the back of the chunk.
    ///
    /// Panics if the capacity of the chunk is exceeded.
    ///
    /// Time: O(1) if there's room at the back, O(n) otherwise
    pub fn push_back(&mut self, value: A) {
        if self.is_full() {
            panic!("Chunk::push_back: can't push to full chunk");
        }
        if self.is_empty() {
            self.left = 0;
            self.right = 0;
        } else if self.right == N::USIZE {
            unsafe { Chunk::force_copy(self.left, 0, self.len(), self) };
            self.right = N::USIZE - self.left;
            self.left = 0;
        }
        unsafe { Chunk::force_write(self.right, value, self) }
        self.right += 1;
    }

    /// Pop an item off the front of the chunk.
    ///
    /// Panics if the chunk is empty.
    ///
    /// Time: O(1)
    pub fn pop_front(&mut self) -> A {
        if self.is_empty() {
            panic!("Chunk::pop_front: can't pop from empty chunk");
        } else {
            let value = unsafe { Chunk::force_read(self.left, self) };
            self.left += 1;
            value
        }
    }

    /// Pop an item off the back of the chunk.
    ///
    /// Panics if the chunk is empty.
    ///
    /// Time: O(1)
    pub fn pop_back(&mut self) -> A {
        if self.is_empty() {
            panic!("Chunk::pop_back: can't pop from empty chunk");
        } else {
            self.right -= 1;
            unsafe { Chunk::force_read(self.right, self) }
        }
    }

    /// Discard all items up to but not including `index`.
    ///
    /// Panics if `index` is out of bounds.
    ///
    /// Time: O(n) for the number of items dropped
    pub fn drop_left(&mut self, index: usize) {
        if index > 0 {
            if index > self.len() {
                panic!("Chunk::drop_left: index out of bounds");
            }
            let start = self.left;
            for i in start..(start + index) {
                unsafe { Chunk::force_drop(i, self) }
            }
            self.left += index;
        }
    }

    /// Discard all items from `index` onward.
    ///
    /// Panics if `index` is out of bounds.
    ///
    /// Time: O(n) for the number of items dropped
    pub fn drop_right(&mut self, index: usize) {
        if index > self.len() {
            panic!("Chunk::drop_right: index out of bounds");
        }
        if index == self.len() {
            return;
        }
        let start = self.left + index;
        for i in start..self.right {
            unsafe { Chunk::force_drop(i, self) }
        }
        self.right = start;
    }

    /// Split a chunk into two, the original chunk containing
    /// everything up to `index` and the returned chunk containing
    /// everything from `index` onwards.
    ///
    /// Panics if `index` is out of bounds.
    ///
    /// Time: O(n) for the number of items in the new chunk
    pub fn split_off(&mut self, index: usize) -> Self {
        if index > self.len() {
            panic!("Chunk::split: index out of bounds");
        }
        if index == self.len() {
            return Self::new();
        }
        let mut right_chunk = Self::new();
        let start = self.left + index;
        let len = self.right - start;
        unsafe { Chunk::force_copy_to(start, 0, len, self, &mut right_chunk) };
        right_chunk.right = len;
        self.right = start;
        right_chunk
    }

    /// Remove all items from `other` and append them to the back of `self`.
    ///
    /// Panics if the capacity of the chunk is exceeded.
    ///
    /// Time: O(n) for the number of items moved
    pub fn append(&mut self, other: &mut Self) {
        let self_len = self.len();
        let other_len = other.len();
        if self_len + other_len > N::USIZE {
            panic!("Chunk::append: chunk size overflow");
        }
        if self.right + other_len > N::USIZE {
            unsafe { Chunk::force_copy(self.left, 0, self_len, self) };
            self.right -= self.left;
            self.left = 0;
        }
        unsafe { Chunk::force_copy_to(other.left, self.right, other_len, other, self) };
        self.right += other_len;
        other.left = 0;
        other.right = 0;
    }

    /// Remove `count` items from the front of `other` and append them to the
    /// back of `self`.
    ///
    /// Panics if `self` doesn't have `count` items left, or if `other` has
    /// fewer than `count` items.
    ///
    /// Time: O(n) for the number of items moved
    pub fn drain_from_front(&mut self, other: &mut Self, count: usize) {
        let self_len = self.len();
        let other_len = other.len();
        debug_assert!(self_len + count <= N::USIZE);
        debug_assert!(other_len >= count);
        if self.right + count > N::USIZE {
            unsafe { Chunk::force_copy(self.left, 0, self_len, self) };
            self.right -= self.left;
            self.left = 0;
        }
        unsafe { Chunk::force_copy_to(other.left, self.right, count, other, self) };
        self.right += count;
        other.left += count;
    }

    /// Remove `count` items from the back of `other` and append them to the
    /// front of `self`.
    ///
    /// Panics if `self` doesn't have `count` items left, or if `other` has
    /// fewer than `count` items.
    ///
    /// Time: O(n) for the number of items moved
    pub fn drain_from_back(&mut self, other: &mut Self, count: usize) {
        let self_len = self.len();
        let other_len = other.len();
        debug_assert!(self_len + count <= N::USIZE);
        debug_assert!(other_len >= count);
        if self.left < count {
            unsafe { Chunk::force_copy(self.left, N::USIZE - self_len, self_len, self) };
            self.left = N::USIZE - self_len;
            self.right = N::USIZE;
        }
        unsafe { Chunk::force_copy_to(other.right - count, self.left - count, count, other, self) };
        self.left -= count;
        other.right -= count;
    }

    /// Update the value at index `index`, returning the old value.
    ///
    /// Panics if `index` is out of bounds.
    ///
    /// Time: O(1)
    pub fn set(&mut self, index: usize, value: A) -> A {
        replace(&mut self[index], value)
    }

    /// Insert a new value at index `index`, shifting all the following values
    /// to the right.
    ///
    /// Panics if the index is out of bounds.
    ///
    /// Time: O(n) for the number of items shifted
    pub fn insert(&mut self, index: usize, value: A) {
        if self.is_full() {
            panic!("Chunk::insert: chunk is full");
        }
        if index > self.len() {
            panic!("Chunk::insert: index out of bounds");
        }
        let real_index = index + self.left;
        let left_size = index;
        let right_size = self.right - real_index;
        if self.right == N::USIZE || (self.left > 0 && left_size < right_size) {
            unsafe {
                Chunk::force_copy(self.left, self.left - 1, left_size, self);
                Chunk::force_write(real_index - 1, value, self);
            }
            self.left -= 1;
        } else {
            unsafe {
                Chunk::force_copy(real_index, real_index + 1, right_size, self);
                Chunk::force_write(real_index, value, self);
            }
            self.right += 1;
        }
    }

    /// Remove the value at index `index`, shifting all the following values to
    /// the left.
    ///
    /// Returns the removed value.
    ///
    /// Panics if the index is out of bounds.
    ///
    /// Time: O(n) for the number of items shifted
    pub fn remove(&mut self, index: usize) -> A {
        if index >= self.len() {
            panic!("Chunk::remove: index out of bounds");
        }
        let real_index = index + self.left;
        let value = unsafe { Chunk::force_read(real_index, self) };
        let left_size = index;
        let right_size = self.right - real_index - 1;
        if left_size < right_size {
            unsafe { Chunk::force_copy(self.left, self.left + 1, left_size, self) };
            self.left += 1;
        } else {
            unsafe { Chunk::force_copy(real_index + 1, real_index, right_size, self) };
            self.right -= 1;
        }
        value
    }

    /// Construct an iterator that drains values from the front of the chunk.
    pub fn drain(&mut self) -> Drain<'_, A, N> {
        Drain { chunk: self }
    }

    /// Discard the contents of the chunk.
    ///
    /// Time: O(n)
    pub fn clear(&mut self) {
        for i in self.left..self.right {
            unsafe { Chunk::force_drop(i, self) }
        }
        self.left = 0;
        self.right = 0;
    }

    /// Get a reference to the contents of the chunk as a slice.
    pub fn as_slice(&self) -> &[A] {
        unsafe {
            from_raw_parts(
                (&self.data as *const ManuallyDrop<N::SizedType> as *const A).add(self.left),
                self.len(),
            )
        }
    }

    /// Get a reference to the contents of the chunk as a mutable slice.
    pub fn as_mut_slice(&mut self) -> &mut [A] {
        unsafe {
            from_raw_parts_mut(
                (&mut self.data as *mut ManuallyDrop<N::SizedType> as *mut A).add(self.left),
                self.len(),
            )
        }
    }
}

impl<A, N> Default for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn default() -> Self {
        Self::new()
    }
}

impl<A, N, I> Index<I> for Chunk<A, N>
where
    I: SliceIndex<[A]>,
    N: ChunkLength<A>,
{
    type Output = I::Output;
    fn index(&self, index: I) -> &Self::Output {
        self.as_slice().index(index)
    }
}

impl<A, N, I> IndexMut<I> for Chunk<A, N>
where
    I: SliceIndex<[A]>,
    N: ChunkLength<A>,
{
    fn index_mut(&mut self, index: I) -> &mut Self::Output {
        self.as_mut_slice().index_mut(index)
    }
}

impl<A, N> Debug for Chunk<A, N>
where
    A: Debug,
    N: ChunkLength<A>,
{
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        f.write_str("Chunk")?;
        f.debug_list().entries(self.iter()).finish()
    }
}

impl<A, N> Hash for Chunk<A, N>
where
    A: Hash,
    N: ChunkLength<A>,
{
    fn hash<H>(&self, hasher: &mut H)
    where
        H: Hasher,
    {
        for item in self {
            item.hash(hasher)
        }
    }
}

impl<A, N, Slice> PartialEq<Slice> for Chunk<A, N>
where
    Slice: Borrow<[A]>,
    A: PartialEq,
    N: ChunkLength<A>,
{
    fn eq(&self, other: &Slice) -> bool {
        self.as_slice() == other.borrow()
    }
}

impl<A, N> Eq for Chunk<A, N>
where
    A: Eq,
    N: ChunkLength<A>,
{
}

impl<A, N> PartialOrd for Chunk<A, N>
where
    A: PartialOrd,
    N: ChunkLength<A>,
{
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        self.iter().partial_cmp(other.iter())
    }
}

impl<A, N> Ord for Chunk<A, N>
where
    A: Ord,
    N: ChunkLength<A>,
{
    fn cmp(&self, other: &Self) -> Ordering {
        self.iter().cmp(other.iter())
    }
}

impl<N> io::Write for Chunk<u8, N>
where
    N: ChunkLength<u8>,
{
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        let old_len = self.len();
        self.extend(buf.iter().cloned().take(N::USIZE - old_len));
        Ok(self.len() - old_len)
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

impl<A, N> Borrow<[A]> for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn borrow(&self) -> &[A] {
        self.as_slice()
    }
}

impl<A, N> BorrowMut<[A]> for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn borrow_mut(&mut self) -> &mut [A] {
        self.as_mut_slice()
    }
}

impl<A, N> AsRef<[A]> for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn as_ref(&self) -> &[A] {
        self.as_slice()
    }
}

impl<A, N> AsMut<[A]> for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn as_mut(&mut self) -> &mut [A] {
        self.as_mut_slice()
    }
}

impl<A, N> Deref for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    type Target = [A];

    fn deref(&self) -> &Self::Target {
        self.as_slice()
    }
}

impl<A, N> DerefMut for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.as_mut_slice()
    }
}

impl<A, N> FromIterator<A> for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    fn from_iter<I>(it: I) -> Self
    where
        I: IntoIterator<Item = A>,
    {
        let mut chunk = Self::new();
        for item in it {
            chunk.push_back(item);
        }
        chunk
    }
}

impl<'a, A, N> IntoIterator for &'a Chunk<A, N>
where
    N: ChunkLength<A>,
{
    type Item = &'a A;
    type IntoIter = SliceIter<'a, A>;
    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

impl<'a, A, N> IntoIterator for &'a mut Chunk<A, N>
where
    N: ChunkLength<A>,
{
    type Item = &'a mut A;
    type IntoIter = SliceIterMut<'a, A>;
    fn into_iter(self) -> Self::IntoIter {
        self.iter_mut()
    }
}

impl<A, N> Extend<A> for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    /// Append the contents of the iterator to the back of the chunk.
    ///
    /// Panics if the chunk exceeds its capacity.
    ///
    /// Time: O(n) for the length of the iterator
    fn extend<I>(&mut self, it: I)
    where
        I: IntoIterator<Item = A>,
    {
        for item in it {
            self.push_back(item);
        }
    }
}

impl<'a, A, N> Extend<&'a A> for Chunk<A, N>
where
    A: 'a + Copy,
    N: ChunkLength<A>,
{
    /// Append the contents of the iterator to the back of the chunk.
    ///
    /// Panics if the chunk exceeds its capacity.
    ///
    /// Time: O(n) for the length of the iterator
    fn extend<I>(&mut self, it: I)
    where
        I: IntoIterator<Item = &'a A>,
    {
        for item in it {
            self.push_back(*item);
        }
    }
}

pub struct Iter<A, N>
where
    N: ChunkLength<A>,
{
    chunk: Chunk<A, N>,
}

impl<A, N> Iterator for Iter<A, N>
where
    N: ChunkLength<A>,
{
    type Item = A;
    fn next(&mut self) -> Option<Self::Item> {
        if self.chunk.is_empty() {
            None
        } else {
            Some(self.chunk.pop_front())
        }
    }

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

impl<A, N> DoubleEndedIterator for Iter<A, N>
where
    N: ChunkLength<A>,
{
    fn next_back(&mut self) -> Option<Self::Item> {
        if self.chunk.is_empty() {
            None
        } else {
            Some(self.chunk.pop_back())
        }
    }
}

impl<A, N> ExactSizeIterator for Iter<A, N> where N: ChunkLength<A> {}

impl<A, N> FusedIterator for Iter<A, N> where N: ChunkLength<A> {}

impl<A, N> IntoIterator for Chunk<A, N>
where
    N: ChunkLength<A>,
{
    type Item = A;
    type IntoIter = Iter<A, N>;

    fn into_iter(self) -> Self::IntoIter {
        Iter { chunk: self }
    }
}

pub struct Drain<'a, A, N>
where
    A: 'a,
    N: ChunkLength<A> + 'a,
{
    chunk: &'a mut Chunk<A, N>,
}

impl<'a, A, N> Iterator for Drain<'a, A, N>
where
    A: 'a,
    N: ChunkLength<A> + 'a,
{
    type Item = A;

    fn next(&mut self) -> Option<Self::Item> {
        if self.chunk.is_empty() {
            None
        } else {
            Some(self.chunk.pop_front())
        }
    }

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

impl<'a, A, N> ExactSizeIterator for Drain<'a, A, N>
where
    A: 'a,
    N: ChunkLength<A> + 'a,
{
}

impl<'a, A, N> FusedIterator for Drain<'a, A, N>
where
    A: 'a,
    N: ChunkLength<A> + 'a,
{
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn is_full() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..64 {
            assert_eq!(false, chunk.is_full());
            chunk.push_back(i);
        }
        assert_eq!(true, chunk.is_full());
    }

    #[test]
    fn push_back_front() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 12..20 {
            chunk.push_back(i);
        }
        assert_eq!(8, chunk.len());
        for i in (0..12).rev() {
            chunk.push_front(i);
        }
        assert_eq!(20, chunk.len());
        for i in 20..32 {
            chunk.push_back(i);
        }
        assert_eq!(32, chunk.len());
        let right: Vec<i32> = chunk.into_iter().collect();
        let left: Vec<i32> = (0..32).collect();
        assert_eq!(left, right);
    }

    #[test]
    fn push_and_pop() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..64 {
            chunk.push_back(i);
        }
        for i in 0..64 {
            assert_eq!(i, chunk.pop_front());
        }
        for i in 0..64 {
            chunk.push_front(i);
        }
        for i in 0..64 {
            assert_eq!(i, chunk.pop_back());
        }
    }

    #[test]
    fn drop_left() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..6 {
            chunk.push_back(i);
        }
        chunk.drop_left(3);
        let vec: Vec<i32> = chunk.into_iter().collect();
        assert_eq!(vec![3, 4, 5], vec);
    }

    #[test]
    fn drop_right() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..6 {
            chunk.push_back(i);
        }
        chunk.drop_right(3);
        let vec: Vec<i32> = chunk.into_iter().collect();
        assert_eq!(vec![0, 1, 2], vec);
    }

    #[test]
    fn split_off() {
        let mut left = Chunk::<_, U64>::new();
        for i in 0..6 {
            left.push_back(i);
        }
        let right = left.split_off(3);
        let left_vec: Vec<i32> = left.into_iter().collect();
        let right_vec: Vec<i32> = right.into_iter().collect();
        assert_eq!(vec![0, 1, 2], left_vec);
        assert_eq!(vec![3, 4, 5], right_vec);
    }

    #[test]
    fn append() {
        let mut left = Chunk::<_, U64>::new();
        for i in 0..32 {
            left.push_back(i);
        }
        let mut right = Chunk::<_, U64>::new();
        for i in (32..64).rev() {
            right.push_front(i);
        }
        left.append(&mut right);
        let out_vec: Vec<i32> = left.into_iter().collect();
        let should_vec: Vec<i32> = (0..64).collect();
        assert_eq!(should_vec, out_vec);
    }

    #[test]
    fn ref_iter() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..64 {
            chunk.push_back(i);
        }
        let out_vec: Vec<&i32> = chunk.iter().collect();
        let should_vec_p: Vec<i32> = (0..64).collect();
        let should_vec: Vec<&i32> = should_vec_p.iter().collect();
        assert_eq!(should_vec, out_vec);
    }

    #[test]
    fn mut_ref_iter() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..64 {
            chunk.push_back(i);
        }
        let out_vec: Vec<&mut i32> = chunk.iter_mut().collect();
        let mut should_vec_p: Vec<i32> = (0..64).collect();
        let should_vec: Vec<&mut i32> = should_vec_p.iter_mut().collect();
        assert_eq!(should_vec, out_vec);
    }

    #[test]
    fn consuming_iter() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..64 {
            chunk.push_back(i);
        }
        let out_vec: Vec<i32> = chunk.into_iter().collect();
        let should_vec: Vec<i32> = (0..64).collect();
        assert_eq!(should_vec, out_vec);
    }

    #[test]
    fn insert_middle() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..32 {
            chunk.push_back(i);
        }
        for i in 33..64 {
            chunk.push_back(i);
        }
        chunk.insert(32, 32);
        let out_vec: Vec<i32> = chunk.into_iter().collect();
        let should_vec: Vec<i32> = (0..64).collect();
        assert_eq!(should_vec, out_vec);
    }

    #[test]
    fn insert_back() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..63 {
            chunk.push_back(i);
        }
        chunk.insert(63, 63);
        let out_vec: Vec<i32> = chunk.into_iter().collect();
        let should_vec: Vec<i32> = (0..64).collect();
        assert_eq!(should_vec, out_vec);
    }

    #[test]
    fn insert_front() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 1..64 {
            chunk.push_front(64 - i);
        }
        chunk.insert(0, 0);
        let out_vec: Vec<i32> = chunk.into_iter().collect();
        let should_vec: Vec<i32> = (0..64).collect();
        assert_eq!(should_vec, out_vec);
    }

    #[test]
    fn remove_value() {
        let mut chunk = Chunk::<_, U64>::new();
        for i in 0..64 {
            chunk.push_back(i);
        }
        chunk.remove(32);
        let out_vec: Vec<i32> = chunk.into_iter().collect();
        let should_vec: Vec<i32> = (0..32).chain(33..64).collect();
        assert_eq!(should_vec, out_vec);
    }

    use std::sync::atomic::{AtomicUsize, Ordering};

    struct DropTest<'a> {
        counter: &'a AtomicUsize,
    }

    impl<'a> DropTest<'a> {
        fn new(counter: &'a AtomicUsize) -> Self {
            counter.fetch_add(1, Ordering::Relaxed);
            DropTest { counter }
        }
    }

    impl<'a> Drop for DropTest<'a> {
        fn drop(&mut self) {
            self.counter.fetch_sub(1, Ordering::Relaxed);
        }
    }

    #[test]
    fn dropping() {
        let counter = AtomicUsize::new(0);
        {
            let mut chunk: Chunk<DropTest> = Chunk::new();
            for _i in 0..20 {
                chunk.push_back(DropTest::new(&counter))
            }
            for _i in 0..20 {
                chunk.push_front(DropTest::new(&counter))
            }
            assert_eq!(40, counter.load(Ordering::Relaxed));
            for _i in 0..10 {
                chunk.pop_back();
            }
            assert_eq!(30, counter.load(Ordering::Relaxed));
        }
        assert_eq!(0, counter.load(Ordering::Relaxed));
    }
}