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
//! A lock-free concurrent slab.
//!
//! Slabs provide pre-allocated storage for many instances of a single data
//! type. When a large number of values of a single type are required,
//! this can be more efficient than allocating each item individually. Since the
//! allocated items are the same size, memory fragmentation is reduced, and
//! creating and removing new items can be very cheap.
//!
//! This crate implements a lock-free concurrent slab, indexed by `usize`s.
//!
//! ## Usage
//!
//! First, add this to your `Cargo.toml`:
//!
//! ```toml
//! sharded-slab = "0.1.1"
//! ```
//!
//! This crate provides two  types, [`Slab`] and [`Pool`], which provide
//! slightly different APIs for using a sharded slab.
//!
//! [`Slab`] implements a slab for _storing_ small types, sharing them between
//! threads, and accessing them by index. New entries are allocated by
//! [inserting] data, moving it in by value. Similarly, entries may be
//! deallocated by [taking] from the slab, moving the value out. This API is
//! similar to a `Vec<Option<T>>`, but allowing lock-free concurrent insertion
//! and removal.
//!
//! In contrast, the [`Pool`] type provides an [object pool] style API for
//! _reusing storage_. Rather than constructing values and moving them into the
//! pool, as with [`Slab`], [allocating an entry][create] from the pool takes a
//! closure that's provided with a mutable reference to initialize the entry in
//! place. When entries are deallocated, they are [cleared] in place. Types
//! which own a heap allocation can be cleared by dropping any _data_ they
//! store, but retaining any previously-allocated capacity. This means that a
//! [`Pool`] may be used to reuse a set of existing heap allocations, reducing
//! allocator load.
//!
//! [inserting]: Slab::insert
//! [taking]: Slab::take
//! [create]: Pool::create
//! [cleared]: Clear
//! [object pool]: https://en.wikipedia.org/wiki/Object_pool_pattern
//!
//! # Examples
//!
//! Inserting an item into the slab, returning an index:
//! ```rust
//! # use sharded_slab::Slab;
//! let slab = Slab::new();
//!
//! let key = slab.insert("hello world").unwrap();
//! assert_eq!(slab.get(key).unwrap(), "hello world");
//! ```
//!
//! To share a slab across threads, it may be wrapped in an `Arc`:
//! ```rust
//! # use sharded_slab::Slab;
//! use std::sync::Arc;
//! let slab = Arc::new(Slab::new());
//!
//! let slab2 = slab.clone();
//! let thread2 = std::thread::spawn(move || {
//!     let key = slab2.insert("hello from thread two").unwrap();
//!     assert_eq!(slab2.get(key).unwrap(), "hello from thread two");
//!     key
//! });
//!
//! let key1 = slab.insert("hello from thread one").unwrap();
//! assert_eq!(slab.get(key1).unwrap(), "hello from thread one");
//!
//! // Wait for thread 2 to complete.
//! let key2 = thread2.join().unwrap();
//!
//! // The item inserted by thread 2 remains in the slab.
//! assert_eq!(slab.get(key2).unwrap(), "hello from thread two");
//!```
//!
//! If items in the slab must be mutated, a `Mutex` or `RwLock` may be used for
//! each item, providing granular locking of items rather than of the slab:
//!
//! ```rust
//! # use sharded_slab::Slab;
//! use std::sync::{Arc, Mutex};
//! let slab = Arc::new(Slab::new());
//!
//! let key = slab.insert(Mutex::new(String::from("hello world"))).unwrap();
//!
//! let slab2 = slab.clone();
//! let thread2 = std::thread::spawn(move || {
//!     let hello = slab2.get(key).expect("item missing");
//!     let mut hello = hello.lock().expect("mutex poisoned");
//!     *hello = String::from("hello everyone!");
//! });
//!
//! thread2.join().unwrap();
//!
//! let hello = slab.get(key).expect("item missing");
//! let mut hello = hello.lock().expect("mutex poisoned");
//! assert_eq!(hello.as_str(), "hello everyone!");
//! ```
//!
//! # Configuration
//!
//! For performance reasons, several values used by the slab are calculated as
//! constants. In order to allow users to tune the slab's parameters, we provide
//! a [`Config`] trait which defines these parameters as associated `consts`.
//! The `Slab` type is generic over a `C: Config` parameter.
//!
//! [`Config`]: trait.Config.html
//!
//! # Comparison with Similar Crates
//!
//! - [`slab`]: Carl Lerche's `slab` crate provides a slab implementation with a
//!   similar API, implemented by storing all data in a single vector.
//!
//!   Unlike `sharded_slab`, inserting and removing elements from the slab
//!   requires  mutable access. This means that if the slab is accessed
//!   concurrently by multiple threads, it is necessary for it to be protected
//!   by a `Mutex` or `RwLock`. Items may not be inserted or removed (or
//!   accessed, if a `Mutex` is used) concurrently, even when they are
//!   unrelated. In many cases, the lock can become a significant bottleneck. On
//!   the other hand, this crate allows separate indices in the slab to be
//!   accessed, inserted, and removed concurrently without requiring a global
//!   lock. Therefore, when the slab is shared across multiple threads, this
//!   crate offers significantly better performance than `slab`.
//!
//!   However, the lock free slab introduces some additional constant-factor
//!   overhead. This means that in use-cases where a slab is _not_ shared by
//!   multiple threads and locking is not required, this crate will likely offer
//!   slightly worse performance.
//!
//!   In summary: `sharded-slab` offers significantly improved performance in
//!   concurrent use-cases, while `slab` should be preferred in single-threaded
//!   use-cases.
//!
//! [`slab`]: https://crates.io/crates/loom
//!
//! # Safety and Correctness
//!
//! Most implementations of lock-free data structures in Rust require some
//! amount of unsafe code, and this crate is not an exception. In order to catch
//! potential bugs in this unsafe code, we make use of [`loom`], a
//! permutation-testing tool for concurrent Rust programs. All `unsafe` blocks
//! this crate occur in accesses to `loom` `UnsafeCell`s. This means that when
//! those accesses occur in this crate's tests, `loom` will assert that they are
//! valid under the C11 memory model across multiple permutations of concurrent
//! executions of those tests.
//!
//! In order to guard against the [ABA problem][aba], this crate makes use of
//! _generational indices_. Each slot in the slab tracks a generation counter
//! which is incremented every time a value is inserted into that slot, and the
//! indices returned by [`Slab::insert`] include the generation of the slot when
//! the value was inserted, packed into the high-order bits of the index. This
//! ensures that if a value is inserted, removed,  and a new value is inserted
//! into the same slot in the slab, the key returned by the first call to
//! `insert` will not map to the new value.
//!
//! Since a fixed number of bits are set aside to use for storing the generation
//! counter, the counter will wrap  around after being incremented a number of
//! times. To avoid situations where a returned index lives long enough to see the
//! generation counter wrap around to the same value, it is good to be fairly
//! generous when configuring the allocation of index bits.
//!
//! [`loom`]: https://crates.io/crates/loom
//! [aba]: https://en.wikipedia.org/wiki/ABA_problem
//! [`Slab::insert`]: struct.Slab.html#method.insert
//!
//! # Performance
//!
//! These graphs were produced by [benchmarks] of the sharded slab implementation,
//! using the [`criterion`] crate.
//!
//! The first shows the results of a benchmark where an increasing number of
//! items are inserted and then removed into a slab concurrently by five
//! threads. It compares the performance of the sharded slab implementation
//! with a `RwLock<slab::Slab>`:
//!
//! <img width="1124" alt="Screen Shot 2019-10-01 at 5 09 49 PM" src="https://user-images.githubusercontent.com/2796466/66078398-cd6c9f80-e516-11e9-9923-0ed6292e8498.png">
//!
//! The second graph shows the results of a benchmark where an increasing
//! number of items are inserted and then removed by a _single_ thread. It
//! compares the performance of the sharded slab implementation with an
//! `RwLock<slab::Slab>` and a `mut slab::Slab`.
//!
//! <img width="925" alt="Screen Shot 2019-10-01 at 5 13 45 PM" src="https://user-images.githubusercontent.com/2796466/66078469-f0974f00-e516-11e9-95b5-f65f0aa7e494.png">
//!
//! These benchmarks demonstrate that, while the sharded approach introduces
//! a small constant-factor overhead, it offers significantly better
//! performance across concurrent accesses.
//!
//! [benchmarks]: https://github.com/hawkw/sharded-slab/blob/master/benches/bench.rs
//! [`criterion`]: https://crates.io/crates/criterion
//!
//! # Implementation Notes
//!
//! See [this page](crate::implementation) for details on this crate's design
//! and implementation.
//!
#![doc(html_root_url = "https://docs.rs/sharded-slab/0.1.4")]
#![warn(missing_debug_implementations, missing_docs)]
#![cfg_attr(docsrs, warn(rustdoc::broken_intra_doc_links))]
#[macro_use]
mod macros;

pub mod implementation;
pub mod pool;

pub(crate) mod cfg;
pub(crate) mod sync;

mod clear;
mod iter;
mod page;
mod shard;
mod tid;

pub use self::{
    cfg::{Config, DefaultConfig},
    clear::Clear,
    iter::UniqueIter,
};
#[doc(inline)]
pub use pool::Pool;

pub(crate) use tid::Tid;

use cfg::CfgPrivate;
use shard::Shard;
use std::{fmt, marker::PhantomData, ptr, sync::Arc};

/// A sharded slab.
///
/// See the [crate-level documentation](crate) for details on using this type.
pub struct Slab<T, C: cfg::Config = DefaultConfig> {
    shards: shard::Array<Option<T>, C>,
    _cfg: PhantomData<C>,
}

/// A handle that allows access to an occupied entry in a [`Slab`].
///
/// While the guard exists, it indicates to the slab that the item the guard
/// references is currently being accessed. If the item is removed from the slab
/// while a guard exists, the removal will be deferred until all guards are
/// dropped.
pub struct Entry<'a, T, C: cfg::Config = DefaultConfig> {
    inner: page::slot::Guard<Option<T>, C>,
    value: ptr::NonNull<T>,
    shard: &'a Shard<Option<T>, C>,
    key: usize,
}

/// A handle to a vacant entry in a [`Slab`].
///
/// `VacantEntry` allows constructing values with the key that they will be
/// assigned to.
///
/// # Examples
///
/// ```
/// # use sharded_slab::Slab;
/// let mut slab = Slab::new();
///
/// let hello = {
///     let entry = slab.vacant_entry().unwrap();
///     let key = entry.key();
///
///     entry.insert((key, "hello"));
///     key
/// };
///
/// assert_eq!(hello, slab.get(hello).unwrap().0);
/// assert_eq!("hello", slab.get(hello).unwrap().1);
/// ```
#[derive(Debug)]
pub struct VacantEntry<'a, T, C: cfg::Config = DefaultConfig> {
    inner: page::slot::InitGuard<Option<T>, C>,
    key: usize,
    _lt: PhantomData<&'a ()>,
}

/// An owned reference to an occupied entry in a [`Slab`].
///
/// While the guard exists, it indicates to the slab that the item the guard
/// references is currently being accessed. If the item is removed from the slab
/// while the guard exists, the  removal will be deferred until all guards are
/// dropped.
///
/// Unlike [`Entry`], which borrows the slab, an `OwnedEntry` clones the [`Arc`]
/// around the slab. Therefore, it keeps the slab from being dropped until all
/// such guards have been dropped. This means that an `OwnedEntry` may be held for
/// an arbitrary lifetime.
///
/// # Examples
///
/// ```
/// # use sharded_slab::Slab;
/// use std::sync::Arc;
///
/// let slab: Arc<Slab<&'static str>> = Arc::new(Slab::new());
/// let key = slab.insert("hello world").unwrap();
///
/// // Look up the created key, returning an `OwnedEntry`.
/// let value = slab.clone().get_owned(key).unwrap();
///
/// // Now, the original `Arc` clone of the slab may be dropped, but the
/// // returned `OwnedEntry` can still access the value.
/// assert_eq!(value, "hello world");
/// ```
///
/// Unlike [`Entry`], an `OwnedEntry` may be stored in a struct which must live
/// for the `'static` lifetime:
///
/// ```
/// # use sharded_slab::Slab;
/// use sharded_slab::OwnedEntry;
/// use std::sync::Arc;
///
/// pub struct MyStruct {
///     entry: OwnedEntry<&'static str>,
///     // ... other fields ...
/// }
///
/// // Suppose this is some arbitrary function which requires a value that
/// // lives for the 'static lifetime...
/// fn function_requiring_static<T: 'static>(t: &T) {
///     // ... do something extremely important and interesting ...
/// }
///
/// let slab: Arc<Slab<&'static str>> = Arc::new(Slab::new());
/// let key = slab.insert("hello world").unwrap();
///
/// // Look up the created key, returning an `OwnedEntry`.
/// let entry = slab.clone().get_owned(key).unwrap();
/// let my_struct = MyStruct {
///     entry,
///     // ...
/// };
///
/// // We can use `my_struct` anywhere where it is required to have the
/// // `'static` lifetime:
/// function_requiring_static(&my_struct);
/// ```
///
/// `OwnedEntry`s may be sent between threads:
///
/// ```
/// # use sharded_slab::Slab;
/// use std::{thread, sync::Arc};
///
/// let slab: Arc<Slab<&'static str>> = Arc::new(Slab::new());
/// let key = slab.insert("hello world").unwrap();
///
/// // Look up the created key, returning an `OwnedEntry`.
/// let value = slab.clone().get_owned(key).unwrap();
///
/// thread::spawn(move || {
///     assert_eq!(value, "hello world");
///     // ...
/// }).join().unwrap();
/// ```
///
/// [`get`]: Slab::get
/// [`Arc`]: std::sync::Arc
pub struct OwnedEntry<T, C = DefaultConfig>
where
    C: cfg::Config,
{
    inner: page::slot::Guard<Option<T>, C>,
    value: ptr::NonNull<T>,
    slab: Arc<Slab<T, C>>,
    key: usize,
}

impl<T> Slab<T> {
    /// Returns a new slab with the default configuration parameters.
    pub fn new() -> Self {
        Self::new_with_config()
    }

    /// Returns a new slab with the provided configuration parameters.
    pub fn new_with_config<C: cfg::Config>() -> Slab<T, C> {
        C::validate();
        Slab {
            shards: shard::Array::new(),
            _cfg: PhantomData,
        }
    }
}

impl<T, C: cfg::Config> Slab<T, C> {
    /// The number of bits in each index which are used by the slab.
    ///
    /// If other data is packed into the `usize` indices returned by
    /// [`Slab::insert`], user code is free to use any bits higher than the
    /// `USED_BITS`-th bit freely.
    ///
    /// This is determined by the [`Config`] type that configures the slab's
    /// parameters. By default, all bits are used; this can be changed by
    /// overriding the [`Config::RESERVED_BITS`][res] constant.
    ///
    /// [res]: crate::Config#RESERVED_BITS
    pub const USED_BITS: usize = C::USED_BITS;

    /// Inserts a value into the slab, returning the integer index at which that
    /// value was inserted. This index can then be used to access the entry.
    ///
    /// If this function returns `None`, then the shard for the current thread
    /// is full and no items can be added until some are removed, or the maximum
    /// number of shards has been reached.
    ///
    /// # Examples
    /// ```rust
    /// # use sharded_slab::Slab;
    /// let slab = Slab::new();
    ///
    /// let key = slab.insert("hello world").unwrap();
    /// assert_eq!(slab.get(key).unwrap(), "hello world");
    /// ```
    pub fn insert(&self, value: T) -> Option<usize> {
        let (tid, shard) = self.shards.current();
        test_println!("insert {:?}", tid);
        let mut value = Some(value);
        shard
            .init_with(|idx, slot| {
                let gen = slot.insert(&mut value)?;
                Some(gen.pack(idx))
            })
            .map(|idx| tid.pack(idx))
    }

    /// Return a handle to a vacant entry allowing for further manipulation.
    ///
    /// This function is useful when creating values that must contain their
    /// slab index. The returned [`VacantEntry`] reserves a slot in the slab and
    /// is able to return the index of the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// # use sharded_slab::Slab;
    /// let mut slab = Slab::new();
    ///
    /// let hello = {
    ///     let entry = slab.vacant_entry().unwrap();
    ///     let key = entry.key();
    ///
    ///     entry.insert((key, "hello"));
    ///     key
    /// };
    ///
    /// assert_eq!(hello, slab.get(hello).unwrap().0);
    /// assert_eq!("hello", slab.get(hello).unwrap().1);
    /// ```
    pub fn vacant_entry(&self) -> Option<VacantEntry<'_, T, C>> {
        let (tid, shard) = self.shards.current();
        test_println!("vacant_entry {:?}", tid);
        shard.init_with(|idx, slot| {
            let inner = slot.init()?;
            let key = inner.generation().pack(tid.pack(idx));
            Some(VacantEntry {
                inner,
                key,
                _lt: PhantomData,
            })
        })
    }

    /// Remove the value at the given index in the slab, returning `true` if a
    /// value was removed.
    ///
    /// Unlike [`take`], this method does _not_ block the current thread until
    /// the value can be removed. Instead, if another thread is currently
    /// accessing that value, this marks it to be removed by that thread when it
    /// finishes accessing the value.
    ///
    /// # Examples
    ///
    /// ```rust
    /// let slab = sharded_slab::Slab::new();
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// // Remove the item from the slab.
    /// assert!(slab.remove(key));
    ///
    /// // Now, the slot is empty.
    /// assert!(!slab.contains(key));
    /// ```
    ///
    /// ```rust
    /// use std::sync::Arc;
    ///
    /// let slab = Arc::new(sharded_slab::Slab::new());
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// let slab2 = slab.clone();
    /// let thread2 = std::thread::spawn(move || {
    ///     // Depending on when this thread begins executing, the item may
    ///     // or may not have already been removed...
    ///     if let Some(item) = slab2.get(key) {
    ///         assert_eq!(item, "hello world");
    ///     }
    /// });
    ///
    /// // The item will be removed by thread2 when it finishes accessing it.
    /// assert!(slab.remove(key));
    ///
    /// thread2.join().unwrap();
    /// assert!(!slab.contains(key));
    /// ```
    /// [`take`]: Slab::take
    pub fn remove(&self, idx: usize) -> bool {
        // The `Drop` impl for `Entry` calls `remove_local` or `remove_remote` based
        // on where the guard was dropped from. If the dropped guard was the last one, this will
        // call `Slot::remove_value` which actually clears storage.
        let tid = C::unpack_tid(idx);

        test_println!("rm_deferred {:?}", tid);
        let shard = self.shards.get(tid.as_usize());
        if tid.is_current() {
            shard.map(|shard| shard.remove_local(idx)).unwrap_or(false)
        } else {
            shard.map(|shard| shard.remove_remote(idx)).unwrap_or(false)
        }
    }

    /// Removes the value associated with the given key from the slab, returning
    /// it.
    ///
    /// If the slab does not contain a value for that key, `None` is returned
    /// instead.
    ///
    /// If the value associated with the given key is currently being
    /// accessed by another thread, this method will block the current thread
    /// until the item is no longer accessed. If this is not desired, use
    /// [`remove`] instead.
    ///
    /// **Note**: This method blocks the calling thread by spinning until the
    /// currently outstanding references are released. Spinning for long periods
    /// of time can result in high CPU time and power consumption. Therefore,
    /// `take` should only be called when other references to the slot are
    /// expected to be dropped soon (e.g., when all accesses are relatively
    /// short).
    ///
    /// # Examples
    ///
    /// ```rust
    /// let slab = sharded_slab::Slab::new();
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// // Remove the item from the slab, returning it.
    /// assert_eq!(slab.take(key), Some("hello world"));
    ///
    /// // Now, the slot is empty.
    /// assert!(!slab.contains(key));
    /// ```
    ///
    /// ```rust
    /// use std::sync::Arc;
    ///
    /// let slab = Arc::new(sharded_slab::Slab::new());
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// let slab2 = slab.clone();
    /// let thread2 = std::thread::spawn(move || {
    ///     // Depending on when this thread begins executing, the item may
    ///     // or may not have already been removed...
    ///     if let Some(item) = slab2.get(key) {
    ///         assert_eq!(item, "hello world");
    ///     }
    /// });
    ///
    /// // The item will only be removed when the other thread finishes
    /// // accessing it.
    /// assert_eq!(slab.take(key), Some("hello world"));
    ///
    /// thread2.join().unwrap();
    /// assert!(!slab.contains(key));
    /// ```
    /// [`remove`]: Slab::remove
    pub fn take(&self, idx: usize) -> Option<T> {
        let tid = C::unpack_tid(idx);

        test_println!("rm {:?}", tid);
        let shard = self.shards.get(tid.as_usize())?;
        if tid.is_current() {
            shard.take_local(idx)
        } else {
            shard.take_remote(idx)
        }
    }

    /// Return a reference to the value associated with the given key.
    ///
    /// If the slab does not contain a value for the given key, or if the
    /// maximum number of concurrent references to the slot has been reached,
    /// `None` is returned instead.
    ///
    /// # Examples
    ///
    /// ```rust
    /// let slab = sharded_slab::Slab::new();
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// assert_eq!(slab.get(key).unwrap(), "hello world");
    /// assert!(slab.get(12345).is_none());
    /// ```
    pub fn get(&self, key: usize) -> Option<Entry<'_, T, C>> {
        let tid = C::unpack_tid(key);

        test_println!("get {:?}; current={:?}", tid, Tid::<C>::current());
        let shard = self.shards.get(tid.as_usize())?;
        shard.with_slot(key, |slot| {
            let inner = slot.get(C::unpack_gen(key))?;
            let value = ptr::NonNull::from(slot.value().as_ref().unwrap());
            Some(Entry {
                inner,
                value,
                shard,
                key,
            })
        })
    }

    /// Return an owned reference to the value at the given index.
    ///
    /// If the slab does not contain a value for the given key, `None` is
    /// returned instead.
    ///
    /// Unlike [`get`], which borrows the slab, this method _clones_ the [`Arc`]
    /// around the slab. This means that the returned [`OwnedEntry`] can be held
    /// for an arbitrary lifetime. However,  this method requires that the slab
    /// itself be wrapped in an `Arc`.
    ///
    /// # Examples
    ///
    /// ```
    /// # use sharded_slab::Slab;
    /// use std::sync::Arc;
    ///
    /// let slab: Arc<Slab<&'static str>> = Arc::new(Slab::new());
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// // Look up the created key, returning an `OwnedEntry`.
    /// let value = slab.clone().get_owned(key).unwrap();
    ///
    /// // Now, the original `Arc` clone of the slab may be dropped, but the
    /// // returned `OwnedEntry` can still access the value.
    /// assert_eq!(value, "hello world");
    /// ```
    ///
    /// Unlike [`Entry`], an `OwnedEntry` may be stored in a struct which must live
    /// for the `'static` lifetime:
    ///
    /// ```
    /// # use sharded_slab::Slab;
    /// use sharded_slab::OwnedEntry;
    /// use std::sync::Arc;
    ///
    /// pub struct MyStruct {
    ///     entry: OwnedEntry<&'static str>,
    ///     // ... other fields ...
    /// }
    ///
    /// // Suppose this is some arbitrary function which requires a value that
    /// // lives for the 'static lifetime...
    /// fn function_requiring_static<T: 'static>(t: &T) {
    ///     // ... do something extremely important and interesting ...
    /// }
    ///
    /// let slab: Arc<Slab<&'static str>> = Arc::new(Slab::new());
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// // Look up the created key, returning an `OwnedEntry`.
    /// let entry = slab.clone().get_owned(key).unwrap();
    /// let my_struct = MyStruct {
    ///     entry,
    ///     // ...
    /// };
    ///
    /// // We can use `my_struct` anywhere where it is required to have the
    /// // `'static` lifetime:
    /// function_requiring_static(&my_struct);
    /// ```
    ///
    /// [`OwnedEntry`]s may be sent between threads:
    ///
    /// ```
    /// # use sharded_slab::Slab;
    /// use std::{thread, sync::Arc};
    ///
    /// let slab: Arc<Slab<&'static str>> = Arc::new(Slab::new());
    /// let key = slab.insert("hello world").unwrap();
    ///
    /// // Look up the created key, returning an `OwnedEntry`.
    /// let value = slab.clone().get_owned(key).unwrap();
    ///
    /// thread::spawn(move || {
    ///     assert_eq!(value, "hello world");
    ///     // ...
    /// }).join().unwrap();
    /// ```
    ///
    /// [`get`]: Slab::get
    /// [`Arc`]: std::sync::Arc
    pub fn get_owned(self: Arc<Self>, key: usize) -> Option<OwnedEntry<T, C>> {
        let tid = C::unpack_tid(key);

        test_println!("get_owned {:?}; current={:?}", tid, Tid::<C>::current());
        let shard = self.shards.get(tid.as_usize())?;
        shard.with_slot(key, |slot| {
            let inner = slot.get(C::unpack_gen(key))?;
            let value = ptr::NonNull::from(slot.value().as_ref().unwrap());
            Some(OwnedEntry {
                inner,
                value,
                slab: self.clone(),
                key,
            })
        })
    }

    /// Returns `true` if the slab contains a value for the given key.
    ///
    /// # Examples
    ///
    /// ```
    /// let slab = sharded_slab::Slab::new();
    ///
    /// let key = slab.insert("hello world").unwrap();
    /// assert!(slab.contains(key));
    ///
    /// slab.take(key).unwrap();
    /// assert!(!slab.contains(key));
    /// ```
    pub fn contains(&self, key: usize) -> bool {
        self.get(key).is_some()
    }

    /// Returns an iterator over all the items in the slab.
    ///
    /// Because this iterator exclusively borrows the slab (i.e. it holds an
    /// `&mut Slab<T>`), elements will not be added or removed while the
    /// iteration is in progress.
    pub fn unique_iter(&mut self) -> iter::UniqueIter<'_, T, C> {
        let mut shards = self.shards.iter_mut();

        let (pages, slots) = match shards.next() {
            Some(shard) => {
                let mut pages = shard.iter();
                let slots = pages.next().and_then(page::Shared::iter);
                (pages, slots)
            }
            None => ([].iter(), None),
        };

        iter::UniqueIter {
            shards,
            pages,
            slots,
        }
    }
}

impl<T> Default for Slab<T> {
    fn default() -> Self {
        Self::new()
    }
}

impl<T: fmt::Debug, C: cfg::Config> fmt::Debug for Slab<T, C> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Slab")
            .field("shards", &self.shards)
            .field("config", &C::debug())
            .finish()
    }
}

unsafe impl<T: Send, C: cfg::Config> Send for Slab<T, C> {}
unsafe impl<T: Sync, C: cfg::Config> Sync for Slab<T, C> {}

// === impl Entry ===

impl<'a, T, C: cfg::Config> Entry<'a, T, C> {
    /// Returns the key used to access the guard.
    pub fn key(&self) -> usize {
        self.key
    }

    #[inline(always)]
    fn value(&self) -> &T {
        unsafe {
            // Safety: this is always going to be valid, as it's projected from
            // the safe reference to `self.value` --- this is just to avoid
            // having to `expect` an option in the hot path when dereferencing.
            self.value.as_ref()
        }
    }
}

impl<'a, T, C: cfg::Config> std::ops::Deref for Entry<'a, T, C> {
    type Target = T;

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

impl<'a, T, C: cfg::Config> Drop for Entry<'a, T, C> {
    fn drop(&mut self) {
        let should_remove = unsafe {
            // Safety: calling `slot::Guard::release` is unsafe, since the
            // `Guard` value contains a pointer to the slot that may outlive the
            // slab containing that slot. Here, the `Entry` guard owns a
            // borrowed reference to the shard containing that slot, which
            // ensures that the slot will not be dropped while this `Guard`
            // exists.
            self.inner.release()
        };
        if should_remove {
            self.shard.clear_after_release(self.key)
        }
    }
}

impl<'a, T, C> fmt::Debug for Entry<'a, T, C>
where
    T: fmt::Debug,
    C: cfg::Config,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self.value(), f)
    }
}

impl<'a, T, C> PartialEq<T> for Entry<'a, T, C>
where
    T: PartialEq<T>,
    C: cfg::Config,
{
    fn eq(&self, other: &T) -> bool {
        self.value().eq(other)
    }
}

// === impl VacantEntry ===

impl<'a, T, C: cfg::Config> VacantEntry<'a, T, C> {
    /// Insert a value in the entry.
    ///
    /// To get the integer index at which this value will be inserted, use
    /// [`key`] prior to calling `insert`.
    ///
    /// # Examples
    ///
    /// ```
    /// # use sharded_slab::Slab;
    /// let mut slab = Slab::new();
    ///
    /// let hello = {
    ///     let entry = slab.vacant_entry().unwrap();
    ///     let key = entry.key();
    ///
    ///     entry.insert((key, "hello"));
    ///     key
    /// };
    ///
    /// assert_eq!(hello, slab.get(hello).unwrap().0);
    /// assert_eq!("hello", slab.get(hello).unwrap().1);
    /// ```
    ///
    /// [`key`]: VacantEntry::key
    pub fn insert(mut self, val: T) {
        let value = unsafe {
            // Safety: this `VacantEntry` only lives as long as the `Slab` it was
            // borrowed from, so it cannot outlive the entry's slot.
            self.inner.value_mut()
        };
        debug_assert!(
            value.is_none(),
            "tried to insert to a slot that already had a value!"
        );
        *value = Some(val);
        let _released = unsafe {
            // Safety: again, this `VacantEntry` only lives as long as the
            // `Slab` it was borrowed from, so it cannot outlive the entry's
            // slot.
            self.inner.release()
        };
        debug_assert!(
            !_released,
            "removing a value before it was inserted should be a no-op"
        )
    }

    /// Return the integer index at which this entry will be inserted.
    ///
    /// A value stored in this entry will be associated with this key.
    ///
    /// # Examples
    ///
    /// ```
    /// # use sharded_slab::*;
    /// let mut slab = Slab::new();
    ///
    /// let hello = {
    ///     let entry = slab.vacant_entry().unwrap();
    ///     let key = entry.key();
    ///
    ///     entry.insert((key, "hello"));
    ///     key
    /// };
    ///
    /// assert_eq!(hello, slab.get(hello).unwrap().0);
    /// assert_eq!("hello", slab.get(hello).unwrap().1);
    /// ```
    pub fn key(&self) -> usize {
        self.key
    }
}
// === impl OwnedEntry ===

impl<T, C> OwnedEntry<T, C>
where
    C: cfg::Config,
{
    /// Returns the key used to access this guard
    pub fn key(&self) -> usize {
        self.key
    }

    #[inline(always)]
    fn value(&self) -> &T {
        unsafe {
            // Safety: this is always going to be valid, as it's projected from
            // the safe reference to `self.value` --- this is just to avoid
            // having to `expect` an option in the hot path when dereferencing.
            self.value.as_ref()
        }
    }
}

impl<T, C> std::ops::Deref for OwnedEntry<T, C>
where
    C: cfg::Config,
{
    type Target = T;

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

impl<T, C> Drop for OwnedEntry<T, C>
where
    C: cfg::Config,
{
    fn drop(&mut self) {
        test_println!("drop OwnedEntry: try clearing data");
        let should_clear = unsafe {
            // Safety: calling `slot::Guard::release` is unsafe, since the
            // `Guard` value contains a pointer to the slot that may outlive the
            // slab containing that slot. Here, the `OwnedEntry` owns an `Arc`
            // clone of the pool, which keeps it alive as long as the `OwnedEntry`
            // exists.
            self.inner.release()
        };
        if should_clear {
            let shard_idx = Tid::<C>::from_packed(self.key);
            test_println!("-> shard={:?}", shard_idx);
            if let Some(shard) = self.slab.shards.get(shard_idx.as_usize()) {
                shard.clear_after_release(self.key)
            } else {
                test_println!("-> shard={:?} does not exist! THIS IS A BUG", shard_idx);
                debug_assert!(std::thread::panicking(), "[internal error] tried to drop an `OwnedEntry` to a slot on a shard that never existed!");
            }
        }
    }
}

impl<T, C> fmt::Debug for OwnedEntry<T, C>
where
    T: fmt::Debug,
    C: cfg::Config,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self.value(), f)
    }
}

impl<T, C> PartialEq<T> for OwnedEntry<T, C>
where
    T: PartialEq<T>,
    C: cfg::Config,
{
    fn eq(&self, other: &T) -> bool {
        *self.value() == *other
    }
}

unsafe impl<T, C> Sync for OwnedEntry<T, C>
where
    T: Sync,
    C: cfg::Config,
{
}

unsafe impl<T, C> Send for OwnedEntry<T, C>
where
    T: Sync,
    C: cfg::Config,
{
}

// === pack ===

pub(crate) trait Pack<C: cfg::Config>: Sized {
    // ====== provided by each implementation =================================

    /// The number of bits occupied by this type when packed into a usize.
    ///
    /// This must be provided to determine the number of bits into which to pack
    /// the type.
    const LEN: usize;
    /// The type packed on the less significant side of this type.
    ///
    /// If this type is packed into the least significant bit of a usize, this
    /// should be `()`, which occupies no bytes.
    ///
    /// This is used to calculate the shift amount for packing this value.
    type Prev: Pack<C>;

    // ====== calculated automatically ========================================

    /// A number consisting of `Self::LEN` 1 bits, starting at the least
    /// significant bit.
    ///
    /// This is the higest value this type can represent. This number is shifted
    /// left by `Self::SHIFT` bits to calculate this type's `MASK`.
    ///
    /// This is computed automatically based on `Self::LEN`.
    const BITS: usize = {
        let shift = 1 << (Self::LEN - 1);
        shift | (shift - 1)
    };
    /// The number of bits to shift a number to pack it into a usize with other
    /// values.
    ///
    /// This is caculated automatically based on the `LEN` and `SHIFT` constants
    /// of the previous value.
    const SHIFT: usize = Self::Prev::SHIFT + Self::Prev::LEN;

    /// The mask to extract only this type from a packed `usize`.
    ///
    /// This is calculated by shifting `Self::BITS` left by `Self::SHIFT`.
    const MASK: usize = Self::BITS << Self::SHIFT;

    fn as_usize(&self) -> usize;
    fn from_usize(val: usize) -> Self;

    #[inline(always)]
    fn pack(&self, to: usize) -> usize {
        let value = self.as_usize();
        debug_assert!(value <= Self::BITS);

        (to & !Self::MASK) | (value << Self::SHIFT)
    }

    #[inline(always)]
    fn from_packed(from: usize) -> Self {
        let value = (from & Self::MASK) >> Self::SHIFT;
        debug_assert!(value <= Self::BITS);
        Self::from_usize(value)
    }
}

impl<C: cfg::Config> Pack<C> for () {
    const BITS: usize = 0;
    const LEN: usize = 0;
    const SHIFT: usize = 0;
    const MASK: usize = 0;

    type Prev = ();

    fn as_usize(&self) -> usize {
        unreachable!()
    }
    fn from_usize(_val: usize) -> Self {
        unreachable!()
    }

    fn pack(&self, _to: usize) -> usize {
        unreachable!()
    }

    fn from_packed(_from: usize) -> Self {
        unreachable!()
    }
}

#[cfg(test)]
pub(crate) use self::tests::util as test_util;

#[cfg(test)]
mod tests;