      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

    //! A concurrent work-stealing deque.
    //!
    //! This data structure is most commonly used in schedulers. The typical setup involves a number of
    //! threads where each thread has its own deque containing tasks. A thread may push tasks into its
    //! deque as well as pop tasks from it. Once it runs out of tasks, it may steal some from other
    //! threads to help complete tasks more quickly. Therefore, work-stealing deques supports three
    //! essential operations: *push*, *pop*, and *steal*.
    //!
    //! # Types of deques
    //!
    //! There are two types of deques, differing only in which order tasks get pushed and popped. The
    //! two task ordering strategies are:
    //!
    //! * First-in first-out (FIFO)
    //! * Last-in first-out (LIFO)
    //!
    //! A deque is a buffer with two ends, front and back. In a FIFO deque, tasks are pushed into the
    //! back, popped from the front, and stolen from the front. However, in a LIFO deque, tasks are
    //! popped from the back instead - that is the only difference.
    //!
    //! # Workers and stealers
    //!
    //! There are two functions that construct a deque: [`fifo`] and [`lifo`]. These functions return a
    //! [`Worker`] and a [`Stealer`]. The thread which owns the deque is usually called *worker*, while
    //! all other threads are *stealers*.
    //!
    //! [`Worker`] is able to push and pop tasks. It cannot be shared among multiple threads - only
    //! one thread owns it.
    //!
    //! [`Stealer`] can only steal tasks. It can be shared among multiple threads by reference or by
    //! cloning. Cloning a [`Stealer`] simply creates another one associated with the same deque.
    //!
    //! # Examples
    //!
    //! ```
    //! use crossbeam_deque::{self as deque, Pop, Steal};
    //! use std::thread;
    //!
    //! // Create a LIFO deque.
    //! let (w, s) = deque::lifo();
    //!
    //! // Push several elements into the back.
    //! w.push(1);
    //! w.push(2);
    //! w.push(3);
    //!
    //! // This is a LIFO deque, which means an element is popped from the back.
    //! // If it was a FIFO deque, `w.pop()` would return `Some(1)`.
    //! assert_eq!(w.pop(), Pop::Data(3));
    //!
    //! // Create a stealer thread.
    //! thread::spawn(move || {
    //!     assert_eq!(s.steal(), Steal::Data(1));
    //!     assert_eq!(s.steal(), Steal::Data(2));
    //! }).join().unwrap();
    //! ```
    //!
    //! [`Worker`]: struct.Worker.html
    //! [`Stealer`]: struct.Stealer.html
    //! [`fifo`]: fn.fifo.html
    //! [`lifo`]: fn.lifo.html

    #![warn(missing_docs)]
    #![warn(missing_debug_implementations)]

    extern crate crossbeam_epoch as epoch;
    extern crate crossbeam_utils as utils;

    use std::cell::Cell;
    use std::cmp;
    use std::fmt;
    use std::marker::PhantomData;
    use std::mem;
    use std::ptr;
    use std::sync::atomic::{self, AtomicIsize, Ordering};
    use std::sync::Arc;

    use epoch::{Atomic, Owned};
    use utils::CachePadded;

    /// Minimum buffer capacity for a deque.
    const MIN_CAP: usize = 32;

    /// Maximum number of additional elements that can be stolen in `steal_many`.
    const MAX_BATCH: usize = 128;

    /// If a buffer of at least this size is retired, thread-local garbage is flushed so that it gets
    /// deallocated as soon as possible.
    const FLUSH_THRESHOLD_BYTES: usize = 1 << 10;

    /// Creates a work-stealing deque with the first-in first-out strategy.
    ///
    /// Elements are pushed into the back, popped from the front, and stolen from the front. In other
    /// words, the worker side behaves as a FIFO queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_deque::{self as deque, Pop, Steal};
    ///
    /// let (w, s) = deque::fifo::<i32>();
    /// w.push(1);
    /// w.push(2);
    /// w.push(3);
    ///
    /// assert_eq!(s.steal(), Steal::Data(1));
    /// assert_eq!(w.pop(), Pop::Data(2));
    /// assert_eq!(w.pop(), Pop::Data(3));
    /// ```
    pub fn fifo<T>() -> (Worker<T>, Stealer<T>) {
        let buffer = Buffer::alloc(MIN_CAP);

        let inner = Arc::new(CachePadded::new(Inner {
            front: AtomicIsize::new(0),
            back: AtomicIsize::new(0),
            buffer: Atomic::new(buffer),
        }));

        let w = Worker {
            inner: inner.clone(),
            cached_buffer: Cell::new(buffer),
            flavor: Flavor::Fifo,
            _marker: PhantomData,
        };
        let s = Stealer {
            inner,
            flavor: Flavor::Fifo,
        };
        (w, s)
    }

    /// Creates a work-stealing deque with the last-in first-out strategy.
    ///
    /// Elements are pushed into the back, popped from the back, and stolen from the front. In other
    /// words, the worker side behaves as a LIFO stack.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_deque::{self as deque, Pop, Steal};
    ///
    /// let (w, s) = deque::lifo::<i32>();
    /// w.push(1);
    /// w.push(2);
    /// w.push(3);
    ///
    /// assert_eq!(s.steal(), Steal::Data(1));
    /// assert_eq!(w.pop(), Pop::Data(3));
    /// assert_eq!(w.pop(), Pop::Data(2));
    /// ```
    pub fn lifo<T>() -> (Worker<T>, Stealer<T>) {
        let buffer = Buffer::alloc(MIN_CAP);

        let inner = Arc::new(CachePadded::new(Inner {
            front: AtomicIsize::new(0),
            back: AtomicIsize::new(0),
            buffer: Atomic::new(buffer),
        }));

        let w = Worker {
            inner: inner.clone(),
            cached_buffer: Cell::new(buffer),
            flavor: Flavor::Lifo,
            _marker: PhantomData,
        };
        let s = Stealer {
            inner,
            flavor: Flavor::Lifo,
        };
        (w, s)
    }

    /// A buffer that holds elements in a deque.
    ///
    /// This is just a pointer to the buffer and its length - dropping an instance of this struct will
    /// *not* deallocate the buffer.
    struct Buffer<T> {
        /// Pointer to the allocated memory.
        ptr: *mut T,

        /// Capacity of the buffer. Always a power of two.
        cap: usize,
    }

    unsafe impl<T> Send for Buffer<T> {}

    impl<T> Buffer<T> {
        /// Allocates a new buffer with the specified capacity.
        fn alloc(cap: usize) -> Self {
            debug_assert_eq!(cap, cap.next_power_of_two());

            let mut v = Vec::with_capacity(cap);
            let ptr = v.as_mut_ptr();
            mem::forget(v);

            Buffer { ptr, cap }
        }

        /// Deallocates the buffer.
        unsafe fn dealloc(self) {
            drop(Vec::from_raw_parts(self.ptr, 0, self.cap));
        }

        /// Returns a pointer to the element at the specified `index`.
        unsafe fn at(&self, index: isize) -> *mut T {
            // `self.cap` is always a power of two.
            self.ptr.offset(index & (self.cap - 1) as isize)
        }

        /// Writes `value` into the specified `index`.
        ///
        /// Using this concurrently with another `read` or `write` is technically
        /// speaking UB due to data races.  We should be using relaxed accesses, but
        /// that would cost too much performance.  Hence, as a HACK, we use volatile
        /// accesses instead.  Experimental evidence shows that this works.
        unsafe fn write(&self, index: isize, value: T) {
            ptr::write_volatile(self.at(index), value)
        }

        /// Reads a value from the specified `index`.
        ///
        /// Using this concurrently with a `write` is technically speaking UB due to
        /// data races.  We should be using relaxed accesses, but that would cost
        /// too much performance.  Hence, as a HACK, we use volatile accesses
        /// instead.  Experimental evidence shows that this works.
        unsafe fn read(&self, index: isize) -> T {
            ptr::read_volatile(self.at(index))
        }
    }

    impl<T> Clone for Buffer<T> {
        fn clone(&self) -> Buffer<T> {
            Buffer {
                ptr: self.ptr,
                cap: self.cap,
            }
        }
    }

    impl<T> Copy for Buffer<T> {}

    /// Possible outcomes of a pop operation.
    #[must_use]
    #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Copy, Clone)]
    pub enum Pop<T> {
        /// The deque was empty at the time of popping.
        Empty,

        /// Some data has been successfully popped.
        Data(T),

        /// Lost the race for popping data to another concurrent steal operation. Try again.
        Retry,
    }

    /// Possible outcomes of a steal operation.
    #[must_use]
    #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Copy, Clone)]
    pub enum Steal<T> {
        /// The deque was empty at the time of stealing.
        Empty,

        /// Some data has been successfully stolen.
        Data(T),

        /// Lost the race for stealing data to another concurrent steal or pop operation. Try again.
        Retry,
    }

    /// Internal data that is shared between the worker and stealers.
    ///
    /// The implementation is based on the following work:
    ///
    /// 1. [Chase and Lev. Dynamic circular work-stealing deque. SPAA 2005.][chase-lev]
    /// 2. [Le, Pop, Cohen, and Nardelli. Correct and efficient work-stealing for weak memory models.
    ///    PPoPP 2013.][weak-mem]
    /// 3. [Norris and Demsky. CDSchecker: checking concurrent data structures written with C/C++
    ///    atomics. OOPSLA 2013.][checker]
    ///
    /// [chase-lev]: https://dl.acm.org/citation.cfm?id=1073974
    /// [weak-mem]: https://dl.acm.org/citation.cfm?id=2442524
    /// [checker]: https://dl.acm.org/citation.cfm?id=2509514
    struct Inner<T> {
        /// The front index.
        front: AtomicIsize,

        /// The back index.
        back: AtomicIsize,

        /// The underlying buffer.
        buffer: Atomic<Buffer<T>>,
    }

    impl<T> Drop for Inner<T> {
        fn drop(&mut self) {
            // Load the back index, front index, and buffer.
            let b = self.back.load(Ordering::Relaxed);
            let f = self.front.load(Ordering::Relaxed);

            unsafe {
                let buffer = self.buffer.load(Ordering::Relaxed, epoch::unprotected());

                // Go through the buffer from front to back and drop all elements in the deque.
                let mut i = f;
                while i != b {
                    ptr::drop_in_place(buffer.deref().at(i));
                    i = i.wrapping_add(1);
                }

                // Free the memory allocated by the buffer.
                buffer.into_owned().into_box().dealloc();
            }
        }
    }

    /// The flavor of a deque: FIFO or LIFO.
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    enum Flavor {
        /// The first-in first-out flavor.
        Fifo,

        /// The last-in first-out flavor.
        Lifo,
    }

    /// The worker side of a deque.
    ///
    /// Workers push elements into the back and pop elements depending on the strategy:
    ///
    /// * In FIFO deques, elements are popped from the front.
    /// * In LIFO deques, elements are popped from the back.
    ///
    /// A deque has only one worker. Workers are not intended to be shared among multiple threads.
    pub struct Worker<T> {
        /// A reference to the inner representation of the deque.
        inner: Arc<CachePadded<Inner<T>>>,

        /// A copy of `inner.buffer` for quick access.
        cached_buffer: Cell<Buffer<T>>,

        /// The flavor of the deque.
        flavor: Flavor,

        /// Indicates that the worker cannot be shared among threads.
        _marker: PhantomData<*mut ()>, // !Send + !Sync
    }

    unsafe impl<T: Send> Send for Worker<T> {}

    impl<T> Worker<T> {
        /// Resizes the internal buffer to the new capacity of `new_cap`.
        #[cold]
        unsafe fn resize(&self, new_cap: usize) {
            // Load the back index, front index, and buffer.
            let b = self.inner.back.load(Ordering::Relaxed);
            let f = self.inner.front.load(Ordering::Relaxed);
            let buffer = self.cached_buffer.get();

            // Allocate a new buffer.
            let new = Buffer::alloc(new_cap);
            self.cached_buffer.set(new);

            // Copy data from the old buffer to the new one.
            let mut i = f;
            while i != b {
                ptr::copy_nonoverlapping(buffer.at(i), new.at(i), 1);
                i = i.wrapping_add(1);
            }

            let guard = &epoch::pin();

            // Replace the old buffer with the new one.
            let old =
                self.inner
                    .buffer
                    .swap(Owned::new(new).into_shared(guard), Ordering::Release, guard);

            // Destroy the old buffer later.
            guard.defer_unchecked(move || old.into_owned().into_box().dealloc());

            // If the buffer is very large, then flush the thread-local garbage in order to deallocate
            // it as soon as possible.
            if mem::size_of::<T>() * new_cap >= FLUSH_THRESHOLD_BYTES {
                guard.flush();
            }
        }

        /// Reserves enough capacity so that `reserve_cap` elements can be pushed without growing the
        /// buffer.
        fn reserve(&self, reserve_cap: usize) {
            if reserve_cap > 0 {
                // Compute the current length.
                let b = self.inner.back.load(Ordering::Relaxed);
                let f = self.inner.front.load(Ordering::SeqCst);
                let len = b.wrapping_sub(f) as usize;

                // The current capacity.
                let cap = self.cached_buffer.get().cap;

                // Is there enough capacity to push `reserve_cap` elements?
                if cap - len < reserve_cap {
                    // Keep doubling the capacity as much as is needed.
                    let mut new_cap = cap * 2;
                    while new_cap - len < reserve_cap {
                        new_cap *= 2;
                    }

                    // Resize the buffer.
                    unsafe {
                        self.resize(new_cap);
                    }
                }
            }
        }

        /// Returns `true` if the deque is empty.
        ///
        /// ```
        /// use crossbeam_deque as deque;
        ///
        /// let (w, _) = deque::lifo();
        /// assert!(w.is_empty());
        /// w.push(1);
        /// assert!(!w.is_empty());
        /// ```
        pub fn is_empty(&self) -> bool {
            let b = self.inner.back.load(Ordering::Relaxed);
            let f = self.inner.front.load(Ordering::SeqCst);
            b.wrapping_sub(f) <= 0
        }

        /// Pushes an element into the back of the deque.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_deque as deque;
        ///
        /// let (w, _) = deque::lifo();
        /// w.push(1);
        /// w.push(2);
        /// ```
        pub fn push(&self, value: T) {
            // Load the back index, front index, and buffer.
            let b = self.inner.back.load(Ordering::Relaxed);
            let f = self.inner.front.load(Ordering::Acquire);
            let mut buffer = self.cached_buffer.get();

            // Calculate the length of the deque.
            let len = b.wrapping_sub(f);

            // Is the deque full?
            if len >= buffer.cap as isize {
                // Yes. Grow the underlying buffer.
                unsafe {
                    self.resize(2 * buffer.cap);
                }
                buffer = self.cached_buffer.get();
            }

            // Write `value` into the slot.
            unsafe {
                buffer.write(b, value);
            }

            atomic::fence(Ordering::Release);

            // Increment the back index.
            //
            // This ordering could be `Relaxed`, but then thread sanitizer would falsely report data
            // races because it doesn't understand fences.
            self.inner.back.store(b.wrapping_add(1), Ordering::Release);
        }

        /// Pops an element from the deque.
        ///
        /// Which end of the deque is used depends on the strategy:
        ///
        /// * If this is a FIFO deque, an element is popped from the front.
        /// * If this is a LIFO deque, an element is popped from the back.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_deque::{self as deque, Pop};
        ///
        /// let (w, _) = deque::fifo();
        /// w.push(1);
        /// w.push(2);
        ///
        /// assert_eq!(w.pop(), Pop::Data(1));
        /// assert_eq!(w.pop(), Pop::Data(2));
        /// assert_eq!(w.pop(), Pop::Empty);
        /// ```
        pub fn pop(&self) -> Pop<T> {
            // Load the back and front index.
            let b = self.inner.back.load(Ordering::Relaxed);
            let f = self.inner.front.load(Ordering::Relaxed);

            // Calculate the length of the deque.
            let len = b.wrapping_sub(f);

            // Is the deque empty?
            if len <= 0 {
                return Pop::Empty;
            }

            match self.flavor {
                // Pop from the front of the deque.
                Flavor::Fifo => {
                    // Try incrementing the front index to pop the value.
                    if self
                        .inner
                        .front
                        .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed)
                        .is_ok()
                    {
                        unsafe {
                            // Read the popped value.
                            let buffer = self.cached_buffer.get();
                            let data = buffer.read(f);

                            // Shrink the buffer if `len - 1` is less than one fourth of the capacity.
                            if buffer.cap > MIN_CAP && len <= buffer.cap as isize / 4 {
                                self.resize(buffer.cap / 2);
                            }

                            return Pop::Data(data);
                        }
                    }

                    Pop::Retry
                }

                // Pop from the back of the deque.
                Flavor::Lifo => {
                    // Decrement the back index.
                    let b = b.wrapping_sub(1);
                    self.inner.back.store(b, Ordering::Relaxed);

                    atomic::fence(Ordering::SeqCst);

                    // Load the front index.
                    let f = self.inner.front.load(Ordering::Relaxed);

                    // Compute the length after the back index was decremented.
                    let len = b.wrapping_sub(f);

                    if len < 0 {
                        // The deque is empty. Restore the back index to the original value.
                        self.inner.back.store(b.wrapping_add(1), Ordering::Relaxed);
                        Pop::Empty
                    } else {
                        // Read the value to be popped.
                        let buffer = self.cached_buffer.get();
                        let mut value = unsafe { Some(buffer.read(b)) };

                        // Are we popping the last element from the deque?
                        if len == 0 {
                            // Try incrementing the front index.
                            if self
                                .inner
                                .front
                                .compare_exchange(
                                    f,
                                    f.wrapping_add(1),
                                    Ordering::SeqCst,
                                    Ordering::Relaxed,
                                ).is_err()
                            {
                                // Failed. We didn't pop anything.
                                mem::forget(value.take());
                            }

                            // Restore the back index to the original value.
                            self.inner.back.store(b.wrapping_add(1), Ordering::Relaxed);
                        } else {
                            // Shrink the buffer if `len` is less than one fourth of the capacity.
                            if buffer.cap > MIN_CAP && len < buffer.cap as isize / 4 {
                                unsafe {
                                    self.resize(buffer.cap / 2);
                                }
                            }
                        }

                        match value {
                            None => Pop::Empty,
                            Some(data) => Pop::Data(data),
                        }
                    }
                }
            }
        }
    }

    impl<T> fmt::Debug for Worker<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Worker { .. }")
        }
    }

    /// The stealer side of a deque.
    ///
    /// Stealers can only steal elements from the front of the deque.
    ///
    /// Stealers are cloneable so that they can be easily shared among multiple threads.
    pub struct Stealer<T> {
        /// A reference to the inner representation of the deque.
        inner: Arc<CachePadded<Inner<T>>>,

        /// The flavor of the deque.
        flavor: Flavor,
    }

    unsafe impl<T: Send> Send for Stealer<T> {}
    unsafe impl<T: Send> Sync for Stealer<T> {}

    impl<T> Stealer<T> {
        /// Returns `true` if the deque is empty.
        ///
        /// ```
        /// use crossbeam_deque as deque;
        ///
        /// let (w, s) = deque::lifo();
        /// assert!(s.is_empty());
        /// w.push(1);
        /// assert!(!s.is_empty());
        /// ```
        pub fn is_empty(&self) -> bool {
            let f = self.inner.front.load(Ordering::Acquire);
            atomic::fence(Ordering::SeqCst);
            let b = self.inner.back.load(Ordering::Acquire);
            b.wrapping_sub(f) <= 0
        }

        /// Steals an element from the front of the deque.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_deque::{self as deque, Steal};
        ///
        /// let (w, s) = deque::lifo();
        /// w.push(1);
        /// w.push(2);
        ///
        /// assert_eq!(s.steal(), Steal::Data(1));
        /// assert_eq!(s.steal(), Steal::Data(2));
        /// assert_eq!(s.steal(), Steal::Empty);
        /// ```
        pub fn steal(&self) -> Steal<T> {
            // Load the front index.
            let f = self.inner.front.load(Ordering::Acquire);

            // A SeqCst fence is needed here.
            //
            // If the current thread is already pinned (reentrantly), we must manually issue the
            // fence. Otherwise, the following pinning will issue the fence anyway, so we don't
            // have to.
            if epoch::is_pinned() {
                atomic::fence(Ordering::SeqCst);
            }

            let guard = &epoch::pin();

            // Load the back index.
            let b = self.inner.back.load(Ordering::Acquire);

            // Is the deque empty?
            if b.wrapping_sub(f) <= 0 {
                return Steal::Empty;
            }

            // Load the buffer and read the value at the front.
            let buffer = self.inner.buffer.load(Ordering::Acquire, guard);
            let value = unsafe { buffer.deref().read(f) };

            // Try incrementing the front index to steal the value.
            if self
                .inner
                .front
                .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed)
                .is_err()
            {
                // We didn't steal this value, forget it.
                mem::forget(value);
                return Steal::Retry;
            }

            // Return the stolen value.
            Steal::Data(value)
        }

        /// Steals elements from the front of the deque.
        ///
        /// If at least one element can be stolen, it will be returned. Additionally, some of the
        /// remaining elements will be stolen and pushed into the back of worker `dest` in order to
        /// balance the work among deques. There is no hard guarantee on exactly how many elements will
        /// be stolen, but it should be around half of the deque.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_deque::{self as deque, Steal};
        ///
        /// let (w1, s1) = deque::fifo();
        /// let (w2, s2) = deque::fifo();
        ///
        /// w1.push(1);
        /// w1.push(2);
        /// w1.push(3);
        /// w1.push(4);
        ///
        /// assert_eq!(s1.steal_many(&w2), Steal::Data(1));
        /// assert_eq!(s2.steal(), Steal::Data(2));
        /// ```
        pub fn steal_many(&self, dest: &Worker<T>) -> Steal<T> {
            // Load the front index.
            let mut f = self.inner.front.load(Ordering::Acquire);

            // A SeqCst fence is needed here.
            //
            // If the current thread is already pinned (reentrantly), we must manually issue the
            // fence. Otherwise, the following pinning will issue the fence anyway, so we don't
            // have to.
            if epoch::is_pinned() {
                atomic::fence(Ordering::SeqCst);
            }

            let guard = &epoch::pin();

            // Load the back index.
            let b = self.inner.back.load(Ordering::Acquire);

            // Is the deque empty?
            let len = b.wrapping_sub(f);
            if len <= 0 {
                return Steal::Empty;
            }

            // Reserve capacity for the stolen additional elements.
            let additional = cmp::min((len as usize - 1) / 2, MAX_BATCH);
            dest.reserve(additional);
            let additional = additional as isize;

            // Get the destination buffer and back index.
            let dest_buffer = dest.cached_buffer.get();
            let mut dest_b = dest.inner.back.load(Ordering::Relaxed);

            // Load the buffer and read the value at the front.
            let buffer = self.inner.buffer.load(Ordering::Acquire, guard);
            let value = unsafe { buffer.deref().read(f) };

            match self.flavor {
                // Steal a batch of elements from the front at once.
                Flavor::Fifo => {
                    // Copy the additional elements from the source to the destination buffer.
                    for i in 0..additional {
                        unsafe {
                            let value = buffer.deref().read(f.wrapping_add(i + 1));
                            dest_buffer.write(dest_b.wrapping_add(i), value);
                        }
                    }

                    // Try incrementing the front index to steal the batch.
                    if self
                        .inner
                        .front
                        .compare_exchange(
                            f,
                            f.wrapping_add(additional + 1),
                            Ordering::SeqCst,
                            Ordering::Relaxed,
                        ).is_err()
                    {
                        // We didn't steal this value, forget it.
                        mem::forget(value);
                        return Steal::Retry;
                    }

                    atomic::fence(Ordering::Release);

                    // Success! Update the back index in the destination deque.
                    //
                    // This ordering could be `Relaxed`, but then thread sanitizer would falsely report
                    // data races because it doesn't understand fences.
                    dest.inner
                        .back
                        .store(dest_b.wrapping_add(additional), Ordering::Release);

                    // Return the first stolen value.
                    Steal::Data(value)
                }

                // Steal a batch of elements from the front one by one.
                Flavor::Lifo => {
                    // Try incrementing the front index to steal the value.
                    if self
                        .inner
                        .front
                        .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed)
                        .is_err()
                    {
                        // We didn't steal this value, forget it.
                        mem::forget(value);
                        return Steal::Retry;
                    }

                    // Move the front index one step forward.
                    f = f.wrapping_add(1);

                    // Repeat the same procedure for the additional steals.
                    for _ in 0..additional {
                        // We've already got the current front index. Now execute the fence to
                        // synchronize with other threads.
                        atomic::fence(Ordering::SeqCst);

                        // Load the back index.
                        let b = self.inner.back.load(Ordering::Acquire);

                        // Is the deque empty?
                        if b.wrapping_sub(f) <= 0 {
                            break;
                        }

                        // Read the value at the front.
                        let value = unsafe { buffer.deref().read(f) };

                        // Try incrementing the front index to steal the value.
                        if self
                            .inner
                            .front
                            .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed)
                            .is_err()
                        {
                            // We didn't steal this value, forget it and break from the loop.
                            mem::forget(value);
                            break;
                        }

                        // Write the stolen value into the destination buffer.
                        unsafe {
                            dest_buffer.write(dest_b, value);
                        }

                        // Move the source front index and the destination back index one step forward.
                        f = f.wrapping_add(1);
                        dest_b = dest_b.wrapping_add(1);

                        atomic::fence(Ordering::Release);

                        // Update the destination back index.
                        //
                        // This ordering could be `Relaxed`, but then thread sanitizer would falsely
                        // report data races because it doesn't understand fences.
                        dest.inner.back.store(dest_b, Ordering::Release);
                    }

                    // Return the first stolen value.
                    Steal::Data(value)
                }
            }
        }
    }

    impl<T> Clone for Stealer<T> {
        fn clone(&self) -> Stealer<T> {
            Stealer {
                inner: self.inner.clone(),
                flavor: self.flavor,
            }
        }
    }

    impl<T> fmt::Debug for Stealer<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Stealer { .. }")
        }
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)