       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

    // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use alloc::{Global, Alloc, Layout, LayoutErr, handle_alloc_error};
    use collections::CollectionAllocErr;
    use hash::{BuildHasher, Hash, Hasher};
    use marker;
    use mem::{size_of, needs_drop};
    use mem;
    use ops::{Deref, DerefMut};
    use ptr::{self, Unique, NonNull};
    use hint;

    use self::BucketState::*;

    /// Integer type used for stored hash values.
    ///
    /// No more than bit_width(usize) bits are needed to select a bucket.
    ///
    /// The most significant bit is ours to use for tagging `SafeHash`.
    ///
    /// (Even if we could have usize::MAX bytes allocated for buckets,
    /// each bucket stores at least a `HashUint`, so there can be no more than
    /// usize::MAX / size_of(usize) buckets.)
    type HashUint = usize;

    const EMPTY_BUCKET: HashUint = 0;
    const EMPTY: usize = 1;

    /// Special `Unique<HashUint>` that uses the lower bit of the pointer
    /// to expose a boolean tag.
    /// Note: when the pointer is initialized to EMPTY `.ptr()` will return
    /// null and the tag functions shouldn't be used.
    struct TaggedHashUintPtr(Unique<HashUint>);

    impl TaggedHashUintPtr {
        #[inline]
        unsafe fn new(ptr: *mut HashUint) -> Self {
            debug_assert!(ptr as usize & 1 == 0 || ptr as usize == EMPTY as usize);
            TaggedHashUintPtr(Unique::new_unchecked(ptr))
        }

        #[inline]
        fn set_tag(&mut self, value: bool) {
            let mut usize_ptr = self.0.as_ptr() as usize;
            unsafe {
                if value {
                    usize_ptr |= 1;
                } else {
                    usize_ptr &= !1;
                }
                self.0 = Unique::new_unchecked(usize_ptr as *mut HashUint)
            }
        }

        #[inline]
        fn tag(&self) -> bool {
            (self.0.as_ptr() as usize) & 1 == 1
        }

        #[inline]
        fn ptr(&self) -> *mut HashUint {
            (self.0.as_ptr() as usize & !1) as *mut HashUint
        }
    }

    /// The raw hashtable, providing safe-ish access to the unzipped and highly
    /// optimized arrays of hashes, and key-value pairs.
    ///
    /// This design is a lot faster than the naive
    /// `Vec<Option<(u64, K, V)>>`, because we don't pay for the overhead of an
    /// option on every element, and we get a generally more cache-aware design.
    ///
    /// Essential invariants of this structure:
    ///
    ///   - if `t.hashes[i] == EMPTY_BUCKET`, then `Bucket::at_index(&t, i).raw`
    ///     points to 'undefined' contents. Don't read from it. This invariant is
    ///     enforced outside this module with the `EmptyBucket`, `FullBucket`,
    ///     and `SafeHash` types.
    ///
    ///   - An `EmptyBucket` is only constructed at an index with
    ///     a hash of EMPTY_BUCKET.
    ///
    ///   - A `FullBucket` is only constructed at an index with a
    ///     non-EMPTY_BUCKET hash.
    ///
    ///   - A `SafeHash` is only constructed for non-`EMPTY_BUCKET` hash. We get
    ///     around hashes of zero by changing them to 0x8000_0000_0000_0000,
    ///     which will likely map to the same bucket, while not being confused
    ///     with "empty".
    ///
    ///   - Both "arrays represented by pointers" are the same length:
    ///     `capacity`. This is set at creation and never changes. The arrays
    ///     are unzipped and are more cache aware (scanning through 8 hashes
    ///     brings in at most 2 cache lines, since they're all right beside each
    ///     other). This layout may waste space in padding such as in a map from
    ///     u64 to u8, but is a more cache conscious layout as the key-value pairs
    ///     are only very shortly probed and the desired value will be in the same
    ///     or next cache line.
    ///
    /// You can kind of think of this module/data structure as a safe wrapper
    /// around just the "table" part of the hashtable. It enforces some
    /// invariants at the type level and employs some performance trickery,
    /// but in general is just a tricked out `Vec<Option<(u64, K, V)>>`.
    ///
    /// The hashtable also exposes a special boolean tag. The tag defaults to false
    /// when the RawTable is created and is accessible with the `tag` and `set_tag`
    /// functions.
    pub struct RawTable<K, V> {
        capacity_mask: usize,
        size: usize,
        hashes: TaggedHashUintPtr,

        // Because K/V do not appear directly in any of the types in the struct,
        // inform rustc that in fact instances of K and V are reachable from here.
        marker: marker::PhantomData<(K, V)>,
    }

    // An unsafe view of a RawTable bucket
    // Valid indexes are within [0..table_capacity)
    pub struct RawBucket<K, V> {
        hash_start: *mut HashUint,
        // We use *const to ensure covariance with respect to K and V
        pair_start: *const (K, V),
        idx: usize,
        _marker: marker::PhantomData<(K, V)>,
    }

    impl<K, V> Copy for RawBucket<K, V> {}
    impl<K, V> Clone for RawBucket<K, V> {
        fn clone(&self) -> RawBucket<K, V> {
            *self
        }
    }

    pub struct Bucket<K, V, M> {
        raw: RawBucket<K, V>,
        table: M,
    }

    impl<K, V, M: Copy> Copy for Bucket<K, V, M> {}
    impl<K, V, M: Copy> Clone for Bucket<K, V, M> {
        fn clone(&self) -> Bucket<K, V, M> {
            *self
        }
    }

    pub struct EmptyBucket<K, V, M> {
        raw: RawBucket<K, V>,
        table: M,
    }

    pub struct FullBucket<K, V, M> {
        raw: RawBucket<K, V>,
        table: M,
    }

    pub type FullBucketMut<'table, K, V> = FullBucket<K, V, &'table mut RawTable<K, V>>;

    pub enum BucketState<K, V, M> {
        Empty(EmptyBucket<K, V, M>),
        Full(FullBucket<K, V, M>),
    }

    // A GapThenFull encapsulates the state of two consecutive buckets at once.
    // The first bucket, called the gap, is known to be empty.
    // The second bucket is full.
    pub struct GapThenFull<K, V, M> {
        gap: EmptyBucket<K, V, ()>,
        full: FullBucket<K, V, M>,
    }

    /// A hash that is not zero, since we use a hash of zero to represent empty
    /// buckets.
    #[derive(PartialEq, Copy, Clone)]
    pub struct SafeHash {
        hash: HashUint,
    }

    impl SafeHash {
        /// Peek at the hash value, which is guaranteed to be non-zero.
        #[inline(always)]
        pub fn inspect(&self) -> HashUint {
            self.hash
        }

        #[inline(always)]
        pub fn new(hash: u64) -> Self {
            // We need to avoid 0 in order to prevent collisions with
            // EMPTY_HASH. We can maintain our precious uniform distribution
            // of initial indexes by unconditionally setting the MSB,
            // effectively reducing the hashes by one bit.
            //
            // Truncate hash to fit in `HashUint`.
            let hash_bits = size_of::<HashUint>() * 8;
            SafeHash { hash: (1 << (hash_bits - 1)) | (hash as HashUint) }
        }
    }

    /// We need to remove hashes of 0. That's reserved for empty buckets.
    /// This function wraps up `hash_keyed` to be the only way outside this
    /// module to generate a SafeHash.
    pub fn make_hash<T: ?Sized, S>(hash_state: &S, t: &T) -> SafeHash
        where T: Hash,
              S: BuildHasher
    {
        let mut state = hash_state.build_hasher();
        t.hash(&mut state);
        SafeHash::new(state.finish())
    }

    // `replace` casts a `*HashUint` to a `*SafeHash`. Since we statically
    // ensure that a `FullBucket` points to an index with a non-zero hash,
    // and a `SafeHash` is just a `HashUint` with a different name, this is
    // safe.
    //
    // This test ensures that a `SafeHash` really IS the same size as a
    // `HashUint`. If you need to change the size of `SafeHash` (and
    // consequently made this test fail), `replace` needs to be
    // modified to no longer assume this.
    #[test]
    fn can_alias_safehash_as_hash() {
        assert_eq!(size_of::<SafeHash>(), size_of::<HashUint>())
    }

    // RawBucket methods are unsafe as it's possible to
    // make a RawBucket point to invalid memory using safe code.
    impl<K, V> RawBucket<K, V> {
        unsafe fn hash(&self) -> *mut HashUint {
            self.hash_start.offset(self.idx as isize)
        }
        unsafe fn pair(&self) -> *mut (K, V) {
            self.pair_start.offset(self.idx as isize) as *mut (K, V)
        }
        unsafe fn hash_pair(&self) -> (*mut HashUint, *mut (K, V)) {
            (self.hash(), self.pair())
        }
    }

    // Buckets hold references to the table.
    impl<K, V, M> FullBucket<K, V, M> {
        /// Borrow a reference to the table.
        pub fn table(&self) -> &M {
            &self.table
        }
        /// Borrow a mutable reference to the table.
        pub fn table_mut(&mut self) -> &mut M {
            &mut self.table
        }
        /// Move out the reference to the table.
        pub fn into_table(self) -> M {
            self.table
        }
        /// Get the raw index.
        pub fn index(&self) -> usize {
            self.raw.idx
        }
        /// Get the raw bucket.
        pub fn raw(&self) -> RawBucket<K, V> {
            self.raw
        }
    }

    impl<K, V, M> EmptyBucket<K, V, M> {
        /// Borrow a reference to the table.
        pub fn table(&self) -> &M {
            &self.table
        }
        /// Borrow a mutable reference to the table.
        pub fn table_mut(&mut self) -> &mut M {
            &mut self.table
        }
    }

    impl<K, V, M> Bucket<K, V, M> {
        /// Get the raw index.
        pub fn index(&self) -> usize {
            self.raw.idx
        }
        /// get the table.
        pub fn into_table(self) -> M {
            self.table
        }
    }

    impl<K, V, M> Deref for FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>>
    {
        type Target = RawTable<K, V>;
        fn deref(&self) -> &RawTable<K, V> {
            &self.table
        }
    }

    /// `Put` is implemented for types which provide access to a table and cannot be invalidated
    ///  by filling a bucket. A similar implementation for `Take` is possible.
    pub trait Put<K, V> {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V>;
    }

    impl<'t, K, V> Put<K, V> for &'t mut RawTable<K, V> {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V> {
            *self
        }
    }

    impl<K, V, M> Put<K, V> for Bucket<K, V, M>
        where M: Put<K, V>
    {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V> {
            self.table.borrow_table_mut()
        }
    }

    impl<K, V, M> Put<K, V> for FullBucket<K, V, M>
        where M: Put<K, V>
    {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V> {
            self.table.borrow_table_mut()
        }
    }

    impl<K, V, M: Deref<Target = RawTable<K, V>>> Bucket<K, V, M> {
        pub fn new(table: M, hash: SafeHash) -> Bucket<K, V, M> {
            Bucket::at_index(table, hash.inspect() as usize)
        }

        pub fn new_from(r: RawBucket<K, V>, t: M)
            -> Bucket<K, V, M>
        {
            Bucket {
                raw: r,
                table: t,
            }
        }

        pub fn at_index(table: M, ib_index: usize) -> Bucket<K, V, M> {
            // if capacity is 0, then the RawBucket will be populated with bogus pointers.
            // This is an uncommon case though, so avoid it in release builds.
            debug_assert!(table.capacity() > 0,
                          "Table should have capacity at this point");
            let ib_index = ib_index & table.capacity_mask;
            Bucket {
                raw: table.raw_bucket_at(ib_index),
                table,
            }
        }

        pub fn first(table: M) -> Bucket<K, V, M> {
            Bucket {
                raw: table.raw_bucket_at(0),
                table,
            }
        }

        // "So a few of the first shall be last: for many be called,
        // but few chosen."
        //
        // We'll most likely encounter a few buckets at the beginning that
        // have their initial buckets near the end of the table. They were
        // placed at the beginning as the probe wrapped around the table
        // during insertion. We must skip forward to a bucket that won't
        // get reinserted too early and won't unfairly steal others spot.
        // This eliminates the need for robin hood.
        pub fn head_bucket(table: M) -> Bucket<K, V, M> {
            let mut bucket = Bucket::first(table);

            loop {
                bucket = match bucket.peek() {
                    Full(full) => {
                        if full.displacement() == 0 {
                            // This bucket occupies its ideal spot.
                            // It indicates the start of another "cluster".
                            bucket = full.into_bucket();
                            break;
                        }
                        // Leaving this bucket in the last cluster for later.
                        full.into_bucket()
                    }
                    Empty(b) => {
                        // Encountered a hole between clusters.
                        b.into_bucket()
                    }
                };
                bucket.next();
            }
            bucket
        }

        /// Reads a bucket at a given index, returning an enum indicating whether
        /// it's initialized or not. You need to match on this enum to get
        /// the appropriate types to call most of the other functions in
        /// this module.
        pub fn peek(self) -> BucketState<K, V, M> {
            match unsafe { *self.raw.hash() } {
                EMPTY_BUCKET => {
                    Empty(EmptyBucket {
                        raw: self.raw,
                        table: self.table,
                    })
                }
                _ => {
                    Full(FullBucket {
                        raw: self.raw,
                        table: self.table,
                    })
                }
            }
        }

        /// Modifies the bucket in place to make it point to the next slot.
        pub fn next(&mut self) {
            self.raw.idx = self.raw.idx.wrapping_add(1) & self.table.capacity_mask;
        }

        /// Modifies the bucket in place to make it point to the previous slot.
        pub fn prev(&mut self) {
            self.raw.idx = self.raw.idx.wrapping_sub(1) & self.table.capacity_mask;
        }
    }

    impl<K, V, M: Deref<Target = RawTable<K, V>>> EmptyBucket<K, V, M> {
        #[inline]
        pub fn next(self) -> Bucket<K, V, M> {
            let mut bucket = self.into_bucket();
            bucket.next();
            bucket
        }

        #[inline]
        pub fn into_bucket(self) -> Bucket<K, V, M> {
            Bucket {
                raw: self.raw,
                table: self.table,
            }
        }

        pub fn gap_peek(self) -> Result<GapThenFull<K, V, M>, Bucket<K, V, M>> {
            let gap = EmptyBucket {
                raw: self.raw,
                table: (),
            };

            match self.next().peek() {
                Full(bucket) => {
                    Ok(GapThenFull {
                        gap,
                        full: bucket,
                    })
                }
                Empty(e) => Err(e.into_bucket()),
            }
        }
    }

    impl<K, V, M> EmptyBucket<K, V, M>
        where M: Put<K, V>
    {
        /// Puts given key and value pair, along with the key's hash,
        /// into this bucket in the hashtable. Note how `self` is 'moved' into
        /// this function, because this slot will no longer be empty when
        /// we return! A `FullBucket` is returned for later use, pointing to
        /// the newly-filled slot in the hashtable.
        ///
        /// Use `make_hash` to construct a `SafeHash` to pass to this function.
        pub fn put(mut self, hash: SafeHash, key: K, value: V) -> FullBucket<K, V, M> {
            unsafe {
                *self.raw.hash() = hash.inspect();
                ptr::write(self.raw.pair(), (key, value));

                self.table.borrow_table_mut().size += 1;
            }

            FullBucket {
                raw: self.raw,
                table: self.table,
            }
        }
    }

    impl<K, V, M: Deref<Target = RawTable<K, V>>> FullBucket<K, V, M> {
        #[inline]
        pub fn next(self) -> Bucket<K, V, M> {
            let mut bucket = self.into_bucket();
            bucket.next();
            bucket
        }

        #[inline]
        pub fn into_bucket(self) -> Bucket<K, V, M> {
            Bucket {
                raw: self.raw,
                table: self.table,
            }
        }

        /// Duplicates the current position. This can be useful for operations
        /// on two or more buckets.
        pub fn stash(self) -> FullBucket<K, V, Self> {
            FullBucket {
                raw: self.raw,
                table: self,
            }
        }

        /// Get the distance between this bucket and the 'ideal' location
        /// as determined by the key's hash stored in it.
        ///
        /// In the cited blog posts above, this is called the "distance to
        /// initial bucket", or DIB. Also known as "probe count".
        pub fn displacement(&self) -> usize {
            // Calculates the distance one has to travel when going from
            // `hash mod capacity` onwards to `idx mod capacity`, wrapping around
            // if the destination is not reached before the end of the table.
            (self.raw.idx.wrapping_sub(self.hash().inspect() as usize)) & self.table.capacity_mask
        }

        #[inline]
        pub fn hash(&self) -> SafeHash {
            unsafe { SafeHash { hash: *self.raw.hash() } }
        }

        /// Gets references to the key and value at a given index.
        pub fn read(&self) -> (&K, &V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&(*pair_ptr).0, &(*pair_ptr).1)
            }
        }
    }

    // We take a mutable reference to the table instead of accepting anything that
    // implements `DerefMut` to prevent fn `take` from being called on `stash`ed
    // buckets.
    impl<'t, K, V> FullBucket<K, V, &'t mut RawTable<K, V>> {
        /// Removes this bucket's key and value from the hashtable.
        ///
        /// This works similarly to `put`, building an `EmptyBucket` out of the
        /// taken bucket.
        pub fn take(self) -> (EmptyBucket<K, V, &'t mut RawTable<K, V>>, K, V) {
            self.table.size -= 1;

            unsafe {
                *self.raw.hash() = EMPTY_BUCKET;
                let (k, v) = ptr::read(self.raw.pair());
                (EmptyBucket {
                     raw: self.raw,
                     table: self.table,
                 },
                k,
                v)
            }
        }
    }

    // This use of `Put` is misleading and restrictive, but safe and sufficient for our use cases
    // where `M` is a full bucket or table reference type with mutable access to the table.
    impl<K, V, M> FullBucket<K, V, M>
        where M: Put<K, V>
    {
        pub fn replace(&mut self, h: SafeHash, k: K, v: V) -> (SafeHash, K, V) {
            unsafe {
                let old_hash = ptr::replace(self.raw.hash() as *mut SafeHash, h);
                let (old_key, old_val) = ptr::replace(self.raw.pair(), (k, v));

                (old_hash, old_key, old_val)
            }
        }
    }

    impl<K, V, M> FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>> + DerefMut
    {
        /// Gets mutable references to the key and value at a given index.
        pub fn read_mut(&mut self) -> (&mut K, &mut V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&mut (*pair_ptr).0, &mut (*pair_ptr).1)
            }
        }
    }

    impl<'t, K, V, M> FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>> + 't
    {
        /// Exchange a bucket state for immutable references into the table.
        /// Because the underlying reference to the table is also consumed,
        /// no further changes to the structure of the table are possible;
        /// in exchange for this, the returned references have a longer lifetime
        /// than the references returned by `read()`.
        pub fn into_refs(self) -> (&'t K, &'t V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&(*pair_ptr).0, &(*pair_ptr).1)
            }
        }
    }

    impl<'t, K, V, M> FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>> + DerefMut + 't
    {
        /// This works similarly to `into_refs`, exchanging a bucket state
        /// for mutable references into the table.
        pub fn into_mut_refs(self) -> (&'t mut K, &'t mut V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&mut (*pair_ptr).0, &mut (*pair_ptr).1)
            }
        }
    }

    impl<K, V, M> GapThenFull<K, V, M>
        where M: Deref<Target = RawTable<K, V>>
    {
        #[inline]
        pub fn full(&self) -> &FullBucket<K, V, M> {
            &self.full
        }

        pub fn into_table(self) -> M {
            self.full.into_table()
        }

        pub fn shift(mut self) -> Result<GapThenFull<K, V, M>, Bucket<K, V, M>> {
            unsafe {
                let (gap_hash, gap_pair) = self.gap.raw.hash_pair();
                let (full_hash, full_pair) = self.full.raw.hash_pair();
                *gap_hash = mem::replace(&mut *full_hash, EMPTY_BUCKET);
                ptr::copy_nonoverlapping(full_pair, gap_pair, 1);
            }

            let FullBucket { raw: prev_raw, .. } = self.full;

            match self.full.next().peek() {
                Full(bucket) => {
                    self.gap.raw = prev_raw;

                    self.full = bucket;

                    Ok(self)
                }
                Empty(b) => Err(b.into_bucket()),
            }
        }
    }

    // Returns a Layout which describes the allocation required for a hash table,
    // and the offset of the array of (key, value) pairs in the allocation.
    fn calculate_layout<K, V>(capacity: usize) -> Result<(Layout, usize), LayoutErr> {
        let hashes = Layout::array::<HashUint>(capacity)?;
        let pairs = Layout::array::<(K, V)>(capacity)?;
        hashes.extend(pairs).map(|(layout, _)| {
            // LLVM seems to have trouble properly const-propagating pairs.align(),
            // possibly due to the use of NonZeroUsize. This little hack allows it
            // to generate optimal code.
            //
            // See https://github.com/rust-lang/rust/issues/51346 for more details.
            (
                layout,
                hashes.size() + hashes.padding_needed_for(mem::align_of::<(K, V)>()),
            )
        })
    }

    pub(crate) enum Fallibility {
        Fallible,
        Infallible,
    }

    use self::Fallibility::*;

    impl<K, V> RawTable<K, V> {
        /// Does not initialize the buckets. The caller should ensure they,
        /// at the very least, set every hash to EMPTY_BUCKET.
        /// Returns an error if it cannot allocate or capacity overflows.
        unsafe fn new_uninitialized_internal(
            capacity: usize,
            fallibility: Fallibility,
        ) -> Result<RawTable<K, V>, CollectionAllocErr> {
            if capacity == 0 {
                return Ok(RawTable {
                    size: 0,
                    capacity_mask: capacity.wrapping_sub(1),
                    hashes: TaggedHashUintPtr::new(EMPTY as *mut HashUint),
                    marker: marker::PhantomData,
                });
            }

            // Allocating hashmaps is a little tricky. We need to allocate two
            // arrays, but since we know their sizes and alignments up front,
            // we just allocate a single array, and then have the subarrays
            // point into it.
            let (layout, _) = calculate_layout::<K, V>(capacity)?;
            let buffer = Global.alloc(layout).map_err(|e| match fallibility {
                Infallible => handle_alloc_error(layout),
                Fallible => e,
            })?;

            Ok(RawTable {
                capacity_mask: capacity.wrapping_sub(1),
                size: 0,
                hashes: TaggedHashUintPtr::new(buffer.cast().as_ptr()),
                marker: marker::PhantomData,
            })
        }

        /// Does not initialize the buckets. The caller should ensure they,
        /// at the very least, set every hash to EMPTY_BUCKET.
        unsafe fn new_uninitialized(capacity: usize) -> RawTable<K, V> {
            match Self::new_uninitialized_internal(capacity, Infallible) {
                Err(CollectionAllocErr::CapacityOverflow) => panic!("capacity overflow"),
                Err(CollectionAllocErr::AllocErr) => unreachable!(),
                Ok(table) => { table }
            }
        }

        fn raw_bucket_at(&self, index: usize) -> RawBucket<K, V> {
            let (_, pairs_offset) = calculate_layout::<K, V>(self.capacity())
                .unwrap_or_else(|_| unsafe { hint::unreachable_unchecked() });
            let buffer = self.hashes.ptr() as *mut u8;
            unsafe {
                RawBucket {
                    hash_start: buffer as *mut HashUint,
                    pair_start: buffer.add(pairs_offset) as *const (K, V),
                    idx: index,
                    _marker: marker::PhantomData,
                }
            }
        }

        fn new_internal(
            capacity: usize,
            fallibility: Fallibility,
        ) -> Result<RawTable<K, V>, CollectionAllocErr> {
            unsafe {
                let ret = RawTable::new_uninitialized_internal(capacity, fallibility)?;
                ptr::write_bytes(ret.hashes.ptr(), 0, capacity);
                Ok(ret)
            }
        }

        /// Tries to create a new raw table from a given capacity. If it cannot allocate,
        /// it returns with AllocErr.
        pub fn try_new(capacity: usize) -> Result<RawTable<K, V>, CollectionAllocErr> {
            Self::new_internal(capacity, Fallible)
        }

        /// Creates a new raw table from a given capacity. All buckets are
        /// initially empty.
        pub fn new(capacity: usize) -> RawTable<K, V> {
            match Self::new_internal(capacity, Infallible) {
                Err(CollectionAllocErr::CapacityOverflow) => panic!("capacity overflow"),
                Err(CollectionAllocErr::AllocErr) => unreachable!(),
                Ok(table) => { table }
            }
        }

        /// The hashtable's capacity, similar to a vector's.
        pub fn capacity(&self) -> usize {
            self.capacity_mask.wrapping_add(1)
        }

        /// The number of elements ever `put` in the hashtable, minus the number
        /// of elements ever `take`n.
        pub fn size(&self) -> usize {
            self.size
        }

        fn raw_buckets(&self) -> RawBuckets<K, V> {
            RawBuckets {
                raw: self.raw_bucket_at(0),
                elems_left: self.size,
                marker: marker::PhantomData,
            }
        }

        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.raw_buckets(),
            }
        }

        pub fn iter_mut(&mut self) -> IterMut<K, V> {
            IterMut {
                iter: self.raw_buckets(),
                _marker: marker::PhantomData,
            }
        }

        pub fn into_iter(self) -> IntoIter<K, V> {
            let RawBuckets { raw, elems_left, .. } = self.raw_buckets();
            // Replace the marker regardless of lifetime bounds on parameters.
            IntoIter {
                iter: RawBuckets {
                    raw,
                    elems_left,
                    marker: marker::PhantomData,
                },
                table: self,
            }
        }

        pub fn drain(&mut self) -> Drain<K, V> {
            let RawBuckets { raw, elems_left, .. } = self.raw_buckets();
            // Replace the marker regardless of lifetime bounds on parameters.
            Drain {
                iter: RawBuckets {
                    raw,
                    elems_left,
                    marker: marker::PhantomData,
                },
                table: NonNull::from(self),
                marker: marker::PhantomData,
            }
        }

        /// Drops buckets in reverse order. It leaves the table in an inconsistent
        /// state and should only be used for dropping the table's remaining
        /// entries. It's used in the implementation of Drop.
        unsafe fn rev_drop_buckets(&mut self) {
            // initialize the raw bucket past the end of the table
            let mut raw = self.raw_bucket_at(self.capacity());
            let mut elems_left = self.size;

            while elems_left != 0 {
                raw.idx -= 1;

                if *raw.hash() != EMPTY_BUCKET {
                    elems_left -= 1;
                    ptr::drop_in_place(raw.pair());
                }
            }
        }

        /// Set the table tag
        pub fn set_tag(&mut self, value: bool) {
            self.hashes.set_tag(value)
        }

        /// Get the table tag
        pub fn tag(&self) -> bool {
            self.hashes.tag()
        }
    }

    /// A raw iterator. The basis for some other iterators in this module. Although
    /// this interface is safe, it's not used outside this module.
    struct RawBuckets<'a, K, V> {
        raw: RawBucket<K, V>,
        elems_left: usize,

        // Strictly speaking, this should be &'a (K,V), but that would
        // require that K:'a, and we often use RawBuckets<'static...> for
        // move iterations, so that messes up a lot of other things. So
        // just use `&'a (K,V)` as this is not a publicly exposed type
        // anyway.
        marker: marker::PhantomData<&'a ()>,
    }

    // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
    impl<'a, K, V> Clone for RawBuckets<'a, K, V> {
        fn clone(&self) -> RawBuckets<'a, K, V> {
            RawBuckets {
                raw: self.raw,
                elems_left: self.elems_left,
                marker: marker::PhantomData,
            }
        }
    }

    impl<'a, K, V> Iterator for RawBuckets<'a, K, V> {
        type Item = RawBucket<K, V>;

        fn next(&mut self) -> Option<RawBucket<K, V>> {
            if self.elems_left == 0 {
                return None;
            }

            loop {
                unsafe {
                    let item = self.raw;
                    self.raw.idx += 1;
                    if *item.hash() != EMPTY_BUCKET {
                        self.elems_left -= 1;
                        return Some(item);
                    }
                }
            }
        }

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

    impl<'a, K, V> ExactSizeIterator for RawBuckets<'a, K, V> {
        fn len(&self) -> usize {
            self.elems_left
        }
    }

    /// Iterator over shared references to entries in a table.
    pub struct Iter<'a, K: 'a, V: 'a> {
        iter: RawBuckets<'a, K, V>,
    }

    unsafe impl<'a, K: Sync, V: Sync> Sync for Iter<'a, K, V> {}
    unsafe impl<'a, K: Sync, V: Sync> Send for Iter<'a, K, V> {}

    // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
    impl<'a, K, V> Clone for Iter<'a, K, V> {
        fn clone(&self) -> Iter<'a, K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    /// Iterator over mutable references to entries in a table.
    pub struct IterMut<'a, K: 'a, V: 'a> {
        iter: RawBuckets<'a, K, V>,
        // To ensure invariance with respect to V
        _marker: marker::PhantomData<&'a mut V>,
    }

    unsafe impl<'a, K: Sync, V: Sync> Sync for IterMut<'a, K, V> {}
    // Both K: Sync and K: Send are correct for IterMut's Send impl,
    // but Send is the more useful bound
    unsafe impl<'a, K: Send, V: Send> Send for IterMut<'a, K, V> {}

    impl<'a, K: 'a, V: 'a> IterMut<'a, K, V> {
        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    /// Iterator over the entries in a table, consuming the table.
    pub struct IntoIter<K, V> {
        table: RawTable<K, V>,
        iter: RawBuckets<'static, K, V>,
    }

    unsafe impl<K: Sync, V: Sync> Sync for IntoIter<K, V> {}
    unsafe impl<K: Send, V: Send> Send for IntoIter<K, V> {}

    impl<K, V> IntoIter<K, V> {
        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    /// Iterator over the entries in a table, clearing the table.
    pub struct Drain<'a, K: 'a, V: 'a> {
        table: NonNull<RawTable<K, V>>,
        iter: RawBuckets<'static, K, V>,
        marker: marker::PhantomData<&'a RawTable<K, V>>,
    }

    unsafe impl<'a, K: Sync, V: Sync> Sync for Drain<'a, K, V> {}
    unsafe impl<'a, K: Send, V: Send> Send for Drain<'a, K, V> {}

    impl<'a, K, V> Drain<'a, K, V> {
        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    impl<'a, K, V> Iterator for Iter<'a, K, V> {
        type Item = (&'a K, &'a V);

        fn next(&mut self) -> Option<(&'a K, &'a V)> {
            self.iter.next().map(|raw| unsafe {
                let pair_ptr = raw.pair();
                (&(*pair_ptr).0, &(*pair_ptr).1)
            })
        }

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

    impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }

    impl<'a, K, V> Iterator for IterMut<'a, K, V> {
        type Item = (&'a K, &'a mut V);

        fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
            self.iter.next().map(|raw| unsafe {
                let pair_ptr = raw.pair();
                (&(*pair_ptr).0, &mut (*pair_ptr).1)
            })
        }

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

    impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }

    impl<K, V> Iterator for IntoIter<K, V> {
        type Item = (SafeHash, K, V);

        fn next(&mut self) -> Option<(SafeHash, K, V)> {
            self.iter.next().map(|raw| {
                self.table.size -= 1;
                unsafe {
                    let (k, v) = ptr::read(raw.pair());
                    (SafeHash { hash: *raw.hash() }, k, v)
                }
            })
        }

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

    impl<K, V> ExactSizeIterator for IntoIter<K, V> {
        fn len(&self) -> usize {
            self.iter().len()
        }
    }

    impl<'a, K, V> Iterator for Drain<'a, K, V> {
        type Item = (SafeHash, K, V);

        #[inline]
        fn next(&mut self) -> Option<(SafeHash, K, V)> {
            self.iter.next().map(|raw| {
                unsafe {
                    self.table.as_mut().size -= 1;
                    let (k, v) = ptr::read(raw.pair());
                    (SafeHash { hash: ptr::replace(&mut *raw.hash(), EMPTY_BUCKET) }, k, v)
                }
            })
        }

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

    impl<'a, K, V> ExactSizeIterator for Drain<'a, K, V> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }

    impl<'a, K: 'a, V: 'a> Drop for Drain<'a, K, V> {
        fn drop(&mut self) {
            self.for_each(drop);
        }
    }

    impl<K: Clone, V: Clone> Clone for RawTable<K, V> {
        fn clone(&self) -> RawTable<K, V> {
            unsafe {
                let cap = self.capacity();
                let mut new_ht = RawTable::new_uninitialized(cap);

                let mut new_buckets = new_ht.raw_bucket_at(0);
                let mut buckets = self.raw_bucket_at(0);
                while buckets.idx < cap {
                    *new_buckets.hash() = *buckets.hash();
                    if *new_buckets.hash() != EMPTY_BUCKET {
                        let pair_ptr = buckets.pair();
                        let kv = ((*pair_ptr).0.clone(), (*pair_ptr).1.clone());
                        ptr::write(new_buckets.pair(), kv);
                    }
                    buckets.idx += 1;
                    new_buckets.idx += 1;
                }

                new_ht.size = self.size();
                new_ht.set_tag(self.tag());

                new_ht
            }
        }
    }

    unsafe impl<#[may_dangle] K, #[may_dangle] V> Drop for RawTable<K, V> {
        fn drop(&mut self) {
            if self.capacity() == 0 {
                return;
            }

            // This is done in reverse because we've likely partially taken
            // some elements out with `.into_iter()` from the front.
            // Check if the size is 0, so we don't do a useless scan when
            // dropping empty tables such as on resize.
            // Also avoid double drop of elements that have been already moved out.
            unsafe {
                if needs_drop::<(K, V)>() {
                    // avoid linear runtime for types that don't need drop
                    self.rev_drop_buckets();
                }
            }

            let (layout, _) = calculate_layout::<K, V>(self.capacity())
                .unwrap_or_else(|_| unsafe { hint::unreachable_unchecked() });
            unsafe {
                Global.dealloc(NonNull::new_unchecked(self.hashes.ptr()).cast(), layout);
                // Remember how everything was allocated out of one buffer
                // during initialization? We only need one call to free here.
            }
        }
    }

# 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`)