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
use std::cmp;
use std::mem;

use aho_corasick::{self, packed, AhoCorasick, AhoCorasickBuilder};
use memchr::{memchr, memchr2, memchr3};
use syntax::hir::literal::{Literal, Literals};

use freqs::BYTE_FREQUENCIES;

/// A prefix extracted from a compiled regular expression.
///
/// A regex prefix is a set of literal strings that *must* be matched at the
/// beginning of a regex in order for the entire regex to match. Similarly
/// for a regex suffix.
#[derive(Clone, Debug)]
pub struct LiteralSearcher {
    complete: bool,
    lcp: FreqyPacked,
    lcs: FreqyPacked,
    matcher: Matcher,
}

#[derive(Clone, Debug)]
enum Matcher {
    /// No literals. (Never advances through the input.)
    Empty,
    /// A set of four or more single byte literals.
    Bytes(SingleByteSet),
    /// A single substring, find using memchr and frequency analysis.
    FreqyPacked(FreqyPacked),
    /// A single substring, find using Boyer-Moore.
    BoyerMoore(BoyerMooreSearch),
    /// An Aho-Corasick automaton.
    AC { ac: AhoCorasick<u32>, lits: Vec<Literal> },
    /// A packed multiple substring searcher, using SIMD.
    ///
    /// Note that Aho-Corasick will actually use this packed searcher
    /// internally automatically, however, there is some overhead associated
    /// with going through the Aho-Corasick machinery. So using the packed
    /// searcher directly results in some gains.
    Packed { s: packed::Searcher, lits: Vec<Literal> },
}

impl LiteralSearcher {
    /// Returns a matcher that never matches and never advances the input.
    pub fn empty() -> Self {
        Self::new(Literals::empty(), Matcher::Empty)
    }

    /// Returns a matcher for literal prefixes from the given set.
    pub fn prefixes(lits: Literals) -> Self {
        let matcher = Matcher::prefixes(&lits);
        Self::new(lits, matcher)
    }

    /// Returns a matcher for literal suffixes from the given set.
    pub fn suffixes(lits: Literals) -> Self {
        let matcher = Matcher::suffixes(&lits);
        Self::new(lits, matcher)
    }

    fn new(lits: Literals, matcher: Matcher) -> Self {
        let complete = lits.all_complete();
        LiteralSearcher {
            complete: complete,
            lcp: FreqyPacked::new(lits.longest_common_prefix().to_vec()),
            lcs: FreqyPacked::new(lits.longest_common_suffix().to_vec()),
            matcher: matcher,
        }
    }

    /// Returns true if all matches comprise the entire regular expression.
    ///
    /// This does not necessarily mean that a literal match implies a match
    /// of the regular expression. For example, the regular expresison `^a`
    /// is comprised of a single complete literal `a`, but the regular
    /// expression demands that it only match at the beginning of a string.
    pub fn complete(&self) -> bool {
        self.complete && !self.is_empty()
    }

    /// Find the position of a literal in `haystack` if it exists.
    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub fn find(&self, haystack: &[u8]) -> Option<(usize, usize)> {
        use self::Matcher::*;
        match self.matcher {
            Empty => Some((0, 0)),
            Bytes(ref sset) => sset.find(haystack).map(|i| (i, i + 1)),
            FreqyPacked(ref s) => s.find(haystack).map(|i| (i, i + s.len())),
            BoyerMoore(ref s) => s.find(haystack).map(|i| (i, i + s.len())),
            AC { ref ac, .. } => {
                ac.find(haystack).map(|m| (m.start(), m.end()))
            }
            Packed { ref s, .. } => {
                s.find(haystack).map(|m| (m.start(), m.end()))
            }
        }
    }

    /// Like find, except matches must start at index `0`.
    pub fn find_start(&self, haystack: &[u8]) -> Option<(usize, usize)> {
        for lit in self.iter() {
            if lit.len() > haystack.len() {
                continue;
            }
            if lit == &haystack[0..lit.len()] {
                return Some((0, lit.len()));
            }
        }
        None
    }

    /// Like find, except matches must end at index `haystack.len()`.
    pub fn find_end(&self, haystack: &[u8]) -> Option<(usize, usize)> {
        for lit in self.iter() {
            if lit.len() > haystack.len() {
                continue;
            }
            if lit == &haystack[haystack.len() - lit.len()..] {
                return Some((haystack.len() - lit.len(), haystack.len()));
            }
        }
        None
    }

    /// Returns an iterator over all literals to be matched.
    pub fn iter(&self) -> LiteralIter {
        match self.matcher {
            Matcher::Empty => LiteralIter::Empty,
            Matcher::Bytes(ref sset) => LiteralIter::Bytes(&sset.dense),
            Matcher::FreqyPacked(ref s) => LiteralIter::Single(&s.pat),
            Matcher::BoyerMoore(ref s) => LiteralIter::Single(&s.pattern),
            Matcher::AC { ref lits, .. } => LiteralIter::AC(lits),
            Matcher::Packed { ref lits, .. } => LiteralIter::Packed(lits),
        }
    }

    /// Returns a matcher for the longest common prefix of this matcher.
    pub fn lcp(&self) -> &FreqyPacked {
        &self.lcp
    }

    /// Returns a matcher for the longest common suffix of this matcher.
    pub fn lcs(&self) -> &FreqyPacked {
        &self.lcs
    }

    /// Returns true iff this prefix is empty.
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns the number of prefixes in this machine.
    pub fn len(&self) -> usize {
        use self::Matcher::*;
        match self.matcher {
            Empty => 0,
            Bytes(ref sset) => sset.dense.len(),
            FreqyPacked(_) => 1,
            BoyerMoore(_) => 1,
            AC { ref ac, .. } => ac.pattern_count(),
            Packed { ref lits, .. } => lits.len(),
        }
    }

    /// Return the approximate heap usage of literals in bytes.
    pub fn approximate_size(&self) -> usize {
        use self::Matcher::*;
        match self.matcher {
            Empty => 0,
            Bytes(ref sset) => sset.approximate_size(),
            FreqyPacked(ref single) => single.approximate_size(),
            BoyerMoore(ref single) => single.approximate_size(),
            AC { ref ac, .. } => ac.heap_bytes(),
            Packed { ref s, .. } => s.heap_bytes(),
        }
    }
}

impl Matcher {
    fn prefixes(lits: &Literals) -> Self {
        let sset = SingleByteSet::prefixes(lits);
        Matcher::new(lits, sset)
    }

    fn suffixes(lits: &Literals) -> Self {
        let sset = SingleByteSet::suffixes(lits);
        Matcher::new(lits, sset)
    }

    fn new(lits: &Literals, sset: SingleByteSet) -> Self {
        if lits.literals().is_empty() {
            return Matcher::Empty;
        }
        if sset.dense.len() >= 26 {
            // Avoid trying to match a large number of single bytes.
            // This is *very* sensitive to a frequency analysis comparison
            // between the bytes in sset and the composition of the haystack.
            // No matter the size of sset, if its members all are rare in the
            // haystack, then it'd be worth using it. How to tune this... IDK.
            // ---AG
            return Matcher::Empty;
        }
        if sset.complete {
            return Matcher::Bytes(sset);
        }
        if lits.literals().len() == 1 {
            let lit = lits.literals()[0].to_vec();
            if BoyerMooreSearch::should_use(lit.as_slice()) {
                return Matcher::BoyerMoore(BoyerMooreSearch::new(lit));
            } else {
                return Matcher::FreqyPacked(FreqyPacked::new(lit));
            }
        }

        let pats = lits.literals().to_owned();
        let is_aho_corasick_fast = sset.dense.len() <= 1 && sset.all_ascii;
        if lits.literals().len() <= 100 && !is_aho_corasick_fast {
            let mut builder = packed::Config::new()
                .match_kind(packed::MatchKind::LeftmostFirst)
                .builder();
            if let Some(s) = builder.extend(&pats).build() {
                return Matcher::Packed { s, lits: pats };
            }
        }
        let ac = AhoCorasickBuilder::new()
            .match_kind(aho_corasick::MatchKind::LeftmostFirst)
            .dfa(true)
            .build_with_size::<u32, _, _>(&pats)
            .unwrap();
        Matcher::AC { ac, lits: pats }
    }
}

pub enum LiteralIter<'a> {
    Empty,
    Bytes(&'a [u8]),
    Single(&'a [u8]),
    AC(&'a [Literal]),
    Packed(&'a [Literal]),
}

impl<'a> Iterator for LiteralIter<'a> {
    type Item = &'a [u8];

    fn next(&mut self) -> Option<Self::Item> {
        match *self {
            LiteralIter::Empty => None,
            LiteralIter::Bytes(ref mut many) => {
                if many.is_empty() {
                    None
                } else {
                    let next = &many[0..1];
                    *many = &many[1..];
                    Some(next)
                }
            }
            LiteralIter::Single(ref mut one) => {
                if one.is_empty() {
                    None
                } else {
                    let next = &one[..];
                    *one = &[];
                    Some(next)
                }
            }
            LiteralIter::AC(ref mut lits) => {
                if lits.is_empty() {
                    None
                } else {
                    let next = &lits[0];
                    *lits = &lits[1..];
                    Some(&**next)
                }
            }
            LiteralIter::Packed(ref mut lits) => {
                if lits.is_empty() {
                    None
                } else {
                    let next = &lits[0];
                    *lits = &lits[1..];
                    Some(&**next)
                }
            }
        }
    }
}

#[derive(Clone, Debug)]
struct SingleByteSet {
    sparse: Vec<bool>,
    dense: Vec<u8>,
    complete: bool,
    all_ascii: bool,
}

impl SingleByteSet {
    fn new() -> SingleByteSet {
        SingleByteSet {
            sparse: vec![false; 256],
            dense: vec![],
            complete: true,
            all_ascii: true,
        }
    }

    fn prefixes(lits: &Literals) -> SingleByteSet {
        let mut sset = SingleByteSet::new();
        for lit in lits.literals() {
            sset.complete = sset.complete && lit.len() == 1;
            if let Some(&b) = lit.get(0) {
                if !sset.sparse[b as usize] {
                    if b > 0x7F {
                        sset.all_ascii = false;
                    }
                    sset.dense.push(b);
                    sset.sparse[b as usize] = true;
                }
            }
        }
        sset
    }

    fn suffixes(lits: &Literals) -> SingleByteSet {
        let mut sset = SingleByteSet::new();
        for lit in lits.literals() {
            sset.complete = sset.complete && lit.len() == 1;
            if let Some(&b) = lit.get(lit.len().checked_sub(1).unwrap()) {
                if !sset.sparse[b as usize] {
                    if b > 0x7F {
                        sset.all_ascii = false;
                    }
                    sset.dense.push(b);
                    sset.sparse[b as usize] = true;
                }
            }
        }
        sset
    }

    /// Faster find that special cases certain sizes to use memchr.
    #[cfg_attr(feature = "perf-inline", inline(always))]
    fn find(&self, text: &[u8]) -> Option<usize> {
        match self.dense.len() {
            0 => None,
            1 => memchr(self.dense[0], text),
            2 => memchr2(self.dense[0], self.dense[1], text),
            3 => memchr3(self.dense[0], self.dense[1], self.dense[2], text),
            _ => self._find(text),
        }
    }

    /// Generic find that works on any sized set.
    fn _find(&self, haystack: &[u8]) -> Option<usize> {
        for (i, &b) in haystack.iter().enumerate() {
            if self.sparse[b as usize] {
                return Some(i);
            }
        }
        None
    }

    fn approximate_size(&self) -> usize {
        (self.dense.len() * mem::size_of::<u8>())
            + (self.sparse.len() * mem::size_of::<bool>())
    }
}

/// Provides an implementation of fast subtring search using frequency
/// analysis.
///
/// memchr is so fast that we do everything we can to keep the loop in memchr
/// for as long as possible. The easiest way to do this is to intelligently
/// pick the byte to send to memchr. The best byte is the byte that occurs
/// least frequently in the haystack. Since doing frequency analysis on the
/// haystack is far too expensive, we compute a set of fixed frequencies up
/// front and hard code them in src/freqs.rs. Frequency analysis is done via
/// scripts/frequencies.py.
#[derive(Clone, Debug)]
pub struct FreqyPacked {
    /// The pattern.
    pat: Vec<u8>,
    /// The number of Unicode characters in the pattern. This is useful for
    /// determining the effective length of a pattern when deciding which
    /// optimizations to perform. A trailing incomplete UTF-8 sequence counts
    /// as one character.
    char_len: usize,
    /// The rarest byte in the pattern, according to pre-computed frequency
    /// analysis.
    rare1: u8,
    /// The offset of the rarest byte in `pat`.
    rare1i: usize,
    /// The second rarest byte in the pattern, according to pre-computed
    /// frequency analysis. (This may be equivalent to the rarest byte.)
    ///
    /// The second rarest byte is used as a type of guard for quickly detecting
    /// a mismatch after memchr locates an instance of the rarest byte. This
    /// is a hedge against pathological cases where the pre-computed frequency
    /// analysis may be off. (But of course, does not prevent *all*
    /// pathological cases.)
    rare2: u8,
    /// The offset of the second rarest byte in `pat`.
    rare2i: usize,
}

impl FreqyPacked {
    fn new(pat: Vec<u8>) -> FreqyPacked {
        if pat.is_empty() {
            return FreqyPacked::empty();
        }

        // Find the rarest two bytes. Try to make them distinct (but it's not
        // required).
        let mut rare1 = pat[0];
        let mut rare2 = pat[0];
        for b in pat[1..].iter().cloned() {
            if freq_rank(b) < freq_rank(rare1) {
                rare1 = b;
            }
        }
        for &b in &pat {
            if rare1 == rare2 {
                rare2 = b
            } else if b != rare1 && freq_rank(b) < freq_rank(rare2) {
                rare2 = b;
            }
        }

        // And find the offsets of their last occurrences.
        let rare1i = pat.iter().rposition(|&b| b == rare1).unwrap();
        let rare2i = pat.iter().rposition(|&b| b == rare2).unwrap();

        let char_len = char_len_lossy(&pat);
        FreqyPacked {
            pat: pat,
            char_len: char_len,
            rare1: rare1,
            rare1i: rare1i,
            rare2: rare2,
            rare2i: rare2i,
        }
    }

    fn empty() -> FreqyPacked {
        FreqyPacked {
            pat: vec![],
            char_len: 0,
            rare1: 0,
            rare1i: 0,
            rare2: 0,
            rare2i: 0,
        }
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub fn find(&self, haystack: &[u8]) -> Option<usize> {
        let pat = &*self.pat;
        if haystack.len() < pat.len() || pat.is_empty() {
            return None;
        }
        let mut i = self.rare1i;
        while i < haystack.len() {
            i += match memchr(self.rare1, &haystack[i..]) {
                None => return None,
                Some(i) => i,
            };
            let start = i - self.rare1i;
            let end = start + pat.len();
            if end > haystack.len() {
                return None;
            }
            let aligned = &haystack[start..end];
            if aligned[self.rare2i] == self.rare2 && aligned == &*self.pat {
                return Some(start);
            }
            i += 1;
        }
        None
    }

    #[cfg_attr(feature = "perf-inline", inline(always))]
    pub fn is_suffix(&self, text: &[u8]) -> bool {
        if text.len() < self.len() {
            return false;
        }
        text[text.len() - self.len()..] == *self.pat
    }

    pub fn len(&self) -> usize {
        self.pat.len()
    }

    pub fn char_len(&self) -> usize {
        self.char_len
    }

    fn approximate_size(&self) -> usize {
        self.pat.len() * mem::size_of::<u8>()
    }
}

fn char_len_lossy(bytes: &[u8]) -> usize {
    String::from_utf8_lossy(bytes).chars().count()
}

/// An implementation of Tuned Boyer-Moore as laid out by
/// Andrew Hume and Daniel Sunday in "Fast String Searching".
/// O(n) in the size of the input.
///
/// Fast string searching algorithms come in many variations,
/// but they can generally be described in terms of three main
/// components.
///
/// The skip loop is where the string searcher wants to spend
/// as much time as possible. Exactly which character in the
/// pattern the skip loop examines varies from algorithm to
/// algorithm, but in the simplest case this loop repeated
/// looks at the last character in the pattern and jumps
/// forward in the input if it is not in the pattern.
/// Robert Boyer and J Moore called this the "fast" loop in
/// their original paper.
///
/// The match loop is responsible for actually examining the
/// whole potentially matching substring. In order to fail
/// faster, the match loop sometimes has a guard test attached.
/// The guard test uses frequency analysis of the different
/// characters in the pattern to choose the least frequency
/// occurring character and use it to find match failures
/// as quickly as possible.
///
/// The shift rule governs how the algorithm will shuffle its
/// test window in the event of a failure during the match loop.
/// Certain shift rules allow the worst-case run time of the
/// algorithm to be shown to be O(n) in the size of the input
/// rather than O(nm) in the size of the input and the size
/// of the pattern (as naive Boyer-Moore is).
///
/// "Fast String Searching", in addition to presenting a tuned
/// algorithm, provides a comprehensive taxonomy of the many
/// different flavors of string searchers. Under that taxonomy
/// TBM, the algorithm implemented here, uses an unrolled fast
/// skip loop with memchr fallback, a forward match loop with guard,
/// and the mini Sunday's delta shift rule. To unpack that you'll have to
/// read the paper.
#[derive(Clone, Debug)]
pub struct BoyerMooreSearch {
    /// The pattern we are going to look for in the haystack.
    pattern: Vec<u8>,

    /// The skip table for the skip loop.
    ///
    /// Maps the character at the end of the input
    /// to a shift.
    skip_table: Vec<usize>,

    /// The guard character (least frequently occurring char).
    guard: u8,
    /// The reverse-index of the guard character in the pattern.
    guard_reverse_idx: usize,

    /// Daniel Sunday's mini generalized delta2 shift table.
    ///
    /// We use a skip loop, so we only have to provide a shift
    /// for the skip char (last char). This is why it is a mini
    /// shift rule.
    md2_shift: usize,
}

impl BoyerMooreSearch {
    /// Create a new string searcher, performing whatever
    /// compilation steps are required.
    fn new(pattern: Vec<u8>) -> Self {
        debug_assert!(!pattern.is_empty());

        let (g, gi) = Self::select_guard(pattern.as_slice());
        let skip_table = Self::compile_skip_table(pattern.as_slice());
        let md2_shift = Self::compile_md2_shift(pattern.as_slice());
        BoyerMooreSearch {
            pattern: pattern,
            skip_table: skip_table,
            guard: g,
            guard_reverse_idx: gi,
            md2_shift: md2_shift,
        }
    }

    /// Find the pattern in `haystack`, returning the offset
    /// of the start of the first occurrence of the pattern
    /// in `haystack`.
    #[inline]
    fn find(&self, haystack: &[u8]) -> Option<usize> {
        if haystack.len() < self.pattern.len() {
            return None;
        }

        let mut window_end = self.pattern.len() - 1;

        // Inspired by the grep source. It is a way
        // to do correct loop unrolling without having to place
        // a crashpad of terminating charicters at the end in
        // the way described in the Fast String Searching paper.
        const NUM_UNROLL: usize = 10;
        // 1 for the initial position, and 1 for the md2 shift
        let short_circut = (NUM_UNROLL + 2) * self.pattern.len();

        if haystack.len() > short_circut {
            // just 1 for the md2 shift
            let backstop =
                haystack.len() - ((NUM_UNROLL + 1) * self.pattern.len());
            loop {
                window_end =
                    match self.skip_loop(haystack, window_end, backstop) {
                        Some(i) => i,
                        None => return None,
                    };
                if window_end >= backstop {
                    break;
                }

                if self.check_match(haystack, window_end) {
                    return Some(window_end - (self.pattern.len() - 1));
                } else {
                    let skip = self.skip_table[haystack[window_end] as usize];
                    window_end +=
                        if skip == 0 { self.md2_shift } else { skip };
                    continue;
                }
            }
        }

        // now process the input after the backstop
        while window_end < haystack.len() {
            let mut skip = self.skip_table[haystack[window_end] as usize];
            if skip == 0 {
                if self.check_match(haystack, window_end) {
                    return Some(window_end - (self.pattern.len() - 1));
                } else {
                    skip = self.md2_shift;
                }
            }
            window_end += skip;
        }

        None
    }

    fn len(&self) -> usize {
        return self.pattern.len();
    }

    /// The key heuristic behind which the BoyerMooreSearch lives.
    ///
    /// See `rust-lang/regex/issues/408`.
    ///
    /// Tuned Boyer-Moore is actually pretty slow! It turns out a handrolled
    /// platform-specific memchr routine with a bit of frequency
    /// analysis sprinkled on top actually wins most of the time.
    /// However, there are a few cases where Tuned Boyer-Moore still
    /// wins.
    ///
    /// If the haystack is random, frequency analysis doesn't help us,
    /// so Boyer-Moore will win for sufficiently large needles.
    /// Unfortunately, there is no obvious way to determine this
    /// ahead of time.
    ///
    /// If the pattern itself consists of very common characters,
    /// frequency analysis won't get us anywhere. The most extreme
    /// example of this is a pattern like `eeeeeeeeeeeeeeee`. Fortunately,
    /// this case is wholly determined by the pattern, so we can actually
    /// implement the heuristic.
    ///
    /// A third case is if the pattern is sufficiently long. The idea
    /// here is that once the pattern gets long enough the Tuned
    /// Boyer-Moore skip loop will start making strides long enough
    /// to beat the asm deep magic that is memchr.
    fn should_use(pattern: &[u8]) -> bool {
        // The minimum pattern length required to use TBM.
        const MIN_LEN: usize = 9;
        // The minimum frequency rank (lower is rarer) that every byte in the
        // pattern must have in order to use TBM. That is, if the pattern
        // contains _any_ byte with a lower rank, then TBM won't be used.
        const MIN_CUTOFF: usize = 150;
        // The maximum frequency rank for any byte.
        const MAX_CUTOFF: usize = 255;
        // The scaling factor used to determine the actual cutoff frequency
        // to use (keeping in mind that the minimum frequency rank is bounded
        // by MIN_CUTOFF). This scaling factor is an attempt to make TBM more
        // likely to be used as the pattern grows longer. That is, longer
        // patterns permit somewhat less frequent bytes than shorter patterns,
        // under the assumption that TBM gets better as the pattern gets
        // longer.
        const LEN_CUTOFF_PROPORTION: usize = 4;

        let scaled_rank = pattern.len().wrapping_mul(LEN_CUTOFF_PROPORTION);
        let cutoff = cmp::max(
            MIN_CUTOFF,
            MAX_CUTOFF - cmp::min(MAX_CUTOFF, scaled_rank),
        );
        // The pattern must be long enough to be worthwhile. e.g., memchr will
        // be faster on `e` because it is short even though e is quite common.
        pattern.len() > MIN_LEN
            // all the bytes must be more common than the cutoff.
            && pattern.iter().all(|c| freq_rank(*c) >= cutoff)
    }

    /// Check to see if there is a match at the given position
    #[inline]
    fn check_match(&self, haystack: &[u8], window_end: usize) -> bool {
        // guard test
        if haystack[window_end - self.guard_reverse_idx] != self.guard {
            return false;
        }

        // match loop
        let window_start = window_end - (self.pattern.len() - 1);
        for i in 0..self.pattern.len() {
            if self.pattern[i] != haystack[window_start + i] {
                return false;
            }
        }

        true
    }

    /// Skip forward according to the shift table.
    ///
    /// Returns the offset of the next occurrence
    /// of the last char in the pattern, or the none
    /// if it never reappears. If `skip_loop` hits the backstop
    /// it will leave early.
    #[inline]
    fn skip_loop(
        &self,
        haystack: &[u8],
        mut window_end: usize,
        backstop: usize,
    ) -> Option<usize> {
        let window_end_snapshot = window_end;
        let skip_of = |we: usize| -> usize {
            // Unsafe might make this faster, but the benchmarks
            // were hard to interpret.
            self.skip_table[haystack[we] as usize]
        };

        loop {
            let mut skip = skip_of(window_end);
            window_end += skip;
            skip = skip_of(window_end);
            window_end += skip;
            if skip != 0 {
                skip = skip_of(window_end);
                window_end += skip;
                skip = skip_of(window_end);
                window_end += skip;
                skip = skip_of(window_end);
                window_end += skip;
                if skip != 0 {
                    skip = skip_of(window_end);
                    window_end += skip;
                    skip = skip_of(window_end);
                    window_end += skip;
                    skip = skip_of(window_end);
                    window_end += skip;
                    if skip != 0 {
                        skip = skip_of(window_end);
                        window_end += skip;
                        skip = skip_of(window_end);
                        window_end += skip;

                        // If ten iterations did not make at least 16 words
                        // worth of progress, we just fall back on memchr.
                        if window_end - window_end_snapshot
                            > 16 * mem::size_of::<usize>()
                        {
                            // Returning a window_end >= backstop will
                            // immediatly break us out of the inner loop in
                            // `find`.
                            if window_end >= backstop {
                                return Some(window_end);
                            }

                            continue; // we made enough progress
                        } else {
                            // In case we are already there, and so that
                            // we will catch the guard char.
                            window_end = window_end
                                .checked_sub(1 + self.guard_reverse_idx)
                                .unwrap_or(0);

                            match memchr(self.guard, &haystack[window_end..]) {
                                None => return None,
                                Some(g_idx) => {
                                    return Some(
                                        window_end
                                            + g_idx
                                            + self.guard_reverse_idx,
                                    );
                                }
                            }
                        }
                    }
                }
            }

            return Some(window_end);
        }
    }

    /// Compute the ufast skip table.
    fn compile_skip_table(pattern: &[u8]) -> Vec<usize> {
        let mut tab = vec![pattern.len(); 256];

        // For every char in the pattern, we write a skip
        // that will line us up with the rightmost occurrence.
        //
        // N.B. the sentinel (0) is written by the last
        // loop iteration.
        for (i, c) in pattern.iter().enumerate() {
            tab[*c as usize] = (pattern.len() - 1) - i;
        }

        tab
    }

    /// Select the guard character based off of the precomputed
    /// frequency table.
    fn select_guard(pattern: &[u8]) -> (u8, usize) {
        let mut rarest = pattern[0];
        let mut rarest_rev_idx = pattern.len() - 1;
        for (i, c) in pattern.iter().enumerate() {
            if freq_rank(*c) < freq_rank(rarest) {
                rarest = *c;
                rarest_rev_idx = (pattern.len() - 1) - i;
            }
        }

        (rarest, rarest_rev_idx)
    }

    /// If there is another occurrence of the skip
    /// char, shift to it, otherwise just shift to
    /// the next window.
    fn compile_md2_shift(pattern: &[u8]) -> usize {
        let shiftc = *pattern.last().unwrap();

        // For a pattern of length 1 we will never apply the
        // shift rule, so we use a poison value on the principle
        // that failing fast is a good thing.
        if pattern.len() == 1 {
            return 0xDEADBEAF;
        }

        let mut i = pattern.len() - 2;
        while i > 0 {
            if pattern[i] == shiftc {
                return (pattern.len() - 1) - i;
            }
            i -= 1;
        }

        // The skip char never re-occurs in the pattern, so
        // we can just shift the whole window length.
        pattern.len() - 1
    }

    fn approximate_size(&self) -> usize {
        (self.pattern.len() * mem::size_of::<u8>())
            + (256 * mem::size_of::<usize>()) // skip table
    }
}

fn freq_rank(b: u8) -> usize {
    BYTE_FREQUENCIES[b as usize] as usize
}

#[cfg(test)]
mod tests {
    use super::{BoyerMooreSearch, FreqyPacked};

    //
    // Unit Tests
    //

    // The "hello, world" of string searching
    #[test]
    fn bm_find_subs() {
        let searcher = BoyerMooreSearch::new(Vec::from(&b"pattern"[..]));
        let haystack = b"I keep seeing patterns in this text";
        assert_eq!(14, searcher.find(haystack).unwrap());
    }

    #[test]
    fn bm_find_no_subs() {
        let searcher = BoyerMooreSearch::new(Vec::from(&b"pattern"[..]));
        let haystack = b"I keep seeing needles in this text";
        assert_eq!(None, searcher.find(haystack));
    }

    //
    // Regression Tests
    //

    #[test]
    fn bm_skip_reset_bug() {
        let haystack = vec![0, 0, 0, 0, 0, 1, 1, 0];
        let needle = vec![0, 1, 1, 0];

        let searcher = BoyerMooreSearch::new(needle);
        let offset = searcher.find(haystack.as_slice()).unwrap();
        assert_eq!(4, offset);
    }

    #[test]
    fn bm_backstop_underflow_bug() {
        let haystack = vec![0, 0];
        let needle = vec![0, 0];

        let searcher = BoyerMooreSearch::new(needle);
        let offset = searcher.find(haystack.as_slice()).unwrap();
        assert_eq!(0, offset);
    }

    #[test]
    fn bm_naive_off_by_one_bug() {
        let haystack = vec![91];
        let needle = vec![91];

        let naive_offset = naive_find(&needle, &haystack).unwrap();
        assert_eq!(0, naive_offset);
    }

    #[test]
    fn bm_memchr_fallback_indexing_bug() {
        let mut haystack = vec![
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 87, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        ];
        let needle = vec![1, 1, 1, 1, 32, 32, 87];
        let needle_start = haystack.len();
        haystack.extend(needle.clone());

        let searcher = BoyerMooreSearch::new(needle);
        assert_eq!(needle_start, searcher.find(haystack.as_slice()).unwrap());
    }

    #[test]
    fn bm_backstop_boundary() {
        let haystack = b"\
// aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
e_data.clone_created(entity_id, entity_to_add.entity_id);
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
"
        .to_vec();
        let needle = b"clone_created".to_vec();

        let searcher = BoyerMooreSearch::new(needle);
        let result = searcher.find(&haystack);
        assert_eq!(Some(43), result);
    }

    #[test]
    fn bm_win_gnu_indexing_bug() {
        let haystack_raw = vec![
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        ];
        let needle = vec![1, 1, 1, 1, 1, 1, 1];
        let haystack = haystack_raw.as_slice();

        BoyerMooreSearch::new(needle.clone()).find(haystack);
    }

    //
    // QuickCheck Properties
    //

    use quickcheck::TestResult;

    fn naive_find(needle: &[u8], haystack: &[u8]) -> Option<usize> {
        assert!(needle.len() <= haystack.len());

        for i in 0..(haystack.len() - (needle.len() - 1)) {
            if haystack[i] == needle[0]
                && &haystack[i..(i + needle.len())] == needle
            {
                return Some(i);
            }
        }

        None
    }

    quickcheck! {
        fn qc_bm_equals_nieve_find(pile1: Vec<u8>, pile2: Vec<u8>) -> TestResult {
            if pile1.len() == 0 || pile2.len() == 0 {
                return TestResult::discard();
            }

            let (needle, haystack) = if pile1.len() < pile2.len() {
                (pile1, pile2.as_slice())
            } else {
                (pile2, pile1.as_slice())
            };

            let searcher = BoyerMooreSearch::new(needle.clone());
            TestResult::from_bool(
                searcher.find(haystack) == naive_find(&needle, haystack))
        }

        fn qc_bm_equals_single(pile1: Vec<u8>, pile2: Vec<u8>) -> TestResult {
            if pile1.len() == 0 || pile2.len() == 0 {
                return TestResult::discard();
            }

            let (needle, haystack) = if pile1.len() < pile2.len() {
                (pile1, pile2.as_slice())
            } else {
                (pile2, pile1.as_slice())
            };

            let bm_searcher = BoyerMooreSearch::new(needle.clone());
            let freqy_memchr = FreqyPacked::new(needle);
            TestResult::from_bool(
                bm_searcher.find(haystack) == freqy_memchr.find(haystack))
        }

        fn qc_bm_finds_trailing_needle(
            haystack_pre: Vec<u8>,
            needle: Vec<u8>
        ) -> TestResult {
            if needle.len() == 0 {
                return TestResult::discard();
            }

            let mut haystack = haystack_pre.clone();
            let searcher = BoyerMooreSearch::new(needle.clone());

            if haystack.len() >= needle.len() &&
                searcher.find(haystack.as_slice()).is_some() {
                return TestResult::discard();
            }

            haystack.extend(needle.clone());

            // What if the the tail of the haystack can start the
            // needle?
            let start = haystack_pre.len()
                .checked_sub(needle.len())
                .unwrap_or(0);
            for i in 0..(needle.len() - 1) {
                if searcher.find(&haystack[(i + start)..]).is_some() {
                    return TestResult::discard();
                }
            }

            TestResult::from_bool(
                searcher.find(haystack.as_slice())
                        .map(|x| x == haystack_pre.len())
                        .unwrap_or(false))
        }

        // qc_equals_* is only testing the negative case as @burntsushi
        // pointed out in https://github.com/rust-lang/regex/issues/446.
        // This quickcheck prop represents an effort to force testing of
        // the positive case. qc_bm_finds_first and qc_bm_finds_trailing_needle
        // already check some of the positive cases, but they don't cover
        // cases where the needle is in the middle of haystack. This prop
        // fills that hole.
        fn qc_bm_finds_subslice(
            haystack: Vec<u8>,
            needle_start: usize,
            needle_length: usize
        ) -> TestResult {
            if haystack.len() == 0 {
                return TestResult::discard();
            }

            let needle_start = needle_start % haystack.len();
            let needle_length = needle_length % (haystack.len() - needle_start);

            if needle_length == 0 {
                return TestResult::discard();
            }

            let needle = &haystack[needle_start..(needle_start + needle_length)];

            let bm_searcher = BoyerMooreSearch::new(needle.to_vec());

            let start = naive_find(&needle, &haystack);
            match start {
                None => TestResult::from_bool(false),
                Some(nf_start) =>
                    TestResult::from_bool(
                        nf_start <= needle_start
                            && bm_searcher.find(&haystack) == start
                    )
            }
        }

        fn qc_bm_finds_first(needle: Vec<u8>) -> TestResult {
            if needle.len() == 0 {
                return TestResult::discard();
            }

            let mut haystack = needle.clone();
            let searcher = BoyerMooreSearch::new(needle.clone());
            haystack.extend(needle);

            TestResult::from_bool(
                searcher.find(haystack.as_slice())
                        .map(|x| x == 0)
                        .unwrap_or(false))
        }
    }
}