1//===-- TrigramIndex.h - a heuristic for SpecialCaseList --------*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//===----------------------------------------------------------------------===//
7//
8// TrigramIndex implements a heuristic for SpecialCaseList that allows to
9// filter out ~99% incoming queries when all regular expressions in the
10// SpecialCaseList are simple wildcards with '*' and '.'. If rules are more
11// complicated, the check is defeated and it will always pass the queries to a
12// full regex.
13//
14// The basic idea is that in order for a wildcard to match a query, the query
15// needs to have all trigrams which occur in the wildcard. We create a trigram
16// index (trigram -> list of rules with it) and then count trigrams in the query
17// for each rule. If the count for one of the rules reaches the expected value,
18// the check passes the query to a regex. If none of the rules got enough
19// trigrams, the check tells that the query is definitely not matched by any
20// of the rules, and no regex matching is needed.
21// A similar idea was used in Google Code Search as described in the blog post:
22// https://swtch.com/~rsc/regexp/regexp4.html
23//
24//===----------------------------------------------------------------------===//
25
26#ifndef LLVM_SUPPORT_TRIGRAMINDEX_H
27#define LLVM_SUPPORT_TRIGRAMINDEX_H
28
29#include "llvm/ADT/SmallVector.h"
30#include "llvm/ADT/StringMap.h"
31
32#include <string>
33#include <unordered_map>
34#include <vector>
35
36namespace llvm {
37class StringRef;
38
39class TrigramIndex {
40 public:
41  /// Inserts a new Regex into the index.
42  void insert(std::string Regex);
43
44  /// Returns true, if special case list definitely does not have a line
45  /// that matches the query. Returns false, if it's not sure.
46  bool isDefinitelyOut(StringRef Query) const;
47
48  /// Returned true, iff the heuristic is defeated and not useful.
49  /// In this case isDefinitelyOut always returns false.
50  bool isDefeated() { return Defeated; }
51 private:
52  // If true, the rules are too complicated for the check to work, and full
53  // regex matching is needed for every rule.
54  bool Defeated = false;
55  // The minimum number of trigrams which should match for a rule to have a
56  // chance to match the query. The number of elements equals the number of
57  // regex rules in the SpecialCaseList.
58  std::vector<unsigned> Counts;
59  // Index holds a list of rules indices for each trigram. The same indices
60  // are used in Counts to store per-rule limits.
61  // If a trigram is too common (>4 rules with it), we stop tracking it,
62  // which increases the probability for a need to match using regex, but
63  // decreases the costs in the regular case.
64  std::unordered_map<unsigned, SmallVector<size_t, 4>> Index{256};
65};
66
67}  // namespace llvm
68
69#endif  // LLVM_SUPPORT_TRIGRAMINDEX_H
70