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