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69 lines
2.9 KiB
C++
69 lines
2.9 KiB
C++
//===-- TrigramIndex.h - a heuristic for SpecialCaseList --------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//===----------------------------------------------------------------------===//
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//
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// TrigramIndex implements a heuristic for SpecialCaseList that allows to
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// filter out ~99% incoming queries when all regular expressions in the
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// SpecialCaseList are simple wildcards with '*' and '.'. If rules are more
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// complicated, the check is defeated and it will always pass the queries to a
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// full regex.
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//
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// The basic idea is that in order for a wildcard to match a query, the query
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// needs to have all trigrams which occur in the wildcard. We create a trigram
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// index (trigram -> list of rules with it) and then count trigrams in the query
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// for each rule. If the count for one of the rules reaches the expected value,
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// the check passes the query to a regex. If none of the rules got enough
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// trigrams, the check tells that the query is definitely not matched by any
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// of the rules, and no regex matching is needed.
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// A similar idea was used in Google Code Search as described in the blog post:
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// https://swtch.com/~rsc/regexp/regexp4.html
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_SUPPORT_TRIGRAMINDEX_H
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#define LLVM_SUPPORT_TRIGRAMINDEX_H
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/StringRef.h"
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#include <string>
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#include <unordered_map>
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#include <vector>
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namespace llvm {
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class StringRef;
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class TrigramIndex {
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public:
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/// Inserts a new Regex into the index.
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void insert(const std::string &Regex);
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/// Returns true, if special case list definitely does not have a line
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/// that matches the query. Returns false, if it's not sure.
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bool isDefinitelyOut(StringRef Query) const;
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/// Returned true, iff the heuristic is defeated and not useful.
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/// In this case isDefinitelyOut always returns false.
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bool isDefeated() { return Defeated; }
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private:
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// If true, the rules are too complicated for the check to work, and full
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// regex matching is needed for every rule.
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bool Defeated = false;
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// The minimum number of trigrams which should match for a rule to have a
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// chance to match the query. The number of elements equals the number of
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// regex rules in the SpecialCaseList.
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std::vector<unsigned> Counts;
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// Index holds a list of rules indices for each trigram. The same indices
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// are used in Counts to store per-rule limits.
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// If a trigram is too common (>4 rules with it), we stop tracking it,
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// which increases the probability for a need to match using regex, but
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// decreases the costs in the regular case.
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std::unordered_map<unsigned, SmallVector<size_t, 4>> Index{256};
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};
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} // namespace llvm
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#endif // LLVM_SUPPORT_TRIGRAMINDEX_H
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