llvm/lib/Support/TrigramIndex.cpp

   1 //===-- TrigramIndex.cpp - a heuristic for SpecialCaseList ----------------===//
   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 //
   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
  13 // full regex.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "llvm/Support/TrigramIndex.h"
  18 #include <set>
  19
  20 using namespace llvm;
  21
  22 static const char RegexAdvancedMetachars[] = "()^$|+?[]\\{}";
  23
  24 static bool isAdvancedMetachar(unsigned Char) {
  25   return strchr(RegexAdvancedMetachars, Char) != nullptr;
  26 }
  27
  28 void TrigramIndex::insert(const std::string &Regex) {
  29   if (Defeated) return;
  30   std::set<unsigned> Was;
  31   unsigned Cnt = 0;
  32   unsigned Tri = 0;
  33   unsigned Len = 0;
  34   bool Escaped = false;
  35   for (unsigned Char : Regex) {
  36     if (!Escaped) {
  37       // Regular expressions allow escaping symbols by preceding it with '\'.
  38       if (Char == '\\') {
  39         Escaped = true;
  40         continue;
  41       }
  42       if (isAdvancedMetachar(Char)) {
  43         // This is a more complicated regex than we can handle here.
  44         Defeated = true;
  45         return;
  46       }
  47       if (Char == '.' || Char == '*') {
  48         Tri = 0;
  49         Len = 0;
  50         continue;
  51       }
  52     }
  53     if (Escaped && Char >= '1' && Char <= '9') {
  54       Defeated = true;
  55       return;
  56     }
  57     // We have already handled escaping and can reset the flag.
  58     Escaped = false;
  59     Tri = ((Tri << 8) + Char) & 0xFFFFFF;
  60     Len++;
  61     if (Len < 3)
  62       continue;
  63     // We don't want the index to grow too much for the popular trigrams,
  64     // as they are weak signals. It's ok to still require them for the
  65     // rules we have already processed. It's just a small additional
  66     // computational cost.
  67     if (Index[Tri].size() >= 4)
  68       continue;
  69     Cnt++;
  70     if (!Was.count(Tri)) {
  71       // Adding the current rule to the index.
  72       Index[Tri].push_back(Counts.size());
  73       Was.insert(Tri);
  74     }
  75   }
  76   if (!Cnt) {
  77     // This rule does not have remarkable trigrams to rely on.
  78     // We have to always call the full regex chain.
  79     Defeated = true;
  80     return;
  81   }
  82   Counts.push_back(Cnt);
  83 }
  84
  85 bool TrigramIndex::isDefinitelyOut(StringRef Query) const {
  86   if (Defeated)
  87     return false;
  88   std::vector<unsigned> CurCounts(Counts.size());
  89   unsigned Tri = 0;
  90   for (size_t I = 0; I < Query.size(); I++) {
  91     Tri = ((Tri << 8) + Query[I]) & 0xFFFFFF;
  92     if (I < 2)
  93       continue;
  94     const auto &II = Index.find(Tri);
  95     if (II == Index.end())
  96       continue;
  97     for (size_t J : II->second) {
  98       CurCounts[J]++;
  99       // If we have reached a desired limit, we have to look at the query
 100       // more closely by running a full regex.
 101       if (CurCounts[J] >= Counts[J])
 102         return false;
 103     }
 104   }
 105   return true;
 106 }