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//===-- TrigramIndex.cpp - a heuristic for SpecialCaseList ----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// TrigramIndex implements a heuristic for SpecialCaseList that allows to
// filter out ~99% incoming queries when all regular expressions in the
// SpecialCaseList are simple wildcards with '*' and '.'. If rules are more
// complicated, the check is defeated and it will always pass the queries to a
// full regex.
//
//===----------------------------------------------------------------------===//
#include "llvm/Support/TrigramIndex.h"
#include <set>
using namespace llvm;
static const char RegexAdvancedMetachars[] = "()^$|+?[]\\{}";
static bool isAdvancedMetachar(unsigned Char) {
return strchr(RegexAdvancedMetachars, Char) != nullptr;
}
void TrigramIndex::insert(const std::string &Regex) {
if (Defeated) return;
std::set<unsigned> Was;
unsigned Cnt = 0;
unsigned Tri = 0;
unsigned Len = 0;
bool Escaped = false;
for (unsigned Char : Regex) {
if (!Escaped) {
// Regular expressions allow escaping symbols by preceding it with '\'.
if (Char == '\\') {
Escaped = true;
continue;
}
if (isAdvancedMetachar(Char)) {
// This is a more complicated regex than we can handle here.
Defeated = true;
return;
}
if (Char == '.' || Char == '*') {
Tri = 0;
Len = 0;
continue;
}
}
if (Escaped && Char >= '1' && Char <= '9') {
Defeated = true;
return;
}
// We have already handled escaping and can reset the flag.
Escaped = false;
Tri = ((Tri << 8) + Char) & 0xFFFFFF;
Len++;
if (Len < 3)
continue;
// We don't want the index to grow too much for the popular trigrams,
// as they are weak signals. It's ok to still require them for the
// rules we have already processed. It's just a small additional
// computational cost.
if (Index[Tri].size() >= 4)
continue;
Cnt++;
if (!Was.count(Tri)) {
// Adding the current rule to the index.
Index[Tri].push_back(Counts.size());
Was.insert(Tri);
}
}
if (!Cnt) {
// This rule does not have remarkable trigrams to rely on.
// We have to always call the full regex chain.
Defeated = true;
return;
}
Counts.push_back(Cnt);
}
bool TrigramIndex::isDefinitelyOut(StringRef Query) const {
if (Defeated)
return false;
std::vector<unsigned> CurCounts(Counts.size());
unsigned Tri = 0;
for (size_t I = 0; I < Query.size(); I++) {
Tri = ((Tri << 8) + Query[I]) & 0xFFFFFF;
if (I < 2)
continue;
const auto &II = Index.find(Tri);
if (II == Index.end())
continue;
for (size_t J : II->second) {
CurCounts[J]++;
// If we have reached a desired limit, we have to look at the query
// more closely by running a full regex.
if (CurCounts[J] >= Counts[J])
return false;
}
}
return true;
}
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