utils/TableGen/StringMatcher.cpp

//===- StringMatcher.cpp - Generate a matcher for input strings -----------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the StringMatcher class.
//
//===----------------------------------------------------------------------===//

#include "StringMatcher.h"
#include "llvm/Support/raw_ostream.h"
#include <map>
using namespace llvm;

/// FindFirstNonCommonLetter - Find the first character in the keys of the
/// string pairs that is not shared across the whole set of strings.  All
/// strings are assumed to have the same length.
static unsigned 
FindFirstNonCommonLetter(const std::vector<const
                              StringMatcher::StringPair*> &Matches) {
  assert(!Matches.empty());
  for (unsigned i = 0, e = Matches[0]->first.size(); i != e; ++i) {
    // Check to see if letter i is the same across the set.
    char Letter = Matches[0]->first[i];
    
    for (unsigned str = 0, e = Matches.size(); str != e; ++str)
      if (Matches[str]->first[i] != Letter)
        return i;
  }
  
  return Matches[0]->first.size();
}

/// EmitStringMatcherForChar - Given a set of strings that are known to be the
/// same length and whose characters leading up to CharNo are the same, emit
/// code to verify that CharNo and later are the same.
///
/// \return - True if control can leave the emitted code fragment.
bool StringMatcher::
EmitStringMatcherForChar(const std::vector<const StringPair*> &Matches,
                         unsigned CharNo, unsigned IndentCount) const {
  assert(!Matches.empty() && "Must have at least one string to match!");
  std::string Indent(IndentCount*2+4, ' ');
  
  // If we have verified that the entire string matches, we're done: output the
  // matching code.
  if (CharNo == Matches[0]->first.size()) {
    assert(Matches.size() == 1 && "Had duplicate keys to match on");
    
    // FIXME: If Matches[0].first has embeded \n, this will be bad.
    OS << Indent << Matches[0]->second << "\t // \"" << Matches[0]->first
    << "\"\n";
    return false;
  }
  
  // Bucket the matches by the character we are comparing.
  std::map<char, std::vector<const StringPair*> > MatchesByLetter;
  
  for (unsigned i = 0, e = Matches.size(); i != e; ++i)
    MatchesByLetter[Matches[i]->first[CharNo]].push_back(Matches[i]);
  
  
  // If we have exactly one bucket to match, see how many characters are common
  // across the whole set and match all of them at once.
  if (MatchesByLetter.size() == 1) {
    unsigned FirstNonCommonLetter = FindFirstNonCommonLetter(Matches);
    unsigned NumChars = FirstNonCommonLetter-CharNo;
    
    // Emit code to break out if the prefix doesn't match.
    if (NumChars == 1) {
      // Do the comparison with if (Str[1] != 'f')
      // FIXME: Need to escape general characters.
      OS << Indent << "if (" << StrVariableName << "[" << CharNo << "] != '"
      << Matches[0]->first[CharNo] << "')\n";
      OS << Indent << "  break;\n";
    } else {
      // Do the comparison with if (Str.substr(1,3) != "foo").    
      // FIXME: Need to escape general strings.
      OS << Indent << "if (" << StrVariableName << ".substr(" << CharNo << ","
      << NumChars << ") != \"";
      OS << Matches[0]->first.substr(CharNo, NumChars) << "\")\n";
      OS << Indent << "  break;\n";
    }
    
    return EmitStringMatcherForChar(Matches, FirstNonCommonLetter, IndentCount);
  }
  
  // Otherwise, we have multiple possible things, emit a switch on the
  // character.
  OS << Indent << "switch (" << StrVariableName << "[" << CharNo << "]) {\n";
  OS << Indent << "default: break;\n";
  
  for (std::map<char, std::vector<const StringPair*> >::iterator LI = 
       MatchesByLetter.begin(), E = MatchesByLetter.end(); LI != E; ++LI) {
    // TODO: escape hard stuff (like \n) if we ever care about it.
    OS << Indent << "case '" << LI->first << "':\t // "
       << LI->second.size() << " string";
    if (LI->second.size() != 1) OS << 's';
    OS << " to match.\n";
    if (EmitStringMatcherForChar(LI->second, CharNo+1, IndentCount+1))
      OS << Indent << "  break;\n";
  }
  
  OS << Indent << "}\n";
  return true;
}


/// Emit - Top level entry point.
///
void StringMatcher::Emit() const {
  // First level categorization: group strings by length.
  std::map<unsigned, std::vector<const StringPair*> > MatchesByLength;
  
  for (unsigned i = 0, e = Matches.size(); i != e; ++i)
    MatchesByLength[Matches[i].first.size()].push_back(&Matches[i]);
  
  // Output a switch statement on length and categorize the elements within each
  // bin.
  OS << "  switch (" << StrVariableName << ".size()) {\n";
  OS << "  default: break;\n";
  
  for (std::map<unsigned, std::vector<const StringPair*> >::iterator LI =
       MatchesByLength.begin(), E = MatchesByLength.end(); LI != E; ++LI) {
    OS << "  case " << LI->first << ":\t // " << LI->second.size()
    << " strings to match.\n";
    if (EmitStringMatcherForChar(LI->second, 0, 0))
      OS << "    break;\n";
  }
  
  OS << "  }\n";
}