Method PossibleMatchRange

extern/re2/re2/dfa.cc:1975–2109

Computes min and max for matching string. Won't return strings bigger than maxlen.

// Computes min and max for matching string.
// Won't return strings bigger than maxlen.
bool DFA::PossibleMatchRange(std::string* min, std::string* max, int maxlen) {
  if (!ok())
    return false;

  // NOTE: if future users of PossibleMatchRange want more precision when
  // presented with infinitely repeated elements, consider making this a
  // parameter to PossibleMatchRange.
  static int kMaxEltRepetitions = 0;

  // Keep track of the number of times we've visited states previously. We only
  // revisit a given state if it's part of a repeated group, so if the value
  // portion of the map tuple exceeds kMaxEltRepetitions we bail out and set
  // |*max| to |PrefixSuccessor(*max)|.
  //
  // Also note that previously_visited_states[UnseenStatePtr] will, in the STL
  // tradition, implicitly insert a '0' value at first use. We take advantage
  // of that property below.
  std::unordered_map<State*, int> previously_visited_states;

  // Pick out start state for anchored search at beginning of text.
  RWLocker l(&cache_mutex_);
  SearchParams params(StringPiece(), StringPiece(), &l);
  params.anchored = true;
  if (!AnalyzeSearch(&params))
    return false;
  if (params.start == DeadState) {  // No matching strings
    *min = "";
    *max = "";
    return true;
  }
  if (params.start == FullMatchState)  // Every string matches: no max
    return false;

  // The DFA is essentially a big graph rooted at params.start,
  // and paths in the graph correspond to accepted strings.
  // Each node in the graph has potentially 256+1 arrows
  // coming out, one for each byte plus the magic end of
  // text character kByteEndText.

  // To find the smallest possible prefix of an accepted
  // string, we just walk the graph preferring to follow
  // arrows with the lowest bytes possible.  To find the
  // largest possible prefix, we follow the largest bytes
  // possible.

  // The test for whether there is an arrow from s on byte j is
  //    ns = RunStateOnByteUnlocked(s, j);
  //    if (ns == NULL)
  //      return false;
  //    if (ns != DeadState && ns->ninst > 0)
  // The RunStateOnByteUnlocked call asks the DFA to build out the graph.
  // It returns NULL only if the DFA has run out of memory,
  // in which case we can't be sure of anything.
  // The second check sees whether there was graph built
  // and whether it is interesting graph.  Nodes might have
  // ns->ninst == 0 if they exist only to represent the fact
  // that a match was found on the previous byte.