return false;
}
+ // This function operates in different modes, DFS mode or BFS mode, indicated
+ // by template parameter __dfs_mode. See _M_main for details.
+ //
+ // ------------------------------------------------------------
+ //
+ // DFS mode:
+ //
+ // It applies a Depth-First-Search (aka backtracking) on given NFA and input
+ // string.
+ // At the very beginning the executor stands in the start state, then it tries
+ // every possible state transition in current state recursively. Some state
+ // transitions consume input string, say, a single-char-matcher or a
+ // back-reference matcher; some don't, like assertion or other anchor nodes.
+ // When the input is exhausted and/or the current state is an accepting state,
+ // the whole executor returns true.
+ //
+ // TODO: This approach is exponentially slow for certain input.
+ // Try to compile the NFA to a DFA.
+ //
+ // Time complexity: o(match_length), O(2^(_M_nfa.size()))
+ // Space complexity: \theta(match_results.size() + match_length)
+ //
+ // ------------------------------------------------------------
+ //
+ // BFS mode:
+ //
+ // Russ Cox's article (http://swtch.com/~rsc/regexp/regexp1.html)
+ // explained this algorithm clearly.
+ //
+ // It first computes epsilon closure for every state that's still matching,
+ // using the same DFS algorithm, but doesn't reenter states (set true in
+ // _M_visited), nor follows _S_opcode_match.
+ //
+ // Then apply DFS using every _S_opcode_match (in _M_match_queue) as the start
+ // state.
+ //
+ // It significantly reduces potential duplicate states, so has a better
+ // upper bound; but it requires more overhead.
+ //
+ // Time complexity: o(match_length * match_results.size())
+ // O(match_length * _M_nfa.size() * match_results.size())
+ // Space complexity: o(_M_nfa.size() + match_results.size())
+ // O(_M_nfa.size() * match_results.size())
template<typename _BiIter, typename _Alloc, typename _TraitsT,
bool __dfs_mode>
template<bool __match_mode>
}
else
{
- // Like the DFS approach, it try every possible state transition;
- // Unlike DFS, it uses a queue instead of a stack to store matching
- // states. It's a BFS approach.
- //
- // Russ Cox's article(http://swtch.com/~rsc/regexp/regexp1.html)
- // explained this algorithm clearly.
- //
- // Time complexity: o(match_length * match_results.size())
- // O(match_length * _M_nfa.size()
- // * match_results.size())
- // Space complexity: o(_M_nfa.size() + match_results.size())
- // O(_M_nfa.size() * match_results.size())
_M_match_queue->push(make_pair(_M_start_state, _M_results));
bool __ret = false;
while (1)
return false;
}
- // A _DFSExecutor perform a DFS on given NFA and input string. At the very
- // beginning the executor stands in the start state, then it try every
- // possible state transition in current state recursively. Some state
- // transitions consume input string, say, a single-char-matcher or a
- // back-reference matcher; some not, like assertion or other anchor nodes.
- // When the input is exhausted and the current state is an accepting state,
- // the whole executor return true.
- //
- // TODO: This approach is exponentially slow for certain input.
- // Try to compile the NFA to a DFA.
- //
- // Time complexity: o(match_length), O(2^(_M_nfa.size()))
- // Space complexity: \theta(match_results.size() + match_length)
- //
template<typename _BiIter, typename _Alloc, typename _TraitsT,
bool __dfs_mode>
template<bool __match_mode>
}
const auto& __state = _M_nfa[__i];
+ // Every change on _M_cur_results and _M_current will be rolled back after
+ // finishing the recursion step.
switch (__state._M_opcode)
{
+ // _M_alt branch is "match once more", while _M_next is "get me out
+ // of this quantifier". Executing _M_next first or _M_alt first don't
+ // mean the same thing, and we need to choose the correct order under
+ // given greedy mode.
case _S_opcode_alternative:
- // Greedy or not, this is a question ;)
+ // Greedy.
if (!__state._M_neg)
{
+ // "Once more" is preferred in greedy mode.
_M_dfs<__match_mode>(__state._M_alt);
+ // If it's DFS executor and already accepted, we're done.
if (!__dfs_mode || !_M_has_sol)
_M_dfs<__match_mode>(__state._M_next);
}
- else
+ else // Non-greedy mode
{
if (__dfs_mode)
{
+ // vice-versa.
_M_dfs<__match_mode>(__state._M_next);
if (!_M_has_sol)
_M_dfs<__match_mode>(__state._M_alt);
}
else
{
+ // DON'T attempt anything, because there's already another
+ // state with higher priority accepted. This state cannot be
+ // better by attempting its next node.
if (!_M_has_sol)
{
_M_dfs<__match_mode>(__state._M_next);
+ // DON'T attempt anything if it's already accepted. An
+ // accepted state *must* be better than a solution that
+ // matches a non-greedy quantifier one more time.
if (!_M_has_sol)
_M_dfs<__match_mode>(__state._M_alt);
}
}
break;
case _S_opcode_subexpr_begin:
- // Here's the critical part: if there's nothing changed since last
- // visit, do NOT continue. This prevents the executor from get into
- // infinite loop when use "()*" to match "".
- //
- // Every change on _M_cur_results will be roll back after the
- // recursion step finished.
+ // If there's nothing changed since last visit, do NOT continue.
+ // This prevents the executor from get into infinite loop when using
+ // "()*" to match "".
if (!_M_cur_results[__state._M_subexpr].matched
|| _M_cur_results[__state._M_subexpr].first != _M_current)
{
if (_M_word_boundry(__state) == !__state._M_neg)
_M_dfs<__match_mode>(__state._M_next);
break;
- // Here __state._M_alt offers a single start node for a sub-NFA.
- // We recursivly invoke our algorithm to match the sub-NFA.
+ // Here __state._M_alt offers a single start node for a sub-NFA.
+ // We recursively invoke our algorithm to match the sub-NFA.
case _S_opcode_subexpr_lookahead:
if (_M_lookahead(__state) == !__state._M_neg)
_M_dfs<__match_mode>(__state._M_next);
break;
// First fetch the matched result from _M_cur_results as __submatch;
// then compare it with
- // (_M_current, _M_current + (__submatch.second - __submatch.first))
- // If matched, keep going; else just return to try another state.
+ // (_M_current, _M_current + (__submatch.second - __submatch.first)).
+ // If matched, keep going; else just return and try another state.
case _S_opcode_backref:
{
_GLIBCXX_DEBUG_ASSERT(__dfs_mode);
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