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Commit caaf33fa by Tim Shen Committed by Tim Shen
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regex_executor.tcc: Add comments. 

2013-10-28  Tim Shen  <timshen91@gmail.com>

	* regex_executor.tcc: Add comments.

From-SVN: r204117
parent 58fe50d5
Showing with 71 additions and 38 deletions
libstdc++-v3/ChangeLog
2013-10-28 Tim Shen <timshen91@gmail.com>
* regex_executor.tcc: Add comments.
2013-10-26 Tim Shen <timshen91@gmail.com>
* include/bits/regex.h: Remove unnecessary friends.
......
libstdc++-v3/include/bits/regex_executor.tcc
......@@ -53,6 +53,49 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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>
......@@ -68,18 +111,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
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)
......@@ -132,20 +163,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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>
......@@ -160,29 +177,44 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
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);
}
......@@ -190,12 +222,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
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)
{
......@@ -232,8 +261,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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);
......@@ -254,8 +283,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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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