regex_executor.tcc

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// class template regex -*- C++ -*-

// Copyright (C) 2013-2025 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/**
 *  @file bits/regex_executor.tcc
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{regex}
 */

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
namespace __detail
{
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    bool _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_search()
    {
      if (_M_search_from_first())
        return true;
      if (_M_flags & regex_constants::match_continuous)
        return false;
      _M_flags |= regex_constants::match_prev_avail;
      while (_M_begin != _M_end)
        {
          ++_M_begin;
          if (_M_search_from_first())
            return true;
        }
      return false;
    }

  // The _M_main function operates in different modes, DFS mode or BFS mode,
  // indicated by template parameter __dfs_mode, and dispatches to one of the
  // _M_main_dispatch overloads.
  //
  // ------------------------------------------------------------
  //
  // 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: \Omega(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>
    bool _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_main_dispatch(_Match_mode __match_mode, __dfs)
    {
      _M_has_sol = false;
      *_M_states._M_get_sol_pos() = _BiIter();
      _M_cur_results = _M_results;
      _M_dfs(__match_mode, _M_states._M_start);
      return _M_has_sol;
    }

  // ------------------------------------------------------------
  //
  // BFS mode:
  //
  // Russ Cox's article (http://swtch.com/~rsc/regexp/regexp1.html)
  // explained this algorithm clearly.
  //
  // It first computes epsilon closure (states that can be achieved without
  // consuming characters) for every state that's still matching,
  // using the same DFS algorithm, but doesn't re-enter states (using
  // _M_states._M_visited to check), nor follow _S_opcode_match.
  //
  // Then apply DFS using every _S_opcode_match (in _M_states._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: \Omega(match_length * match_results.size())
  //                  O(match_length * _M_nfa.size() * match_results.size())
  // Space complexity: \Omega(_M_nfa.size() + match_results.size())
  //                   O(_M_nfa.size() * match_results.size())
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    bool _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_main_dispatch(_Match_mode __match_mode, __bfs)
    {
      _M_states._M_queue(_M_states._M_start, _M_results);
      bool __ret = false;
      while (1)
        {
          _M_has_sol = false;
          if (_M_states._M_match_queue.empty())
            break;
          std::fill_n(_M_states._M_visited_states, _M_nfa.size(), false);
          auto __old_queue = std::move(_M_states._M_match_queue);
          auto __alloc = _M_cur_results.get_allocator();
          for (auto& __task : __old_queue)
            {
              _M_cur_results = _ResultsVec(std::move(__task.second), __alloc);
              _M_dfs(__match_mode, __task.first);
            }
          if (__match_mode == _Match_mode::_Prefix)
            __ret |= _M_has_sol;
          if (_M_current == _M_end)
            break;
          ++_M_current;
        }
      if (__match_mode == _Match_mode::_Exact)
        __ret = _M_has_sol;
      _M_states._M_match_queue.clear();
      return __ret;
    }

  // Return whether now match the given sub-NFA.
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    bool _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_lookahead(_StateIdT __next)
    {
      // Backreferences may refer to captured content.
      // We may want to make this faster by not copying,
      // but let's not be clever prematurely.
      _ResultsVec __what(_M_cur_results);
      _Executor __sub(_M_current, _M_end, __what, _M_re, _M_flags);
      __sub._M_states._M_start = __next;
      if (__sub._M_search_from_first())
        {
          for (size_t __i = 0; __i < __what.size(); __i++)
            if (__what[__i].matched)
              _M_cur_results[__i] = __what[__i];
          return true;
        }
      return false;
    }

  // __rep_count records how many times (__rep_count.second)
  // this node is visited under certain input iterator
  // (__rep_count.first). This prevent the executor from entering
  // infinite loop by refusing to continue when it's already been
  // visited more than twice. It's `twice` instead of `once` because
  // we need to spare one more time for potential group capture.
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_rep_once_more(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      auto& __rep_count = _M_rep_count[__i];
      if (__rep_count.second == 0 || __rep_count.first != _M_current)
        {
          auto __back = __rep_count;
          __rep_count.first = _M_current;
          __rep_count.second = 1;
          _M_dfs(__match_mode, __state._M_alt);
          __rep_count = __back;
        }
      else
        {
          if (__rep_count.second < 2)
            {
              __rep_count.second++;
              _M_dfs(__match_mode, __state._M_alt);
              __rep_count.second--;
            }
        }
    }

  // _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.
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_repeat(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];

      // Greedy.
      if (!__state._M_neg)
        {
          _M_rep_once_more(__match_mode, __i);
          // 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 // Non-greedy mode
        {
          if constexpr (__dfs_mode)
            {
              // vice-versa.
              _M_dfs(__match_mode, __state._M_next);
              if (!_M_has_sol)
                _M_rep_once_more(__match_mode, __i);
            }
          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_rep_once_more(__match_mode, __i);
                }
            }
        }
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_subexpr_begin(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];

      auto& __res = _M_cur_results[__state._M_subexpr];
      auto __back = __res.first;
      __res.first = _M_current;
      _M_dfs(__match_mode, __state._M_next);
      __res.first = __back;
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_subexpr_end(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];

      auto& __res = _M_cur_results[__state._M_subexpr];
      auto __back = __res;
      __res.second = _M_current;
      __res.matched = true;
      _M_dfs(__match_mode, __state._M_next);
      __res = __back;
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    inline void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_line_begin_assertion(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_at_begin())
        _M_dfs(__match_mode, __state._M_next);
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    inline void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_line_end_assertion(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_at_end())
        _M_dfs(__match_mode, __state._M_next);
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    inline void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_word_boundary(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_word_boundary() == !__state._M_neg)
        _M_dfs(__match_mode, __state._M_next);
    }

  // Here __state._M_alt offers a single start node for a sub-NFA.
  // We recursively invoke our algorithm to match the sub-NFA.
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_subexpr_lookahead(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_lookahead(__state._M_alt) == !__state._M_neg)
        _M_dfs(__match_mode, __state._M_next);
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_match(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];

      if (_M_current == _M_end)
        return;
      if constexpr (__dfs_mode)
        {
          if (__state._M_matches(*_M_current))
            {
              ++_M_current;
              _M_dfs(__match_mode, __state._M_next);
              --_M_current;
            }
        }
      else
        if (__state._M_matches(*_M_current))
          _M_states._M_queue(__state._M_next, _M_cur_results);
    }

  template<typename _BiIter, typename _TraitsT>
    struct _Backref_matcher
    {
      _Backref_matcher(bool /* __icase */, const _TraitsT& __traits)
      : _M_traits(__traits) { }

      bool
      _M_apply(_BiIter __expected_begin,
               _BiIter __expected_end, _BiIter __actual_begin,
               _BiIter __actual_end)
      {
        return _M_traits.transform(__expected_begin, __expected_end)
            == _M_traits.transform(__actual_begin, __actual_end);
      }

      const _TraitsT& _M_traits;
    };

  template<typename _BiIter, typename _CharT>
    struct _Backref_matcher<_BiIter, std::regex_traits<_CharT>>
    {
      using _TraitsT = std::regex_traits<_CharT>;
      _Backref_matcher(bool __icase, const _TraitsT& __traits)
      : _M_icase(__icase), _M_traits(__traits) { }

      bool
      _M_apply(_BiIter __expected_begin,
               _BiIter __expected_end, _BiIter __actual_begin,
               _BiIter __actual_end)
      {
        if (!_M_icase)
          return _GLIBCXX_STD_A::__equal4(__expected_begin, __expected_end,
                               __actual_begin, __actual_end);
        typedef std::ctype<_CharT> __ctype_type;
        const auto& __fctyp = use_facet<__ctype_type>(_M_traits.getloc());
        return _GLIBCXX_STD_A::__equal4(__expected_begin, __expected_end,
                             __actual_begin, __actual_end,
                             [this, &__fctyp](_CharT __lhs, _CharT __rhs)
                             {
                               return __fctyp.tolower(__lhs)
                                 == __fctyp.tolower(__rhs);
                             });
      }

      bool _M_icase;
      const _TraitsT& _M_traits;
    };

  // 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 and try another state.
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_backref(_Match_mode __match_mode, _StateIdT __i)
    {
      static_assert(__dfs_mode, "this should never be instantiated");

      const auto& __state = _M_nfa[__i];
      auto& __submatch = _M_cur_results[__state._M_backref_index];
      if (!__submatch.matched)
        return;
      auto __last = _M_current;
      for (auto __tmp = __submatch.first;
           __last != _M_end && __tmp != __submatch.second;
           ++__tmp)
        ++__last;
      if (_Backref_matcher<_BiIter, _TraitsT>(
              _M_re.flags() & regex_constants::icase,
              _M_re._M_automaton->_M_traits)._M_apply(
                  __submatch.first, __submatch.second, _M_current, __last))
        {
          if (__last != _M_current)
            {
              auto __backup = _M_current;
              _M_current = __last;
              _M_dfs(__match_mode, __state._M_next);
              _M_current = __backup;
            }
          else
            _M_dfs(__match_mode, __state._M_next);
        }
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_accept(_Match_mode __match_mode, _StateIdT)
    {
      if constexpr (__dfs_mode)
        {
          __glibcxx_assert(!_M_has_sol);
          if (__match_mode == _Match_mode::_Exact)
            _M_has_sol = _M_current == _M_end;
          else
            _M_has_sol = true;
          if (_M_current == _M_begin
              && (_M_flags & regex_constants::match_not_null))
            _M_has_sol = false;
          if (_M_has_sol)
            {
              if (_M_nfa._M_flags & regex_constants::ECMAScript)
                _M_results = _M_cur_results;
              else // POSIX
                {
                  __glibcxx_assert(_M_states._M_get_sol_pos());
                  // Here's POSIX's logic: match the longest one. However
                  // we never know which one (lhs or rhs of "|") is longer
                  // unless we try both of them and compare the results.
                  // The member variable _M_sol_pos records the end
                  // position of the last successful match. It's better
                  // to be larger, because POSIX regex is always greedy.
                  // TODO: This could be slow.
                  if (*_M_states._M_get_sol_pos() == _BiIter()
                      || std::distance(_M_begin,
                                       *_M_states._M_get_sol_pos())
                         < std::distance(_M_begin, _M_current))
                    {
                      *_M_states._M_get_sol_pos() = _M_current;
                      _M_results = _M_cur_results;
                    }
                }
            }
        }
      else
        {
          if (_M_current == _M_begin
              && (_M_flags & regex_constants::match_not_null))
            return;
          if (__match_mode == _Match_mode::_Prefix || _M_current == _M_end)
            if (!_M_has_sol)
              {
                _M_has_sol = true;
                _M_results = _M_cur_results;
              }
        }
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_handle_alternative(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];

      if (_M_nfa._M_flags & regex_constants::ECMAScript)
        {
          // TODO: Fix BFS support. It is wrong.
          _M_dfs(__match_mode, __state._M_alt);
          // Pick lhs if it matches. Only try rhs if it doesn't.
          if (!_M_has_sol)
            _M_dfs(__match_mode, __state._M_next);
        }
      else
        {
          // Try both and compare the result.
          // See "case _S_opcode_accept:" handling above.
          _M_dfs(__match_mode, __state._M_alt);
          auto __has_sol = _M_has_sol;
          _M_has_sol = false;
          _M_dfs(__match_mode, __state._M_next);
          _M_has_sol |= __has_sol;
        }
    }

  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_dfs(_Match_mode __match_mode, _StateIdT __i)
    {
      if (_M_states._M_visited(__i))
        return;

      switch (_M_nfa[__i]._M_opcode())
        {
        case _S_opcode_repeat:
          _M_handle_repeat(__match_mode, __i); break;
        case _S_opcode_subexpr_begin:
          _M_handle_subexpr_begin(__match_mode, __i); break;
        case _S_opcode_subexpr_end:
          _M_handle_subexpr_end(__match_mode, __i); break;
        case _S_opcode_line_begin_assertion:
          _M_handle_line_begin_assertion(__match_mode, __i); break;
        case _S_opcode_line_end_assertion:
          _M_handle_line_end_assertion(__match_mode, __i); break;
        case _S_opcode_word_boundary:
          _M_handle_word_boundary(__match_mode, __i); break;
        case _S_opcode_subexpr_lookahead:
          _M_handle_subexpr_lookahead(__match_mode, __i); break;
        case _S_opcode_match:
          _M_handle_match(__match_mode, __i); break;
        case _S_opcode_backref:
          if constexpr (__dfs_mode)
            _M_handle_backref(__match_mode, __i);
          else
            __builtin_unreachable();
          break;
        case _S_opcode_accept:
          _M_handle_accept(__match_mode, __i); break;
        case _S_opcode_alternative:
          _M_handle_alternative(__match_mode, __i); break;
        default:
          __glibcxx_assert(false);
        }
    }

  // Return whether now is at some word boundary.
  template<typename _BiIter, typename _Alloc, typename _TraitsT,
           bool __dfs_mode>
    bool _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
    _M_word_boundary() const
    {
      if (_M_current == _M_begin && (_M_flags & regex_constants::match_not_bow))
        return false;
      if (_M_current == _M_end && (_M_flags & regex_constants::match_not_eow))
        return false;

      bool __left_is_word = false;
      if (_M_current != _M_begin
          || (_M_flags & regex_constants::match_prev_avail))
        {
          auto __prev = _M_current;
          if (_M_is_word(*std::prev(__prev)))
            __left_is_word = true;
        }
      bool __right_is_word =
        _M_current != _M_end && _M_is_word(*_M_current);

      return __left_is_word != __right_is_word;
    }
} // namespace __detail
#pragma GCC diagnostic pop

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace