findPreviousReducerPrefix()

std::pair< bool, int > NCF4::findPreviousReducerPrefix ( const Word & w )

private

Definition at line 769 of file NCF4.cpp.

{
  // build the largest proper prefix of m and look it up in previous
  // mColumnMonomials table.  we will return (true, row index) if
  // found and (false, -1) if not.
 
  // note that just because the second coordinate of an
  // mColumnMonomials entry is -1 does not mean there isn't a reducer
  // in the row -- in this case it is a new GB element and already
  // added to the mGroebner list.  In this case, we will be using that
  // entry anyway so no need to make the additional effort to find it
  // in the mRows table.
 
  // get prefix of m as a monom
  // search for prefix in mPreviousColumnMonomials
  // if it.second.second != -1, then return (true,it.second.first)
  // else return (false, -1)
 
  // if the monom is the empty monomial, return false
  if (w.size() == 0) return std::make_pair(false,-1);
  
  std::pair<bool,int> retval;
  
  Word prefix(w.begin(),w.end()-1);
  const auto it = mPreviousColumnMonomials.find(prefix);
  if (it == mPreviousColumnMonomials.end()) // not in table
    retval = std::make_pair(false,-1);
  else
  {
    int colNum = (*it).second.first;
    if (mPreviousColumns[colNum].pivotRow == -1)  // in column table and not a reducer monomial
      retval = std::make_pair(false,-1);
    else  // in table and a reducer monomial
      retval = std::make_pair(true,mPreviousColumns[colNum].pivotRow);
  }
  return retval;
}

References Word::begin(), Word::end(), mPreviousColumnMonomials, mPreviousColumns, prefix(), and Word::size().

Referenced by findDivisor().