find_good_divisor()

int gbA::find_good_divisor ( exponents_t e, int x, int degf, int & result_gap )

private

Definition at line 1710 of file gb-default.cpp.

{
  int n = 0;
  int gap;
  int egap = degf - weightInfo_->exponents_weight(e, x);
 
  VECTOR(MonomialTable::mon_term *) divisors;
 
  if (!is_local_gb && divisor_previous >= 0 && x == divisor_previous_comp)
    {
      gbelem *tg = gb[divisor_previous];
      gap = tg->gap - egap;
      if (gap <= 0 && exponents_divide(_nvars, tg->lead, e))
        {
          result_gap = 0;
          return divisor_previous;
        }
    }
 
  /* First search for ring divisors */
  if (ringtable) n += ringtable->find_divisors(-1, e, 1, &divisors);
 
  /* Next search for GB divisors */
  n += lookup->find_divisors(-1, e, x, &divisors);
 
  if (M2_gbTrace == 15 && n >= 2)
    {
      gbelem *tg = gb[divisors[n - 1]->_val];
      int sz = tg->size;
      if (sz >= 0)  // was 3, why??
        {
          buffer o;
          o << "    reducers: ";
          for (int j = 0; j < n; j++) o << "g" << divisors[j]->_val << " ";
          emit_line(o.str());
        }
    }
  /* Now find the minimal gap value */
  if (n == 0)
    {
      result_gap = 0;
      return -1;
    }
 
  if (is_local_gb)
    {
      // new version, under development
      int i = 0;
      int j = divisors[i]->_val;
      gap = gb[j]->gap - egap;
      if (gap < 0) gap = 0;
      while (true)
        {
          int mingap = gap;
          int best = j;
          do
            {
              if (++i == n)
                {
                  divisor_previous = best;
                  divisor_previous_comp = x;
                  result_gap = mingap;  // a difference between two gaps is no
                                        // longer a "gap"...
                  if (result_gap < 0)
                    result_gap = 0;  // I'm not sure this is needed.
                  return best;
                }
              j = divisors[i]->_val;
              gap = gb[j]->gap - egap;
              if (gap < 0) gap = 0;
            }
          while (gap >= mingap);  // used to be: (! gap < mingap || gap ==
                                  // mingap && false)
        }
    }
  else
    {
      int newgap;
      int result = divisors[n - 1]->_val;
      gbelem *tg = gb[result];
      gap = tg->gap - egap;
      if (gap <= 0)
        {
          gap = 0;
          int minsz = tg->size;
          for (int i = n - 2; i >= 0; i--)
            {
              int new_val = divisors[i]->_val;
              tg = gb[new_val];
              int sz = tg->size;
              if (sz < minsz)
                {
                  if (tg->gap <= egap)
                    {
                      minsz = sz;
                      result = new_val;
                    }
                }
            }
        }
      else
        //    for (i=1; i<n; i++)
        for (int i = n - 2; i >= 0; i--)
          {
            int new_val = divisors[i]->_val;
            tg = gb[new_val];
 
            newgap = tg->gap - egap;
            if (newgap <= 0)
              {
                gap = 0;
                result = new_val;
                break;
              }
            else if (newgap < gap)
              {
                result = new_val;
                gap = newgap;
              }
          }
      divisor_previous = result;
      divisor_previous_comp = x;
      result_gap = gap;
      return result;
    }
}

References _nvars, divisor_previous, divisor_previous_comp, emit_line(), exponents_divide(), gbA::gbelem::gap, gb(), is_local_gb, gbA::gbelem::lead, lookup, M2_gbTrace, result(), ringtable, gbA::gbelem::size, buffer::str(), VECTOR, weightInfo_, and x.

Referenced by reduce_kk(), and remainder_non_ZZ().