calc_pfaff() [1/2]

ring_elem PfaffianComputation::calc_pfaff ( size_t * r, int p )

private

Definition at line 62 of file pfaff.cpp.

{
  if (p2 == 2) return M->elem(static_cast<int>(r[0]), static_cast<int>(r[1]));
  ring_elem result = R->from_long(0);
 
  bool negate = true;
  for (int i = p2 - 2; i >= 0; i--)
    {
      std::swap(r[i], r[p2 - 2]);
      negate = !negate;
      ring_elem g =
          M->elem(static_cast<int>(r[p2 - 2]), static_cast<int>(r[p2 - 1]));
      if (R->is_zero(g))
        {
          R->remove(g);
          continue;
        }
      ring_elem h = calc_pfaff(r, p2 - 2);
      ring_elem gh = R->mult(g, h);
      R->remove(g);
      R->remove(h);
      if (negate)
        R->subtract_to(result, gh);
      else
        R->add_to(result, gh);
    }
 
  // pulling out the columns has disordered r. Fix it.
 
  size_t temp = r[p2 - 2];
  for (size_t i = p2 - 2; i > 0; i--) r[i] = r[i - 1];
  r[0] = temp;
 
  return result;
}

References calc_pfaff(), M, R, result(), and std::swap().

Referenced by Matrix::pfaffian().