# Copyright (C) 1996 Auburn University.  All Rights Reserved.
#
# This file is part of Octave. 
#
# Octave 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 2, or (at your option) any 
# later version. 
# 
# Octave 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.
# 
# You should have received a copy of the GNU General Public License 
# along with Octave; see the file COPYING.  If not, write to the Free 
# Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111 USA. 
 
function [zer,pol,k] = ss2zp(a,b,c,d)
# Converts a state space representation to a set of poles and zeros.
#
# [pol,zer,k] = ss2zp(a,b,c,d) returns the poles and zeros of the state space 
# system (a,b,c,d).  K is a gain associated with the zeros.
#
# used internally in system data structure manipulations

# Written by David Clem August 15, 1994
# Hodel: changed order of output arguments to zer, pol, k. July 1996
# a s hodel: added argument checking, allow for pure gain blocks aug 1996

  if(nargin != 4)
    usage("[zer,pol,k] = ss2zp(a,b,c,d)");
  endif

  [n,m,p] = abcddim(a,b,c,d);
  if (n == -1)
    error("ss2tf: Non compatible matrix arguments");
  elseif ( (m != 1) | (p != 1))
    error(["ss2tf: not SISO system: m=",num2str(m)," p=",num2str(p)]);
  endif
 
  if(n == 0)
    # gain block only
    k = d;
    zer = pol = [];
  else
    # First, get the denominator coefficients
    [zer,k] = tzero(a,b,c,d);
    pol = eig(a);
  endif
endfunction