## 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. 

## -*- texinfo -*-
## @deftypefn {Function File } { outputs =} ss2zp ( inputs ) 
## @format
##  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 format manipulations
## 
## 
## @end format
## @end deftypefn

 
function [zer, pol, k] = ss2zp (a, b, c, d)

  ## 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