## 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 0211

## -*- texinfo -*-
## @deftypefn {Function File } { @var{sys} =} sysupdate ( @var{sys}, @var{opt} ) 
##  Update the internal representation of a system.
## 
## @strong{Inputs}
## @table @var
## @item sys:
## system data structure
## @item opt
##  string:  
## @table @code
## @item "tf"
## update transfer function form
## @item "zp" 
## update zero-pole form
## @item "ss" 
## update state space form
## @item "all" 
## all of the above
## @end table
## @end table
## 
## @strong{Outputs}
## @var{retsys}: contains union of data in sys and requested data.
## If requested data in sys is already up to date then retsys=sys.
## 
## Conversion to @code{tf} or @code{zp} exits with an error if the system is 
##  mixed continuous/digital.
## @end deftypefn

## See also: tf2sys, ss2sys, zp2sys, sysout, sys2ss, sys2tf, sys2zp

function sys = sysupdate(sys,opt)

  ## Written by John Ingram  7-9-96

  ## check for correct number of inputs 
  if (nargin != 2)
    usage("newsys = sysupdate(sys,opt)");
  elseif(! is_struct(sys) )
   error("1st argument must be system data structure")
  elseif(! (strcmp(opt,"tf") + strcmp(opt,"zp") + ...
	strcmp(opt,"ss") + strcmp(opt,"all")) )
    error("2nd argument must be \"tf\", \"zp\", \"ss\", or \"all\"");
  endif

  ## check to make sure not trying to make a SISO system out of a MIMO sys
  if ( (strcmp(opt,"tf") + strcmp(opt,"zp") + strcmp(opt,"all")) ...
	& strcmp(sysgettype(sys),"ss") &  (! is_siso(sys) ) )
    error("MIMO -> SISO update requested");
  endif

  ## update transfer function if desired
  if ( (strcmp(opt, "tf") + strcmp(opt,"all"))&&  (!sys.sys(2)))
    ## check to make sure the system is not discrete and continuous
    is_digital(sys);

    ## if original system zero-pole
    if strcmp(sysgettype(sys),"zp")
      [sys.num,sys.den] = zp2tf(sys.zer,sys.pol,sys.k);
      sys.sys(2) = 1;
    ## if original system is state-space
    elseif(sys.sys(1) == 2)
      [sys.num,sys.den] = ss2tf(sys.a,sys.b,sys.c,sys.d);
      sys.sys(2) = 1; 
    endif
  endif


  ## update zero-pole if desired
  if ( (strcmp(opt, "zp") + strcmp(opt,"all")) && (! sys.sys(3)) )
    ## check to make sure the system is not discrete and continuous
    is_digital(sys);

    ## original system is transfer function
    if (sys.sys(1) == 0)
      [sys.zer,sys.pol,sys.k] = tf2zp(sys.num,sys.den);
      sys.sys(3) = 1;
    ## original system is state-space

    elseif(sys.sys(1) == 2)
      [sys.zer,sys.pol,sys.k] = ss2zp(sys.a,sys.b,sys.c,sys.d);
      sys.sys(3) = 1; 
    endif

  endif

  ## update state-space if desired
  if ( (strcmp(opt, "ss") + strcmp(opt,"all")) && (! sys.sys(4)) )
    ## original system is transfer function
    if (sys.sys(1) == 0)
      [sys.a,sys.b,sys.c,sys.d] = tf2ss(sys.num,sys.den);
      sys.sys(4) = 1;
    ## original system is zero-pole
    elseif(sys.sys(1) == 1)
      [sys.a,sys.b,sys.c,sys.d] = zp2ss(sys.zer,sys.pol,sys.k);
      sys.sys(4) = 1; 
    endif

    ## create new state names
    sys.stname = sysdefstname(sys.n, sys.nz);
  endif
  

endfunction