## Copyright (C) 1993, 1994, 1995 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, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301 USA.

## -*- texinfo -*-
## @deftypefn {Function File} {[@var{n}, @var{m}, @var{p}] =} abcddim (@var{a}, @var{b}, @var{c}, @var{d})
## Check for compatibility of the dimensions of the matrices defining
## the linear system
## @iftex
## @tex
## $[A, B, C, D]$ corresponding to
## $$
## \eqalign{
##  {dx\over dt} &= A x + B u\cr
##             y &= C x + D u}
## $$
## @end tex
## @end iftex
## @ifinfo
## [A, B, C, D] corresponding to
##
## @example
## dx/dt = a x + b u
## y = c x + d u
## @end example
##
## @end ifinfo
## or a similar discrete-time system.
##
## If the matrices are compatibly dimensioned, then @code{abcddim} returns
##
## @table @var
## @item n
## The number of system states.
##
## @item m
## The number of system inputs.
##
## @item p
## The number of system outputs.
## @end table
##
## Otherwise @code{abcddim} returns @var{n} = @var{m} = @var{p} = @minus{}1.
##
## Note: n = 0 (pure gain block) is returned without warning.
##
## @end deftypefn
##
## @seealso{is_abcd}

## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
## Created: August 1993.
## a s hodel: modified to accept pure-gain systems aug 1996

function [n, m, p] = abcddim (a, b, c, d)

  if (nargin != 4)
    error ("abcddim: four arguments required");
  endif

  n = m = p = -1;

  [a, an, am] = __abcddims__ (a);
  [b, bn, bm] = __abcddims__ (b);
  [c, cn, cm] = __abcddims__ (c);
  [d, dn, dm] = __abcddims__ (d);

  if ( (!issquare(a)) & (!isempty(a)) )
    warning (["abcddim: a is not square (",num2str(an),"x",num2str(am),")"]);
    return
  endif

  if( (bm == 0) & (dm == 0) )
    warning("abcddim: no inputs");
  elseif (bn != am)
    warning (["abcddim: a(",num2str(an),"x",num2str(am), ...
      " and b(",num2str(bn),"x",num2str(bm),") are not compatible"]);
    return
  endif

  if( (cn == 0) & (dn == 0 ) )
    warning("abcddim: no outputs");
  elseif (cm != an)
    warning (["abcddim: a(",num2str(an),"x",num2str(am), ...
        " and c(",num2str(cn),"x",num2str(cm),") are not compatible"]);
    return
  endif

  have_connections = (bn*cn != 0);

  if( (dn == 0) & have_connections)
    warning("abcddim: empty d matrix passed; setting compatibly with b, c");
    [d, dn, dm] = __abcddims__ (zeros (cn, bm));
  endif

  if(an > 0)
    [dn, dm] = size(d);
    if ( (cn != dn) & have_connections )
      warning (["abcddim: c(",num2str(cn),"x",num2str(cm), ...
        " and d(",num2str(dn),"x",num2str(dm),") are not compatible"]);
      return
    endif

    if ( (bm != dm) & have_connections )
      warning (["abcddim: b(",num2str(bn),"x",num2str(bm), ...
          " and d(",num2str(dn),"x",num2str(dm),") are not compatible"]);
      return
    endif

    m = bm;
    p = cn;
  else
    [p,m] = size(d);
  endif
  n = an;
endfunction