### Copyright (C) 1996 John W. Eaton
###
### This file is part of Octave.
###
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### 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.
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### 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
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### 02111-1307, USA.

## Usage: [k, p, e] = dlqr (A, B, Q, R {,Z})
##
## Linear quadratic regulator design for the discrete time system
##
##   x[k+1] = A x[k] + B u[k]
##
## to minimize the cost functional
##
##  J = Sum { x' Q x + u' R u } 			Z omitted
##
## or
##
##  J = Sum { x' Q x + u' R u +2 x' Z u}		Z included
##
## Returns:
##
##   k = state feedback gain, (A - B K) is stable
##   p = solution of algebraic Riccati equation
##   e = closed loop poles of (A - B K)

function [k, p, e] = dlqr (a, b, q, r, zz)

  ## Written by A. S. Hodel (scotte@eng.auburn.edu) August 1993.
  ## Converted to discrete time by R. B. Tenison
  ## (btenison@eng.auburn.edu) October 1993

  if (nargin != 4 && nargin != 5)
    error ("dlqr: invalid number of arguments");
  endif

  ## Check a.
  if ((n = is_square (a)) == 0)
    error ("dlqr: requires 1st parameter(a) to be square");
  endif

  ## Check b.
  [n1, m] = size (b);
  if (n1 != n)
    error ("dlqr: a,b not conformal");
  endif

  ## Check q.
  
  if ((n1 = is_square (q)) == 0 || n1 != n)
    error ("dlqr: q must be square and conformal with a");
  endif

  ## Check r.
  if((m1 = is_square(r)) == 0 || m1 != m)
    error ("dlqr: r must be square and conformal with column dimension of b");
  endif

  ## Check if n is there.
  if (nargin == 5)
    [n1, m1] = size (zz);
    if (n1 != n || m1 != m)
      error ("dlqr: z must be identically dimensioned with b");
    endif

    ## Incorporate cross term into a and q.

    ao = a - (b/r)*zz';
    qo = q - (zz/r)*zz';
  else
    zz = zeros (n, m);
    ao = a;
    qo = q;
  endif

  ## Check that q, (r) are symmetric, positive (semi)definite

  if (is_symmetric (q) && is_symmetric (r) ...
      && all (eig (q) >= 0) && all (eig (r) > 0))
    p = dare (ao, b, qo, r);
    k = (r+b'*p*b)\b'*p*a + r\zz';
    e = eig (a - b*k);
  else
    error ("dlqr: q (r) must be symmetric positive (semi) definite");
  endif

endfunction