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##
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## later version. 
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## -*- texinfo -*-
## @deftypefn {Function File } {[y, t] = } stepimp(@var{sitype},@var{sys}[, @var{inp}, @var{tstop}, @var{n}]) 
## Impulse or step response for a linear system.
##       The system can be discrete or multivariable (or both).
##       This m-file contains the "common code" of step and impulse.
## 
## Produces a plot or the response data for system sys.
## 
## Limited argument checking; "do not attempt to do this at home".
## Used internally in @code{impulse}, @code{step}. Use @code{step}
## or @code{impulse} instead.
## 
## @end deftypefn

## See also: step, impulse

function [y, t] = stepimp(sitype, sys, inp, tstop, n)

  ## Written by Kai P. Mueller October 2, 1997
  ## based on lsim.m of Scottedward Hodel

  if (sitype == 1)         IMPULSE = 0;
  elseif (sitype == 2)     IMPULSE = 1;
  else		   	   error("stepimp: illegal sitype argument.")
  endif
  sys = sysupdate(sys,"ss");

  USE_DEF = 0;   # default tstop and n if we have to give up
  N_MIN = 50;    # minimum number of points
  N_MAX = 2000;  # maximum number of points
  T_DEF = 10.0;  # default simulation time

  ## collect useful information about the system
  [ncstates,ndstates,NIN,NOUT] = sysdimensions(sys);
  TSAMPLE = sysgettsam(sys);

  if (nargin < 3)                      inp = 1;
  elseif (inp < 1 | inp > NIN)         error("Argument inp out of range")
  endif

  DIGITAL = is_digital(sys);
  if (DIGITAL)
    NSTATES = ndstates;
    if (TSAMPLE < eps)
      error("stepimp: sampling time of discrete system too small.")
    endif
  else        NSTATES = ncstates;       endif
  if (NSTATES < 1)
    error("step: pure gain block (n_states < 1), step response is trivial");
  endif
  if (nargin < 5)
    ## we have to compute the time when the system reaches steady state
    ## and the step size
    ev = eig(sys2ss(sys));
    if (DIGITAL)
      ## perform bilinear transformation on poles in z
      for i = 1:NSTATES
        pole = ev(i);
	if (abs(pole + 1) < 1.0e-10)
	  ev(i) = 0;
	else
	  ev(i) = 2 / TSAMPLE * (pole - 1) / (pole + 1);
	endif
      endfor
    endif
    ## remove poles near zero from eigenvalue array ev
    nk = NSTATES;
    for i = 1:NSTATES
      if (abs(ev(i)) < 1.0e-10)
        ev(i) = 0;
        nk = nk - 1;
      endif
    endfor
    if (nk == 0)
      USE_DEF = 1;
      ## printf("##STEPIMP-DEBUG: using defaults.\n");
    else
      ev = ev(find(ev));
      x = max(abs(ev));
      t_step = 0.2 * pi / x;
      x = min(abs(real(ev)));
      t_sim = 5.0 / x;
      ## round up
      yy = 10^(ceil(log10(t_sim)) - 1);
      t_sim = yy * ceil(t_sim / yy);
      ## printf("##STEPIMP-DEBUG: nk=%d   t_step=%f  t_sim=%f\n",
      ##   nk, t_step, t_sim);  
    endif
  endif

  if (DIGITAL)
    ## ---- sampled system
    if (nargin == 5)
      n = round(n);
      if (n < 2)
        error("stepimp: n must not be less than 2.")
      endif
    else
      if (nargin == 4)
        ## n is unknown
      elseif (nargin >= 1)
        ## tstop and n are unknown
        if (USE_DEF)
          tstop = (N_MIN - 1) * TSAMPLE;
        else
          tstop = t_sim;
        endif
      endif
      n = floor(tstop / TSAMPLE) + 1;
      if (n < 2)  n = 2;  endif
      if (n > N_MAX)
	n = N_MAX;
	printf("Hint: number of samples limited to %d by default.\n", \
	       N_MAX);
	printf("  ==> increase \"n\" parameter for longer simulations.\n");
      endif
    endif
    tstop = (n - 1) * TSAMPLE;
    t_step = TSAMPLE;
  else
    ## ---- continuous system
    if (nargin == 5)
      n = round(n);
      if (n < 2)
        error("step: n must not be less than 2.")
      endif
      t_step = tstop / (n - 1);
    else
      if (nargin == 4)
        ## only n in unknown
        if (USE_DEF)
          n = N_MIN;
	  t_step = tstop / (n - 1);
        else
          n = floor(tstop / t_step) + 1;
        endif
      else
        ## tstop and n are unknown
        if (USE_DEF)
          tstop = T_DEF;
	  n = N_MIN;
	  t_step = tstop / (n - 1);
        else
          tstop = t_sim;
          n = floor(tstop / t_step) + 1;
        endif
      endif
      if (n < N_MIN)
	n = N_MIN;
        t_step = tstop / (n - 1);
      endif
      if (n > N_MAX)
    	tstop = (n - 1) * t_step;
	t_step = tstop / (N_MAX - 1);
	n = N_MAX;
      endif
    endif
    tstop = (n - 1) * t_step;
    [jnk,B] = sys2ss(sys);
    B = B(:,inp);
    sys = c2d(sys, t_step);
  endif
  ## printf("##STEPIMP-DEBUG: t_step=%f n=%d  tstop=%f\n", t_step, n, tstop);

  F = sys.a;
  G = sys.b(:,inp);
  C = sys.c;
  D = sys.d(:,inp);
  y = zeros(NOUT, n);
  t = linspace(0, tstop, n);

  if (IMPULSE)
    if (!DIGITAL && (D'*D > 0))
      error("impulse: D matrix is nonzero, impulse response infinite.")
    endif
    if (DIGITAL)
      y(:,1) = D / t_step;
      x = G / t_step;
    else
      x = B;
      y(:,1) = C * x;
      x = F * x;
    endif
    for i = 2:n
      y(:,i) = C * x;
      x = F * x;
    endfor
  else
    x = zeros(NSTATES, 1);
    for i = 1:n
      y(:,i) = C * x + D;
      x = F * x + G;
    endfor
  endif

  if(nargout == 0)
    ## Plot the information
    oneplot();
    gset nogrid
    gset nologscale
    gset autoscale
    gset nokey
    clearplot();
    if (gnuplot_has_multiplot)
      if (IMPULSE)
        gm = zeros(NOUT, 1);
	tt = "impulse";
      else
        ssys = ss2sys(F, G, C, D, t_step);
        gm = dcgain(ssys);
	tt = "step";
      endif
      ncols = floor(sqrt(NOUT));
      nrows = ceil(NOUT / ncols);
      for i = 1:NOUT
        subplot(nrows, ncols, i);
	title(sprintf("%s: | %s -> %s", tt,sysgetsignals(sys,"in",inp,1), ...
	  sysgetsignals(sys,"out",i,1)));
	if (DIGITAL)
	  [ts, ys] = stairs(t, y(i,:));
	  ts = ts(1:2*n-2)';  ys = ys(1:2*n-2)';
	  if (length(gm) > 0)
	    yy = [ys; gm(i)*ones(size(ts))];
	  else
	    yy = ys;
	  endif
	  grid("on");
	  xlabel("time [s]");
	  ylabel("y(t)");
          plot(ts, yy);
	else
	  if (length(gm) > 0)
	    yy = [y(i,:); gm(i)*ones(size(t))];
	  else
	    yy = y(i,:);
	  endif
	  grid("on");
	  xlabel("time [s]");
	  ylabel("y(t)");
	  plot(t, yy);
	endif
      endfor
      ## leave gnuplot in multiplot mode is bad style
      oneplot();
    else
      ## plot everything in one diagram
      title([tt, " response | ", sysgetsignals(sys,"in",inp,1), ...
	" -> all outputs"]);
      if (DIGITAL)
        stairs(t, y(i,:));
      else
	grid("on");
	xlabel("time [s]");
	ylabel("y(t)");
	plot(t, y(i,:));
      endif
    endif
    y=[];
    t=[];
  endif
  ## printf("##STEPIMP-DEBUG: gratulations, successfull completion.\n");
endfunction