## Copyright (C) 1999 Paul Kienzle <pkienzle@users.sf.net>
##
## This program 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 3 of the License, or
## (at your option) any later version.
##
## This program 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 this program; see the file COPYING. If not, see
## <https://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn  {Function File} {[@var{x}, @var{t}] =} impz (@var{b})
## @deftypefnx {Function File} {[@var{x}, @var{t}] =} impz (@var{b}, @var{a})
## @deftypefnx {Function File} {[@var{x}, @var{t}] =} impz (@var{b}, @var{a}, @var{n})
## @deftypefnx {Function File} {[@var{x}, @var{t}] =} impz (@var{b}, @var{a}, @var{n}, @var{fs})
## @deftypefnx {Function File} {} impz (@dots{})
##
## Generate impulse-response characteristics of the filter. The filter
## coefficients correspond to the the z-plane rational function with
## numerator b and denominator a.  If a is not specified, it defaults to
## 1. If n is not specified, or specified as [], it will be chosen such
## that the signal has a chance to die down to -120dB, or to not explode
## beyond 120dB, or to show five periods if there is no significant
## damping. If no return arguments are requested, plot the results.
##
## @seealso{freqz, zplane}
## @end deftypefn

## FIXME: Call equivalent function from control toolbox since it is
##        probably more sophisticated than this one, and since it
##        is silly to maintain two different versions of essentially
##        the same thing.

function [x_r, t_r] = impz(b, a = [1], n = [], fs = 1)

  if nargin == 0 || nargin > 4
    print_usage;
  endif

  if isempty(n) && length(a) > 1
    precision = 1e-6;
    r = roots(a);
    maxpole = max(abs(r));
    if (maxpole > 1+precision)     # unstable -- cutoff at 120 dB
      n = floor(6/log10(maxpole));
    elseif (maxpole < 1-precision) # stable -- cutoff at -120 dB
      n = floor(-6/log10(maxpole));
    else                           # periodic -- cutoff after 5 cycles
      n = 30;

      ## find longest period less than infinity
      ## cutoff after 5 cycles (w=10*pi)
      rperiodic = r(find(abs(r)>=1-precision & abs(arg(r))>0));
      if !isempty(rperiodic)
        n_periodic = ceil(10*pi./min(abs(arg(rperiodic))));
        if (n_periodic > n)
          n = n_periodic;
        endif
      endif

      ## find most damped pole
      ## cutoff at -60 dB
      rdamped = r(find(abs(r)<1-precision));
      if !isempty(rdamped)
        n_damped = floor(-3/log10(max(abs(rdamped))));
        if (n_damped > n)
          n = n_damped;
        endif
      endif
    endif
    n = n + length(b);
  elseif isempty(n)
    n = length(b);
  elseif (! isscalar (n))
    ## TODO: missing option of having N as a vector of values to
    ##       compute the impulse response.
    error ("impz: N must be empty or a scalar");
  endif

  if length(a) == 1
    x = fftfilt(b/a, [1, zeros(1,n-1)]');
  else
    x = filter(b, a, [1, zeros(1,n-1)]');
  endif
  t = [0:n-1]/fs;

  if nargout >= 1 x_r = x; endif
  if nargout >= 2 t_r = t; endif
  if nargout == 0
    # scale and work out xlabel
    if nargin == 4
      if max(t) < 1
        label_text = "Time (msec)";
        t = t * 1000;
      else
        label_text = "Time (sec)";
      endif
    else
      label_text = "samples";
    endif

    stem(t, x);
    title("Impulse Response");
    xlabel(label_text);
    ylabel("Amplitude");
  endif

endfunction

%!assert (size (impz (1, [1 -1 0.9], 100)), [100 1])

## Missing functionality
%!xtest
%! [h, t] = impz (1, [1 -1 0.9], 0:101);
%! assert (size (h), [101 1])
%! assert (t, 0:101)
