## Copyright (C) 2005 Julius O. Smith III <jos@ccrma.stanford.edu>
##
## 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{r}, @var{p}, @var{f}, @var{m}] =} residuez (@var{b}, @var{a})
## Compute the partial fraction expansion of filter @math{H(z) = B(z)/A(z)}.
##
## INPUTS:
## @var{b} and @var{a} are vectors specifying the digital filter
## @math{H(z) = B(z)/A(z)}. See @code{help filter} for documentation of the
## @var{b} and @var{a} filter coefficients.
##
## RETURNED:
## @itemize
## @item @var{r} = column vector containing the filter-pole residues
## @item @var{p} = column vector containing the filter poles
## @item @var{f} = row vector containing the FIR part, if any
## @item @var{m} = column vector of pole multiplicities
## @end itemize
##
## EXAMPLES:
## @example
## See @code{test residuez verbose} to see a number of examples.
## @end example
##
## For the theory of operation, see
## @indicateurl{http://ccrma.stanford.edu/~jos/filters/residuez.html}
##
## @seealso{residue, residued}
## @end deftypefn

function [r, p, f, m] = residuez(B, A, tol)

  ## RESIDUEZ - return residues, poles, and FIR part of B(z)/A(z)
  ##
  ## Let nb = length(b), na = length(a), and N=na-1 = no. of poles.
  ## If nb<na, then f will be empty, and the returned filter is
  ##
  ##             r(1)                      r(N)
  ## H(z) = ----------------  + ... + ----------------- = R(z)
  ##        [ 1-p(1)/z ]^m(1)         [ 1-p(N)/z ]^m(N)
  ##
  ## If, on the other hand, nb >= na, the FIR part f will not be empty.
  ## Let M = nb-na+1 = order of f = length(f)-1). Then the returned filter is
  ##
  ## H(z) = f(1) + f(2)/z + f(3)/z^2 + ... + f(M+1)/z^M + R(z)
  ##
  ## where R(z) is the parallel one-pole filter bank defined above.
  ## Note, in particular, that the impulse-response of the one-pole
  ## filter bank is in parallel with that of the the FIR part.  This can
  ## be wasteful when matching the initial impulse response is important,
  ## since F(z) can already match the first N terms of the impulse
  ## response. To obtain a decomposition in which the impulse response of
  ## the IIR part R(z) starts after that of the FIR part F(z), use RESIDUED.
  ##
  ## J.O. Smith, 9/19/05

  if nargin==3
    warning("tolerance ignored");
  endif
  NUM = B(:)'; DEN = A(:)';
  ## Matlab's residue does not return m (since it is implied by p):
  [r,p,f,m]=residue(conj(fliplr(NUM)),conj(fliplr(DEN)));
  p = 1 ./ p;
  r = r .* ((-p) .^m);
  if f, f = conj(fliplr(f)); endif

endfunction

%!test
%! B=[1 -2 1]; A=[1 -1];
%! [r,p,f,m] = residuez(B,A);
%! assert(r,0,100*eps);
%! assert(p,1,100*eps);
%! assert(f,[1 -1],100*eps);
%! assert(m,1,100*eps);

%!test
%! B=1; A=[1 -1j];
%! [r,p,f,m] = residuez(B,A);
%! assert(r,1,100*eps);
%! assert(p,1j,100*eps);
%! assert(f,[],100*eps);
%! assert(m,1,100*eps);

%!test
%! B=1; A=[1 -1 .25];
%! [r,p,f,m] = residuez(B,A);
%! [rs,is] = sort(r);
%! assert(rs,[0;1],1e-7);
%! assert(p(is),[0.5;0.5],1e-8);
%! assert(f,[],100*eps);
%! assert(m(is),[1;2],100*eps);

%!test
%! B=1; A=[1 -0.75 .125];
%! [r,p,f,m] = residuez(B,A);
%! [rs,is] = sort(r);
%! assert(rs,[-1;2],100*eps);
%! assert(p(is),[0.25;0.5],100*eps);
%! assert(f,[],100*eps);
%! assert(m(is),[1;1],100*eps);

%!test
%! B=[1,6,2]; A=[1,-2,1];
%! [r,p,f,m] = residuez(B,A);
%! [rs,is] = sort(r);
%! assert(rs,[-10;9],1e-7);
%! assert(p(is),[1;1],1e-8);
%! assert(f,[2],100*eps);
%! assert(m(is),[1;2],100*eps);

%!test
%! B=[6,2]; A=[1,-2,1];
%! [r,p,f,m] = residuez(B,A);
%! [rs,is] = sort(r);
%! assert(rs,[-2;8],1e-7);
%! assert(p(is),[1;1],1e-8);
%! assert(f,[],100*eps);
%! assert(m(is),[1;2],100*eps);

%!test
%! B=[1,6,6,2]; A=[1,-2,1];
%! [r,p,f,m] = residuez(B,A);
%! [rs,is] = sort(r);
%! assert(rs,[-24;15],2e-7);
%! assert(p(is),[1;1],1e-8);
%! assert(f,[10,2],100*eps);
%! assert(m(is),[1;2],100*eps);

%!test
%! B=[1,6,6,2]; A=[1,-(2+j),(1+2j),-j];
%! [r,p,f,m] = residuez(B,A);
%! [rs,is] = sort(r);
%! assert(rs,[-2+2.5j;7.5+7.5j;-4.5-12j],1E-6);
%! assert(p(is),[1j;1;1],1E-6);
%! assert(f,-2j,1E-6);
%! assert(m(is),[1;2;1],1E-6);

%!test
%! B=[1,0,1]; A=[1,0,0,0,0,-1];
%! [r,p,f,m] = residuez(B,A);
%! [as,is] = sort(angle(p));
%! rise = [ ...
%!  0.26180339887499 - 0.19021130325903i; ...
%!  0.03819660112501 + 0.11755705045849i; ...
%!  0.4; ...
%!  0.03819660112501 - 0.11755705045849i; ...
%!  0.26180339887499 + 0.19021130325903i;];
%! pise = [ ...
%! -0.80901699437495 - 0.58778525229247i; ...
%!  0.30901699437495 - 0.95105651629515i; ...
%!  1; ...
%!  0.30901699437495 + 0.95105651629515i; ...
%! -0.80901699437495 + 0.58778525229247i];
%! assert(r(is),rise,100*eps);
%! assert(p(is),pise,100*eps);
%! assert(f,[],100*eps);
%! assert(m,[1;1;1;1;1],100*eps);
