### Copyright (C) 1996 John W. Eaton
###
### 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, 59 Temple Place - Suite 330, Boston, MA
### 02111-1307, USA.

## usage: fftconv (a, b [, N])
##
## c = fftconv (a, b) returns the convolution of the vectors a and b,
## a vector with length equal to length (a) + length (b) - 1.  
## If a and b are the coefficient vectors of two polynomials, c is
## the coefficient vector of the product polynomial.
##
## The computation uses the FFT by calling fftfilt.  If the optional
## argument N is specified, an N-point FFT is used.

function c = fftconv (a, b, N)

  ## Written by KH (Kurt.Hornik@ci.tuwien.ac.at) on Sep 3, 1994.
  
  if (nargin < 2 || nargin > 3)
    usage ("fftconv (b, x [, N])");
  endif
  
  if (is_matrix (a) || is_matrix (b))
    error ("fftconv:  both a and b should be vectors");
  endif
  la = length (a);
  lb = length (b);
  if ((la == 1) || (lb == 1))
    c = a * b;
  else
    lc = la + lb - 1;
    a(lc) = 0;
    b(lc) = 0;
    if (nargin == 2)
      c = fftfilt (a, b);
    else
      if !(is_scalar (N))
	error ("fftconv: N has to be a scalar");
      endif
      c = fftfilt (a, b, N);
    endif
  endif

endfunction