## Copyright (C) 2003 David Bateman
##
## 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; if not, see <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {} convmtx (@var{a}, @var{n})
##
## Create matrix to perform repeated convolutions with the same vector
## in a Galois Field. If @var{a} is a column vector and @var{x} is  a 
## column vector of length @var{n}, in a Galois Field then 
##
## @code{convmtx(@var{a}, @var{n}) * @var{x}}
##
## gives the convolution of of @var{a} and @var{x} and is the
## same as @code{conv(@var{a}, @var{x})}. The difference is if
## many vectors are to be convolved with the same vector, then
## this technique is possibly faster.
##
## Similarly, if @var{a} is a row vector and @var{x} is a row 
## vector of length @var{n}, then
##
## @code{@var{x} * convmtx(@var{a}, @var{n})}
##
## is the same as @code{conv(@var{x}, @var{a})}.
## @end deftypefn
## @seealso{conv}

function b = convmtx (a, n)

  if (!isgalois (a))
    error("convmtx: argument must be a galois variable");
  endif

  b = gf(convmtx(a.x,n), a.m, a.prim_poly);

endfunction