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rsdecof.m

## 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 2 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} {} rsdecof (@var{in},@var{out})
## @deftypefnx {Function File} {} rsdecof (@var{in},@var{out},@var{t})
##
## Decodes an ascii file using a Reed-Solomon coder. The input file is
## defined by @var{in} and the result is written to the output file @var{out}.
## The type of coding to use is determined by whether the input file is 7-
## or 8-bit. If the input file is 7-bit, the default coding is [127,117].
## while the default coding for an 8-bit file is a [255, 235]. This allows
## for 5 or 10 error characters in 127 or 255 symbols to be corrected 
## respectively. The number of errors that can be corrected can be overridden 
## by the variable @var{t}.
##
## If the file is not an integer multiple of the message size (127 or 255)
## in length, then the file is padded with the EOT (ascii character 4)
## character before decoding.
##
## @end deftypefn
## @seealso{rsencof}

function rsdecof(in, out, t)

  if ((nargin < 2) || (nargin > 3))
    usage("rsdecof (in, out [, t])");
  endif

  if (!ischar(in) || !ischar(out))
    error ("rsdecof: input and output filenames must be strings");
  endif

  if (nargin != 3)
    t = 0;
  else
    if (!isscalar(t) || (t != floor(d)) || (t < 1))
      error ("rsdecof: t must be a postive, non-zero integer");
    endif
  endif

  try fid = fopen(in, "rt");
  catch
    error ("rsdecof: can not open input file");
  end
  [code, count] = fread(fid, Inf, "char");
  fclose(fid);

  is8bit = (max(code) > 127);

  if (is8bit)
    m = 8;
    n = 255;
    if (t == 0)
      t = 10;
    endif
  else
    m = 7;
    n = 127;
    if (t == 0)
      t = 5;
    endif
  endif
  k = n - 2 * t;

  ncodewords = ceil(count / n);
  npad = n * ncodewords - count;
  code = reshape([code ; 4 * ones(npad,1)],n,ncodewords)';

  msg = rsdec(gf(code,m), n, k, "beginning")';
  msg = msg(:);

  try fid = fopen(out, "w+t");
  catch
    error ("rsdecof; can not open output file");
  end
  fwrite(fid, msg(1:(ncodewords*k-npad)), "char");
  fclose(fid);

endfunction

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