#!/usr/bin/perl
###############################################################################
#                                                                             #
#                         ---===  softDVB-T  ===---                           #
#                   (software generated DVB-T transmitter)                    #
#                                                                             #
###############################################################################
#                                                                             #
# Authors: Marcin Szelest SQ9DJJ    Marcin.Szelest@gmail.com                  #
#          Slawomir Lyszkowski      slyszkowski@yahoo.pl                      #
#                                                                             #
# File contents:                                                              #
#          Behaviaral model of Reed-Solomon encoder.                          #
#                                                                             #
# File purpose:                                                               #
#          Verification of gnuradio Reed-Solomon encoder block.               #
###############################################################################


# RS(204,188, t=8) is derived from RS(255,239,t=8) by adding 51bytes of zeros before message.
# Added zeros must be removed from the output stream!
$PARITY_BYTES = 204-188;


# For calculation purposes we are using GNU Octave 3.2.3 with octave-communication-common package

# Let's generate Galois fied with use of p(x) polynominal
# according ETSI EN 300 744:
#
#         8    4    3    2
# p(x) = x  + x  + x  + x  + 1     , so p(x) = 0x11D
#
# Octave command which generates Galois field for p(x):
# galois_field = gf(0:255, 8, 0x11D);
#
#
# Let's calculate g(x) now:
#
#              0       1       2          15
# g(x) = (x + a )(x + a )(x + a )...(x + a  )     , where a = 0x02
#
# Octave command which simplify polynominal above:
# g_x_ = rsgenpoly(255, 239, 0x11D, 0)
#@g_x_ = (1, 59, 13, 104, 189, 68, 209, 30, 8, 163, 65, 41, 229, 98, 50, 36, 59);
@g_x_ = reverse (59, 13, 104, 189, 68, 209, 30, 8, 163, 65, 41, 229, 98, 50, 36, 59);

# we need two arithmetic opperation on Galois field: addition and multiplication
sub gf_add ($$) {
  return (($_[0]^$_[1]) & 0xff)
}

# Algorithm according http://en.wikipedia.org/wiki/Finite_field_arithmetic
sub gf_mult ($$) {
  $l = $_[0];
  $m = $_[1];
  $p = 0;
  for (1..8) {
    if ($m & 0x01) { $p = $p ^ $l }
    $hi = $l & 0x80;
    $l = $l << 1;
    if ($hi)       { $l = $l ^ 0x11d }
    $m = $m >> 1;
  }
 return ($p & 0xff)
}

# RS(204,188, t=8) is derived from RS(255,239,t=8)
sub encoder_RS_204 ($) {
  @in_msg = ( $_[0] =~ m/../g );
  if (scalar(@in_msg) != 188) { print "Error: Insuficient number of bytes (should be 188, is ".scalar(@in_msg).")\nExiting...\n"; exit }
  @tab = split(" ",encoder_RS_255(("00" x 51).(join "",@in_msg)));
  return (join " ",@tab[51..255])
}

# Algorithm according RSCODE version 1.3 (Henry Minsky)
sub encoder_RS_255 ($) {
  @in_msg =  ( $_[0] =~ m/../g );
  for ($i=0; $i<(scalar @in_msg); $i++) { @in_msg[$i] = sprintf("%d",hex @in_msg[$i]) }
  if (scalar(@in_msg) != 239) { print "Error: Insuficient number of bytes (should be 239, is ".scalar(@in_msg).")\nExiting...\n"; exit }

  # RS endode starts here
  @lsfr[15] = 0;
  for ($i=0; $i<239; $i++) {
    $fdbk = gf_add(@in_msg[$i], @lsfr[15]);
    for ($j = $PARITY_BYTES-1; $j > 0; $j--) {
      @lsfr[$j] = gf_add(@lsfr[$j-1], gf_mult(@g_x_[$j], $fdbk));
    }
    @lsfr[0] = gf_mult(@g_x_[0], $fdbk);
  }
 return (join " ",@in_msg)." ".(join " ",reverse @lsfr[0..$PARITY_BYTES-1])
}



$msg = "fa" x 188;
$res = encoder_RS_204($msg);


# print results
print "\tsrc_data = (";
@arr = ( $msg =~ m/../g );
for ($i=0; $i<(scalar @arr)-1; $i++) { printf("0x%02x, ", hex @arr[$i]) }
printf("0x%02x)\n\n", hex @arr[$i]);

print "\texpected_result = (";
@arr = split(" ", $res);
for ($i=0; $i<(scalar @arr)-1; $i++) { printf("0x%02x, ", @arr[$i]) }
printf("0x%02x)\n\n",          @arr[$i]);