#!/usr/bin/perl -w
# Shift the alpha characters based on bayer matrix.

use strict;

# 4x4 Bayer dither matrix.
my @bayer =
(
   [ 0,  8,  2, 10],
   [12,  4, 14,  6],
   [ 3, 11,  1,  9],
   [15,  7, 13,  5]
);


# Rotate an alpha character by the specified amount, preserving case.
sub rot($$)
{
   my ($c, $s) = @_;

   my $base = ($c =~ /[a-z]/) ? ord('a')
                              : ($c =~ /[A-Z]/) ? ord('A') : undef;
   return defined($base) ? chr((ord($c) - $base + $s) % 26 + $base) : $c;
}


while( my $line = <> )
{
   my $x = 0;
   foreach my $c (map {chr $_} unpack "C*", $line)
   {
      print rot($c, $bayer[$. & 3][$x & 3]);
      $x++;
   }
}