#!/usr/bin/perl -w
# shift_encode.pl - Don Yang (uguu.org)
#
# Encode text using constants found with shift_constants.c

use strict;
use constant BOUNDARY => 40;
use constant MUL1 => 3;
use constant MUL2 => 255;
use constant SHIFT => 158;

# Build translation table
my %table = ();
for(my $y = 33; $y < 127; $y++)
{
   next if $y == ord('\"') || $y == ord('\\');

   my $x = ($y * ($y < BOUNDARY ? MUL1 : MUL2) + SHIFT) % 256;
   push @{$table{$x}}, $y;
}

# Build indices so that we alternate between choices for the same
# input character, where available.  In practice, there is usually
# only one choice.
my %index = ();
$index{$_} = 0 foreach keys %table;

# Encode input
my $input = join '', <>;
foreach my $c (unpack 'C*', $input)
{
   die "No translation for $c\n" unless exists $table{$c};

   print chr($table{$c}[$index{$c}]);
   $index{$c} = ($index{$c} + 1) % (scalar @{$table{$c}});
}