#!/usr/bin/perl -w
# mix.pl - Don Yang (uguu.org)
#
# For each character in background file, mix in non-whitespace characters
# from overlay.
#
# 11/27/11

use strict;
die "$0 <background.txt> <overlay.txt>\n" unless $#ARGV == 1;

# Load input
my @background;
if( $ARGV[0] eq "-" )
{
   push @background, $_ foreach <STDIN>;
}
else
{
   open my $file, "< $ARGV[0]" or die $!;
   @background = <$file>;
   close $file;
}
chomp foreach @background;

my @overlay;
if( $ARGV[1] eq "-" )
{
   push @overlay, $_ foreach <STDIN>;
}
else
{
   open my $file, "< $ARGV[1]" or die $!;
   @overlay = <$file>;
   close $file;
}
chomp foreach @overlay;

# Mix inputs
for(my $i = 0; $i <= $#background; $i++)
{
   if( $i > $#overlay )
   {
      print $background[$i], "\n";
      next;
   }

   for(my $j = 0; $j < length($background[$i]); $j++)
   {
      if( $j < length($overlay[$i]) &&
          substr($overlay[$i], $j, 1) !~ /\s/s )
      {
         print substr($overlay[$i], $j, 1);
      }
      else
      {
         print substr($background[$i], $j, 1);
      }
   }
   print "\n";
}