#!/usr/bin/perl -w
# overlay.pl - Don Yang (uguu.org)
#
# 2014-03-16

use strict;

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

sub LoadFile($)
{
   my ($filename) = @_;
   my @lines = ();
   if( $filename eq "-" )
   {
      @lines = <STDIN>;
   }
   else
   {
      open my $file, "<$filename" or die $!;
      @lines = <$file>;
      close $file;
   }
   chomp foreach @lines;
   return @lines;
}


my @bg = LoadFile($ARGV[0]);
my @overlay = LoadFile($ARGV[1]);

for(my $y = 0; $y <= $#bg; $y++)
{
   if( $y >= scalar @overlay )
   {
      print $bg[$y], "\n";
      next;
   }
   for(my $x = 0; $x < length($bg[$y]); $x++)
   {
      if( $x >= length($overlay[$y]) )
      {
         print substr($bg[$y], $x);
         last;
      }
      if( substr($bg[$y], $x, 1) =~ /\s/ )
      {
         print substr($overlay[$y], $x, 1);
      }
      else
      {
         print substr($bg[$y], $x, 1);
      }
   }
   print "\n";
}