#!/usr/bin/perl -w
# merge.pl - Don Yang (uguu.org)
#
# Usage:
#  <base.txt> <overlay.txt>
#
# Read input lines from base.txt, and replace any whitespaces in
# base.txt with characters from overlay.txt.
#
# 2019-02-24


use strict;

die "$0 <base.txt> <overlay.txt>\n" unless $#ARGV == 1;
my ($base, $overlay) = @ARGV;

open my $base_file, "<$base" or die $!;
open my $overlay_file, "<$overlay" or die $!;
while( my $base_line = <$base_file> )
{
   # Load input lines
   chomp $base_line;

   my $overlay_line = <$overlay_file>;
   if( defined($overlay_line) )
   {
      chomp $overlay_line;
   }
   else
   {
      $overlay_line = "";
   }

   # Copy non-whitespace characters from base, and all other
   # characters from overlay.
   my $output = "";
   my $i = 0;
   for(; $i < length($base_line); $i++)
   {
      if( substr($base_line, $i, 1) eq " " && $i < length($overlay_line) )
      {
         $output .= substr($overlay_line, $i, 1);
      }
      else
      {
         $output .= substr($base_line, $i, 1);
      }
   }
   if( length($overlay_line) > length($base_line) )
   {
      $output .= substr($overlay_line, length($base_line));
   }
   print $output, "\n";
}
close $base_file;
close $overlay_file;