# updated_line_number.pl
# updated_line_number.pl old_file revised_file line_number
#
# compares old_file to revised_file and will determine what a line number in old_file will be for revised_file
#
use strict;
my ( $old_file, $new_file, $line_number ) = @ARGV;
my @new_to_old;
my @old_to_new;
my $lines_in_new = 0;
my $lines_in_old = 0;

initialize_arrays();
determine_differences();
print "$old_to_new[$line_number]\n";

sub initialize_arrays{
    # Initialize arrays. Size is old + new to give shrink and grow room during calculations.
    # Just ignore extra when done.
    open my $new_file_handle, "<", $new_file or die "Unable to open $new_file\n";
    $lines_in_new++ while ( <$new_file_handle> );
    open my $old_file_handle, "<", $old_file or die "Unable to open $old_file\n";
    $lines_in_old++ while ( <$old_file_handle> );
    $old_to_new[$_] = 0 for ( 1 .. $lines_in_new + $lines_in_old );
    $new_to_old[$_] = $_ for ( 1 .. $lines_in_new + $lines_in_old );
}

sub determine_differences{
    for my $capture_line ( `diff $old_file $new_file` ) {
        # Only process lines starting with number. Ignore <, >, or --- lines.
        if ( $capture_line =~ /^(?:(\d+),)?(\d+)([acd])(?:(\d+),)?(\d+)$/ ) {
            $3 ne 'd' and $new_to_old[$_] = 0 for ( ( $4 ? $4 : $5 ) .. $5 );
            $new_to_old[$_] += ( $4 ? $4 : $5 ) - $5 + $2 - ( $1 : $1 ? $2 ) + ( $3 cmp 'c' ) 
                for ( $5 + 1 .. $lines_in_old + $lines_in_new );
        }
    }
    # Now get inverse to go from old to new.
    # There will be complete arrays going old to new and new to old just to get one answer.
    for my $n ( 0 .. $lines_in_old + $lines_in_new ) {
        if ( $new_to_old[$n] > 0 ) {
            $old_to_new[$new_to_old[$n]] = $n;
        }
        else {
            $old_to_new[$n] = 0; # If no match, make line number zero.
        }
    }
}