#!/usr/bin/perl

#
# https://www.linux.org.ru/forum/development/12024408
#

use strict;
use warnings;


my $positions_file = 'positions.txt';


if ( @ARGV > 0 ) {
    &restore_positions();
    &compare();
    &save_positions();
} else {
    &print_usage();
    exit 1;
}

exit 0;


{
    my %positions;


    sub restore_positions() {
        -e $positions_file or return;

        open my $fh, "<", $positions_file
            or die "open $positions_file: $!";

        while ( <$fh> ) {
            chomp;
            my ( $file, $pos ) = split /:/;
            $positions{ $file } = $pos;
        }

        close $fh
            or die "close $positions_file: $!";
    }

    sub save_positions() {
        open my $fh, ">", $positions_file
            or die "open $positions_file: $!";

        while ( my ( $file, $pos ) = each %positions ) {
            syswrite $fh, join( ':', $file, $pos ) . "\n";
        }

        close $fh
            or die "close $positions_file: !";
    }

    sub get_position($) {
        my ( $file ) = @_;


        return ( exists $positions{ $file } )
            ? $positions{ $file }
            : 0;
    }

    sub set_position($$) {
        my ( $file, $pos ) = @_;

        $positions{ $file } = $pos;
    }
}


sub compare() {
    my @FHS;

    for ( my $filenum = 0; $filenum < @ARGV; $filenum++ ) {
        my $file = $ARGV[ $filenum ];
        open my $fh, "<", $file or die "open $file: $!";
        push @FHS, $fh;
        my $pos = &get_position( $file );
        seek( $fh, $pos, 0 ) or die "seek $file: $!";
    }

    my %uniq;

    IN: while ( my $in = <STDIN> ) {
        if ( exists $uniq{ $in } ) {
            delete $uniq{ $in };
            next;
        }

        for ( my $filenum = 0; $filenum < @ARGV; $filenum++ ) {
            my $file = $ARGV[ $filenum ];
            my $fh = $FHS[ $filenum ];
            my $curpos = 0;

            for ( $curpos = tell( $fh );
                my $line = <$fh>;
                $curpos = tell( $fh ) )
            {
                if ( $line eq $in ) {
                    &set_position( $file, $curpos );
                    next IN;
                }

                if ( not exists $uniq{ $line } ) {
                    $uniq{ $line } = 1;
                }
            }

            &set_position( $file, $curpos );
        }

        print $in;
    }
}

sub print_usage {
    print <<'EOF';
Usage:
    echo urls | xcmp.pl file1 [ file2 file 3 ... fileN ]
EOF
}