use strict;
use Getopt::Long;
use IO::File;
use Data::Dumper;

GetOptions
(
    'i|input=s'    => \( my $INPUT    = "./access.log" ),
    'l|lastpos=s'  => \( my $LASTPOS  = "./lastpos.txt" ),
    'f|feedback'   => \( my $FEEDBACK = undef ),
);

unless ( defined $INPUT && defined $LASTPOS )
{
    print <<EOF;
Logfile Parser - Parse input log efficiently

Usage: $0 [option]
Options:
  -i|--input   [filename]     Specify the input log file name.
  -l|--lastpos [filename]     Specify the name of last pos file.
  -f|--feedback               Let the program print progress prompt.

EOF
    exit(1);
}

# load the last pos information
my $lastinfo;
$lastinfo = ReadLastPosFile($LASTPOS) if -f $LASTPOS;
print "Last position:\n", Dumper($lastinfo) if ($FEEDBACK);

# verify the log file
my $begin_pos = VerifyLastPosition($INPUT, $lastinfo);

# process the log file
my $f = new IO::File $INPUT, "r" or die "Could not open log file";

if ($begin_pos == -1) {
    die "Log file has not been changed since last run";
} else {
    seek $f, $begin_pos, 0; # seek to start of next line
}

my $next_pos = $begin_pos;
my $next_line;
while ($next_line = <$f>)
{
    $begin_pos = $next_pos;
    $next_pos = tell;

    # process the log file here
    chomp($next_line);
    print "$next_line\n";
}

# at here, begin pos is the position of the last line
seek $f, $begin_pos, 0;
chomp($next_line = <$f>);
$lastinfo->{pos} = $begin_pos;
$lastinfo->{text} = $next_line;

print "Last Pos Info:\n", Dumper($lastinfo) if ($FEEDBACK);

# ok, write the last info back to file
WriteLastPosFile($LASTPOS, $lastinfo);

exit(0);


sub ReadLastPosFile
{
    # last pos file format - <pos>|<last-line-seen>
    my $filename = shift;
    my $f = new IO::File $filename, "r"
        or die "Could not open lastpos file";
    chomp(my $info = <$f>);

    my %lastinfo;
    ($lastinfo{pos}, $lastinfo{text}) = $info =~ /(\d+)\|(.*)/;

    return \%lastinfo;
}

sub WriteLastPosFile
{
    my ($filename, $lastinfo) = @_;
    my $f = new IO::File $filename, "w"
        or die "Could not write to lastpos file";
    printf $f "%s|%s\n", $lastinfo->{pos}, $lastinfo->{text};
}

sub VerifyLastPosition
{
    my ($logfile, $lastinfo) = @_;
    my $f = new IO::File $logfile, "r" or die "Could not open log file";

    seek $f, 0, 2;  # seek to the end of the file
    my $eof = tell $f;

    return 0 if $lastinfo->{pos} >= $eof; # ok, file has been trimmed

    seek $f, $lastinfo->{pos}, 0;
    chomp(my $line = <$f>); # retrieve what we believe was the last line

    return 0 if $line ne $lastinfo->{text}; # ok, file has been trimmed

    my $begin_pos = tell $f;  # otherwise start from next line

    # -1 means the file has not been changed since
    # last time it was parsed.
    return $eof == $begin_pos ? -1 : $begin_pos;
}