Further to my last post here is an implemetation of the halve the difference method. Uncomment the 3 lines at the start to generate an 8MB test.file. We find our desired reference point in 20 tries (worst case scenario) in a few milliseconds and then dump the rest of the file (3 lines). Assuming you are going to have to work with dates you will need to modify this of course so you can compare if you are before or after your desired start but the principle holds. The total run time should be only a fraction over the time it take to write your output file. Rename it and you are done. You *will* get an infinite loop if your $find_this is not in the file so we abort if $count > $max_tries. With $maxtries set to 100 you are ok for a file with up to 2**100 lines (10**30 in rough terms :-)

my $file = 'c:/test.file';

#open F, ">$file" or $!;
#print F "$_\n" for 1..1000000;
#exit;

my $find_this  = 999997;
my $file_size  = -s $file;
my $top        = 0;
my $bot        = $file_size;
my $count      = 0;
my $max_tries  = 100;

open OLD, $file or die $!;

while (++$count) {
    my $middle = int(($top+$bot)/2);
    seek OLD, $middle , 0;
    my $partial = <OLD>;
    my $full_line = <OLD>;
    chomp $full_line;
    if ($full_line eq $find_this) {
        print "Took $count tries\n";
        print while <OLD>;
        exit;
    }
    if ($full_line < $find_this) {
        $top = $middle;
    }
    else {
        $bot = $middle;
    }
    die "Ark, baling out of infinite loop" if $count > $max_tries;
}
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Let us know how you get on.

cheers

tachyon

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