# Usage: record_search($search_string, $filename, 11);
# Returns the record number if the record is found, undef otherwise.
# Searched file must be sorted.

sub record_search {
    my ($key, $filename, $recordsize) = @_;
    my $fh = IO::File->new($filename, 'r')
        or croak("Could not open `$filename': $!");

    my $hi = ($fh->stat)[7] / $recordsize;
    my $lo = 0;

    while ($lo < $hi) {
        my $mid = int(($lo + $hi) / 2);
        $fh->seek($mid * $recordsize, 0);
        my $target = $fh->getline();
        chomp($target);

        if ($target < $key) {
            $lo = $mid + 1;
        } elsif ($target == $key) {
            return $mid;
        } else {
            $hi = $mid;
        }
    }
}