# Find a good place to start traversing a large
# file for sorted data.

return 0 if $filelen < 2000000;  # A few MB? Please... :-)
my $safe_offset = 0;
my $jump = 0.5;
my $step = 0.25;
for(1..8) {
  # See if 
  my($testoff) = int($filelen * $jump);
  seek(FILE, $testoff, 0);

  if ( do_test(*FILE, $threshold) ) {
    $safe_offset = $testoff; # Point to offset for 
    $jump = $jump + $step;
  } else {
    $jump = $jump - $step;
  }
  $step = $step / 2.0;
}

# Go to selected place:
seek(FILE, $safe_offset);
<FILE> if $safe_offset > 0;

return $safe_offset;

sub do_test {
  my(*FILE, $compare) = @_;
  # Has moved somewhere in file. Skip partial line of log:
  <FILE>;
  my $log = <FILE>;
  # You write this (you know the format). Return true if OK:
  return date_test($log, $compare);
}