use constant EARTH_RADIUS => 3956;
use constant PI           => 4 * atan2 1, 1;

sub make_quick_reject {
  # Args: lat (radians), long (radians), dist
  my($lat,$long,$dist) = @_;
  my($distcos) = cos($dist / EARTH_RADIUS);
  my($maxlatdiff) = atan2(sqrt(1 - ($distcos * $distcos)), $distcos);
  if (abs($lat) + $maxlatdiff > PI/2) {
    # special case - near poles
    return sub { abs($lat - $_[0]) > $maxlatdiff; };
  }
  my($longmult);
  $longmult = 1 / cos(abs($lat) + $maxlatdiff);
  my($maxlongdiff) = $maxlatdiff * $longmult;
  return sub { abs($lat - $_[0]) > $maxlatdiff or abs($long - $_[1]) > $maxlongdiff; }
}

sub make_quick_accept {
  # Args: lat (radians), long (radians), dist
  my($lat,$long,$dist) = @_;
  my($distcos) = cos($dist / EARTH_RADIUS);
  my($maxlatdiff) = atan2(sqrt(1 - ($distcos * $distcos)), $distcos) / sqrt(2);
  my($longmult);
  if (abs($lat) < $maxlatdiff ) {
    # special case - near equator
    $longmult = 1;
  } else {
    $longmult = 1 / cos(abs($lat) - $maxlatdiff);
  }
  my($maxlongdiff) = $maxlatdiff * $longmult;
  return sub { abs($lat - $_[0]) < $maxlatdiff and 
               abs($long - $_[1]) < $maxlongdiff; }
}