/**
* Return the maximum prefix length that would fit the IP address given.
*
* This is useful to determine how my bit would be needed to store the IP
* address when you don't already have a prefix for the IP.
*
* @example 216.240.32.0 would return 27
*
* @param string $ip IP address without prefix
* @param integer $bits Maximum bits to check; defaults to 32 for IPv4 and 128 for IPv6
*/
public static function max_prefix($ip, $bits = null)
{
static $mask = array();
$ver = false === filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4) ? 6 : 4;
$max = $ver == 6 ? 128 : 32;
if ($bits === null) {
$bits = $max;
}
$int = IP::inet_ptod($ip);
while ($bits > 0) {
// micro-optimization; calculate mask once ...
if (!isset($mask[$ver][$bits - 1])) {
// 2^$max - 2^($max - $bits);
if ($ver == 4) {
$mask[$ver][$bits - 1] = pow(2, $max) - pow(2, $max - ($bits - 1));
} else {
$mask[$ver][$bits - 1] = bcsub(bcpow(2, $max), bcpow(2, $max - ($bits - 1)));
}
}
$m = $mask[$ver][$bits - 1];
//printf("%s/%d: %s & %s == %s\n", $ip, $bits-1, BC::bcdecbin($m, 32), BC::bcdecbin($int, 32), BC::bcdecbin(BC::bcand($int, $m)));
//echo "$ip/", $bits-1, ": ", IP::inet_dtop($m), " ($m) & $int == ", BC::bcand($int, $m), "\n";
if (bccomp(BC::bcand($int, $m), $int) != 0) {
return $bits;
}
$bits--;
}
return $bits;
}
