php - How do I alphabetize a list of domains in this way? -

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so if wanted organize list of websites alphabetically, , there of form: example1.com, test.com, stackoverflow.com, google.com, easy. however, want organize subdomains. consider following 3 domains:

a.domain.com domain.com anotherdomain.com

if handed them on software alphabetize, alphabetized this:

a.domain.com anotherdomain.com domain.com

however, not how want them alphabetized. want them alphabetized domain, , subdomain "tiebreaker," in other words, this:

anotherdomain.com domain.com a.domain.com

could tell me how code php (or javascript) this? (you can assume each "website" on fresh line of code.)

$array = array(     'b.domain.com',     'a.domain.com',     'domain.com',     'anotherdomain.com',     'php.net',     'example.com' );  function sort_domains($domain1, $domain2) {     $domain1 = array_reverse(explode('.', $domain1));     $domain2 = array_reverse(explode('.', $domain2));     // set $i 0 if want tld sorted     for($i = 1; ; $i++)     {         // might idea store value of issets here         if(isset($domain1[$i]) && isset($domain2[$i]))         {             $difference = strcmp($domain1[$i], $domain2[$i]);             if($difference != 0)             {                 return $difference;             }             continue;         }         if(!isset($domain1[$i]) && !isset($domain2[$i]))         {             return 0;         }         return isset($domain1[$i]) ? 1 : -1;     } }  usort($array, 'sort_domains');  /* array (     [0] => anotherdomain.com     [1] => domain.com     [2] => a.domain.com     [3] => b.domain.com     [4] => example.com     [5] => php.net ) */

edit:

as per suggestion of alnitak, here version of sort_domains caches pieces of each domain name:

function sort_domains($domain1, $domain2) {     static $cache = array();     if(!array_key_exists($domain1, $cache))     {         $cache[$domain1] = array_reverse(explode('.', $domain1));     }     if(!array_key_exists($domain2, $cache))     {         $cache[$domain2] = array_reverse(explode('.', $domain2));     }     // set $i 0 if want tld sorted     for($i = 1; ; $i++)     {         $isset_1 = isset($cache[$domain1][$i]);         $isset_2 = isset($cache[$domain2][$i]);         if($isset_1 && $isset_2)         {             $difference = strcmp($cache[$domain1][$i], $cache[$domain2][$i]);             if($difference != 0)             {                 return $difference;             }             continue;         }         if(!$isset_1 && !$isset_2)         {             return 0;         }         return $isset_1 ? 1 : -1;     } }

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