如何以这种方式按字母顺序排列域名列表？

Question

So if I wanted to organize a list of websites alphabetically, and there were all of this form: example1.com, test.com, stackoverflow.com, google.com, it would be easy. However, I want to also organize subdomains. Consider the following three domains:

a.domain.com
domain.com
anotherdomain.com

If I handed them over to software to alphabetize, they would be alphabetized like this:

a.domain.com
anotherdomain.com
domain.com

However, this is not how I want them alphabetized. I want them to be alphabetized by domain, and then by subdomain as a "tiebreaker," in other words, like this:

anotherdomain.com
domain.com
a.domain.com

Could someone tell me how to code PHP (or JavaScript) to do this? (You can assume that each "website" is on a fresh line of code.)

Answer 1

$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 $i to 0 if you want the TLD to be sorted
    for($i = 1; ; $i++)
    {
        // Might be a good idea to store the value of the issets up here
        if(isset($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 the suggestion of Alnitak, here is a version of sort_domains which caches the 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 $i to 0 if you want the TLD to be sorted
    for($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;
    }
}