silverstripe-googlesitemaps/code/extensions/GoogleSitemapExtension.php

<?php

/**
 * Decorate the page object to provide google sitemaps with 
 * additionally options and configuration.
 * 
 * @package googlesitemaps
 */
class GoogleSitemapExtension extends DataExtension {

	/**
	 * @return boolean
	 */
	public function canIncludeInGoogleSitemap() {
		$can = true;

		if($this->owner->hasMethod('AbsoluteLink')) {
			$hostHttp = parse_url(Director::protocolAndHost(), PHP_URL_HOST);
			$objHttp = parse_url($this->owner->AbsoluteLink(), PHP_URL_HOST);

			if($objHttp != $hostHttp) {
				$can = false;
			}
		}
		
		if($can) {
			$can = $this->owner->canView();
		}

		if($can) {
			$can = $this->owner->getGooglePriority();
		}

		$this->owner->invokeWithExtensions('alterCanIncludeInGoogleSitemap', $can);

		return $can;
	}


	/**
	 * @return void
	 */
	public function onAfterPublish() {
		GoogleSitemap::ping();
	}

	/**
	 * @return void
	 */
	public function onAfterUnpublish() {
		GoogleSitemap::ping();
	}


	/**
	 * The default value of the priority field depends on the depth of the page in
	 * the site tree, so it must be calculated dynamically.
	 *
	 * @return mixed
	 */
	public function getGooglePriority() {
		$field = $this->owner->hasField('Priority');

		if(isset($this->Priority) || ($field && $this->Priority = $this->owner->getField('Priority'))) {
			return ($this->Priority < 0) ? false : $this->Priority;
		}

		return GoogleSitemap::get_priority_for_class($this->owner->class);
	}

	/**
	 * Returns a pages change frequency calculated by pages age and number of 
	 * versions. Google expects always, hourly, daily, weekly, monthly, yearly 
	 * or never as values.
	 * 
	 * @see http://support.google.com/webmasters/bin/answer.py?hl=en&answer=183668&topic=8476&ctx=topic
	 *
	 * @return SS_Datetime
	 */
	public function getChangeFrequency() {
		if($freq = GoogleSitemap::get_frequency_for_class($this->owner->class)) {
			return $freq;
		}

		$date = date('Y-m-d H:i:s');

		$created = new SS_Datetime();
		$created->value = ($this->owner->Created) ? $this->owner->Created : $date;

		$now = new SS_Datetime();
		$now->value = $date;
		
		$versions = ($this->owner->Version) ? $this->owner->Version : 1;
		$timediff = $now->format('U') - $created->format('U');

		// Check how many revisions have been made over the lifetime of the
		// Page for a rough estimate of it's changing frequency.
		$period = $timediff / ($versions + 1);

		if ($period > 60 * 60 * 24 * 365) {
			$freq = 'yearly';
		} elseif ($period > 60 * 60 * 24 * 30) {
			$freq = 'monthly';
		} elseif ($period > 60 * 60 * 24 * 7) {
			$freq = 'weekly';
		} elseif ($period > 60 * 60 * 24) {
			$freq = 'daily';
		} elseif ($period > 60 * 60) {
			$freq = 'hourly';
		} else {
			$freq = 'always';
		}

		return $freq;
	}
}