<?php
/**
 * CacheDependency class file.
 *
 * @link http://www.yiiframework.com/
 * @copyright Copyright &copy; 2008-2012 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */

namespace yii\caching;

/**
 * CacheDependency is the base class for cache dependency classes.
 *
 * CacheDependency implements the {@link ICacheDependency} interface.
 * Child classes should override its {@link generateDependentData} for
 * actual dependency checking.
 *
 * @property boolean $hasChanged Whether the dependency has changed.
 * @property mixed $dependentData The data used to determine if dependency has been changed.
 * This data is available after {@link evaluateDependency} is called.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @since 2.0
 */
class CacheDependency extends \yii\base\Object
{
	/**
	 * @var boolean Whether this dependency is reusable or not.
	 * If set to true, dependent data for this cache dependency will only be generated once per request.
	 * You can then use the same cache dependency for multiple separate cache calls on the same page
	 * without the overhead of re-evaluating the dependency each time.
	 * Defaults to false;
	 * @since 1.1.11
	 */
	public $reuseDependentData = false;

	/**
	 * @var array cached data for reusable dependencies.
	 * @since 1.1.11
	 */
	private static $_reusableData = array();

	private $_hash;
	private $_data;

	/**
	 * Evaluates the dependency by generating and saving the data related with dependency.
	 * This method is invoked by cache before writing data into it.
	 */
	public function evaluateDependency()
	{
		if ($this->reuseDependentData) {
			$hash = $this->getHash();
			if (!isset(self::$_reusableData[$hash]['dependentData'])) {
				self::$_reusableData[$hash]['dependentData'] = $this->generateDependentData();
			}
			$this->_data = self::$_reusableData[$hash]['dependentData'];
		} else {
			$this->_data = $this->generateDependentData();
		}
	}

	/**
	 * @return boolean whether the dependency has changed.
	 */
	public function getHasChanged()
	{
		if ($this->reuseDependentData) {
			$hash = $this->getHash();
			if (!isset(self::$_reusableData[$hash]['hasChanged'])) {
				if (!isset(self::$_reusableData[$hash]['dependentData'])) {
					self::$_reusableData[$hash]['dependentData'] = $this->generateDependentData();
				}
				self::$_reusableData[$hash]['hasChanged'] = self::$_reusableData[$hash]['dependentData'] != $this->_data;
			}
			return self::$_reusableData[$hash]['hasChanged'];
		} else {
			return $this->generateDependentData() != $this->_data;
		}
	}

	/**
	 * @return mixed the data used to determine if dependency has been changed.
	 * This data is available after {@link evaluateDependency} is called.
	 */
	public function getDependentData()
	{
		return $this->_data;
	}

	/**
	 * Generates the data needed to determine if dependency has been changed.
	 * Derived classes should override this method to generate actual dependent data.
	 * @return mixed the data needed to determine if dependency has been changed.
	 */
	protected function generateDependentData()
	{
		return null;
	}

	/**
	 * Generates a unique hash that identifies this cache dependency.
	 * @return string the hash for this cache dependency
	 */
	private function getHash()
	{
		if ($this->_hash === null) {
			$this->_hash = sha1(serialize($this));
		}
		return $this->_hash;
	}
}