<?php
/**
 * @link http://www.yiiframework.com/
 * @copyright Copyright (c) 2008 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */

namespace yii\caching;

/**
 * Dependency is the base class for cache dependency classes.
 *
 * Child classes should override its [[generateDependencyData()]] for generating
 * the actual dependency data.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @since 2.0
 */
abstract class Dependency extends \yii\base\Object
{
    /**
     * @var mixed the dependency data that is saved in cache and later is compared with the
     * latest dependency data.
     */
    public $data;
    /**
     * @var boolean whether this dependency is reusable or not. True value means that dependent
     * data for this cache dependency will be generated only once per request. This allows you
     * to use the same cache dependency for multiple separate cache calls while generating the same
     * page without an overhead of re-evaluating dependency data each time. Defaults to false.
     */
    public $reusable = false;

    /**
     * @var array static storage of cached data for reusable dependencies.
     */
    private static $_reusableData = [];
    /**
     * @var string a unique hash value for this cache dependency.
     */
    private $_hash;

    /**
     * Evaluates the dependency by generating and saving the data related with dependency.
     * This method is invoked by cache before writing data into it.
     * @param Cache $cache the cache component that is currently evaluating this dependency
     */
    public function evaluateDependency($cache)
    {
        if (!$this->reusable) {
            $this->data = $this->generateDependencyData($cache);
        } else {
            if ($this->_hash === null) {
                $this->_hash = sha1(serialize($this));
            }
            if (!array_key_exists($this->_hash, self::$_reusableData)) {
                self::$_reusableData[$this->_hash] = $this->generateDependencyData($cache);
            }
            $this->data = self::$_reusableData[$this->_hash];
        }
    }

    /**
     * Returns a value indicating whether the dependency has changed.
     * @param  Cache   $cache the cache component that is currently evaluating this dependency
     * @return boolean whether the dependency has changed.
     */
    public function getHasChanged($cache)
    {
        if (!$this->reusable) {
            return $this->generateDependencyData($cache) !== $this->data;
        } else {
            if ($this->_hash === null) {
                $this->_hash = sha1(serialize($this));
            }
            if (!array_key_exists($this->_hash, self::$_reusableData)) {
                self::$_reusableData[$this->_hash] = $this->generateDependencyData($cache);
            }

            return self::$_reusableData[$this->_hash] !== $this->data;
        }
    }

    /**
     * Resets all cached data for reusable dependencies.
     */
    public static function resetReusableData()
    {
        self::$_reusableData = [];
    }

    /**
     * Generates the data needed to determine if dependency has been changed.
     * Derived classes should override this method to generate the actual dependency data.
     * @param  Cache $cache the cache component that is currently evaluating this dependency
     * @return mixed the data needed to determine if dependency has been changed.
     */
    abstract protected function generateDependencyData($cache);
}
