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

namespace yii\web;

use Yii;
use yii\base\ActionFilter;
use yii\base\Action;

/**
 * The HttpCache provides functionality for caching via HTTP Last-Modified and Etag headers.
 *
 * It is an action filter that can be added to a controller and handles the `beforeAction` event.
 *
 * To use AccessControl, declare it in the `behaviors()` method of your controller class.
 * In the following example the filter will be applied to the `list`-action and
 * the Last-Modified header will contain the date of the last update to the user table in the database.
 *
 * ~~~
 * public function behaviors()
 * {
 *     return [
 *         'httpCache' => [
 *             'class' => \yii\web\HttpCache::className(),
 *             'only' => ['list'],
 *             'lastModified' => function ($action, $params) {
 *                 $q = new Query();
 *                 return strtotime($q->from('users')->max('updated_timestamp'));
 *             },
 * //            'etagSeed' => function ($action, $params) {
 * //                return // generate etag seed here
 * //            }
 *         ],
 *     ];
 * }
 * ~~~
 *
 * @author Da:Sourcerer <webmaster@dasourcerer.net>
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @since 2.0
 */
class HttpCache extends ActionFilter
{
    /**
     * @var callable a PHP callback that returns the UNIX timestamp of the last modification time.
     * The callback's signature should be:
     *
     * ~~~
     * function ($action, $params)
     * ~~~
     *
     * where `$action` is the [[Action]] object that this filter is currently handling;
     * `$params` takes the value of [[params]]. The callback should return a UNIX timestamp.
     */
    public $lastModified;
    /**
     * @var callable a PHP callback that generates the Etag seed string.
     * The callback's signature should be:
     *
     * ~~~
     * function ($action, $params)
     * ~~~
     *
     * where `$action` is the [[Action]] object that this filter is currently handling;
     * `$params` takes the value of [[params]]. The callback should return a string serving
     * as the seed for generating an Etag.
     */
    public $etagSeed;
    /**
     * @var mixed additional parameters that should be passed to the [[lastModified]] and [[etagSeed]] callbacks.
     */
    public $params;
    /**
     * @var string HTTP cache control header. If null, the header will not be sent.
     */
    public $cacheControlHeader = 'max-age=3600, public';

    /**
     * This method is invoked right before an action is to be executed (after all possible filters.)
     * You may override this method to do last-minute preparation for the action.
     * @param  Action  $action the action to be executed.
     * @return boolean whether the action should continue to be executed.
     */
    public function beforeAction($action)
    {
        $verb = Yii::$app->getRequest()->getMethod();
        if ($verb !== 'GET' && $verb !== 'HEAD' || $this->lastModified === null && $this->etagSeed === null) {
            return true;
        }

        $lastModified = $etag = null;
        if ($this->lastModified !== null) {
            $lastModified = call_user_func($this->lastModified, $action, $this->params);
        }
        if ($this->etagSeed !== null) {
            $seed = call_user_func($this->etagSeed, $action, $this->params);
            $etag = $this->generateEtag($seed);
        }

        $this->sendCacheControlHeader();
        $response = Yii::$app->getResponse();
        if ($etag !== null) {
            $response->getHeaders()->set('Etag', $etag);
        }

        if ($this->validateCache($lastModified, $etag)) {
            $response->setStatusCode(304);

            return false;
        }

        if ($lastModified !== null) {
            $response->getHeaders()->set('Last-Modified', gmdate('D, d M Y H:i:s', $lastModified) . ' GMT');
        }

        return true;
    }

    /**
     * Validates if the HTTP cache contains valid content.
     * @param  integer $lastModified the calculated Last-Modified value in terms of a UNIX timestamp.
     *                               If null, the Last-Modified header will not be validated.
     * @param  string  $etag         the calculated ETag value. If null, the ETag header will not be validated.
     * @return boolean whether the HTTP cache is still valid.
     */
    protected function validateCache($lastModified, $etag)
    {
        if ($lastModified !== null && (!isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) || @strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) < $lastModified)) {
            return false;
        } else {
            return $etag === null || isset($_SERVER['HTTP_IF_NONE_MATCH']) && $_SERVER['HTTP_IF_NONE_MATCH'] === $etag;
        }
    }

    /**
     * Sends the cache control header to the client
     * @see cacheControl
     */
    protected function sendCacheControlHeader()
    {
        session_cache_limiter('public');
        $headers = Yii::$app->getResponse()->getHeaders();
        $headers->set('Pragma');
        if ($this->cacheControlHeader !== null) {
            $headers->set('Cache-Control', $this->cacheControlHeader);
        }
    }

    /**
     * Generates an Etag from the given seed string.
     * @param  string $seed Seed for the ETag
     * @return string the generated Etag
     */
    protected function generateEtag($seed)
    {
        return '"' . base64_encode(sha1($seed, true)) . '"';
    }
}