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<?php

/*

 * Copyright 2012 Google Inc.

 *

 * Licensed under the Apache License, Version 2.0 (the "License");

 * you may not use this file except in compliance with the License.

 * You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

/**

 * Implement the caching directives specified in rfc2616. This

 * implementation is guided by the guidance offered in rfc2616-sec13.

 * @author Chirag Shah <chirags@google.com>

 */

class Google_CacheParser {

  public static $CACHEABLE_HTTP_METHODS = array('GET', 'HEAD');

  public static $CACHEABLE_STATUS_CODES = array('200', '203', '300', '301');

  private function __construct() {}

  /**

   * Check if an HTTP request can be cached by a private local cache.

   *

   * @static

   * @param Google_HttpRequest $resp

   * @return bool True if the request is cacheable.

   * False if the request is uncacheable.

   */

  public static function isRequestCacheable (Google_HttpRequest $resp) {

    $method = $resp->getRequestMethod();

    if (! in_array($method, self::$CACHEABLE_HTTP_METHODS)) {

      return false;

    }

    // Don't cache authorized requests/responses.

    // [rfc2616-14.8] When a shared cache receives a request containing an

    // Authorization field, it MUST NOT return the corresponding response

    // as a reply to any other request...

    if ($resp->getRequestHeader("authorization")) {

      return false;

    }

    return true;

  }

  /**

   * Check if an HTTP response can be cached by a private local cache.

   *

   * @static

   * @param Google_HttpRequest $resp

   * @return bool True if the response is cacheable.

   * False if the response is un-cacheable.

   */

  public static function isResponseCacheable (Google_HttpRequest $resp) {

    // First, check if the HTTP request was cacheable before inspecting the

    // HTTP response.

    if (false == self::isRequestCacheable($resp)) {

      return false;

    }

    $code = $resp->getResponseHttpCode();

    if (! in_array($code, self::$CACHEABLE_STATUS_CODES)) {

      return false;

    }

    // The resource is uncacheable if the resource is already expired and

    // the resource doesn't have an ETag for revalidation.

    $etag = $resp->getResponseHeader("etag");

    if (self::isExpired($resp) && $etag == false) {

      return false;

    }

    // [rfc2616-14.9.2]  If [no-store is] sent in a response, a cache MUST NOT

    // store any part of either this response or the request that elicited it.

    $cacheControl = $resp->getParsedCacheControl();

    if (isset($cacheControl['no-store'])) {

      return false;

    }

    // Pragma: no-cache is an http request directive, but is occasionally

    // used as a response header incorrectly.

    $pragma = $resp->getResponseHeader('pragma');

    if ($pragma == 'no-cache' || strpos($pragma, 'no-cache') !== false) {

      return false;

    }

    // [rfc2616-14.44] Vary: * is extremely difficult to cache. "It implies that

    // a cache cannot determine from the request headers of a subsequent request

    // whether this response is the appropriate representation."

    // Given this, we deem responses with the Vary header as uncacheable.

    $vary = $resp->getResponseHeader('vary');

    if ($vary) {

      return false;

    }

    return true;

  }

  /**

   * @static

   * @param Google_HttpRequest $resp

   * @return bool True if the HTTP response is considered to be expired.

   * False if it is considered to be fresh.

   */

  public static function isExpired(Google_HttpRequest $resp) {

    // HTTP/1.1 clients and caches MUST treat other invalid date formats,

    // especially including the value “0”, as in the past.

    $parsedExpires = false;

    $responseHeaders = $resp->getResponseHeaders();

    if (isset($responseHeaders['expires'])) {

      $rawExpires = $responseHeaders['expires'];

      // Check for a malformed expires header first.

      if (empty($rawExpires) || (is_numeric($rawExpires) && $rawExpires <= 0)) {

        return true;

      }

      // See if we can parse the expires header.

      $parsedExpires = strtotime($rawExpires);

      if (false == $parsedExpires || $parsedExpires <= 0) {

        return true;

      }

    }

    // Calculate the freshness of an http response.

    $freshnessLifetime = false;

    $cacheControl = $resp->getParsedCacheControl();

    if (isset($cacheControl['max-age'])) {

      $freshnessLifetime = $cacheControl['max-age'];

    }

    $rawDate = $resp->getResponseHeader('date');

    $parsedDate = strtotime($rawDate);

    if (empty($rawDate) || false == $parsedDate) {

      $parsedDate = time();

    }

    if (false == $freshnessLifetime && isset($responseHeaders['expires'])) {

      $freshnessLifetime = $parsedExpires - $parsedDate;

    }

    if (false == $freshnessLifetime) {

      return true;

    }

    // Calculate the age of an http response.

    $age = max(0, time() - $parsedDate);

    if (isset($responseHeaders['age'])) {

      $age = max($age, strtotime($responseHeaders['age']));

    }

    return $freshnessLifetime <= $age;

  }

  /**

   * Determine if a cache entry should be revalidated with by the origin.

   *

   * @param Google_HttpRequest $response

   * @return bool True if the entry is expired, else return false.

   */

  public static function mustRevalidate(Google_HttpRequest $response) {

    // [13.3] When a cache has a stale entry that it would like to use as a

    // response to a client's request, it first has to check with the origin

    // server to see if its cached entry is still usable.

    return self::isExpired($response);

  }

}