<?php
/**
 * Class HTTP_ConditionalGet 
 * @package Minify
 * @subpackage HTTP
 */

/**
 * Implement conditional GET via a timestamp or hash of content
 *
 * E.g. Content from DB with update time:
 * <code>
 * list($updateTime, $content) = getDbUpdateAndContent();
 * $cg = new HTTP_ConditionalGet(array(
 * 'lastModifiedTime' => $updateTime
 * ,'isPublic' => true
 * ));
 * $cg->sendHeaders();
 * if ($cg->cacheIsValid) {
 * exit();
 * }
 * echo $content;
 * </code>
 * 
 * E.g. Shortcut for the above
 * <code>
 * HTTP_ConditionalGet::check($updateTime, true); // exits if client has cache
 * echo $content;
 * </code>
 *
 * E.g. Content from DB with no update time:
 * <code>
 * $content = getContentFromDB();
 * $cg = new HTTP_ConditionalGet(array(
 * 'contentHash' => md5($content)
 * ));
 * $cg->sendHeaders();
 * if ($cg->cacheIsValid) {
 * exit();
 * }
 * echo $content;
 * </code>
 * 
 * E.g. Static content with some static includes:
 * <code>
 * // before content
 * $cg = new HTTP_ConditionalGet(array(
 * 'lastUpdateTime' => max(
 * filemtime(__FILE__)
 * ,filemtime('/path/to/header.inc')
 * ,filemtime('/path/to/footer.inc')
 * )
 * ));
 * $cg->sendHeaders();
 * if ($cg->cacheIsValid) {
 * exit();
 * }
 * </code>
 * @package Minify
 * @subpackage HTTP
 * @author Stephen Clay <steve@mrclay.org>
 */
class HTTP_ConditionalGet {

 /**
 * Does the client have a valid copy of the requested resource?
 * 
 * You'll want to check this after instantiating the object. If true, do
 * not send content, just call sendHeaders() if you haven't already.
 *
 * @var bool
 */
 public $cacheIsValid = null;

 /**
 * @param array $spec options
 * 
 * 'isPublic': (bool) if false, the Cache-Control header will contain
 * "private", allowing only browser caching. (default false)
 * 
 * 'lastModifiedTime': (int) if given, both ETag AND Last-Modified headers
 * will be sent with content. This is recommended.
 *
 * 'encoding': (string) if set, the header "Vary: Accept-Encoding" will
 * always be sent and a truncated version of the encoding will be appended
 * to the ETag. E.g. "pub123456;gz". This will also trigger a more lenient 
 * checking of the client's If-None-Match header, as the encoding portion of
 * the ETag will be stripped before comparison.
 * 
 * 'contentHash': (string) if given, only the ETag header can be sent with
 * content (only HTTP1.1 clients can conditionally GET). The given string 
 * should be short with no quote characters and always change when the 
 * resource changes (recommend md5()). This is not needed/used if 
 * lastModifiedTime is given.
 * 
 * 'eTag': (string) if given, this will be used as the ETag header rather
 * than values based on lastModifiedTime or contentHash. Also the encoding
 * string will not be appended to the given value as described above.
 * 
 * 'invalidate': (bool) if true, the client cache will be considered invalid
 * without testing. Effectively this disables conditional GET. 
 * (default false)
 * 
 * 'maxAge': (int) if given, this will set the Cache-Control max-age in 
 * seconds, and also set the Expires header to the equivalent GMT date. 
 * After the max-age period has passed, the browser will again send a 
 * conditional GET to revalidate its cache.
 */
 public function __construct($spec)
 {
 $scope = (isset($spec['isPublic']) && $spec['isPublic'])
 ? 'public'
 : 'private';
 $maxAge = 0;
 // backwards compatibility (can be removed later)
 if (isset($spec['setExpires']) 
 && is_numeric($spec['setExpires'])
 && ! isset($spec['maxAge'])) {
 $spec['maxAge'] = $spec['setExpires'] - $_SERVER['REQUEST_TIME'];
 }
 if (isset($spec['maxAge'])) {
 $maxAge = $spec['maxAge'];
 $this->_headers['Expires'] = self::gmtDate(
 $_SERVER['REQUEST_TIME'] + $spec['maxAge'] 
 );
 }
 $etagAppend = '';
 if (isset($spec['encoding'])) {
 $this->_stripEtag = true;
 $this->_headers['Vary'] = 'Accept-Encoding';
 if ('' !== $spec['encoding']) {
 if (0 === strpos($spec['encoding'], 'x-')) {
 $spec['encoding'] = substr($spec['encoding'], 2);
 }
 $etagAppend = ';' . substr($spec['encoding'], 0, 2);
 }
 }
 if (isset($spec['lastModifiedTime'])) {
 $this->_setLastModified($spec['lastModifiedTime']);
 if (isset($spec['eTag'])) { // Use it
 $this->_setEtag($spec['eTag'], $scope);
 } else { // base both headers on time
 $this->_setEtag($spec['lastModifiedTime'] . $etagAppend, $scope);
 }
 } elseif (isset($spec['eTag'])) { // Use it
 $this->_setEtag($spec['eTag'], $scope);
 } elseif (isset($spec['contentHash'])) { // Use the hash as the ETag
 $this->_setEtag($spec['contentHash'] . $etagAppend, $scope);
 }
 $privacy = ($scope === 'private')
 ? ', private'
 : '';
 $this->_headers['Cache-Control'] = "max-age={$maxAge}{$privacy}";
 // invalidate cache if disabled, otherwise check
 $this->cacheIsValid = (isset($spec['invalidate']) && $spec['invalidate'])
 ? false
 : $this->_isCacheValid();
 }
 
 /**
 * Get array of output headers to be sent
 * 
 * In the case of 304 responses, this array will only contain the response
 * code header: array('_responseCode' => 'HTTP/1.0 304 Not Modified')
 * 
 * Otherwise something like: 
 * <code>
 * array(
 * 'Cache-Control' => 'max-age=0, public'
 * ,'ETag' => '"foobar"'
 * )
 * </code>
 *
 * @return array 
 */
 public function getHeaders()
 {
 return $this->_headers;
 }

 /**
 * Set the Content-Length header in bytes
 * 
 * With most PHP configs, as long as you don't flush() output, this method
 * is not needed and PHP will buffer all output and set Content-Length for 
 * you. Otherwise you'll want to call this to let the client know up front.
 * 
 * @param int $bytes
 * 
 * @return int copy of input $bytes
 */
 public function setContentLength($bytes)
 {
 return $this->_headers['Content-Length'] = $bytes;
 }

 /**
 * Send headers
 * 
 * @see getHeaders()
 * 
 * Note this doesn't "clear" the headers. Calling sendHeaders() will
 * call header() again (but probably have not effect) and getHeaders() will
 * still return the headers.
 *
 * @return null
 */
 public function sendHeaders()
 {
 $headers = $this->_headers;
 if (array_key_exists('_responseCode', $headers)) {
 // FastCGI environments require 3rd arg to header() to be set
 list(, $code) = explode(' ', $headers['_responseCode'], 3);
 header($headers['_responseCode'], true, $code);
 unset($headers['_responseCode']);
 }
 foreach ($headers as $name => $val) {
 header($name . ': ' . $val);
 }
 }
 
 /**
 * Exit if the client's cache is valid for this resource
 *
 * This is a convenience method for common use of the class
 *
 * @param int $lastModifiedTime if given, both ETag AND Last-Modified headers
 * will be sent with content. This is recommended.
 *
 * @param bool $isPublic (default false) if true, the Cache-Control header 
 * will contain "public", allowing proxies to cache the content. Otherwise 
 * "private" will be sent, allowing only browser caching.
 *
 * @param array $options (default empty) additional options for constructor
 */
 public static function check($lastModifiedTime = null, $isPublic = false, $options = array())
 {
 if (null !== $lastModifiedTime) {
 $options['lastModifiedTime'] = (int)$lastModifiedTime;
 }
 $options['isPublic'] = (bool)$isPublic;
 $cg = new HTTP_ConditionalGet($options);
 $cg->sendHeaders();
 if ($cg->cacheIsValid) {
 exit();
 }
 }
 
 
 /**
 * Get a GMT formatted date for use in HTTP headers
 * 
 * <code>
 * header('Expires: ' . HTTP_ConditionalGet::gmtdate($time));
 * </code> 
 *
 * @param int $time unix timestamp
 * 
 * @return string
 */
 public static function gmtDate($time)
 {
 return gmdate('D, d M Y H:i:s \G\M\T', $time);
 }
 
 protected $_headers = array();
 protected $_lmTime = null;
 protected $_etag = null;
 protected $_stripEtag = false;

 /**
 * @param string $hash
 *
 * @param string $scope
 */
 protected function _setEtag($hash, $scope)
 {
 $this->_etag = '"' . substr($scope, 0, 3) . $hash . '"';
 $this->_headers['ETag'] = $this->_etag;
 }

 /**
 * @param int $time
 */
 protected function _setLastModified($time)
 {
 $this->_lmTime = (int)$time;
 $this->_headers['Last-Modified'] = self::gmtDate($time);
 }

 /**
 * Determine validity of client cache and queue 304 header if valid
 *
 * @return bool
 */
 protected function _isCacheValid()
 {
 if (null === $this->_etag) {
 // lmTime is copied to ETag, so this condition implies that the
 // server sent neither ETag nor Last-Modified, so the client can't 
 // possibly has a valid cache.
 return false;
 }
 $isValid = ($this->resourceMatchedEtag() || $this->resourceNotModified());
 if ($isValid) {
 $this->_headers['_responseCode'] = 'HTTP/1.0 304 Not Modified';
 }
 return $isValid;
 }

 /**
 * @return bool
 */
 protected function resourceMatchedEtag()
 {
 if (!isset($_SERVER['HTTP_IF_NONE_MATCH'])) {
 return false;
 }
 $clientEtagList = get_magic_quotes_gpc()
 ? stripslashes($_SERVER['HTTP_IF_NONE_MATCH'])
 : $_SERVER['HTTP_IF_NONE_MATCH'];
 $clientEtags = explode(',', $clientEtagList);
 
 $compareTo = $this->normalizeEtag($this->_etag);
 foreach ($clientEtags as $clientEtag) {
 if ($this->normalizeEtag($clientEtag) === $compareTo) {
 // respond with the client's matched ETag, even if it's not what
 // we would've sent by default
 $this->_headers['ETag'] = trim($clientEtag);
 return true;
 }
 }
 return false;
 }

 /**
 * @param string $etag
 *
 * @return string
 */
 protected function normalizeEtag($etag) {
 $etag = trim($etag);
 return $this->_stripEtag
 ? preg_replace('/;\\w\\w"$/', '"', $etag)
 : $etag;
 }

 /**
 * @return bool
 */
 protected function resourceNotModified()
 {
 if (!isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
 return false;
 }
 // strip off IE's extra data (semicolon)
 list($ifModifiedSince) = explode(';', $_SERVER['HTTP_IF_MODIFIED_SINCE'], 2);
 if (strtotime($ifModifiedSince) >= $this->_lmTime) {
 // Apache 2.2's behavior. If there was no ETag match, send the 
 // non-encoded version of the ETag value.
 $this->_headers['ETag'] = $this->normalizeEtag($this->_etag);
 return true;
 }
 return false;
 }
}