<?php
/**
 * HTTP Caching adaptor class that provides caching services to the
 * [Request_Client] class, using HTTP cache control logic as defined in
 * RFC 2616.
 *
 * @package    Kohana
 * @category   Base
 * @author     Kohana Team
 * @copyright  (c) Kohana Team
 * @license    https://koseven.ga/LICENSE.md
 * @since      3.2.0
 */
class KO7_HTTP_Cache {

	const CACHE_STATUS_KEY    = 'x-cache-status';
	const CACHE_STATUS_SAVED  = 'SAVED';
	const CACHE_STATUS_HIT    = 'HIT';
	const CACHE_STATUS_MISS   = 'MISS';
	const CACHE_HIT_KEY       = 'x-cache-hits';

	/**
	 * Factory method for HTTP_Cache that provides a convenient dependency
	 * injector for the Cache library.
	 *
	 *      // Create HTTP_Cache with named cache engine
	 *      $http_cache = HTTP_Cache::factory('memcache', array(
	 *          'allow_private_cache' => FALSE
	 *          )
	 *      );
	 *
	 *      // Create HTTP_Cache with supplied cache engine
	 *      $http_cache = HTTP_Cache::factory(Cache::instance('memcache'),
	 *          array(
	 *              'allow_private_cache' => FALSE
	 *          )
	 *      );
	 *
	 * @uses    Cache
	 * @param   mixed   $cache      cache engine to use
	 * @param   array   $options    options to set to this class
	 * @return  HTTP_Cache
	 */
	public static function factory($cache, array $options = [])
	{
		if ( ! $cache instanceof Cache)
		{
			$cache = Cache::instance($cache);
		}

		$options['cache'] = $cache;

		return new HTTP_Cache($options);
	}

	/**
	 * Basic cache key generator that hashes the entire request and returns
	 * it. This is fine for static content, or dynamic content where user
	 * specific information is encoded into the request.
	 *
	 *      // Generate cache key
	 *      $cache_key = HTTP_Cache::basic_cache_key_generator($request);
	 *
	 * @param   Request $request
	 * @return  string
	 */
	public static function basic_cache_key_generator(Request $request)
	{
		$uri     = $request->uri();
		$query   = $request->query();
		$headers = $request->headers()->getArrayCopy();
		$body    = $request->body();

		return sha1($uri.'?'.http_build_query($query, NULL, '&').'~'.implode('~', $headers).'~'.$body);
	}

	/**
	 * @var     Cache    cache driver to use for HTTP caching
	 */
	protected $_cache;

	/**
	 * @var    callback  Cache key generator callback
	 */
	protected $_cache_key_callback;

	/**
	 * @var    boolean   Defines whether this client should cache `private` cache directives
	 * @link   http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9
	 */
	protected $_allow_private_cache = FALSE;

	/**
	 * @var    int       The timestamp of the request
	 */
	protected $_request_time;

	/**
	 * @var    int       The timestamp of the response
	 */
	protected $_response_time;

	/**
	 * Constructor method for this class. Allows dependency injection of the
	 * required components such as `Cache` and the cache key generator.
	 *
	 * @param   array $options
	 */
	public function __construct(array $options = [])
	{
		foreach ($options as $key => $value)
		{
			if (method_exists($this, $key))
			{
				$this->$key($value);
			}
		}

		if ($this->_cache_key_callback === NULL)
		{
			$this->cache_key_callback('HTTP_Cache::basic_cache_key_generator');
		}
	}

	/**
	 * Executes the supplied [Request] with the supplied [Request_Client].
	 * Before execution, the HTTP_Cache adapter checks the request type,
	 * destructive requests such as `POST`, `PUT` and `DELETE` will bypass
	 * cache completely and ensure the response is not cached. All other
	 * Request methods will allow caching, if the rules are met.
	 *
	 * @param   Request_Client  $client     client to execute with Cache-Control
	 * @param   Request         $request    request to execute with client
	 * @return  [Response]
	 */
	public function execute(Request_Client $client, Request $request, Response $response)
	{
		if ( ! $this->_cache instanceof Cache)
			return $client->execute_request($request, $response);

		// If this is a destructive request, by-pass cache completely
		if (in_array($request->method(), [
			HTTP_Request::POST,
			HTTP_Request::PUT,
			HTTP_Request::DELETE]))
		{
			// Kill existing caches for this request
			$this->invalidate_cache($request);

			$response = $client->execute_request($request, $response);

			$cache_control = HTTP_Header::create_cache_control([
				'no-cache',
				'must-revalidate'
			]);

			// Ensure client respects destructive action
			return $response->headers('cache-control', $cache_control);
		}

		// Create the cache key
		$cache_key = $this->create_cache_key($request, $this->_cache_key_callback);

		// Try and return cached version
		if (($cached_response = $this->cache_response($cache_key, $request)) instanceof Response)
			return $cached_response;

		// Start request time
		$this->_request_time = time();

		// Execute the request with the Request client
		$response = $client->execute_request($request, $response);

		// Stop response time
		$this->_response_time = (time() - $this->_request_time);

		// Cache the response
		$this->cache_response($cache_key, $request, $response);

		$response->headers(HTTP_Cache::CACHE_STATUS_KEY,
			HTTP_Cache::CACHE_STATUS_MISS);

		return $response;
	}

	/**
	 * Invalidate a cached response for the [Request] supplied.
	 * This has the effect of deleting the response from the
	 * [Cache] entry.
	 *
	 * @param   Request  $request Response to remove from cache
	 * @return  void
	 */
	public function invalidate_cache(Request $request)
	{
		if (($cache = $this->cache()) instanceof Cache)
		{
			$cache->delete($this->create_cache_key($request, $this->_cache_key_callback));
		}

		return;
	}

	/**
	 * Getter and setter for the internal caching engine,
	 * used to cache responses if available and valid.
	 *
	 * @param   KO7_Cache  $cache    engine to use for caching
	 * @return  KO7_Cache
	 * @return  KO7_Request_Client
	 */
	public function cache(Cache $cache = NULL)
	{
		if ($cache === NULL)
			return $this->_cache;

		$this->_cache = $cache;
		return $this;
	}

	/**
	 * Gets or sets the [Request_Client::allow_private_cache] setting.
	 * If set to `TRUE`, the client will also cache cache-control directives
	 * that have the `private` setting.
	 *
	 * @link    http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9
	 * @param   boolean $setting    allow caching of privately marked responses
	 * @return  boolean
	 * @return  [Request_Client]
	 */
	public function allow_private_cache($setting = NULL)
	{
		if ($setting === NULL)
			return $this->_allow_private_cache;

		$this->_allow_private_cache = (bool) $setting;
		return $this;
	}

	/**
	 * Sets or gets the cache key generator callback for this caching
	 * class. The cache key generator provides a unique hash based on the
	 * `Request` object passed to it.
	 *
	 * The default generator is [HTTP_Cache::basic_cache_key_generator()], which
	 * serializes the entire `HTTP_Request` into a unique sha1 hash. This will
	 * provide basic caching for static and simple dynamic pages. More complex
	 * algorithms can be defined and then passed into `HTTP_Cache` using this
	 * method.
	 *
	 *      // Get the cache key callback
	 *      $callback = $http_cache->cache_key_callback();
	 *
	 *      // Set the cache key callback
	 *      $http_cache->cache_key_callback('Foo::cache_key');
	 *
	 *      // Alternatively, in PHP 5.3 use a closure
	 *      $http_cache->cache_key_callback(function (Request $request) {
	 *            return sha1($request->render());
	 *      });
	 *
	 * @param   callback    $callback
	 * @return  mixed
	 * @throws  HTTP_Exception
	 */
	public function cache_key_callback($callback = NULL)
	{
		if ($callback === NULL)
			return $this->_cache_key_callback;

		if ( ! is_callable($callback))
			throw new KO7_Exception('cache_key_callback must be callable!');

		$this->_cache_key_callback = $callback;
		return $this;
	}

	/**
	 * Creates a cache key for the request to use for caching
	 * [KO7_Response] returned by [Request::execute].
	 *
	 * This is the default cache key generating logic, but can be overridden
	 * by setting [HTTP_Cache::cache_key_callback()].
	 *
	 * @param   Request     $request    request to create key for
	 * @param   callback    $callback   optional callback to use instead of built-in method
	 * @return  string
	 */
	public function create_cache_key(Request $request, $callback = FALSE)
	{
		if (is_callable($callback))
			return call_user_func($callback, $request);
		else
			return HTTP_Cache::basic_cache_key_generator($request);
	}

	/**
	 * Controls whether the response can be cached. Uses HTTP
	 * protocol to determine whether the response can be cached.
	 *
	 * @link    http://www.w3.org/Protocols/rfc2616/rfc2616.html RFC 2616
	 * @param   Response  $response The Response
	 * @return  boolean
	 */
	public function set_cache(Response $response)
	{
		$headers = $response->headers()->getArrayCopy();

		if ($cache_control = Arr::get($headers, 'cache-control'))
		{
			// Parse the cache control
			$cache_control = HTTP_Header::parse_cache_control($cache_control);

			// If the no-cache or no-store directive is set, return
			if (array_intersect($cache_control, ['no-cache', 'no-store']))
				return FALSE;

			// Check for private cache and get out of here if invalid
			if ( ! $this->_allow_private_cache AND in_array('private', $cache_control))
			{
				if ( ! isset($cache_control['s-maxage']))
					return FALSE;

				// If there is a s-maxage directive we can use that
				$cache_control['max-age'] = $cache_control['s-maxage'];
			}

			// Check that max-age has been set and if it is valid for caching
			if (isset($cache_control['max-age']) AND $cache_control['max-age'] < 1)
				return FALSE;
		}

		if ($expires = Arr::get($headers, 'expires') AND ! isset($cache_control['max-age']))
		{
			// Can't cache things that have expired already
			if (strtotime($expires) <= time())
				return FALSE;
		}

		return TRUE;
	}

	/**
	 * Caches a [Response] using the supplied [Cache]
	 * and the key generated by [Request_Client::_create_cache_key].
	 *
	 * If not response is supplied, the cache will be checked for an existing
	 * one that is available.
	 *
	 * @param   string      $key        the cache key to use
	 * @param   Request     $request    the HTTP Request
	 * @param   Response    $response   the HTTP Response
	 * @return  mixed
	 */
	public function cache_response($key, Request $request, Response $response = NULL)
	{
		if ( ! $this->_cache instanceof Cache)
			return FALSE;

		// Check for Pragma: no-cache
		if ($pragma = $request->headers('pragma'))
		{
			if ($pragma  == 'no-cache')
				return FALSE;
			elseif (is_array($pragma) AND in_array('no-cache', $pragma))
				return FALSE;
		}

		// If there is no response, lookup an existing cached response
		if ($response === NULL)
		{
			$response = $this->_cache->get($key);

			if ( ! $response instanceof Response)
				return FALSE;

			// Do cache hit arithmetic, using fast arithmetic if available
			if ($this->_cache instanceof Cache_Arithmetic)
			{
				$hit_count = $this->_cache->increment(HTTP_Cache::CACHE_HIT_KEY.$key);
			}
			else
			{
				$hit_count = $this->_cache->get(HTTP_Cache::CACHE_HIT_KEY.$key);
				$this->_cache->set(HTTP_Cache::CACHE_HIT_KEY.$key, ++$hit_count);
			}

			// Update the header to have correct HIT status and count
			$response->headers(HTTP_Cache::CACHE_STATUS_KEY,
				HTTP_Cache::CACHE_STATUS_HIT)
				->headers(HTTP_Cache::CACHE_HIT_KEY, $hit_count);

			return $response;
		}
		else
		{
			if (($ttl = $this->cache_lifetime($response)) === FALSE)
				return FALSE;

			$response->headers(HTTP_Cache::CACHE_STATUS_KEY,
				HTTP_Cache::CACHE_STATUS_SAVED);

			// Set the hit count to zero
			$this->_cache->set(HTTP_Cache::CACHE_HIT_KEY.$key, 0);

			return $this->_cache->set($key, $response, $ttl);
		}
	}

	/**
	 * Calculates the total Time To Live based on the specification
	 * RFC 2616 cache lifetime rules.
	 *
	 * @param   Response  $response  Response to evaluate
	 * @return  mixed  TTL value or false if the response should not be cached
	 */
	public function cache_lifetime(Response $response)
	{
		// Get out of here if this cannot be cached
		if ( ! $this->set_cache($response))
			return FALSE;

		// Calculate apparent age
		if ($date = $response->headers('date'))
		{
			$apparent_age = max(0, $this->_response_time - strtotime($date));
		}
		else
		{
			$apparent_age = max(0, $this->_response_time);
		}

		// Calculate corrected received age
		if ($age = $response->headers('age'))
		{
			$corrected_received_age = max($apparent_age, intval($age));
		}
		else
		{
			$corrected_received_age = $apparent_age;
		}

		// Corrected initial age
		$corrected_initial_age = $corrected_received_age + $this->request_execution_time();

		// Resident time
		$resident_time = time() - $this->_response_time;

		// Current age
		$current_age = $corrected_initial_age + $resident_time;

		// Prepare the cache freshness lifetime
		$ttl = NULL;

		// Cache control overrides
		if ($cache_control = $response->headers('cache-control'))
		{
			// Parse the cache control header
			$cache_control = HTTP_Header::parse_cache_control($cache_control);

			if (isset($cache_control['max-age']))
			{
				$ttl = $cache_control['max-age'];
			}

			if (isset($cache_control['s-maxage']) AND isset($cache_control['private']) AND $this->_allow_private_cache)
			{
				$ttl = $cache_control['s-maxage'];
			}

			if (isset($cache_control['max-stale']) AND ! isset($cache_control['must-revalidate']))
			{
				$ttl = $current_age + $cache_control['max-stale'];
			}
		}

		// If we have a TTL at this point, return
		if ($ttl !== NULL)
			return $ttl;

		if ($expires = $response->headers('expires'))
			return strtotime($expires) - $current_age;

		return FALSE;
	}

	/**
	 * Returns the duration of the last request execution.
	 * Either returns the time of completed requests or
	 * `FALSE` if the request hasn't finished executing, or
	 * is yet to be run.
	 *
	 * @return  mixed
	 */
	public function request_execution_time()
	{
		if ($this->_request_time === NULL OR $this->_response_time === NULL)
			return FALSE;

		return $this->_response_time - $this->_request_time;
	}

} // End KO7_HTTP_Cache
