class Cache

class Cache {
    
    /**
     * Indicates that the item should never be removed unless explicitly deleted.
     */
    const PERMANENT = CacheBackendInterface::CACHE_PERMANENT;
    
    /**
     * Merges arrays of cache contexts and removes duplicates.
     *
     * @param string[] ...
     *   Cache contexts arrays to merge.
     *
     * @return string[]
     *   The merged array of cache contexts.
     */
    public static function mergeContexts(array ...$cache_contexts) {
        $cache_contexts = array_unique(array_merge(...$cache_contexts));
        assert(\Drupal::service('cache_contexts_manager')->assertValidTokens($cache_contexts), sprintf('Failed to assert that "%s" are valid cache contexts.', implode(', ', $cache_contexts)));
        return $cache_contexts;
    }
    
    /**
     * Merges arrays of cache tags and removes duplicates.
     *
     * The cache tags array is returned in a format that is valid for
     * \Drupal\Core\Cache\CacheBackendInterface::set().
     *
     * When caching elements, it is necessary to collect all cache tags into a
     * single array, from both the element itself and all child elements. This
     * allows items to be invalidated based on all tags attached to the content
     * they're constituted from.
     *
     * @param string[] ...
     *   Cache tags arrays to merge.
     *
     * @return string[]
     *   The merged array of cache tags.
     */
    public static function mergeTags(array ...$cache_tags) {
        $cache_tags = array_unique(array_merge(...$cache_tags));
        assert(Inspector::assertAllStrings($cache_tags), 'Cache tags must be valid strings');
        return $cache_tags;
    }
    
    /**
     * Merges max-age values (expressed in seconds), finds the lowest max-age.
     *
     * Ensures infinite max-age (Cache::PERMANENT) is taken into account.
     *
     * @param int ...
     *   Max age values to merge.
     *
     * @return int
     *   The minimum max-age value.
     */
    public static function mergeMaxAges(...$max_ages) {
        // Remove Cache::PERMANENT values to return the correct minimum value.
        $max_ages = array_filter($max_ages, function ($max_age) {
            return $max_age !== Cache::PERMANENT;
        });
        // If there are no max ages left return Cache::PERMANENT, otherwise return
        // the minimum value.
        return empty($max_ages) ? Cache::PERMANENT : min($max_ages);
    }
    
    /**
     * Build an array of cache tags from a given prefix and an array of suffixes.
     *
     * Each suffix will be converted to a cache tag by appending it to the prefix,
     * with a colon between them.
     *
     * @param string $prefix
     *   A prefix string.
     * @param array $suffixes
     *   An array of suffixes. Will be cast to strings.
     * @param string $glue
     *   A string to be used as glue for concatenation. Defaults to a colon.
     *
     * @return string[]
     *   An array of cache tags.
     */
    public static function buildTags($prefix, array $suffixes, $glue = ':') {
        $tags = [];
        foreach ($suffixes as $suffix) {
            $tags[] = $prefix . $glue . $suffix;
        }
        return $tags;
    }
    
    /**
     * Marks cache items from all bins with any of the specified tags as invalid.
     *
     * @param string[] $tags
     *   The list of tags to invalidate cache items for.
     */
    public static function invalidateTags(array $tags) {
        \Drupal::service('cache_tags.invalidator')->invalidateTags($tags);
    }
    
    /**
     * Gets all cache bin services.
     *
     * @return \Drupal\Core\Cache\CacheBackendInterface[]
     *   An array of cache backend objects keyed by cache bins.
     */
    public static function getBins() {
        $bins = [];
        $container = \Drupal::getContainer();
        foreach ($container->getParameter('cache_bins') as $service_id => $bin) {
            $bins[$bin] = $container->get($service_id);
        }
        return $bins;
    }
    
    /**
     * Generates a hash from a query object, to be used as part of the cache key.
     *
     * This smart caching strategy saves Drupal from querying and rendering to
     * HTML when the underlying query is unchanged.
     *
     * Expensive queries should use the query builder to create the query and then
     * call this function. Executing the query and formatting results should
     * happen in a #pre_render callback.
     *
     * @param \Drupal\Core\Database\Query\SelectInterface $query
     *   A select query object.
     *
     * @return string
     *   A hash of the query arguments.
     */
    public static function keyFromQuery(SelectInterface $query) {
        $query->preExecute();
        $keys = [
            (string) $query,
            $query->getArguments(),
        ];
        return hash('sha256', serialize($keys));
    }

}