class VariationCache

class VariationCache implements VariationCacheInterface {
    
    /**
     * Constructs a new VariationCache object.
     *
     * @param \Symfony\Component\HttpFoundation\RequestStack $requestStack
     *   The request stack.
     * @param \Drupal\Core\Cache\CacheBackendInterface $cacheBackend
     *   The cache backend to wrap.
     * @param \Drupal\Core\Cache\Context\CacheContextsManager $cacheContextsManager
     *   The cache contexts manager.
     */
    public function __construct(RequestStack $requestStack, CacheBackendInterface $cacheBackend, CacheContextsManager $cacheContextsManager) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function get(array $keys, CacheableDependencyInterface $initial_cacheability) {
        $chain = $this->getRedirectChain($keys, $initial_cacheability);
        return array_pop($chain);
    }
    
    /**
     * {@inheritdoc}
     */
    public function set(array $keys, $data, CacheableDependencyInterface $cacheability, CacheableDependencyInterface $initial_cacheability) : void {
        $initial_contexts = $initial_cacheability->getCacheContexts();
        $contexts = $cacheability->getCacheContexts();
        if ($missing_contexts = array_diff($initial_contexts, $contexts)) {
            throw new \LogicException(sprintf('The complete set of cache contexts for a variation cache item must contain all of the initial cache contexts, missing: %s.', implode(', ', $missing_contexts)));
        }
        // Don't store uncacheable items.
        if ($cacheability->getCacheMaxAge() === 0) {
            return;
        }
        // Track the potential effect of cache context optimization on cache tags.
        $optimized_cacheability = CacheableMetadata::createFromObject($cacheability);
        $cid = $this->createCacheId($keys, $optimized_cacheability);
        // Check whether we had any cache redirects leading to the cache ID already.
        // If there are none, we know that there is no proper redirect path to the
        // cache ID we're trying to store the data at. This may be because there is
        // either no full redirect path yet or there is one that is too specific at
        // a given step of the way. In case of the former, we simply need to store a
        // redirect. In case of the latter, we need to replace the overly specific
        // step with a simpler one.
        $chain = $this->getRedirectChain($keys, $initial_cacheability);
        if (!array_key_exists($cid, $chain)) {
            // We can easily find overly specific redirects by comparing their cache
            // contexts to the ones we have here. If a redirect has more or different
            // contexts, it needs to be replaced with a simplified version.
            //
            // Simplifying overly specific redirects can be done in two ways:
            //
            // -------
            //
            // Problem: The redirect is a superset of the current cache contexts.
            // Solution: We replace the redirect with the current contexts.
            //
            // Example: Suppose we try to store an object with context A, whereas we
            // already have a redirect that uses A and B. In this case we simply store
            // the object at the address designated by context A and next time someone
            // tries to load the initial AB object, it will restore its redirect path
            // by adding an AB redirect step after A.
            //
            // -------
            //
            // Problem: The redirect overlaps, with both options having unique values.
            // Solution: Find the common contexts and use those for a new redirect.
            //
            // Example: Suppose we try to store an object with contexts A and C, but
            // we already have a redirect that uses A and B. In this case we find A to
            // be the common cache context and replace the redirect with one only
            // using A, immediately followed by one for AC so there is a full path to
            // the data we're trying to set. Next time someone tries to load the
            // initial AB object, it will restore its redirect path by adding an AB
            // redirect step after A.
            foreach ($chain as $chain_cid => $result) {
                if ($result && $result->data instanceof CacheRedirect) {
                    $result_contexts = $result->data
                        ->getCacheContexts();
                    if (array_diff($result_contexts, $contexts)) {
                        // Check whether we have an overlap scenario as we need to manually
                        // create an extra redirect in that case.
                        $common_contexts = array_intersect($result_contexts, $contexts);
                        // != is the most appropriate comparison operator here, since we
                        // only want to know if any keys or values don't match.
                        if ($common_contexts != $contexts) {
                            // Set the redirect to the common contexts at the current address.
                            // In the above example this is essentially overwriting the
                            // redirect to AB with a redirect to A.
                            $common_cacheability = (new CacheableMetadata())->setCacheContexts($common_contexts);
                            $this->cacheBackend
                                ->set($chain_cid, new CacheRedirect($common_cacheability));
                            // Before breaking the loop, set the current address to the next
                            // one in line so that we can store the full redirect as well. In
                            // the above example, this is the part where we immediately also
                            // store a redirect to AC at the CID that A pointed to.
                            $chain_cid = $this->createCacheIdFast($keys, $common_cacheability);
                        }
                        break;
                    }
                }
            }
            // The loop above either broke at an overly specific step or completed
            // without any problem. In both cases, $chain_cid ended up with the value
            // that we should store the new redirect at.
            //
            // Cache redirects are stored indefinitely and without tags as they never
            // need to be cleared. If they ever end up leading to a stale cache item
            // that now uses different contexts then said item will either follow an
            // existing path of redirects or carve its own over the old one.
            
            /** @phpstan-ignore-next-line */
            $this->cacheBackend
                ->set($chain_cid, new CacheRedirect($cacheability));
        }
        $this->cacheBackend
            ->set($cid, $data, $this->maxAgeToExpire($cacheability->getCacheMaxAge()), $optimized_cacheability->getCacheTags());
    }
    
    /**
     * {@inheritdoc}
     */
    public function delete(array $keys, CacheableDependencyInterface $initial_cacheability) : void {
        $chain = $this->getRedirectChain($keys, $initial_cacheability);
        $this->cacheBackend
            ->delete(array_key_last($chain));
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidate(array $keys, CacheableDependencyInterface $initial_cacheability) : void {
        $chain = $this->getRedirectChain($keys, $initial_cacheability);
        $this->cacheBackend
            ->invalidate(array_key_last($chain));
    }
    
    /**
     * Performs a full get, returning every step of the way.
     *
     * This will check whether there is a cache redirect and follow it if so. It
     * will keep following redirects until it gets to a cache miss or the actual
     * cache object.
     *
     * @param string[] $keys
     *   The cache keys to retrieve the cache entry for.
     * @param \Drupal\Core\Cache\CacheableDependencyInterface $initial_cacheability
     *   The cache metadata of the data to store before other systems had a chance
     *   to adjust it. This is also commonly known as "pre-bubbling" cacheability.
     *
     * @return array
     *   Every cache get that lead to the final result, keyed by the cache ID used
     *   to query the cache for that result.
     */
    protected function getRedirectChain(array $keys, CacheableDependencyInterface $initial_cacheability) : array {
        $cid = $this->createCacheIdFast($keys, $initial_cacheability);
        $chain[$cid] = $result = $this->cacheBackend
            ->get($cid);
        while ($result && $result->data instanceof CacheRedirect) {
            $cid = $this->createCacheIdFast($keys, $result->data);
            $chain[$cid] = $result = $this->cacheBackend
                ->get($cid);
        }
        return $chain;
    }
    
    /**
     * Maps a max-age value to an "expire" value for the Cache API.
     *
     * @param int $max_age
     *   A max-age value.
     *
     * @return int
     *   A corresponding "expire" value.
     *
     * @see \Drupal\Core\Cache\CacheBackendInterface::set()
     */
    protected function maxAgeToExpire($max_age) {
        if ($max_age !== Cache::PERMANENT) {
            return (int) $this->requestStack
                ->getMainRequest()->server
                ->get('REQUEST_TIME') + $max_age;
        }
        return $max_age;
    }
    
    /**
     * Creates a cache ID based on cache keys and cacheable metadata.
     *
     * If cache contexts are optimized during the creating of the cache ID, then
     * the effect of said optimization on the cache contexts will be reflected in
     * the provided cacheable metadata.
     *
     * @param string[] $keys
     *   The cache keys of the data to store.
     * @param \Drupal\Core\Cache\CacheableMetadata $cacheable_metadata
     *   The cacheable metadata of the data to store.
     *
     * @return string
     *   The cache ID.
     */
    protected function createCacheId(array $keys, CacheableMetadata &$cacheable_metadata) {
        if ($contexts = $cacheable_metadata->getCacheContexts()) {
            $context_cache_keys = $this->cacheContextsManager
                ->convertTokensToKeys($contexts);
            $keys = array_merge($keys, $context_cache_keys->getKeys());
            $cacheable_metadata = $cacheable_metadata->merge($context_cache_keys);
        }
        return implode(':', $keys);
    }
    
    /**
     * Creates a cache ID based on cache keys and cacheable metadata.
     *
     * This is a simpler, faster version of ::createCacheID() to be used when you
     * do not care about how cache context optimization affects the cache tags.
     *
     * @param string[] $keys
     *   The cache keys of the data to store.
     * @param \Drupal\Core\Cache\CacheableDependencyInterface $cacheability
     *   The cache metadata of the data to store.
     *
     * @return string
     *   The cache ID for the redirect.
     */
    protected function createCacheIdFast(array $keys, CacheableDependencyInterface $cacheability) {
        if ($contexts = $cacheability->getCacheContexts()) {
            $context_cache_keys = $this->cacheContextsManager
                ->convertTokensToKeys($contexts);
            $keys = array_merge($keys, $context_cache_keys->getKeys());
        }
        return implode(':', $keys);
    }

}