class ResponseCacheStrategy

class ResponseCacheStrategy implements ResponseCacheStrategyInterface {
    
    /**
     * Cache-Control headers that are sent to the final response if they appear in ANY of the responses.
     */
    private const OVERRIDE_DIRECTIVES = [
        'private',
        'no-cache',
        'no-store',
        'no-transform',
        'must-revalidate',
        'proxy-revalidate',
    ];
    
    /**
     * Cache-Control headers that are sent to the final response if they appear in ALL of the responses.
     */
    private const INHERIT_DIRECTIVES = [
        'public',
        'immutable',
    ];
    private int $embeddedResponses = 0;
    private bool $isNotCacheableResponseEmbedded = false;
    private int $age = 0;
    private \DateTimeInterface|false|null $lastModified = null;
    private array $flagDirectives = [
        'no-cache' => null,
        'no-store' => null,
        'no-transform' => null,
        'must-revalidate' => null,
        'proxy-revalidate' => null,
        'public' => null,
        'private' => null,
        'immutable' => null,
    ];
    private array $ageDirectives = [
        'max-age' => null,
        's-maxage' => null,
        'expires' => false,
    ];
    public function add(Response $response) : void {
        ++$this->embeddedResponses;
        foreach (self::OVERRIDE_DIRECTIVES as $directive) {
            if ($response->headers
                ->hasCacheControlDirective($directive)) {
                $this->flagDirectives[$directive] = true;
            }
        }
        foreach (self::INHERIT_DIRECTIVES as $directive) {
            if (false !== $this->flagDirectives[$directive]) {
                $this->flagDirectives[$directive] = $response->headers
                    ->hasCacheControlDirective($directive);
            }
        }
        $age = $response->getAge();
        $this->age = max($this->age, $age);
        if ($this->willMakeFinalResponseUncacheable($response)) {
            $this->isNotCacheableResponseEmbedded = true;
            return;
        }
        $maxAge = $response->headers
            ->hasCacheControlDirective('max-age') ? (int) $response->headers
            ->getCacheControlDirective('max-age') : null;
        $sharedMaxAge = $response->headers
            ->hasCacheControlDirective('s-maxage') ? (int) $response->headers
            ->getCacheControlDirective('s-maxage') : $maxAge;
        $expires = $response->getExpires();
        $expires = null !== $expires ? (int) $expires->format('U') - (int) $response->getDate()
            ->format('U') : null;
        // See https://datatracker.ietf.org/doc/html/rfc7234#section-4.2.2
        // If a response is "public" but does not have maximum lifetime, heuristics might be applied.
        // Do not store NULL values so the final response can have more limiting value from other responses.
        $isHeuristicallyCacheable = $response->headers
            ->hasCacheControlDirective('public') && null === $maxAge && null === $sharedMaxAge && null === $expires;
        if (!$isHeuristicallyCacheable || null !== $maxAge || null !== $expires) {
            $this->storeRelativeAgeDirective('max-age', $maxAge, $expires, $age);
        }
        if (!$isHeuristicallyCacheable || null !== $sharedMaxAge || null !== $expires) {
            $this->storeRelativeAgeDirective('s-maxage', $sharedMaxAge, $expires, $age);
        }
        if (null !== $expires) {
            $this->ageDirectives['expires'] = true;
        }
        if (false !== $this->lastModified) {
            $lastModified = $response->getLastModified();
            $this->lastModified = $lastModified ? max($this->lastModified, $lastModified) : false;
        }
    }
    public function update(Response $response) : void {
        // if we have no embedded Response, do nothing
        if (0 === $this->embeddedResponses) {
            return;
        }
        // Remove Etag since it cannot be merged from embedded responses.
        $response->setEtag(null);
        $this->add($response);
        $response->headers
            ->set('Age', $this->age);
        if ($this->isNotCacheableResponseEmbedded) {
            $response->setLastModified(null);
            if ($this->flagDirectives['no-store']) {
                $response->headers
                    ->set('Cache-Control', 'no-cache, no-store, must-revalidate');
            }
            else {
                $response->headers
                    ->set('Cache-Control', 'no-cache, must-revalidate');
            }
            return;
        }
        $response->setLastModified($this->lastModified ?: null);
        $flags = array_filter($this->flagDirectives);
        if (isset($flags['must-revalidate'])) {
            $flags['no-cache'] = true;
        }
        $response->headers
            ->set('Cache-Control', implode(', ', array_keys($flags)));
        $maxAge = null;
        if (is_numeric($this->ageDirectives['max-age'])) {
            $maxAge = $this->ageDirectives['max-age'] + $this->age;
            $response->headers
                ->addCacheControlDirective('max-age', $maxAge);
        }
        if (is_numeric($this->ageDirectives['s-maxage'])) {
            $sMaxage = $this->ageDirectives['s-maxage'] + $this->age;
            if ($maxAge !== $sMaxage) {
                $response->headers
                    ->addCacheControlDirective('s-maxage', $sMaxage);
            }
        }
        if ($this->ageDirectives['expires'] && null !== $maxAge) {
            $date = clone $response->getDate();
            $date = $date->modify('+' . $maxAge . ' seconds');
            $response->setExpires($date);
        }
    }
    
    /**
     * RFC2616, Section 13.4.
     *
     * @see https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4
     */
    private function willMakeFinalResponseUncacheable(Response $response) : bool {
        // RFC2616: A response received with a status code of 200, 203, 300, 301 or 410
        // MAY be stored by a cache […] unless a cache-control directive prohibits caching.
        if ($response->headers
            ->hasCacheControlDirective('no-cache') || $response->headers
            ->hasCacheControlDirective('no-store')) {
            return true;
        }
        // Etag headers cannot be merged, they render the response uncacheable
        // by default (except if the response also has max-age etc.).
        if (null === $response->getEtag() && \in_array($response->getStatusCode(), [
            200,
            203,
            300,
            301,
            410,
        ])) {
            return false;
        }
        // RFC2616: A response received with any other status code (e.g. status codes 302 and 307)
        // MUST NOT be returned in a reply to a subsequent request unless there are
        // cache-control directives or another header(s) that explicitly allow it.
        $cacheControl = [
            'max-age',
            's-maxage',
            'must-revalidate',
            'proxy-revalidate',
            'public',
            'private',
        ];
        foreach ($cacheControl as $key) {
            if ($response->headers
                ->hasCacheControlDirective($key)) {
                return false;
            }
        }
        if ($response->headers
            ->has('Expires')) {
            return false;
        }
        return true;
    }
    
    /**
     * Store lowest max-age/s-maxage/expires for the final response.
     *
     * The response might have been stored in cache a while ago. To keep things comparable,
     * we have to subtract the age so that the value is normalized for an age of 0.
     *
     * If the value is lower than the currently stored value, we update the value, to keep a rolling
     * minimal value of each instruction. If the value is NULL, the directive will not be set on the final response.
     */
    private function storeRelativeAgeDirective(string $directive, ?int $value, ?int $expires, int $age) : void {
        if (null === $value && null === $expires) {
            $this->ageDirectives[$directive] = false;
        }
        if (false !== $this->ageDirectives[$directive]) {
            $value = min($value ?? PHP_INT_MAX, $expires ?? PHP_INT_MAX);
            $value -= $age;
            $this->ageDirectives[$directive] = null !== $this->ageDirectives[$directive] ? min($this->ageDirectives[$directive], $value) : $value;
        }
    }

}