class TraceContextPropagator

final class TraceContextPropagator implements TextMapPropagatorInterface {
    public const TRACEPARENT = 'traceparent';
    public const TRACESTATE = 'tracestate';
    private const VERSION = '00';
    // Currently, only '00' is supported
    public const FIELDS = [
        self::TRACEPARENT,
        self::TRACESTATE,
    ];
    private static ?self $instance = null;
    public static function getInstance() : self {
        if (null === self::$instance) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    
    /** {@inheritdoc} */
    public function fields() : array {
        return self::FIELDS;
    }
    
    /** {@inheritdoc} */
    public function inject(&$carrier, ?PropagationSetterInterface $setter = null, ?ContextInterface $context = null) : void {
        $setter ??= ArrayAccessGetterSetter::getInstance();
        $context ??= Context::getCurrent();
        $spanContext = Span::fromContext($context)->getContext();
        if (!$spanContext->isValid()) {
            return;
        }
        // Build and inject the traceparent header
        $traceparent = self::VERSION . '-' . $spanContext->getTraceId() . '-' . $spanContext->getSpanId() . '-' . ($spanContext->isSampled() ? '01' : '00');
        $setter->set($carrier, self::TRACEPARENT, $traceparent);
        // Build and inject the tracestate header
        // Spec says to avoid sending empty tracestate headers
        if (($tracestate = (string) $spanContext->getTraceState()) !== '') {
            $setter->set($carrier, self::TRACESTATE, $tracestate);
        }
    }
    
    /** {@inheritdoc} */
    public function extract($carrier, ?PropagationGetterInterface $getter = null, ?ContextInterface $context = null) : ContextInterface {
        $getter ??= ArrayAccessGetterSetter::getInstance();
        $context ??= Context::getCurrent();
        $spanContext = self::extractImpl($carrier, $getter);
        if (!$spanContext->isValid()) {
            return $context;
        }
        return $context->withContextValue(Span::wrap($spanContext));
    }
    private static function extractImpl($carrier, PropagationGetterInterface $getter) : SpanContextInterface {
        $traceparent = $getter->get($carrier, self::TRACEPARENT);
        if ($traceparent === null) {
            return SpanContext::getInvalid();
        }
        // traceParent = {version}-{trace-id}-{parent-id}-{trace-flags}
        $pieces = explode('-', $traceparent);
        // If the header does not have at least 4 pieces, it is invalid -- restart the trace.
        if (count($pieces) < 4) {
            return SpanContext::getInvalid();
        }
        [
            $version,
            $traceId,
            $spanId,
            $traceFlags,
        ] = $pieces;
        
        /**
         * Return invalid if:
         * - Version is invalid (not 2 char hex or 'ff')
         * - Trace version, trace ID, span ID or trace flag are invalid
         */
        if (!TraceContextValidator::isValidTraceVersion($version) || !SpanContextValidator::isValidTraceId($traceId) || !SpanContextValidator::isValidSpanId($spanId) || !TraceContextValidator::isValidTraceFlag($traceFlags)) {
            return SpanContext::getInvalid();
        }
        // Return invalid if the trace version is not a future version but still has > 4 pieces.
        $versionIsFuture = hexdec($version) > hexdec(self::VERSION);
        if (count($pieces) > 4 && !$versionIsFuture) {
            return SpanContext::getInvalid();
        }
        // Only the sampled flag is extracted from the traceFlags (00000001)
        $convertedTraceFlags = hexdec($traceFlags);
        $isSampled = ($convertedTraceFlags & TraceFlags::SAMPLED) === TraceFlags::SAMPLED;
        // Tracestate = 'Vendor1=Value1,...,VendorN=ValueN'
        $rawTracestate = $getter->get($carrier, self::TRACESTATE);
        if ($rawTracestate !== null) {
            $tracestate = new TraceState($rawTracestate);
            return SpanContext::createFromRemoteParent($traceId, $spanId, $isSampled ? TraceFlags::SAMPLED : TraceFlags::DEFAULT, $tracestate);
        }
        // Only traceparent header is extracted. No tracestate.
        return SpanContext::createFromRemoteParent($traceId, $spanId, $isSampled ? TraceFlags::SAMPLED : TraceFlags::DEFAULT);
    }

}