feat(vf): add Assembly and InFlightDatagram to Transport

core/include/vf/Transport.h

@@ -157,3 +157,123 @@ public:
     uint32_t nextExpectedMessageId() const { return nextDeliverMessageId; }
     std::size_t reorderBufferSize() const { return completed.size(); }
 
+    static uint64_t fragmentKey(uint32_t messageId, uint32_t fragmentIndex);
+
+private:
+    struct Assembly {
+        uint32_t fragmentCount = 0;
+        uint32_t received = 0;
+        std::vector<ByteBuffer> parts;
+        std::vector<uint8_t> have;
+    };
+
+    void drainCompleted();
+
+    std::map<uint64_t, OutgoingFragment> outgoing;
+    uint32_t nextOutgoingMessageId = 0;
+
+    std::map<uint32_t, Assembly> incoming;
+    std::map<uint32_t, ByteBuffer> completed;
+    uint32_t nextDeliverMessageId = 0;
+    std::deque<ByteBuffer> delivered;
+};
+
+// Unordered, send-once stream for state that ages badly. A message older than
+// the newest one already delivered is thrown away instead of queued, because a
+// stale entity position is worse than no update at all.
+class UnreliableChannel {
+public:
+    void queueMessage(const uint8_t* payload, std::size_t size, std::size_t fragmentPayload);
+
+    const Datagram::Fragment* peekOutgoing() const;
+    void dropOutgoing();
+    std::size_t outgoingFragmentCount() const { return outgoing.size(); }
+
+    bool accept(const Datagram::Fragment& fragment);
+    bool popDelivered(ByteBuffer& out);
+    std::size_t deliveredWaiting() const { return delivered.size(); }
+    uint32_t highestDeliveredMessageId() const { return highestDelivered; }
+    uint64_t staleDroppedCount() const { return stale; }
+
+private:
+    struct Assembly {
+        uint32_t fragmentCount = 0;
+        uint32_t received = 0;
+        std::vector<ByteBuffer> parts;
+        std::vector<uint8_t> have;
+    };
+
+    std::deque<Datagram::Fragment> outgoing;
+    uint32_t nextOutgoingMessageId = 0;
+
+    std::map<uint32_t, Assembly> incoming;
+    uint32_t highestDelivered = 0;
+    bool anyDelivered = false;
+    std::deque<ByteBuffer> delivered;
+    uint64_t stale = 0;
+};
+
+// Reliability layered over the unreliable datagram link: sequencing, acks with
+// a 32-entry bitfield, timeout-driven retransmission, a bounded send window and
+// the two message channels. Time is the tick handed in from outside; the class
+// never reads a clock.
+class Transport {
+public:
+    Transport(LoopbackLink& link, int endpoint, TransportConfig config = {});
+
+    void sendReliable(const uint8_t* payload, std::size_t size);
+    void sendReliable(const ByteBuffer& payload);
+    void sendUnreliable(const uint8_t* payload, std::size_t size);
+    void sendUnreliable(const ByteBuffer& payload);
+
+    // Reads arrivals and applies their acks.
+    void pump(uint64_t tick);
+    // Expires unacked datagrams, then packs and sends whatever fits.
+    void flush(uint64_t tick);
+    void update(uint64_t tick);
+
+    bool popReliable(ByteBuffer& out);
+    bool popUnreliable(ByteBuffer& out);
+
+    const TransportStats& stats() const { return statistics; }
+    const TransportConfig& settings() const { return config; }
+    std::size_t maxFragmentPayload() const;
+    std::size_t inFlightDatagramCount() const { return inFlight.size(); }
+    std::size_t unackedFragmentCount() const { return reliable.unackedFragmentCount(); }
+    std::size_t pendingOutgoingCount() const;
+    ReliableChannel& reliableChannel() { return reliable; }
+    UnreliableChannel& unreliableChannel() { return unreliable; }
+
+private:
+    struct InFlightDatagram {
+        SequenceNumber sequence = 0;
+        uint64_t sentTick = 0;
+        std::vector<uint64_t> reliableKeys;
+    };
+
+    bool registerIncoming(SequenceNumber sequence);
+    void applyAcks(SequenceNumber ackLatest, AckBitfield ackHistory, uint64_t tick);
+    void acknowledge(SequenceNumber sequence, uint64_t tick);
+    bool buildDatagram(Datagram& out);
+    void sampleRtt(uint64_t sample);
+
+    LoopbackLink* link;
+    int endpoint;
+    TransportConfig config;
+
+    ReliableChannel reliable;
+    UnreliableChannel unreliable;
+
+    SequenceNumber nextSequence = 0;
+    std::vector<InFlightDatagram> inFlight;
+
+    SequenceNumber remoteLatest = 0;
+    AckBitfield remoteHistory = 0;
+    bool anyReceived = false;
+    bool ackPending = false;
+
+    std::vector<ByteBuffer> arrivals;
+    TransportStats statistics;
+};
+
+} // namespace vf