Improve lossy connections by sending diff from "known" state if it's easy

src/network/networktransport.cc

@@ -117,8 +117,8 @@ void Transport<MyState, RemoteState>::recv( void )
       }
     }
     if ( verbose ) {
-      fprintf( stderr, "[%u] Received state %d [ack %d]\n",
-	       (unsigned int)(timestamp() % 100000), (int)new_state.num, (int)inst.ack_num() );
+      fprintf( stderr, "[%u] Received state %d [coming from %d, ack %d]\n",
+	       (unsigned int)(timestamp() % 100000), (int)new_state.num, (int)inst.old_num(), (int)inst.ack_num() );
     }
     received_states.push_back( new_state );
     sender.set_ack_num( received_states.back().num );

src/network/transportsender.cc

@@ -150,6 +150,8 @@ void TransportSender<MyState>::tick( void )
     
   string diff = current_state.diff_from( assumed_receiver_state->state );
 
+  attempt_prospective_resend_optimization( diff );
+
   /* verify diff has round-trip identity (modulo Unicode fallback rendering) */
   /*
   MyState newstate( assumed_receiver_state->state );
@@ -349,3 +351,26 @@ void TransportSender<MyState>::set_ack_num( uint64_t s_ack_num )
 {
   ack_num = s_ack_num;
 }
+
+/* Investigate diff against known receiver state instead */
+/* Mutates proposed_diff */
+template <class MyState>
+void TransportSender<MyState>::attempt_prospective_resend_optimization( string &proposed_diff )
+{
+  if ( assumed_receiver_state == sent_states.begin() ) {
+    return;
+  }
+
+  string resend_diff = current_state.diff_from( sent_states.front().state );
+
+  /* We do a prophylactic resend if it would make the diff shorter,
+     or if it would lengthen it by no more than 100 bytes and still be
+     less than 1000 bytes. */
+
+  if ( (resend_diff.size() <= proposed_diff.size())
+       || ( (resend_diff.size() < 1000)
+	    && (resend_diff.size() - proposed_diff.size() < 100) ) ) {
+    assumed_receiver_state = sent_states.begin();
+    proposed_diff = resend_diff;
+  }
+}

src/network/transportsender.h

@@ -48,6 +48,7 @@ namespace Network {
 
     /* helper methods for tick() */
     void update_assumed_receiver_state( void );
+    void attempt_prospective_resend_optimization( string &proposed_diff );
     void rationalize_states( void );
     void send_to_receiver( string diff );
     void send_empty_ack( void );