Check msgid on receive and update caches

ethermess.c

@@ -446,7 +446,42 @@ void handle_message(const unsigned char source_mac[6], const unsigned char *data
 
 	// TODO: Check that the message is valid utf-8 with newline as the only control char
 
-	// TODO: Check whether we've received this message already and update msgid cache
+	// See whether we've already received this message and update the next msgid if so
+	ssize_t cache_index = msgid_cache_lookup(source_mac);
+	if (cache_index == -1) {
+		// No cache entry -> add an empty one
+		cache_index = msgid_cache_add(source_mac);
+	}
+
+	if (!msgid_cache[cache_index].know_receive) {
+		// We have a cache entry, but no idea what next message to receive
+		// Assume this is not a repeat transmission, and accept the message
+		// The easiest way to do this is to set this message's msgid as the
+		// one we're looking to receive
+		msgid_cache[cache_index].next_receive = msgid;
+		msgid_cache[cache_index].know_receive = true;
+	}
+
+	if (msgid_cache[cache_index].next_receive - msgid < 0x8000) {
+		// The msgid counter can wrap around, so a simple larger than
+		// comparison will not work. Instead, we look at the distance
+		// between the message's msgid and the one we're expecting to
+		// receive. If it is from 0 to 0x8000, we consider it to be
+		// in the forwards direction, and if it is more, we consider it
+		// to be in the backwards one. This leaves equally sized (2^15)
+		// ranges for values in both cases.
+
+		// In this case the msgid is considered to be of a message we
+		// haven't yet seen. Therefore we'll continue processing, and
+		// update the next msgid we're expecting to receive to this one
+		// plus 1
+		msgid_cache[cache_index].next_receive = msgid + 1;
+	} else {
+		// In this case we consider the message already received
+		// We will send an ack, but not process it any further
+		send_ack(source_mac, msgid);
+		return;
+	}
 
 	char mac[18];
 	format_mac(source_mac, mac);