timeserver.java added (as found in nodered json)

src/Timeserver/timeserver.java

@@ -0,0 +1,88 @@
+/* LoRaWAN Timeserver
+
+construct 7 byte timesync_answer from gateway timestamp and node's time_sync_req
+
+byte    meaning
+1       sequence number (taken from node's time_sync_req)
+2       timezone in 15 minutes steps
+3..6    current second (from epoch time 1970)
+7       1/250ths fractions of current second
+
+*/
+
+function timecompare(a, b) {
+  
+  const timeA = a.time;
+  const timeB = b.time;
+
+  let comparison = 0;
+  if (timeA > timeB) {
+    comparison = 1;
+  } else if (timeA < timeB) {
+    comparison = -1;
+  }
+  return comparison;
+}
+
+let confidence = 2000; // max millisecond diff gateway time to server time
+
+// guess if we have received a valid time_sync_req command
+if (msg.payload.payload_raw.length != 1)
+  return;
+
+var deviceMsg = { payload: msg.payload.dev_id };
+var seqNo = msg.payload.payload_raw[0];
+var seqNoMsg = { payload: seqNo };
+var gateway_list = msg.payload.metadata.gateways;
+
+// filter all gateway timestamps that have milliseconds part (which we assume have a ".")
+var gateways = gateway_list.filter(function (element) {
+  return (element.time.includes("."));
+});
+
+var gateway_time = gateways.map(gw => {
+    return {
+      time: new Date(gw.time),
+      eui: gw.gtw_id,
+      }
+  });
+var server_time = new Date(msg.payload.metadata.time);
+
+// validate all gateway timestamps against lorawan server_time (which is assumed to be recent)
+var gw_timestamps = gateway_time.filter(function (element) {
+  return ((element.time > (server_time - confidence) && element.time <= server_time));
+});
+
+// if no timestamp left, we have no valid one and exit
+if (gw_timestamps.length === 0) {
+    var notavailMsg = { payload: "n/a" };
+    var notimeMsg = { payload: 0xff };    
+    var buf2 = Buffer.alloc(1);
+    msg.payload = new Buffer(buf2.fill(0xff));
+    msg.port = 9; // Paxcounter TIMEPORT
+    return [notavailMsg, notavailMsg, deviceMsg, seqNoMsg, msg];}
+
+// sort time array in ascending order to find most recent timestamp for time answer
+gw_timestamps.sort(timecompare);
+
+var timestamp = gw_timestamps[0].time;
+var eui = gw_timestamps[0].eui;
+var offset = server_time - timestamp;
+
+var seconds = Math.floor(timestamp/1000);
+var fractions = (timestamp % 1000) / 4;
+
+let buf = new ArrayBuffer(7);
+new DataView(buf).setUint8(0, seqNo);
+// Timezone (in 15min steps)
+var timezone = 8; // CET = UTC+2h
+new DataView(buf).setUint8(1, timezone);
+new DataView(buf).setUint32(2, seconds);
+new DataView(buf).setUint8(6, fractions);
+
+msg.payload = new Buffer(new Uint8Array(buf));
+msg.port = 9; // Paxcounter TIMEPORT
+var euiMsg = { payload: eui };
+var offsetMsg = { payload: offset };
+
+return [euiMsg, offsetMsg, deviceMsg, seqNoMsg, msg];