From e0f15c6a21c188edd61ca78d034aaf1505fb59cb Mon Sep 17 00:00:00 2001
From: Anton Tananaev <anton.tananaev@gmail.com>
Date: Sat, 12 Sep 2020 22:58:40 -0700
Subject: Refactor NMEA checksum

---
 src/main/java/org/traccar/WebDataHandler.java                  | 2 +-
 src/main/java/org/traccar/helper/Checksum.java                 | 9 ++-------
 src/main/java/org/traccar/protocol/GpsGateProtocolDecoder.java | 3 ++-
 src/main/java/org/traccar/protocol/LaipacProtocolDecoder.java  | 4 ++--
 src/main/java/org/traccar/protocol/LaipacProtocolEncoder.java  | 2 +-
 src/test/java/org/traccar/helper/ChecksumTest.java             | 5 ++++-
 6 files changed, 12 insertions(+), 13 deletions(-)

(limited to 'src')

diff --git a/src/main/java/org/traccar/WebDataHandler.java b/src/main/java/org/traccar/WebDataHandler.java
index 3be575b48..b3b83a9cf 100644
--- a/src/main/java/org/traccar/WebDataHandler.java
+++ b/src/main/java/org/traccar/WebDataHandler.java
@@ -117,7 +117,7 @@ public class WebDataHandler extends BaseDataHandler {
             f.format("%1$td%1$tm%1$ty,,", calendar);
         }
 
-        s.append(Checksum.nmea(s.toString()));
+        s.append(Checksum.nmea(s.substring(1)));
 
         return s.toString();
     }
diff --git a/src/main/java/org/traccar/helper/Checksum.java b/src/main/java/org/traccar/helper/Checksum.java
index d41dc2992..c1de983d8 100644
--- a/src/main/java/org/traccar/helper/Checksum.java
+++ b/src/main/java/org/traccar/helper/Checksum.java
@@ -151,13 +151,8 @@ public final class Checksum {
         return checksum;
     }
 
-    public static String nmea(String msg) {
-        int checksum = 0;
-        byte[] bytes = msg.getBytes(StandardCharsets.US_ASCII);
-        for (int i = 1; i < bytes.length; i++) {
-            checksum ^= bytes[i];
-        }
-        return String.format("*%02x", checksum).toUpperCase();
+    public static String nmea(String string) {
+        return String.format("*%02X", xor(string));
     }
 
     public static int sum(ByteBuffer buf) {
diff --git a/src/main/java/org/traccar/protocol/GpsGateProtocolDecoder.java b/src/main/java/org/traccar/protocol/GpsGateProtocolDecoder.java
index cc187225b..c158d3212 100644
--- a/src/main/java/org/traccar/protocol/GpsGateProtocolDecoder.java
+++ b/src/main/java/org/traccar/protocol/GpsGateProtocolDecoder.java
@@ -69,7 +69,8 @@ public class GpsGateProtocolDecoder extends BaseProtocolDecoder {
 
     private void send(Channel channel, SocketAddress remoteAddress, String message) {
         if (channel != null) {
-            channel.writeAndFlush(new NetworkMessage(message + Checksum.nmea(message) + "\r\n", remoteAddress));
+            channel.writeAndFlush(new NetworkMessage(
+                    message + Checksum.nmea(message.substring(1)) + "\r\n", remoteAddress));
         }
     }
 
diff --git a/src/main/java/org/traccar/protocol/LaipacProtocolDecoder.java b/src/main/java/org/traccar/protocol/LaipacProtocolDecoder.java
index 0c72568f3..4abb75025 100644
--- a/src/main/java/org/traccar/protocol/LaipacProtocolDecoder.java
+++ b/src/main/java/org/traccar/protocol/LaipacProtocolDecoder.java
@@ -152,7 +152,7 @@ public class LaipacProtocolDecoder extends BaseProtocolDecoder {
 
         if (responseCode != null) {
             String response = "$AVCFG," + devicePassword + "," + responseCode;
-            response += Checksum.nmea(response) + "\r\n";
+            response += Checksum.nmea(response.substring(1)) + "\r\n";
             channel.writeAndFlush(new NetworkMessage(response, remoteAddress));
         }
 
@@ -163,7 +163,7 @@ public class LaipacProtocolDecoder extends BaseProtocolDecoder {
 
         if (Character.isLowerCase(status.charAt(0))) {
             String response = "$EAVACK," + event + "," + checksum;
-            response += Checksum.nmea(response) + "\r\n";
+            response += Checksum.nmea(response.substring(1)) + "\r\n";
             channel.writeAndFlush(new NetworkMessage(response, remoteAddress));
         }
 
diff --git a/src/main/java/org/traccar/protocol/LaipacProtocolEncoder.java b/src/main/java/org/traccar/protocol/LaipacProtocolEncoder.java
index 0c9f8ebb8..aaa5a70f7 100644
--- a/src/main/java/org/traccar/protocol/LaipacProtocolEncoder.java
+++ b/src/main/java/org/traccar/protocol/LaipacProtocolEncoder.java
@@ -29,7 +29,7 @@ public class LaipacProtocolEncoder extends StringProtocolEncoder {
     @Override
     protected String formatCommand(Command command, String format, String... keys) {
         String sentence = super.formatCommand(command, "$" + format, keys);
-        sentence += Checksum.nmea(sentence) + "\r\n";
+        sentence += Checksum.nmea(sentence.substring(1)) + "\r\n";
         return sentence;
     }
 
diff --git a/src/test/java/org/traccar/helper/ChecksumTest.java b/src/test/java/org/traccar/helper/ChecksumTest.java
index ff48527bc..248f4dcae 100644
--- a/src/test/java/org/traccar/helper/ChecksumTest.java
+++ b/src/test/java/org/traccar/helper/ChecksumTest.java
@@ -42,8 +42,11 @@ public class ChecksumTest {
         assertEquals(0x00, Checksum.modulo256(ByteBuffer.wrap(new byte[] {0x00})));
         assertEquals(0x00, Checksum.modulo256(ByteBuffer.wrap(new byte[] {0x00, 0x00, 0x00})));
         assertEquals(0xca, Checksum.modulo256(ByteBuffer.wrap(new byte[] {0x77, 0x77, 0x77, 0x77, 0x77, 0x77})));
+    }
 
-
+    @Test
+    public void testNmea() {
+        assertEquals("*2A", Checksum.nmea("GSC,011412000010789,M4(Ro=500)"));
     }
 
 }
-- 
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