From fbfe89c70467930bf3f8630de1f16aa8be8e0e9b Mon Sep 17 00:00:00 2001
From: Morten Nielsen <mn@iter.dk>
Date: Sat, 1 Aug 2020 11:03:24 -0700
Subject: [PATCH] Allow skipping checksums and better handle missing checksum
 after `*`

As discussed in #74
---
 src/NmeaParser/Nmea/NmeaMessage.cs            | 15 ++++++----
 .../NmeaParser.Tests/NmeaMessages.cs          | 28 ++++++++++++++++++-
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/src/NmeaParser/Nmea/NmeaMessage.cs b/src/NmeaParser/Nmea/NmeaMessage.cs
index 4f939b9..14db334 100644
--- a/src/NmeaParser/Nmea/NmeaMessage.cs
+++ b/src/NmeaParser/Nmea/NmeaMessage.cs
@@ -138,28 +138,33 @@ namespace NmeaParser.Messages
         /// </summary>
         /// <param name="message">The NMEA message string.</param>
         /// <param name="previousSentence">The previously received message (only used if parsing multi-sentence messages)</param>
+        /// <param name="ignoreChecksum">If <c>true</c> ignores the checksum completely, if <c>false</c> validates the checksum if present.</param>
         /// <returns>The nmea message that was parsed.</returns>
         /// <exception cref="System.ArgumentException">
         /// Invalid nmea message: Missing starting character '$'
         /// or checksum failure
         /// </exception>
-        public static NmeaMessage Parse(string message, IMultiSentenceMessage? previousSentence = null)
+        public static NmeaMessage Parse(string message, IMultiSentenceMessage? previousSentence = null, bool ignoreChecksum = false)
         {
             if (string.IsNullOrEmpty(message))
                 throw new ArgumentNullException(nameof(message));
 
             int checksum = -1;
-            if (message[0] != '$')
-                throw new ArgumentException("Invalid NMEA message: Missing starting character '$'");
             if (message[0] != '$')
                 throw new ArgumentException("Invalid NMEA message: Missing starting character '$'");
             var idx = message.IndexOf('*');
             if (idx >= 0)
             {
-                checksum = Convert.ToInt32(message.Substring(idx + 1), 16);
+                if (message.Length > idx + 1)
+                {
+                    if (int.TryParse(message.Substring(idx + 1), NumberStyles.HexNumber, CultureInfo.InvariantCulture, out int c))
+                        checksum = c;
+                    else
+                        throw new ArgumentException("Invalid checksum string");
+                }
                 message = message.Substring(0, message.IndexOf('*'));
             }
-            if (checksum > -1)
+            if (!ignoreChecksum && checksum > -1)
             {
                 int checksumTest = 0;
                 for (int i = 1; i < message.Length; i++)
diff --git a/src/UnitTests/NmeaParser.Tests/NmeaMessages.cs b/src/UnitTests/NmeaParser.Tests/NmeaMessages.cs
index b7dd7e7..7119721 100644
--- a/src/UnitTests/NmeaParser.Tests/NmeaMessages.cs
+++ b/src/UnitTests/NmeaParser.Tests/NmeaMessages.cs
@@ -97,7 +97,33 @@ namespace NmeaParser.Tests
                 }
             }
         }
-          
+
+        [TestMethod]
+        public void MissingChecksumAfterStar()
+        {
+            string input = "$GPRMA,A,4917.24,S,12309.57,W,1000.0,2000.0,123.4,321.0,10,E,A*";
+            var msg = NmeaMessage.Parse(input);
+            Assert.IsNotNull(msg);
+        }
+
+        [TestMethod]
+        public void MissingChecksum()
+        {
+            string input = "$GPRMA,A,4917.24,S,12309.57,W,1000.0,2000.0,123.4,321.0,10,E,A";
+            var msg = NmeaMessage.Parse(input);
+            Assert.IsNotNull(msg);
+        }
+
+        [TestMethod]
+        public void IgnoreChecksum()
+        {
+            string input = "$GPRMA,A,4917.24,S,12309.57,W,1000.0,2000.0,123.4,321.0,10,E,A*00";
+            var msg = NmeaMessage.Parse(input, ignoreChecksum: true);
+            Assert.IsNotNull(msg);
+
+            Assert.ThrowsException<ArgumentException>(() => NmeaMessage.Parse(input, ignoreChecksum: false));
+        }
+
         [TestMethod]
         public void TestGprma()
         {