Decoding compressed latitude and longitude

src/AprsParser/Position.cs

@@ -1,6 +1,7 @@
 namespace AprsSharp.AprsParser
 {
     using System;
+    using System.Diagnostics;
     using System.Globalization;
     using System.Text;
     using System.Text.RegularExpressions;
@@ -167,23 +168,70 @@
                 throw new ArgumentNullException(nameof(coords));
             }
 
+            Ambiguity = 0;
+
+            // first try uncompressed
             Match match = Regex.Match(coords, RegexStrings.PositionLatLongWithSymbols);
-            match.AssertSuccess("Coordinates", nameof(coords));
 
-            Ambiguity = 0;
-            double latitude = DecodeLatitude(match.Groups[1].Value);
-            double longitude = DecodeLongitude(match.Groups[3].Value);
+            if (match.Success)
+            {
+                double latitude = DecodeLatitude(match.Groups[1].Value);
+                double longitude = DecodeLongitude(match.Groups[3].Value);
 
-            SymbolTableIdentifier = match.Groups[2].Value[0];
-            SymbolCode = match.Groups[4].Value[0];
-            Coordinates = new GeoCoordinate(latitude, longitude);
+                SymbolTableIdentifier = match.Groups[2].Value[0];
+                SymbolCode = match.Groups[4].Value[0];
+                Coordinates = new GeoCoordinate(latitude, longitude);
+                return;
+            }
+
+            // next try compressed
+            match = Regex.Match(coords, RegexStrings.CompressedPosition);
+
+            if (match.Success)
+            {
+                double latitude = DecodeCompressedLatitude(match.Groups[2].Value);
+                double longitude = DecodeCompressedLongitude(match.Groups[3].Value);
+                SymbolTableIdentifier = match.Groups[1].Value[0];
+                SymbolCode = match.Groups[4].Value[0];
+                Coordinates = new GeoCoordinate(latitude, longitude);
+                return;
+            }
+        }
+
+        private static int DecodeBase91(string encoded)
+        {
+            var bytes = Encoding.ASCII.GetBytes(encoded);
+
+            var result = 0;
+            for (var i = 0; i < bytes.Length; i++)
+            {
+                result += i == bytes.Length - 1
+                    ? bytes[i] - 33
+                    : (bytes[i] - 33) * (int)Math.Pow(91, bytes.Length - i - 1);
+            }
+
+            return result;
+        }
+
+        private static double DecodeCompressedLatitude(string coords)
+        {
+            Debug.Assert(coords.Length == 4, "Compressed latitude must be 4 characters");
+            var latitude = 90 - (DecodeBase91(coords) / 380926.0);
+            return Math.Round(latitude, 4);
+        }
+
+        private static double DecodeCompressedLongitude(string coords)
+        {
+            Debug.Assert(coords.Length == 4, "Compressed longitude must be 4 characters");
+            var longitude = -180 + (DecodeBase91(coords) / 190463.0);
+            return Math.Round(longitude, 4);
         }
 
         /// <summary>
         /// Decode from Maidenhead Location System gridsquare to a point in the middle of the gridsquare.
         /// </summary>
         /// <param name="gridsquare">4 or 6 char Maidenhead gridsquare, optionally followed by symbol table and identifier.</param>
         public void DecodeMaidenhead(string gridsquare)
         {
             if (gridsquare == null)
             {

src/AprsParser/RegexStrings.cs

@@ -34,6 +34,18 @@
         /// </summary>
         public const string PositionLatLongWithSymbols = @"([0-9 \.NS]{8})(.)([0-9 \.EW]{9})(.)";
 
+        /// <summary>
+        /// Matchdes a compressed latitude and longitude with optional additional data
+        /// Six matches:
+        ///     Symbol table ID
+        ///     Compressed Latitude
+        ///     Compressed Longitude
+        ///     Symbol code
+        ///     Compressed one of: course/speed, radio range, or altitude
+        ///     Compressed data type
+        /// </summary>
+        public const string CompressedPosition = @"(.)(.{4})(.{4})(.)(.{2})(.)";
+
         /// <summary>
         /// Same as <see cref="MaidenheadGridWithOptionalSymbols"/> but forces full line match.
         /// </summary>

test/AprsParserUnitTests/PositionUnitTests.cs

@@ -293,6 +293,7 @@ public void DecodeLongitudeWrongDecimalPointLocation()
         [InlineData(null, '\\', '.', 0, 0)] // defaults
         [InlineData("4903.50N/07201.75W-", '/', '-', 49.0583, -72.0292)] // from APRS spec
         [InlineData("4903.50S/07201.75E-", '/', '-', -49.0583, 72.0292)] // Ensure south and east work
+        [InlineData("/5L!!<*e7>7P[", '/', '>', 49.5, -72.7500)] // from the APRS spec
         public void Decode(
             string? encodedPosition,
             char expectedSymbolTable,