Added missing Gray coding

Author: consuelita

AudioCoder/src/main/java/org/operatorfoundation/audiocoder/mfsk/MFSKEncoder.kt

@@ -146,7 +146,7 @@ object MFSKEncoder
                 toneIndex = (toneIndex shl 1) or (if (bit) 1 else 0)
             }
 
-            symbols[symbolIndex] = toneIndex
+            symbols[symbolIndex] = toneIndex xor (toneIndex shr 1)
         }
 
         return symbols

AudioCoder/src/main/java/org/operatorfoundation/audiocoder/mfsk/MFSKStation.kt

@@ -189,8 +189,10 @@ class MFSKStation(
                         val energies        = DoubleArray(configuration.mode.toneCount) { i ->
                             GoertzelFilter.energy(samples, toneFrequencies[i], configuration.sampleRate)
                         }
+
                         // maxByOrNull is safe, toneCount is always >= 8 by MFSKMode's design.
-                        val winnerToneIndex = energies.indices.maxByOrNull { energies[it] }!!
+                        val rawToneIndex    = energies.indices.maxByOrNull { energies[it] }!!
+                        val winnerToneIndex = grayDecode(rawToneIndex)
 
                         // Extract bitsPerSymbol bits from the winner, MSB-first,
                         // and feed each into the Varicode decoder.
@@ -303,4 +305,17 @@ class MFSKStation(
 
         return buffer
     }
+
+    private fun grayDecode(gray: Int): Int
+    {
+        var n = gray
+        var mask = n shr 1
+
+        while (mask != 0) {
+            n = n xor mask
+            mask = mask shr 1
+        }
+
+        return n
+    }
 }