From 9e83d23c48bcdba9ca041989f5cc199c79a6f158 Mon Sep 17 00:00:00 2001
From: Jonathan Naylor <naylorjs@yahoo.com>
Date: Thu, 14 Jul 2016 18:58:03 +0100
Subject: [PATCH] Simple minded rate 3/4 FEC decoding.

---
 DMRSlot.cpp    | 12 ++++-----
 DMRTrellis.cpp | 67 ++++++++++++++++++++++++++++++++++++++------------
 DMRTrellis.h   |  1 +
 3 files changed, 58 insertions(+), 22 deletions(-)

diff --git a/DMRSlot.cpp b/DMRSlot.cpp
index 3dc8ecd..12dcbd8 100644
--- a/DMRSlot.cpp
+++ b/DMRSlot.cpp
@@ -365,11 +365,11 @@ void CDMRSlot::writeModem(unsigned char *data)
 				bptc.decode(data + 2U, payload);
 				bptc.encode(payload, data + 2U);
 			} else if (dataType == DT_RATE_34_DATA) {
-				LogDebug("DMR Slot %u, received RF rate 3/4 data", m_slotNo);
 				CDMRTrellis trellis;
 				unsigned char payload[18U];
-				trellis.decode(data + 2U, payload);
-				// trellis.encode(payload, data + 2U);
+				bool ret = trellis.decode(data + 2U, payload);
+				if (ret)
+					trellis.encode(payload, data + 2U);
 			}
 
 			// Regenerate the Slot Type
@@ -1164,11 +1164,11 @@ void CDMRSlot::writeNetwork(const CDMRData& dmrData)
 			bptc.decode(data + 2U, payload);
 			bptc.encode(payload, data + 2U);
 		} else if (dataType == DT_RATE_34_DATA) {
-			LogDebug("DMR Slot %u, received network rate 3/4 data", m_slotNo);
 			CDMRTrellis trellis;
 			unsigned char payload[18U];
-			trellis.decode(data + 2U, payload);
-			// trellis.encode(payload, data + 2U);
+			bool ret = trellis.decode(data + 2U, payload);
+			if (ret)
+				trellis.encode(payload, data + 2U);
 		}
 
 		// Regenerate the Slot Type
diff --git a/DMRTrellis.cpp b/DMRTrellis.cpp
index f2bc0c1..60eb33d 100644
--- a/DMRTrellis.cpp
+++ b/DMRTrellis.cpp
@@ -59,29 +59,38 @@ bool CDMRTrellis::decode(const unsigned char* data, unsigned char* payload)
 	dibitsToPoints(dibits, points);
 
 	unsigned char tribits[49U];
-	unsigned char state = 0U;
 
-	for (unsigned int i = 0U; i < 49U; i++) {
-		tribits[i] = 9U;
+	// Check the original code
+	unsigned int failPos = checkCode(points, tribits);
+	if (failPos == 999U) {
+		tribitsToBits(tribits, payload);
+		return true;
+	}
 
-		for (unsigned int j = 0U; j < 8U; j++) {
-			if (points[i] == ENCODE_TABLE[state * 8U + j]) {
-				tribits[i] = j;
-				break;
+	for (unsigned j = 0U; j < 20U; j++) {
+		unsigned int bestPos = 0U;
+		unsigned int bestVal = 0U;
+
+		for (unsigned int i = 0U; i < 16U; i++) {
+			points[failPos] = i;
+
+			unsigned int pos = checkCode(points, tribits);
+			if (pos == 999U) {
+				tribitsToBits(tribits, payload);
+				return true;
+			}
+
+			if (pos > bestPos) {
+				bestPos = pos;
+				bestVal = i;
 			}
 		}
 
-		if (tribits[i] == 9U) {
-			LogDebug("Decoding failure at position %u", i);
-			return false;
-		}
-
-		state = tribits[i];
+		points[failPos] = bestVal;
+		failPos = bestPos;
 	}
 
-	tribitsToBits(tribits, payload);
-
-	return true;
+	return false;
 }
 
 void CDMRTrellis::encode(const unsigned char* payload, unsigned char* data)
@@ -320,3 +329,29 @@ void CDMRTrellis::tribitsToBits(const unsigned char* tribits, unsigned char* pay
 		WRITE_BIT(payload, n, b3);
 	}
 }
+
+unsigned int CDMRTrellis::checkCode(const unsigned char* points, unsigned char* tribits) const
+{
+	unsigned char state = 0U;
+
+	for (unsigned int i = 0U; i < 49U; i++) {
+		tribits[i] = 9U;
+
+		for (unsigned int j = 0U; j < 8U; j++) {
+			if (points[i] == ENCODE_TABLE[state * 8U + j]) {
+				tribits[i] = j;
+				break;
+			}
+		}
+
+		if (tribits[i] == 9U)
+			return i;
+
+		state = tribits[i];
+	}
+
+	if (tribits[48U] != 0U)
+		return 48U;
+
+	return 999U;
+}
diff --git a/DMRTrellis.h b/DMRTrellis.h
index a902fd9..a63043b 100644
--- a/DMRTrellis.h
+++ b/DMRTrellis.h
@@ -29,6 +29,7 @@ private:
 	void pointsToDibits(const unsigned char* points, signed char* dibits) const;
 	void bitsToTribits(const unsigned char* payload, unsigned char* tribits) const;
 	void tribitsToBits(const unsigned char* tribits, unsigned char* payload) const;
+	unsigned int checkCode(const unsigned char* points, unsigned char* tribits) const;
 };
 
 #endif