From 1c17757a4eb7bb1d656b7833db7f7ff3420c49f8 Mon Sep 17 00:00:00 2001
From: lwvmobile <lwvmobile@gmail.com>
Date: Sat, 19 Aug 2023 07:43:30 -0400
Subject: [PATCH] DMR 3/4 Simple Error Correction;

---
 src/dmr_34.c     | 37 ++++++++++++++++++++++++++++++++-----
 src/dmr_dburst.c |  6 +++---
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/src/dmr_34.c b/src/dmr_34.c
index ecef764..d68eb2e 100644
--- a/src/dmr_34.c
+++ b/src/dmr_34.c
@@ -60,6 +60,14 @@ uint32_t dmr_34(uint8_t * input, uint8_t treturn[18])
   for (i = 0; i < 49; i++) 
     point[i] = constellation_map[nibs[i]];
 
+  //debug view points -- point[0] should be zero
+  // fprintf (stderr, "\n P =");
+  // for (i = 0; i < 49; i++) 
+  //   fprintf (stderr, " %02d", point[i]);
+
+  //free-bee on err correction, point[0] should always be zero (flush bits)
+  point[0] = 0;
+
   //convert constellation points into tribit values using the FSM
   uint8_t state = 0;
   uint32_t tribits[49];
@@ -82,16 +90,31 @@ uint32_t dmr_34(uint8_t * input, uint8_t treturn[18])
     if (tribits[i] > 7)
     {
       irr_err++; //tally number of errors
-      state = 0; //just using this for now, ideally we want to try to correct errors
-    }
 
-    //TODO: Implement some form/attempt to correct errors here
+      //debug position of error and state value
+      // fprintf (stderr, " %d:%d;", i, state);
+
+      //Make a hard decision and flip point to fit in current state
+      point[i] ^= 7; //lucky number 7 (0111)
+
+      //NOTE: Ideally, a full path metric to find the surviving path
+      //is the best way to correct this, but it is also complex and no garauntee to fix errs
+
+      //decrement one and try again
+      if (i != 0) i--;
+     
+    }
 
   }
 
-  //if there are no errors, then convert tribits into a return payload
+  //debug view tribits/states
+  // fprintf (stderr, "\n T =");
+  // for (i = 0; i < 49; i++) 
+  //   fprintf (stderr, " %02d", tribits[i]);
+
+  //convert tribits into a return payload
   uint32_t temp = 0;
-  if (irr_err == 0)
+  if (1 == 1) //irr_err == 0
   {
     //break into chunks of 24 bit values and shuffle into 8-bit (byte) treturn values
     for (i = 0; i < 6; i++)
@@ -104,6 +127,10 @@ uint32_t dmr_34(uint8_t * input, uint8_t treturn[18])
     }
   }
 
+  //trellis point/state err tally
+  if (irr_err != 0)
+    fprintf (stderr, " ERR = %d", irr_err);
+
   return (irr_err);
 }
 
diff --git a/src/dmr_dburst.c b/src/dmr_dburst.c
index c5ad51c..b6524c1 100644
--- a/src/dmr_dburst.c
+++ b/src/dmr_dburst.c
@@ -436,8 +436,8 @@ void dmr_data_burst_handler(dsd_opts * opts, dsd_state * state, uint8_t info[196
     memset (TrellisReturn, 0, sizeof(TrellisReturn));
     IrrecoverableErrors = dmr_34(tdibits, TrellisReturn);
 
-    //debug -- its a lot easier to hear the number of errors, than to eyeball it
-    // if (IrrecoverableErrors) beeper (opts, state, slot);
+    //NOTE: IrrecoverableErrors in this context are a tally of errors from trellis
+    //they may have been successfully corrected, the CRC will reveal as much
 
     for (i = 0; i < pdu_len; i++)
       DMR_PDU[i] = TrellisReturn[i+pdu_start]; 
@@ -597,7 +597,7 @@ void dmr_data_burst_handler(dsd_opts * opts, dsd_state * state, uint8_t info[196
 
   // } 
 
-  if (IrrecoverableErrors != 0 && databurst != 0x09) //&& databurst != 0x05
+  if (IrrecoverableErrors != 0 && databurst != 0x08 && databurst != 0x09) //&& databurst != 0x05
   {
     fprintf (stderr, "%s", KRED);
     fprintf(stderr, " (FEC ERR)");