Addition of checksum to OTAG packets

Inc/cobs.h

@@ -55,7 +55,7 @@ const uint8_t *COBSgetFrameExtent( const uint8_t *inputEnc, int len );
 bool COBSSimpleDecode( const uint8_t *inputEnc, int len, struct Frame *o );
 bool COBSisEOFRAME( const uint8_t *inputEnc );
 
-void COBSEncode( const uint8_t *frontMsg, int lfront, const uint8_t *inputMsg, int len, struct Frame *o );
+void COBSEncode( const uint8_t *frontMsg, int lfront, const uint8_t *backMsg, int lback, const uint8_t *inputMsg, int lmsg, struct Frame *o );
 
 /* Context free functions */
 void COBSPump( struct COBS *t, const uint8_t *incoming, int len,

Inc/otag.h

@@ -21,8 +21,11 @@ extern "C" {
 struct OTAGFrame
 {
     unsigned int len;                       /* Received length (after pre-processing) */
-    uint8_t tag;                            /* Tag (packet type) */
-    uint64_t tstamp;                        /* Timestamp for the packet */
+    uint8_t      tag;                       /* Tag (packet type) */
+    uint8_t      sum;                       /* Checksum byte */
+    bool         good;                      /* Is the checksum valid? */
+    uint64_t     tstamp;                    /* Timestamp for the packet */
+
     uint8_t *d;                             /* ...pointer to the data itself */
 };
 

Src/cobs.c

@@ -46,15 +46,16 @@ void COBSDelete( struct COBS *t )
 
 // ====================================================================================================
 
-void COBSEncode( const uint8_t *frontMsg, int lfront, const uint8_t *inputMsg, int len, struct Frame *o )
+void COBSEncode( const uint8_t *frontMsg, int lfront, const uint8_t *backMsg, int lback, const uint8_t *inputMsg, int lmsg, struct Frame *o )
 
 /* Encode frame and write into provided output Frame buffer */
 
 {
     uint8_t *wp = o->d;
     o->len = 0;
 
-    len += lfront;
+    /* Get overall message length */
+    int len = lfront + lmsg + lback;
 
     assert( len <= COBS_OVERALL_MAX_PACKET_LEN );
 
@@ -65,8 +66,8 @@ void COBSEncode( const uint8_t *frontMsg, int lfront, const uint8_t *inputMsg, i
 
         for ( int i = 0; len--; i++ )
         {
-            /* Take byte either from frontmatter or main message */
-            const uint8_t *rp = ( i < lfront ) ? &frontMsg[i] : &inputMsg[i - lfront];
+            /* Take byte from frontMatter, main message or backMatter depending on where we are in transmission */
+            const uint8_t *rp = ( i < lfront ) ? &frontMsg[i] : ( i < ( lfront + lmsg ) ) ? &inputMsg[i - lfront] : &backMsg[i - lfront - lmsg];
 
             if ( COBS_SYNC_CHAR != *rp )
             {

Src/otag.c

@@ -55,7 +55,15 @@ void OTAGEncode( const uint8_t channel, const uint64_t tstamp, const uint8_t *in
 
 {
     const uint8_t frontMatter[1] = { channel };
-    COBSEncode( frontMatter, 1, inputMsg, len, o );
+    uint8_t backMatter[1] = { channel };
+
+    /* Calculate packet sum for last byte */
+    for ( int i = 0; i < len; i++ )
+    {
+        backMatter[0] += inputMsg[i];
+    }
+
+    COBSEncode( frontMatter, 1, backMatter, 1, inputMsg, len, o );
 }
 
 // ====================================================================================================
@@ -73,11 +81,22 @@ static void _pumpcb( struct Frame *p, void *param )
     /* Callback function when a COBS packet is complete */
     struct OTAG *t = ( struct OTAG * )param;
 
-    t->f.len = p->len - 1; /* OTAG frames have the first element representing the tag */
-    t->f.tag = p->d[0];    /* First byte of an OTAG frame is the tag */
-    t->f.d = &p->d[1];     /* This is the rest of the data */
-    /* Timestamp was already set for this cluster */
+    t->f.len  = p->len - 2;     /* OTAG frames have the first element representing the tag and last element the checksum */
+    t->f.tag  = p->d[0];        /* First byte of an OTAG frame is the tag */
+    t->f.sum  = p->d[p->len - 1]; /* Last byte of an OTAG frame is the sum */
+    t->f.d    = &p->d[1];       /* This is the rest of the data */
+
+    /* Calculate received packet sum and insert good status into packet */
+    uint8_t sum  = t->f.tag;
+
+    for ( int i = 0; i < t->f.len; i++ )
+    {
+        sum += p->d[i];
+    }
 
+    t->f.good = ( sum == t->f.sum );
+
+    /* Timestamp was already set for this cluster */
     ( t->cb )( &t->f, t->param );
 }
 
@@ -99,17 +118,6 @@ void OTAGPump( struct OTAG *t, const uint8_t *incoming, int len,
 
 // ====================================================================================================
 
-bool OTAGSimpleDecode( const uint8_t *inputEnc, int len, struct Frame *o )
-
-/* Decode frame and write decoded frame into provided Frame buffer           */
-/* Returns FALSE if packet did not decode...and store the fragment.          */
-
-{
-    return COBSSimpleDecode( inputEnc, len, o );
-}
-
-// ====================================================================================================
-
 const uint8_t *OTAGgetFrameExtent( const uint8_t *inputEnc, int len )
 
 /* Look through memory until an end of frame marker is found, or memory is exhausted. */