From e6a3cfbe420c7cbd3b046d6929bf2773b5e054af Mon Sep 17 00:00:00 2001
From: Philip Heron <phil@sanslogic.co.uk>
Date: Mon, 4 Apr 2016 00:28:25 +0100
Subject: [PATCH] Bypass the FEC if the packet has a valid CRC

---
 ssdv.c | 68 +++++++++++++++++++++++++++++++++++++++++++++-------------
 ssdv.h |  5 +++--
 2 files changed, 56 insertions(+), 17 deletions(-)

diff --git a/ssdv.c b/ssdv.c
index 6c33d72..726f5d3 100644
--- a/ssdv.c
+++ b/ssdv.c
@@ -1300,19 +1300,50 @@ char ssdv_dec_is_packet(uint8_t *packet, int *errors)
 	memcpy(pkt, packet, SSDV_PKT_SIZE);
 	pkt[0] = 0x55;
 	
+	type = SSDV_TYPE_INVALID;
+	
 	if(pkt[1] == 0x66 + SSDV_TYPE_NOFEC)
 	{
-		type = SSDV_TYPE_NOFEC;
-		
+		/* Test for a valid NOFEC packet */
 		pkt_size_payload = SSDV_PKT_SIZE - SSDV_PKT_SIZE_HEADER - SSDV_PKT_SIZE_CRC;
 		pkt_size_crcdata = SSDV_PKT_SIZE_HEADER + pkt_size_payload - 1;
 		
+		/* No FEC scan */
 		if(errors) *errors = 0;
-	}
-	else
-	{
-		type = SSDV_TYPE_NORMAL;
 		
+		/* Test the checksum */
+		x = crc32(&pkt[1], pkt_size_crcdata);
+		
+		i = 1 + pkt_size_crcdata;
+		if(x == (pkt[i + 3] | (pkt[i + 2] << 8) | (pkt[i + 1] << 16) | (pkt[i] << 24)))
+		{
+			/* Valid, set the type and continue */
+			type = SSDV_TYPE_NOFEC;
+		}
+	}
+	else if(pkt[1] == 0x66 + SSDV_TYPE_NORMAL)
+	{
+		/* Test for a valid NORMAL packet */
+		pkt_size_payload = SSDV_PKT_SIZE - SSDV_PKT_SIZE_HEADER - SSDV_PKT_SIZE_CRC - SSDV_PKT_SIZE_RSCODES;
+		pkt_size_crcdata = SSDV_PKT_SIZE_HEADER + pkt_size_payload - 1;
+		
+		/* No FEC scan */
+		if(errors) *errors = 0;
+		
+		/* Test the checksum */
+		x = crc32(&pkt[1], pkt_size_crcdata);
+		
+		i = 1 + pkt_size_crcdata;
+		if(x == (pkt[i + 3] | (pkt[i + 2] << 8) | (pkt[i + 1] << 16) | (pkt[i] << 24)))
+		{
+			/* Valid, set the type and continue */
+			type = SSDV_TYPE_NORMAL;
+		}
+	}
+	
+	if(type == SSDV_TYPE_INVALID)
+	{
+		/* Test for a valid NORMAL packet with correctable errors */
 		pkt_size_payload = SSDV_PKT_SIZE - SSDV_PKT_SIZE_HEADER - SSDV_PKT_SIZE_CRC - SSDV_PKT_SIZE_RSCODES;
 		pkt_size_crcdata = SSDV_PKT_SIZE_HEADER + pkt_size_payload - 1;
 		
@@ -1322,6 +1353,22 @@ char ssdv_dec_is_packet(uint8_t *packet, int *errors)
 		
 		if(i < 0) return(-1); /* Reed-solomon decoder failed */
 		if(errors) *errors = i;
+		
+		/* Test the checksum */
+		x = crc32(&pkt[1], pkt_size_crcdata);
+		
+		i = 1 + pkt_size_crcdata;
+		if(x == (pkt[i + 3] | (pkt[i + 2] << 8) | (pkt[i + 1] << 16) | (pkt[i] << 24)))
+		{
+			/* Valid, set the type and continue */
+			type = SSDV_TYPE_NORMAL;
+		}
+	}
+	
+	if(type == SSDV_TYPE_INVALID)
+	{
+		/* All attempts to read the packet have failed */
+		return(-1);
 	}
 	
 	/* Sanity checks */
@@ -1335,15 +1382,6 @@ char ssdv_dec_is_packet(uint8_t *packet, int *errors)
 		if(p.mcu_offset >= pkt_size_payload) return(-1);
 	}
 	
-	/* Test the checksum */
-	x = crc32(&pkt[1], pkt_size_crcdata);
-	
-	i = 1 + pkt_size_crcdata;
-	if(pkt[i++] != ((x >> 24) & 0xFF)) return(-1);
-	if(pkt[i++] != ((x >> 16) & 0xFF)) return(-1);
-	if(pkt[i++] != ((x >> 8) & 0xFF)) return(-1);
-	if(pkt[i++] != (x & 0xFF)) return(-1);
-	
 	/* Appears to be a valid packet! Copy it back */
 	memcpy(packet, pkt, SSDV_PKT_SIZE);
 	
diff --git a/ssdv.h b/ssdv.h
index 8a1f4f8..70f42bf 100644
--- a/ssdv.h
+++ b/ssdv.h
@@ -42,8 +42,9 @@ extern "C" {
 
 #define SSDV_MAX_CALLSIGN (6) /* Maximum number of characters in a callsign */
 
-#define SSDV_TYPE_NORMAL (0)
-#define SSDV_TYPE_NOFEC  (1)
+#define SSDV_TYPE_INVALID (0xFF)
+#define SSDV_TYPE_NORMAL  (0x00)
+#define SSDV_TYPE_NOFEC   (0x01)
 
 typedef struct
 {