NMT: Changes in parity check

src/nmt/frame.c

@@ -879,6 +879,9 @@ static void encode_digits(const uint8_t *digits, char *bits)
 		bits[(i << 1) + 1] = x[i - 6] + '0';
 }
 
+/* debug parity check */
+//#define DEBUG_DECODE
+
 /* decode digits from 140 bits (not including sync) */
 // FIXME: do real convolutional decoding
 static int decode_digits(uint8_t *digits, const char *bits, int callack)
@@ -888,8 +891,12 @@ static int decode_digits(uint8_t *digits, const char *bits, int callack)
 
 	/* information bits */
 	for (i = 0; i < 6; i++) {
-		if (bits[(i << 1) + 1] != '0')
+		if (bits[(i << 1) + 1] != '0') {
+#ifdef DEBUG_DECODE
+			PDEBUG(DFRAME, DEBUG_DEBUG, "Frame bad at information bit #%d.\n", i);
+#endif
 			return -1;
+		}
 	}
 	for (i = 6; i < 70; i++)
 		x[i - 6] = bits[(i << 1) + 1] - '0';
@@ -908,13 +915,22 @@ static int decode_digits(uint8_t *digits, const char *bits, int callack)
 		digits[14] = 0;
 		digits[15] = 0;
 		short_frame = 1;
+#ifdef DEBUG_DECODE
+		PDEBUG(DFRAME, DEBUG_DEBUG, "Received shortened frame\n");
+#endif
 	}
 
 	/* parity check bits */
 	for (i = 0; i < 3; i++) {
-		if (bits[(i << 1)] != '1' - x[i])
+		if (bits[(i << 1)] != '1' - x[i]) {
+#ifdef DEBUG_DECODE
+			PDEBUG(DFRAME, DEBUG_DEBUG, "Frame bad at parity bit #%d.\n", i);
+#endif
 			return -1;
+		}
 	}
+#if 0
+#warning this check does not work, since we get error even at bit 50
 	for (i = 3; i < ((short_frame) ? 52 : 64); i++) {
 		if (bits[(i << 1)] != '1' - (x[i] ^ x[i - 3])) {
 			/* According to NMT Doc 450-3, bits after Y(114) shall
@@ -930,18 +946,43 @@ static int decode_digits(uint8_t *digits, const char *bits, int callack)
 				digits[13] = 0;
 				break;
 			}
+#ifdef DEBUG_DECODE
+			PDEBUG(DFRAME, DEBUG_DEBUG, "Frame bad at parity bit #%d.\n", i);
+#endif
+			return -1;
+		}
+	}
+#endif
+	/* Tests showed corrupt frame at parity #50 (i == 50)
+	 * We just ignore whatever we receive after 48 bits of checksum.
+	 * This is not the correct approach, but in case of a corrupt
+	 * frame 10.a, we would drop it, if it failes later checks.
+	 */
+	for (i = 3; i < ((short_frame) ? 48 : 64); i++) {
+		if (bits[(i << 1)] != '1' - (x[i] ^ x[i - 3])) {
+#ifdef DEBUG_DECODE
+			PDEBUG(DFRAME, DEBUG_DEBUG, "Frame bad at parity bit #%d.\n", i);
+#endif
 			return -1;
 		}
 	}
 	if (short_frame)
 		return 0;
 	for (i = 64; i < 67; i++) {
-		if (bits[(i << 1)] != '1' - x[i - 3])
+		if (bits[(i << 1)] != '1' - x[i - 3]) {
+#ifdef DEBUG_DECODE
+			PDEBUG(DFRAME, DEBUG_DEBUG, "Frame bad at parity bit #%d.\n", i);
+#endif
 			return -1;
+		}
 	}
 	for (i = 67; i < 70; i++) {
-		if (bits[(i << 1)] != '1')
+		if (bits[(i << 1)] != '1') {
+#ifdef DEBUG_DECODE
+			PDEBUG(DFRAME, DEBUG_DEBUG, "Frame bad at parity bit #%d.\n", i);
+#endif
 			return -1;
+		}
 	}
 
 	return 0;