NMT: Implement Hagelbarger Code

This will correct burst errors of received messages. If the message
is too corrupted, it will be ignored, because some element may not
match then. The digits and line signals are checked for consistency,
since they are repeated serveral times in a message.

1 files changed, 87 insertions, 0 deletions

diff --git a/src/nmt/hagelbarger.c b/src/nmt/hagelbarger.c
new file mode 100644
index 0000000..f6ac2cd
--- /dev/null
+++ b/src/nmt/hagelbarger.c
@@ -0,0 +1,87 @@
+/* Hagelbarger (6,19) code
+ *
+ * A burst up to 6 encoded bits may be corrupt, to correct them.
+ * After corrupt bits, a minimum of 19 bits must be correct to correct
+ * another burst of corrupted bits.
+ *
+ * There is no parity check, so it is required to check all information
+ * elements of each message. Messages that contain signals or digits are
+ * protected by repeating the digits in the information element.
+ *
+ * (C) 2017 by Andreas Eversberg <jolly@eversberg.eu>
+ * All Rights Reserved
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "stdint.h"
+
+/* To encode NMT message: (MSB first)
+ * Use input with 9 bytes, the last byte must be 0x00.
+ * Use output with 18 bytes, ignore the last four (lower) bits of last byte.
+ * Use length of 72.
+ */
+void hagelbarger_encode(const uint8_t *input, uint8_t *output, int length)
+{
+	uint8_t reg = 0x00, data, check;
+	int i;
+
+	for (i = 0; i < length; i++) {
+		/* get data from input (MSB first) */
+		data = (input[i / 8] >> (7 - (i & 7))) & 1;
+		/* push data into shift register (LSB first) */
+		reg = (reg << 1) | data;
+		/* get data bit from register */
+		data = (reg >> 6) & 1;
+		/* calc check bit from register */
+		check = (reg + (reg >> 3) + 1) & 1;
+		/* put check bit and data bit to output (MSB first) */
+		output[i / 4] = (output[i / 4] << 2) | (check << 1) | data;
+	}
+}
+
+/* To decode NMT message: (MSB first)
+ * Use input with 19 bytes, the unused last 12 (lower) bits must be zero.
+ * Use output with 8 bytes.
+ * Use length of 64.
+ */
+void hagelbarger_decode(const uint8_t *input, uint8_t *output, int length)
+{
+	uint16_t reg_data = 0x00, reg_check = 0x00, data, check, r_parity, s_parity;
+	int i;
+
+	length += 10;
+
+	for (i = 0; i < length; i++) {
+		/* get check bit from input (MSB first) */
+		check = (input[i / 4] >> (7 - (i & 3) * 2)) & 1;
+		/* get data bit from input (MSB first) */
+		data = (input[i / 4] >> (6 - (i & 3) * 2)) & 1;
+		/* push check bit into shift register (LSB first) */
+		reg_check = (reg_check << 1) | check;
+		/* push data bit into shift register (LSB first) */
+		reg_data = (reg_data << 1) | data;
+		/* calculate parity */
+		r_parity = (reg_data + (reg_data >> 3) + (reg_check >> 6) + 1) & 1;
+		s_parity = ((reg_data >> 3) + (reg_data >> 6) + (reg_check >> 9) + 1) & 1;
+		/* flip message bit, if both parity checks fail */
+		/* use 4th bit that will be shifted to 5th bit next loop */
+		if (r_parity && s_parity)
+			reg_data ^= 0x0008;
+		/* put message bit to output (MSB first) */
+		if (i >= 10)
+			output[(i - 10) / 8] = (output[(i - 10) / 8] << 1) | ((reg_data >> 4) & 1);
+	}
+}
+