From 0cbd9657d20fb300d4fc2039ec137988eb40d21a Mon Sep 17 00:00:00 2001
From: Andreas Eversberg <jolly@eversberg.eu>
Date: Mon, 24 Jul 2017 19:18:01 +0200
Subject: Move Hagelbarger error correction code from NMT to common code

It will be used by Radiocom 2000 also.
Some minor fixes and improvements apply.
---
 src/common/Makefile.am   |   1 +
 src/common/hagelbarger.c | 101 +++++++++++++++++++++++++++++++++++++++++++++++
 src/common/hagelbarger.h |   4 ++
 3 files changed, 106 insertions(+)
 create mode 100644 src/common/hagelbarger.c
 create mode 100644 src/common/hagelbarger.h

(limited to 'src/common')

diff --git a/src/common/Makefile.am b/src/common/Makefile.am
index f719c6e..92447dc 100644
--- a/src/common/Makefile.am
+++ b/src/common/Makefile.am
@@ -25,6 +25,7 @@ libcommon_a_SOURCES = \
 	fft.c \
 	fm_modulation.c \
 	ffsk.c \
+	hagelbarger.c \
 	sender.c \
 	display_wave.c \
 	display_status.c \
diff --git a/src/common/hagelbarger.c b/src/common/hagelbarger.c
new file mode 100644
index 0000000..7d42bbc
--- /dev/null
+++ b/src/common/hagelbarger.c
@@ -0,0 +1,101 @@
+/* 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"
+
+/* enable to debug the process of parity check */
+//#define DEBUG_HAGEL
+
+/* 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 70.
+ */
+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;
+	}
+	/* shift last output byte all the way to MSB */
+	while ((i % 4))
+		output[i++ / 4] <<= 2;
+}
+
+/* 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 = 0xff, data, check, r_parity, s_parity;
+	int i, o;
+
+	length += 10;
+
+	for (i = 0, o = 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;
+#ifdef DEBUG_HAGEL
+		printf("#%d: r=%d s=%d\n", i - 10, r_parity, s_parity);
+#endif
+		/* 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[o / 8] = (output[o / 8] << 1) | ((reg_data >> 4) & 1);
+			o++;
+		}
+	}
+	/* shift last output byte all the way to MSB */
+	while ((o % 8))
+		output[o++ / 8] <<= 1;
+}
+
diff --git a/src/common/hagelbarger.h b/src/common/hagelbarger.h
new file mode 100644
index 0000000..9076afc
--- /dev/null
+++ b/src/common/hagelbarger.h
@@ -0,0 +1,4 @@
+
+void hagelbarger_encode(const uint8_t *input, uint8_t *output, int length);
+void hagelbarger_decode(const uint8_t *input, uint8_t *output, int length);
+
-- 
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