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/* 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;
}
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