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Diffstat (limited to 'src/nmt/hagelbarger.c')
-rw-r--r-- | src/nmt/hagelbarger.c | 87 |
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); + } +} + |