mirror of https://gerrit.osmocom.org/osmo-tetra
107 lines
2.4 KiB
C
107 lines
2.4 KiB
C
/* CRC16 (most likely the ITU variant) working on plain bits as a trial
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* to find out if simple padding to the right is going to be enough
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*/
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/* (C) 2011 by Holger Hans Peter Freyther
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* All Rights Reserved
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation; either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Affero General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*
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*/
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#include <lower_mac/crc_simple.h>
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#include <stdio.h>
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/**
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* X.25 rec 2.2.7.4 Frame Check Sequence. This should be
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* CRC ITU-T from the kernel or such.
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*/
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#define GEN_POLY 0x1021
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uint16_t get_nth_bit(const uint8_t *input, int _bit)
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{
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uint16_t val;
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int byte = _bit / 8;
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int bit = 7 - _bit % 8;
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val = (input[byte] & (1 << bit)) >> bit;
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return val;
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}
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/**
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* This is mostly from http://en.wikipedia.org/wiki/Computation_of_CRC
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* Code fragment 2. Due some stupidity it took longer to implement than
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* it should have taken.
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*/
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uint16_t crc16_itut_bytes(uint16_t crc, const uint8_t *input, int number_bits)
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{
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int i;
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for (i = 0; i < number_bits; ++i) {
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uint16_t bit = get_nth_bit(input, i);
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crc ^= bit << 15;
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if ((crc & 0x8000)) {
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crc <<= 1;
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crc ^= GEN_POLY;
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} else {
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crc <<= 1;
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}
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}
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return crc;
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}
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uint16_t crc16_itut_bits(uint16_t crc, const uint8_t *input, int number_bits)
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{
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int i;
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for (i = 0; i < number_bits; ++i) {
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uint16_t bit = input[i] & 0x1;
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crc ^= bit << 15;
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if ((crc & 0x8000)) {
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crc <<= 1;
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crc ^= GEN_POLY;
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} else {
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crc <<= 1;
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}
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}
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return crc;
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}
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uint16_t crc16_itut_poly(uint16_t crc, uint32_t poly, const uint8_t *input, int number_bits)
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{
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int i;
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for (i = 0; i < number_bits; ++i) {
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uint16_t bit = input[i] & 0x1;
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crc ^= bit << 15;
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if ((crc & 0x8000)) {
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crc <<= 1;
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crc ^= poly;
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} else {
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crc <<= 1;
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}
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}
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return crc;
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}
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uint16_t crc16_ccitt_bits(uint8_t *bits, unsigned int len)
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{
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return crc16_itut_bits(0xffff, bits, len);
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}
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