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annotate libutil/crc8gen.c @ 194:05d01e810217

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libutil: add TFO message gen function based on Osmocom crc8gen
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 17 Mar 2023 16:52:21 -0800
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1 /*! \file crc8gen.c
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2 * Osmocom generic CRC routines (for max 8 bits poly). */
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3 /*
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4 * Copyright (C) 2011 Sylvain Munaut <tnt@246tNt.com>
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5 *
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6 * All Rights Reserved
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7 *
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8 * SPDX-License-Identifier: GPL-2.0+
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9 *
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10 * This program is free software; you can redistribute it and/or modify
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11 * it under the terms of the GNU General Public License as published by
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12 * the Free Software Foundation; either version 2 of the License, or
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13 * (at your option) any later version.
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14 *
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15 * This program is distributed in the hope that it will be useful,
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16 * but WITHOUT ANY WARRANTY; without even the implied warranty of
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17 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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18 * GNU General Public License for more details.
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19 *
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20 * You should have received a copy of the GNU General Public License along
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21 * with this program; if not, write to the Free Software Foundation, Inc.,
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22 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
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23 */
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24
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25 /*! \addtogroup crc
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26 * @{
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27 * Osmocom generic CRC routines (for max 8 bits poly).
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28 *
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29 * \file crc8gen.c.tpl */
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30
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31 #include <stdint.h>
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32 #include "osmo_bits.h"
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33
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34 /*! Compute the CRC value of a given array of hard-bits
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35 * \param[in] code The CRC code description to apply
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36 * \param[in] in Array of hard bits
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37 * \param[in] len Length of the array of hard bits
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38 * \returns The CRC value
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39 */
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40 uint8_t
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41 osmo_crc8gen_compute_bits(const struct osmo_crc8gen_code *code,
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42 const ubit_t *in, int len)
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43 {
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44 const uint8_t poly = code->poly;
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45 uint8_t crc = code->init;
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46 int i, n = code->bits-1;
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47
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48 for (i=0; i<len; i++) {
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49 uint8_t bit = in[i] & 1;
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50 crc ^= (bit << n);
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51 if (crc & ((uint8_t)1 << n)) {
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52 crc <<= 1;
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53 crc ^= poly;
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54 } else {
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55 crc <<= 1;
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56 }
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57 crc &= ((uint8_t)1 << code->bits) - 1;
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58 }
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59
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60 crc ^= code->remainder;
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61
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62 return crc;
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63 }
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64
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65
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66 /*! Checks the CRC value of a given array of hard-bits
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67 * \param[in] code The CRC code description to apply
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68 * \param[in] in Array of hard bits
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69 * \param[in] len Length of the array of hard bits
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70 * \param[in] crc_bits Array of hard bits with the alleged CRC
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71 * \returns 0 if CRC matches. 1 in case of error.
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72 *
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73 * The crc_bits array must have a length of code->len
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74 */
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75 int
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76 osmo_crc8gen_check_bits(const struct osmo_crc8gen_code *code,
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77 const ubit_t *in, int len, const ubit_t *crc_bits)
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78 {
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79 uint8_t crc;
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80 int i;
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81
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82 crc = osmo_crc8gen_compute_bits(code, in, len);
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83
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84 for (i=0; i<code->bits; i++)
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85 if (crc_bits[i] ^ ((crc >> (code->bits-i-1)) & 1))
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86 return 1;
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87
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88 return 0;
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89 }
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90
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91
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92 /*! Computes and writes the CRC value of a given array of bits
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93 * \param[in] code The CRC code description to apply
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94 * \param[in] in Array of hard bits
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95 * \param[in] len Length of the array of hard bits
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96 * \param[in] crc_bits Array of hard bits to write the computed CRC to
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97 *
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98 * The crc_bits array must have a length of code->len
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99 */
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100 void
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101 osmo_crc8gen_set_bits(const struct osmo_crc8gen_code *code,
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102 const ubit_t *in, int len, ubit_t *crc_bits)
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103 {
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104 uint8_t crc;
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105 int i;
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106
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107 crc = osmo_crc8gen_compute_bits(code, in, len);
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108
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109 for (i=0; i<code->bits; i++)
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110 crc_bits[i] = ((crc >> (code->bits-i-1)) & 1);
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111 }
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112
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113 /*! @} */
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114
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115 /* vim: set syntax=c: */