kopia lustrzana https://github.com/OpenRTX/OpenRTX
Implementation of Golay(24,12) decoder, reimplemented Golay(24,12) encoder using generator matrix, unit test for Golay(24,12) encoding and decoding routines
Silvano Seva
rodzic
a3b7b490d4
commit
8a3643ad3a
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@ -1,10 +1,10 @@
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/***************************************************************************
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* Copyright (C) 2021 by Federico Amedeo Izzo IU2NUO, *
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* Niccolò Izzo IU2KIN *
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* Wojciech Kaczmarski SP5WWP *
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* Frederik Saraci IU2NRO *
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* Silvano Seva IU2KWO *
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* *
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* Adapted from original code written by Rob Riggs, Mobilinkd LLC *
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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 General Public License as published by *
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@ -29,9 +29,115 @@
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#include <cstdint>
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namespace Golay24
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{
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/**
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* Compute the Golay(24,12) codeword of a given block of data using the M17
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* Golay polynomial.
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* Data encoding matrix for Golay(24,12) code with generator polynomial 0xC75;
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*/
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static const uint16_t encode_matrix[12] =
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{
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0x8eb, 0x93e, 0xa97, 0xdc6, 0x367, 0x6cd,
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0xd99, 0x3da, 0x7b4, 0xf68, 0x63b, 0xc75
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};
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/**
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* Data decoding matrix for Golay(24,12) code with generator polynomial 0xC75;
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*/
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static const uint16_t decode_matrix[12] =
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{
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0xc75, 0x49f, 0x93e, 0x6e3, 0xdc6, 0xf13,
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0xab9, 0x1ed, 0x3da, 0x7b4, 0xf68, 0xa4f
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};
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/**
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* Function computing the Golay(24,12) checksum of a given 12-bit data block.
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*
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* @param value: input data.
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* @return Golay(24,12) checksum.
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*/
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static uint16_t calcChecksum(const uint16_t& value)
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{
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uint16_t checksum = 0;
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for(uint8_t i = 0; i < 12; i++)
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{
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if(value & (1 << i))
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{
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checksum ^= encode_matrix[i];
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}
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}
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return checksum;
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}
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/**
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* Detect and correct errors in a Golay(24,12) codeword.
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*
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* @param codeword: input codeword.
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* @return bitmask corresponding to detected bit errors in the codeword, or
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* 0xFFFFFFFF if bit errors are unrecoverable.
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*/
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static uint32_t detectErrors(const uint32_t& codeword)
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{
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uint16_t data = codeword >> 12;
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uint16_t parity = codeword & 0xFFF;
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uint16_t syndrome = parity ^ calcChecksum(data);
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if(__builtin_popcount(syndrome) <= 3)
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{
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return syndrome;
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}
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for(uint8_t i = 0; i<12; i++)
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{
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uint16_t e = 1 << i;
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uint16_t coded_error = encode_matrix[i];
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if(__builtin_popcount(syndrome^coded_error) <= 2)
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{
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return (e << 12) | (syndrome ^ coded_error);
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}
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}
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uint16_t inv_syndrome = 0;
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for(uint8_t i = 0; i < 12; i++)
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{
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if(syndrome & (1 << i))
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{
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inv_syndrome ^= decode_matrix[i];
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}
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}
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if(__builtin_popcount(inv_syndrome) <= 3)
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{
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return inv_syndrome << 12;
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}
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for(uint8_t i = 0; i < 12; i++ )
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{
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uint16_t e = 1 << i;
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uint16_t coding_error = decode_matrix[i];
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if(__builtin_popcount(inv_syndrome ^ coding_error) <= 2 )
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{
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return ((inv_syndrome ^ coding_error) << 12) | e;
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}
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}
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return 0xFFFFFFFF;
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}
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} // namespace Golay24
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/**
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* Compute the Golay(24,12) codeword of a given block of data using the
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* generator polynomial 0xC75.
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* Result is composed as follows:
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*
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* +--------+----------+--------+
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@ -43,21 +149,25 @@
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* \param data: input data, upper four bits are discarded.
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* \return resulting 24 bit codeword.
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*/
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static uint32_t golay_encode24(const uint16_t& data)
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static inline uint32_t golay24_encode(const uint16_t& data)
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{
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// Compute [23,12] Golay codeword
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uint32_t codeword = data & 0x0FFFF;
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for(size_t i = 0; i < 12; i++)
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{
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if(codeword & 0x01) codeword ^= 0x0C75;
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codeword >>= 1;
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}
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return (data << 12) | Golay24::calcChecksum(data);
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}
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// Append data to codeword, result is checkbits(11) | data(12)
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codeword |= (data << 11);
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// Compute parity and append it to codeword
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return (codeword << 1) | (__builtin_popcount(codeword) & 0x01);
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/**
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* Decode a Golay(24,12) codeword, correcting eventual bit errors. In case the
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* bit errors are not correctable, the function returns 0xFFFF, a value outside
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* the range allowed for the 12-bit input data required by Golay coding.
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*
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* \param codeword: input Golay(24,12) codeword.
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* \return original data block or 0xFFFF in case of unrecoverable errors.
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*/
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static inline uint16_t golay24_decode(const uint32_t& codeword)
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{
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uint32_t errors = Golay24::detectErrors(codeword);
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if(errors == 0xFFFFFFFF) return 0xFFFF;
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return ((codeword ^ errors) >> 12) & 0x0FFF;
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}
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#endif /* M17_GOLAY_H */
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@ -111,7 +111,7 @@ lich_t M17LinkSetupFrame::generateLichSegment(const uint8_t segmentNum)
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lich_t result;
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for(size_t i = 0; i < blocks.size(); i++)
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{
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uint32_t encoded = golay_encode24(blocks[i]);
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uint32_t encoded = golay24_encode(blocks[i]);
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encoded = __builtin_bswap32(encoded << 8);
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memcpy(&result[3*i], &encoded, 3);
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}
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@ -1 +0,0 @@
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@ -0,0 +1,81 @@
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/***************************************************************************
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* Copyright (C) 2021 by Federico Amedeo Izzo IU2NUO, *
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* Niccolò Izzo IU2KIN *
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* Frederik Saraci IU2NRO *
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* Silvano Seva IU2KWO *
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* *
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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 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 General Public License for more details. *
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* *
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* You should have received a copy of the GNU 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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#include <cstdio>
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#include <cstdint>
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#include <random>
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#include "M17/M17Golay.h"
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using namespace std;
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default_random_engine rng;
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/**
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* Generate a mask with a random number of bit errors in random positions.
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*/
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uint32_t generateErrorMask()
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{
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uint32_t errorMask = 0;
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uniform_int_distribution< uint8_t > numErrs(0, 5);
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uniform_int_distribution< uint8_t > errPos(0, 23);
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for(uint8_t i = 0; i < numErrs(rng); i++)
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{
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errorMask |= 1 << errPos(rng);
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}
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return errorMask;
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}
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int main()
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{
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uniform_int_distribution< uint16_t > rndValue(0, 2047);
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for(uint32_t i = 0; i < 10000; i++)
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{
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uint16_t value = rndValue(rng);
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uint32_t cword = golay24_encode(value);
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uint32_t emask = generateErrorMask();
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// Check for correct encoding/decoding in absence of errors
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bool decoding_ok = (golay24_decode(cword) == value);
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// Check for correct encoding/decoding in presence of errors
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uint16_t decoded = golay24_decode(cword ^ emask);
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bool correcting_ok = false;
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// For four or more bit errors, decode should return 0xFFFF (uncorrectable error)
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if((__builtin_popcount(emask) >= 4) && (decoded == 0xFFFF))
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{
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correcting_ok = true;
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}
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// Less than four errors should be corrected.
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if(decoded == value) correcting_ok = true;
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printf("Value %04x, emask %08x errs %d -> d %s, c %s\n", value, emask,
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__builtin_popcount(emask),
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decoding_ok ? "OK" : "FAIL",
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correcting_ok ? "OK" : "FAIL");
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}
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return 0;
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}
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