169 lines
3.8 KiB
C++
169 lines
3.8 KiB
C++
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/* -*- c++ -*- */
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/*
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* gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
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*
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* Copyright (C) 2016, 2017, 2018
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* Libre Space Foundation <http://librespacefoundation.org/>
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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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#ifdef HAVE_CONFIG_H
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#include "config.h"
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#endif
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#include <gnuradio/io_signature.h>
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#include <satnogs/golay24.h>
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#include <satnogs/utils.h>
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namespace gr
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{
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namespace satnogs
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{
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golay24::golay24 ()
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{
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d_H = { 0x7ff, 0xee2, 0xdc5, 0xb8b, 0xf16, 0xe2d, 0xc5b, 0x8b7, 0x96e, 0xadc,
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0xdb8, 0xb71 };
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d_X = {0x800, 0x400, 0x200, 0x100, 0x080, 0x040, 0x020, 0x010, 0x008, 0x004,
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0x002, 0x001};
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}
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golay24::~golay24 ()
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{
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}
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/**
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* Encodes a 12-bit message
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* @param in the input 12-bit message. The message should be placed at the
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* 12 LS bits
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* @return the coded 24-bit message. The message is placed at the 24 LS bits
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*/
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uint32_t
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golay24::encode12 (uint16_t in)
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{
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uint32_t c[2] =
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{ 0x0, 0x0 };
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c[0] = in & 0xFFF;
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for (size_t i = 0; i < 12; i++) {
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uint32_t tmp = 0;
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for (size_t j = 0; j < 12; j++) {
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tmp ^= (((c[0] & d_H[i]) >> j) & 0x01);
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}
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c[1] = (c[1] << 1) ^ tmp;
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}
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return ((c[0] & 0xFFF) << 12) | (c[1] & 0xFFF);
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}
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static inline uint32_t
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ham_dist(uint32_t x)
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{
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return bit_count(x & 0xFFF);
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}
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/**
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* Decodes a single Golay (24, 12, 8) codeword
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*
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* @param out the 12-bit decoded message. The message is placed at the 12 LS bits
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* @param in the coded 24 bit code word. The message should be placed at the
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* 24 LS bits
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* @return true if the decoding was successful, false in case the error correction
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* could not be performed
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*/
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bool
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golay24::decode24(uint16_t *out, const uint32_t in)
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{
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uint32_t e[2] = {0x0, 0x0};
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uint32_t r[2] = {0x0, 0x0};
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uint32_t c[2] = {0x0, 0x0};
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uint32_t syndrome = 0x0;
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bool found = false;
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uint32_t col = 0;
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uint32_t q = 0;
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r[0] = (in >> 12) & 0xFFF;
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r[1] = in & 0xFFF;
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for(size_t j = 0; j < 12; j++) {
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uint32_t tmp = 0x0;
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for(size_t i = 0; i < 12; i++) {
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tmp ^= (((d_X[j] & r[0]) >> i) & 0x1);
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}
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for(size_t i = 0; i < 12; i++) {
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tmp ^= (((d_H[j] & r[1]) >> i) & 0x1);
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}
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syndrome = (syndrome << 1) ^ tmp;
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}
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if(ham_dist(syndrome) <= 3) {
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e[0] = syndrome;
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e[1] = 0x0;
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}
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else{
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do {
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if (ham_dist (syndrome ^ d_H[col]) <= 2) {
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e[0] = syndrome ^ d_H[col];
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e[1] = d_X[col];
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found = 1;
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}
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col++;
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}
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while ((col < 12) && !found);
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if ((col == 12) && !found) {
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for (size_t j = 0; j < 12; j++) {
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uint32_t tmp = 0x0;
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for (size_t i = 0; i < 12; i++) {
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tmp ^= (((d_H[j] & syndrome) >> i) & 0x1);
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}
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q = (q << 1) ^ tmp;
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}
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}
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if (ham_dist (q) <= 3) {
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e[0] = 0;
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e[1] = q;
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}
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else {
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col = 0;
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found = 0;
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do {
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if (ham_dist (q ^ d_H[col]) <= 2) {
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e[0] = d_X[col];
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e[1] = q ^ d_H[col];
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found = 1;
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}
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col++;
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}
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while ((col < 12) && !found);
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if (col == 12 && !found) {
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return false;
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}
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}
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}
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c[0] = r[0] ^ e[0];
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c[1] = r[1] ^ e[1];
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/* Return the message part only, not the parity */
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*out = c[0];
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return true;
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}
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} /* namespace satnogs */
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} /* namespace gr */
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