[FEC] Added FEC class
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jgromes
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2 changed files with 309 additions and 0 deletions
252
src/utils/FEC.cpp
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252
src/utils/FEC.cpp
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#include "FEC.h"
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#include <string.h>
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RadioLibBCH::RadioLibBCH() {
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}
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/*
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BCH Encoder based on https://www.codeproject.com/articles/13189/pocsag-encoder
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Significantly cleaned up and slightly fixed.
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*/
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void RadioLibBCH::begin(uint8_t n, uint8_t k, uint32_t poly) {
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this->n = n;
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this->k = k;
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this->poly = poly;
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this->alphaTo = new int32_t[n + 1];
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this->indexOf = new int32_t[n + 1];
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this->generator = new int32_t[n - k + 1];
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// find the maximum power of the polynomial
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for(this->m = 0; this->m < 31; this->m++) {
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if((poly >> this->m) == 1) {
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break;
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}
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}
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/*
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* generate GF(2**m) from the irreducible polynomial p(X) in p[0]..p[m]
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* lookup tables: index->polynomial form this->alphaTo[] contains j=alpha**i;
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* polynomial form -> index form this->indexOf[j=alpha**i] = i alpha=2 is the
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* primitive element of GF(2**m)
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*/
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int32_t mask = 1;
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this->alphaTo[this->m] = 0;
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for(uint8_t i = 0; i < this->m; i++) {
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this->alphaTo[i] = mask;
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this->indexOf[this->alphaTo[i]] = i;
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if(this->poly & ((uint32_t)0x01 << i)) {
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this->alphaTo[this->m] ^= mask;
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}
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mask <<= 1;
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}
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this->indexOf[this->alphaTo[this->m]] = this->m;
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mask >>= 1;
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for(uint8_t i = this->m + 1; i < this->n; i++) {
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if(this->alphaTo[i - 1] >= mask) {
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this->alphaTo[i] = this->alphaTo[this->m] ^ ((this->alphaTo[i - 1] ^ mask) << 1);
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} else {
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this->alphaTo[i] = this->alphaTo[i - 1] << 1;
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}
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this->indexOf[this->alphaTo[i]] = i;
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}
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this->indexOf[0] = -1;
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/*
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* Compute generator polynomial of BCH code of length = 31, redundancy = 10
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* (OK, this is not very efficient, but we only do it once, right? :)
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*/
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int32_t ii = 0;
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int32_t jj = 1;
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int32_t ll = 0;
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int32_t kaux = 0;
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bool test = false;
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int32_t aux = 0;
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int32_t cycle[15][6] = { { 0 } };
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int32_t size[15] = { 0 };
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// Generate cycle sets modulo 31
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cycle[0][0] = 0; size[0] = 1;
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cycle[1][0] = 1; size[1] = 1;
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do {
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// Generate the jj-th cycle set
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ii = 0;
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do {
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ii++;
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cycle[jj][ii] = (cycle[jj][ii - 1] * 2) % this->n;
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size[jj]++;
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aux = (cycle[jj][ii] * 2) % this->n;
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} while(aux != cycle[jj][0]);
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// Next cycle set representative
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ll = 0;
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do {
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ll++;
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test = false;
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for(ii = 1; ((ii <= jj) && !test); ii++) {
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// Examine previous cycle sets
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for(kaux = 0; ((kaux < size[ii]) && !test); kaux++) {
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test = (ll == cycle[ii][kaux]);
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}
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}
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} while(test && (ll < (this->n - 1)));
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if(!test) {
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jj++; // next cycle set index
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cycle[jj][0] = ll;
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size[jj] = 1;
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}
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} while(ll < (this->n - 1));
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// Search for roots 1, 2, ..., m-1 in cycle sets
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int32_t rdncy = 0;
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int32_t* min = new int32_t[this->n - this->k + 1];
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kaux = 0;
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for(ii = 1; ii <= jj; ii++) {
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min[kaux] = 0;
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for(jj = 0; jj < size[ii]; jj++) {
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for(uint8_t root = 1; root < this->m; root++) {
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if(root == cycle[ii][jj]) {
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min[kaux] = ii;
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}
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}
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}
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if(min[kaux]) {
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rdncy += size[min[kaux]];
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kaux++;
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}
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}
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int32_t noterms = kaux;
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int32_t* zeros = new int32_t[this->n - this->k + 1];
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kaux = 1;
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for(ii = 0; ii < noterms; ii++) {
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for(jj = 0; jj < size[min[ii]]; jj++) {
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zeros[kaux] = cycle[min[ii]][jj];
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kaux++;
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}
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}
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delete[] min;
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// Compute generator polynomial
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this->generator[0] = this->alphaTo[zeros[1]];
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this->generator[1] = 1; // g(x) = (X + zeros[1]) initially
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for(ii = 2; ii <= rdncy; ii++) {
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this->generator[ii] = 1;
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for(jj = ii - 1; jj > 0; jj--) {
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if(this->generator[jj] != 0) {
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this->generator[jj] = this->generator[jj - 1] ^ this->alphaTo[(this->indexOf[this->generator[jj]] + zeros[ii]) % this->n];
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} else {
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this->generator[jj] = this->generator[jj - 1];
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}
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}
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this->generator[0] = this->alphaTo[(this->indexOf[this->generator[0]] + zeros[ii]) % this->n];
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}
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delete[] zeros;
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}
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/*
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BCH Encoder based on https://www.codeproject.com/articles/13189/pocsag-encoder
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Significantly cleaned up and slightly fixed.
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*/
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uint32_t RadioLibBCH::encode(uint32_t dataword) {
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// we only use the "k" most significant bits
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int32_t* data = new int32_t[this->k];
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int32_t j1 = 0;
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for(int32_t i = this->n; i > (this->n - this->k); i--) {
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if(dataword & ((uint32_t)1<<i)) {
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data[j1++]=1;
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} else {
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data[j1++]=0;
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}
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}
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// reset the M(x)+r array elements
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int32_t* Mr = new int32_t[this->n];
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memset(Mr, 0x00, this->n*sizeof(int32_t));
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// copy the contents of data into Mr and add the zeros
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memcpy(Mr, data, this->k*sizeof(int32_t));
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int32_t j = 0;
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int32_t start = 0;
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int32_t end = this->n - this->k;
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while(end < this->n) {
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for(int32_t i = end; i > start-2; --i) {
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if(Mr[start]) {
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Mr[i] ^= this->generator[j];
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++j;
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} else {
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++start;
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j = 0;
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end = start + this->n - this->k;
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break;
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}
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}
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}
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int32_t* bb = new int32_t[this->n - this->k + 1];
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j = 0;
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for(int32_t i = start; i < end; ++i) {
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bb[j] = Mr[i];
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++j;
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}
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delete[] Mr;
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int32_t iEvenParity = 0;
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int32_t* recd = new int32_t[this->n + 1];
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for(uint8_t i = 0; i < this->k; i++) {
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recd[this->n - i] = data[i];
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if(data[i] == 1) {
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iEvenParity++;
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}
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}
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delete[] data;
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for(uint8_t i = 0; i < this->n - this->k + 1; i++) {
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recd[this->n - this->k - i] = bb[i];
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if(bb[i] == 1) {
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iEvenParity++;
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}
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}
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delete[] bb;
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if((iEvenParity % 2) == 0) {
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recd[0] = 0;
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} else {
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recd[0] = 1;
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}
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int32_t res = 0;
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for(int32_t i = 0; i < this->n + 1; i++) {
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if(recd[i]) {
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res |= ((uint32_t)1<<i);
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}
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}
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return(res);
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}
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RadioLibBCH RadioLibBCHInstance;
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57
src/utils/FEC.h
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57
src/utils/FEC.h
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@ -0,0 +1,57 @@
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#if !defined(_RADIOLIB_FEC_H)
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#define _RADIOLIB_FEC_H
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#include "../TypeDef.h"
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#include "../Module.h"
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#if defined(RADIOLIB_BUILD_ARDUINO)
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#include "../ArduinoHal.h"
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#endif
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// BCH(31, 21) code constants
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#define RADIOLIB_PAGER_BCH_N (31)
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#define RADIOLIB_PAGER_BCH_K (21)
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#define RADIOLIB_PAGER_BCH_PRIMITIVE_POLY (0x25)
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/*!
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\class RadioLibBCH
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\brief Class to calculate Bose–Chaudhuri–Hocquenghem (BCH) class of forward error correction codes.
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In theory, this should be able to calculate an arbitrary BCH(N, K) code,
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but so far it was only tested for BCH(31, 21).
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*/
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class RadioLibBCH {
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public:
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/*!
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\brief Default constructor.
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*/
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RadioLibBCH();
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/*!
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\brief Initialization method.
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\param n Code word length in bits, up to 32.
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\param k Data portion length in bits, up to "n".
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\param poly Powers of the irreducible polynomial.
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*/
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void begin(uint8_t n, uint8_t k, uint32_t poly);
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/*!
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\brief Encoding method - encodes one data word (without check bits) into a code word (with check bits).
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\param dataword Data word without check bits. The caller is responsible to make sure the data is
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on the correct bit positions!
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\returns Code word with error check bits.
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*/
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uint32_t encode(uint32_t dataword);
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private:
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uint8_t n;
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uint8_t k;
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uint32_t poly;
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uint8_t m;
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int32_t* alphaTo;
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int32_t* indexOf;
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int32_t* generator;
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};
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// the global singleton
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extern RadioLibBCH RadioLibBCHInstance;
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#endif
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