/*
sfrsd2.c
A soft-decision decoder for the JT65 (63,12) Reed-Solomon code.
This decoding scheme is built around Phil Karn's Berlekamp-Massey
errors and erasures decoder. The approach is inspired by a number of
publications, including the stochastic Chase decoder described
in "Stochastic Chase Decoding of Reed-Solomon Codes", by Leroux et al.,
IEEE Communications Letters, Vol. 14, No. 9, September 2010 and
"Soft-Decision Decoding of Reed-Solomon Codes Using Successive Error-
and-Erasure Decoding," by Soo-Woong Lee and B. V. K. Vijaya Kumar.
Steve Franke K9AN and Joe Taylor K1JT
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <string.h>
#include "rs2.h"
static void *rs;
void sfrsd2_(int mrsym[], int mrprob[], int mr2sym[], int mr2prob[],
int* ntrials0, int* verbose0, int correct[], int param[],
int indexes[], double tt[], int ntry[])
{
int rxdat[63], rxprob[63], rxdat2[63], rxprob2[63];
int workdat[63],workdat2[63];
int era_pos[51];
int c, i, j, numera, nmr2, nerr, nn=63, kk=12;
FILE *datfile, *logfile;
int ntrials = *ntrials0;
int verbose = *verbose0;
int nhard=0,nhard_min=32768,nsoft=0,nsoft_min=32768, ncandidates;
int ngmd,nera_best;
clock_t t0=0,t1=0;
int perr[8][8] = {
12, 31, 44, 52, 60, 57, 50, 50,
28, 38, 49, 58, 65, 69, 64, 80,
40, 41, 53, 62, 66, 73, 76, 81,
50, 53, 53, 64, 70, 76, 77, 81,
50, 50, 52, 60, 71, 72, 77, 84,
50, 50, 56, 62, 67, 73, 81, 85,
50, 50, 71, 62, 70, 77, 80, 85,
50, 50, 62, 64, 71, 75, 82, 87};
int pmr2[8][8] = {
4, 8, 9, 7, 6, 0, 0, 0,
13, 18, 15, 11, 9, 7, 5, 0,
0, 23, 21, 15, 12, 10, 7, 4,
0, 34, 28, 20, 16, 14, 11, 7,
0, 20, 26, 25, 19, 14, 12, 9,
0, 0, 28, 27, 22, 19, 14, 11,
0, 0, 40, 29, 29, 23, 18, 12,
0, 0, 40, 35, 31, 21, 20, 13};
if(verbose) {
logfile=fopen("sfrsd.log","a");
if( !logfile ) {
printf("Unable to open sfrsd.log\n");
exit(1);
}
}
// Initialize the KA9Q Reed-Solomon encoder/decoder
unsigned int symsize=6, gfpoly=0x43, fcr=3, prim=1, nroots=51;
rs=init_rs_int(symsize, gfpoly, fcr, prim, nroots, 0);
// Reverse the received symbol vector for BM decoder
for (i=0; i<63; i++) {
rxdat[i]=mrsym[62-i];
rxprob[i]=mrprob[62-i];
rxdat2[i]=mr2sym[62-i];
rxprob2[i]=mr2prob[62-i];
}
// Sort the mrsym probabilities to find the least reliable symbols
int k, pass, tmp, nsym=63;
int probs[63];
for (i=0; i<63; i++) {
indexes[i]=i;
probs[i]=rxprob[i];
}
for (pass = 1; pass <= nsym-1; pass++) {
for (k = 0; k < nsym - pass; k++) {
if( probs[k] < probs[k+1] ) {
tmp = probs[k];
probs[k] = probs[k+1];
probs[k+1] = tmp;
tmp = indexes[k];
indexes[k] = indexes[k+1];
indexes[k+1] = tmp;
}
}
}
// See if we can decode using BM HDD, and calculate the syndrome vector.
memset(era_pos,0,51*sizeof(int));
numera=0;
memcpy(workdat,rxdat,sizeof(rxdat));
nerr=decode_rs_int(rs,workdat,era_pos,numera,1);
if( nerr >= 0 ) {
if(verbose) fprintf(logfile," BM decode nerrors= %3d : ",nerr);
memcpy(correct,workdat,63*sizeof(int));
ngmd=-1;
param[0]=0;
param[1]=0;
param[2]=0;
param[3]=0;
param[4]=0;
return;
}
/*
Generate random erasure-locator vectors and see if any of them
decode. This will generate a list of potential codewords. The
"soft" distance between each codeword and the received word is
used to decide which codeword is "best".
*/
#ifdef WIN32
srand(0xdeadbeef);
#else
srandom(0xdeadbeef);
#endif
float ratio, ratio0[63];
int threshe, thresh2, nsum;
int thresh0[63],thresh1[63], mr2flag;
ncandidates=0;
nsum=0;
int ii,jj;
for (i=0; i<nn; i++) {
nsum=nsum+rxprob[i];
j = indexes[62-i];
ratio = (float)rxprob2[j]/(float)rxprob[j];
ratio0[i]=ratio;
ii = 7.999*ratio;
jj = (62-i)/8;
thresh0[i] = 1.3*perr[ii][jj];
thresh1[i] = 0.4*pmr2[ii][jj];
}
if(nsum==0) return;
for( k=0; k<ntrials; k++) {
memset(era_pos,0,51*sizeof(int));
memcpy(workdat,rxdat,sizeof(rxdat));
/*
Mark a subset of the symbols as erasures.
Run through the ranked symbols, starting with the worst, i=0.
NB: j is the symbol-vector index of the symbol with rank i.
*/
numera=0;
nmr2=0;
for (i=0; i<nn; i++) {
j = indexes[62-i];
threshe=thresh0[i];
thresh2=thresh1[i];
long int ir, ir2;
#ifdef WIN32
ir=rand();
ir2=rand();
#else
ir=random();
ir2=random();
#endif
if( ((ir % 100) < threshe ) && (numera+2*nmr2) < 51 ) {
era_pos[numera]=j;
numera=numera+1;
}
if( ((ir2 % 100) < thresh2) && (numera+2*nmr2)<51) {
workdat[j]=rxdat2[j];
nmr2=nmr2+1;
}
}
t0=clock();
// rs=init_rs_int(symsize, gfpoly, fcr, prim, nroots, 1);
nerr=decode_rs_int(rs,workdat,era_pos,numera,1);
t1=clock();
tt[0]+=(double)(t1-t0)/CLOCKS_PER_SEC;
if( nerr >= 0 ) {
ncandidates=ncandidates+1;
nhard=0;
nsoft=0;
for (i=0; i<63; i++) {
if(workdat[i] != rxdat[i]) {
nhard=nhard+1;
if(workdat[i] != rxdat2[i]) {
nsoft=nsoft+rxprob[i];
}
}
}
nsoft=63*nsoft/nsum;
if((nsoft < 33) && (nhard < 43) && (nhard+nsoft) < 74) { //???
if( (nsoft < nsoft_min) ) {
nsoft_min=nsoft;
nhard_min=nhard;
memcpy(correct,workdat,63*sizeof(int));
ngmd=0;
nera_best=numera;
ntry[0]=k;
}
}
if(nsoft_min < 27) break;
if((nsoft_min < 32) && (nhard_min < 43) &&
(nhard_min+nsoft_min) < 74) break;
}
if(k == ntrials-1) ntry[0]=k+1;
}
if(verbose) fprintf(logfile,
"%d trials and %d candidates after stochastic loop\n",k,ncandidates);
if( (ncandidates >= 0) && (nsoft_min < 36) && (nhard_min < 44) ) {
if(verbose) {
for (i=0; i<63; i++) {
fprintf(logfile,"%3d %3d %3d %3d %3d %3d\n",i,correct[i],
rxdat[i],rxprob[i],rxdat2[i],rxprob2[i]);
}
fprintf(logfile,"**** ncandidates %d nhard %d nsoft %d nsum %d\n",
ncandidates,nhard_min,nsoft_min,nsum);
}
} else {
nhard_min=-1;
}
if(verbose) {
fprintf(logfile,"exiting sfrsd\n");
fclose(logfile);
}
param[0]=ncandidates;
param[1]=nhard_min;
param[2]=nsoft_min;
param[3]=nera_best;
param[4]=ngmd;
if(param[0]==0) param[2]=-1;
return;
}