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530 lines
16 KiB
530 lines
16 KiB
#include <string.h>
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#include "codecAO40.h"
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/* ---------------------- */
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/* AO40 Encoder - Decoder */
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/* ---------------------- */
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/* Scramble and RS encode 256 byte blocks of data into 5200 bits
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* or Descramble and RS decode 5200 bits into the 256 bytes of data
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*
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* --------------------------------------------------------------------------
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* This decoder has evolved extensively through the work of Phil Karn. It draws
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* on his own ideas and optimisations, and on the work of others. The lineage
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* is as below, and parts of the authors' notices are included here. (JRM)
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* AO40 encoder / decoder
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* Copyright 2002 Phil Karn, KA9Q
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* May be used under the terms of the GNU General Public License (GPL)
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*
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* Reed-Solomon coding and decoding
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* Phil Karn (karn@ka9q.ampr.org) September 1996
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*
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* This file is derived from the program "new_rs_erasures.c" by Robert
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* Morelos-Zaragoza (robert@spectra.eng.hawaii.edu) and Hari Thirumoorthy
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* (harit@spectra.eng.hawaii.edu), Aug 1995
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* --------------------------------------------------------------------------
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*
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* From the RM-Z & HT program:
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* The encoding and decoding methods are based on the
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* book "Error Control Coding: Fundamentals and Applications",
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* by Lin and Costello, Prentice Hall, 1983, ISBN 0-13-283796-X
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* Portions of this program are from a Reed-Solomon encoder/decoder
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* in C, written by Simon Rockliff (simon@augean.ua.oz.au) on 21/9/89.
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* --------------------------------------------------------------------------
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*
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* From the 1989/1991 SR program (also based on Lin and Costello):
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* This program may be freely modified and/or given to whoever wants it.
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* A condition of such distribution is that the author's contribution be
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* acknowledged by his name being left in the comments heading the program,
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* Simon Rockliff, 26th June 1991
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*
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*/
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/* Defines for RS Decoder(s) */
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#ifndef min
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#define min(a,b) ((a) < (b) ? (a) : (b))
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#endif
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/*
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CCodecAO40::CCodecAO40(void)
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{
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}
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CCodecAO40::~CCodecAO40(void)
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{
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}
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*/
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//int CCodecAO40::mod255(int x) {
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int mod255(int x) {
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while (x >= 255) {
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x -= 255;
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x = (x >> 8) + (x & 255);
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}
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return x;
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}
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//int CCodecAO40::decode_rs_8(char *data, int *eras_pos, int no_eras){
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int decode_rs_8(char *data, int *eras_pos, int no_eras){
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int deg_lambda, el, deg_omega;
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int i, j, r,k;
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unsigned char u,q,tmp,num1,num2,den,discr_r;
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unsigned char lambda[NROOTS+1], s[NROOTS]; /* Err+Eras Locator poly and syndrome poly */
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unsigned char b[NROOTS+1], t[NROOTS+1], omega[NROOTS+1];
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unsigned char root[NROOTS], reg[NROOTS+1], loc[NROOTS];
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int syn_error, count;
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/* form the syndromes; i.e., evaluate data(x) at roots of g(x) */
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for(i=0;i<NROOTS;i++)
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s[i] = data[0];
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for(j=1;j<NN;j++){
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for(i=0;i<NROOTS;i++){
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if(s[i] == 0){
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s[i] = data[j];
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} else {
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s[i] = data[j] ^ ALPHA_TO[mod255(INDEX_OF[s[i]] + (FCR+i)*PRIM)];
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}
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}
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}
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/* Convert syndromes to index form, checking for nonzero condition */
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syn_error = 0;
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for(i=0;i<NROOTS;i++){
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syn_error |= s[i];
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s[i] = INDEX_OF[s[i]];
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}
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if (!syn_error) {
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/* if syndrome is zero, data[] is a codeword and there are no
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* errors to correct. So return data[] unmodified
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*/
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count = 0;
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goto finish;
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}
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memset(&lambda[1],0,NROOTS*sizeof(lambda[0]));
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lambda[0] = 1;
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if (no_eras > 0) {
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/* Init lambda to be the erasure locator polynomial */
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lambda[1] = ALPHA_TO[mod255(PRIM*(NN-1-eras_pos[0]))];
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for (i = 1; i < no_eras; i++) {
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u = mod255(PRIM*(NN-1-eras_pos[i]));
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for (j = i+1; j > 0; j--) {
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tmp = INDEX_OF[lambda[j - 1]];
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if(tmp != A0)
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lambda[j] ^= ALPHA_TO[mod255(u + tmp)];
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}
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}
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}
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for(i=0;i<NROOTS+1;i++)
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b[i] = INDEX_OF[lambda[i]];
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/*
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* Begin Berlekamp-Massey algorithm to determine error+erasure
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* locator polynomial
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*/
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r = no_eras;
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el = no_eras;
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while (++r <= NROOTS) { /* r is the step number */
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/* Compute discrepancy at the r-th step in poly-form */
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discr_r = 0;
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for (i = 0; i < r; i++){
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if ((lambda[i] != 0) && (s[r-i-1] != A0)) {
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discr_r ^= ALPHA_TO[mod255(INDEX_OF[lambda[i]] + s[r-i-1])];
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}
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}
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discr_r = INDEX_OF[discr_r]; /* Index form */
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if (discr_r == A0) {
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/* 2 lines below: B(x) <-- x*B(x) */
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memmove(&b[1],b,NROOTS*sizeof(b[0]));
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b[0] = A0;
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} else {
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/* 7 lines below: T(x) <-- lambda(x) - discr_r*x*b(x) */
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t[0] = lambda[0];
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for (i = 0 ; i < NROOTS; i++) {
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if(b[i] != A0)
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t[i+1] = lambda[i+1] ^ ALPHA_TO[mod255(discr_r + b[i])];
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else
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t[i+1] = lambda[i+1];
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}
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if (2 * el <= r + no_eras - 1) {
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el = r + no_eras - el;
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/*
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* 2 lines below: B(x) <-- inv(discr_r) *
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* lambda(x)
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*/
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for (i = 0; i <= NROOTS; i++)
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b[i] = (lambda[i] == 0) ? A0 : mod255(INDEX_OF[lambda[i]] - discr_r + NN);
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} else {
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/* 2 lines below: B(x) <-- x*B(x) */
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memmove(&b[1],b,NROOTS*sizeof(b[0]));
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b[0] = A0;
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}
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memcpy(lambda,t,(NROOTS+1)*sizeof(t[0]));
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}
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}
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/* Convert lambda to index form and compute deg(lambda(x)) */
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deg_lambda = 0;
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for(i=0;i<NROOTS+1;i++){
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lambda[i] = INDEX_OF[lambda[i]];
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if(lambda[i] != A0)
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deg_lambda = i;
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}
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/* Find roots of the error+erasure locator polynomial by Chien search */
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memcpy(®[1],&lambda[1],NROOTS*sizeof(reg[0]));
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count = 0; /* Number of roots of lambda(x) */
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for (i = 1,k=IPRIM-1; i <= NN; i++,k = mod255(k+IPRIM)) {
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q = 1; /* lambda[0] is always 0 */
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for (j = deg_lambda; j > 0; j--){
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if (reg[j] != A0) {
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reg[j] = mod255(reg[j] + j);
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q ^= ALPHA_TO[reg[j]];
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}
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}
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if (q != 0)
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continue; /* Not a root */
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/* store root (index-form) and error location number */
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root[count] = i;
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loc[count] = k;
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/* If we've already found max possible roots,
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* abort the search to save time
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*/
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if(++count == deg_lambda)
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break;
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}
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if (deg_lambda != count) {
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/*
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* deg(lambda) unequal to number of roots => uncorrectable
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* error detected
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*/
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count = -1;
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goto finish;
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}
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/*
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* Compute err+eras evaluator poly omega(x) = s(x)*lambda(x) (modulo
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* x**NROOTS). in index form. Also find deg(omega).
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*/
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deg_omega = 0;
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for (i = 0; i < NROOTS;i++){
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tmp = 0;
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j = (deg_lambda < i) ? deg_lambda : i;
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for(;j >= 0; j--){
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if ((s[i - j] != A0) && (lambda[j] != A0))
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tmp ^= ALPHA_TO[mod255(s[i - j] + lambda[j])];
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}
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if(tmp != 0)
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deg_omega = i;
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omega[i] = INDEX_OF[tmp];
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}
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omega[NROOTS] = A0;
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/*
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* Compute error values in poly-form. num1 = omega(inv(X(l))), num2 =
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* inv(X(l))**(FCR-1) and den = lambda_pr(inv(X(l))) all in poly-form
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*/
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for (j = count-1; j >=0; j--) {
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num1 = 0;
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for (i = deg_omega; i >= 0; i--) {
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if (omega[i] != A0)
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num1 ^= ALPHA_TO[mod255(omega[i] + i * root[j])];
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}
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num2 = ALPHA_TO[mod255(root[j] * (FCR - 1) + NN)];
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den = 0;
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/* lambda[i+1] for i even is the formal derivative lambda_pr of lambda[i] */
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for (i = min(deg_lambda,NROOTS-1) & ~1; i >= 0; i -=2) {
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if(lambda[i+1] != A0)
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den ^= ALPHA_TO[mod255(lambda[i+1] + i * root[j])];
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}
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if (den == 0) {
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count = -1;
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goto finish;
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}
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/* Apply error to data */
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if (num1 != 0) {
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data[loc[j]] ^= ALPHA_TO[mod255(INDEX_OF[num1] + INDEX_OF[num2] + NN - INDEX_OF[den])];
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}
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}
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finish:
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if(eras_pos != NULL){
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for(i=0;i<count;i++)
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eras_pos[i] = loc[i];
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}
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return count;
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}
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/* ---------- */
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/* Re-encoder */
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/* ---------- */
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/* Reference encoder for proposed coded AO-40 telemetry format - v1.0 7 Jan 2002
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* Copyright 2002, Phil Karn, KA9Q
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* This software may be used under the terms of the GNU Public License (GPL)
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*/
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/* Adapted from the above enc_ref.c as used by the spacecraft (JRM) */
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/* Write one binary channel symbol into the interleaver frame and update the pointers */
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//void CCodecAO40::interleave_symbol(int c)
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void interleave_symbol(int c)
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{
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int row,col;
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col=m_ileaver_index/COLUMNS;
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row=m_ileaver_index%COLUMNS;
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if(c)
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{
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m_encoded[row*ROWS+col] = 1;
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}
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m_ileaver_index++;
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}
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/* Convolutionally encode and interleave one byte */
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//void CCodecAO40::encode_and_interleave(unsigned char c,int cnt){
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void encode_and_interleave(unsigned char c,int cnt){
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while(cnt-- != 0)
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{
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m_conv_sr = (m_conv_sr << 1) | (c >> 7);
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c <<= 1;
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interleave_symbol( Partab[m_conv_sr & CPOLYA]);
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interleave_symbol(!Partab[m_conv_sr & CPOLYB]); /* Second encoder symbol is inverted */
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}
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}
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/* Scramble a byte, convolutionally encode and interleave into frame */
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//void CCodecAO40::scramble_and_encode(unsigned char c){
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void scramble_and_encode(unsigned char c){
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c ^= Scrambler[m_encoded_bytes]; /* Scramble byte */
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encode_and_interleave(c,8); /* RS encode and place into reencode buffer */
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}
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/* Encodes the 256 byte source block RSdecdata[] into 5200 byte block of symbols
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* results stored in m_encoded.
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* On success encoded buffer is returned, an zeroed buffer on failure
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*/
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//const unsigned char *CCodecAO40::encode(unsigned char *source_bytes, int byte_count)
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const unsigned char *encode(unsigned char *source_bytes, int byte_count)
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{
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if(BLOCKSIZE != byte_count || NULL == source_bytes )
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{
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memset(m_encoded, 0, BLOCKSIZE);
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return m_encoded;
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}
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init_encoder();
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for(int i=0;i<BLOCKSIZE;i++)
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{
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encode_byte(source_bytes[i]) ;
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}
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for(int i=0;i<64;i++)
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{
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encode_parity();
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}
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return m_encoded;
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}
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/* The original three C user's entry points now follow. They are:
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init_encoder() Called once before using system.
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encode_byte(unsigned char c) Called with each user byte (i.e. 256 calls)
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encode_parity() Called 64 times to finish off
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*/
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/* This function initializes the encoder. */
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//void CCodecAO40::init_encoder(void){
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void init_encoder(void){
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int i,j,sr;
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m_encoded_bytes = 0;
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m_conv_sr = 0;
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m_ileaver_index = COLUMNS; /* Sync vector is in first column; data starts here */
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for(j=0;j<RSBLOCKS;j++) /* Flush parity array */
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{
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for(i=0;i<NROOTS;i++)
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{
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m_RS_block[j][i] = 0;
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}
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}
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/* Clear re-encoded array */
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for(i=0;i<SYMPBLOCK;i++)
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{
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m_encoded[i] = 0;
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}
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/* Generate sync vector, interleave into re-encode array, 1st column */
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sr = 0x7f;
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for(i=0;i<65;i++)
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{
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if(sr & 64)
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{
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m_encoded[ROWS*i] = 1; /* Every 80th symbol is a sync bit */
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}
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sr = (sr << 1) | Partab[sr & SYNC_POLY];
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}
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}
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/* This function is called with each user data byte to be encoded into the
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* current frame. It should be called in sequence 256 times per frame, followed
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* by 64 calls to encode_parity().
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*/
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//void CCodecAO40::encode_byte(unsigned char c){
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void encode_byte(unsigned char c){
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unsigned char *rp; /* RS block pointer */
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int i;
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unsigned char feedback;
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/* Update the appropriate Reed-Solomon codeword */
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rp = m_RS_block[m_encoded_bytes & 1];
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/* Compute feedback term */
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feedback = INDEX_OF[c ^ rp[0]];
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/* If feedback is non-zero, multiply by each generator polynomial coefficient and
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* add to corresponding shift register elements
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*/
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if(feedback != A0){
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int j;
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/* This loop exploits the palindromic nature of the generator polynomial
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* to halve the number of discrete multiplications
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*/
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for(j=0;j<15;j++){
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unsigned char t;
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t = ALPHA_TO[mod255(feedback + RS_poly[j])];
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rp[j+1] ^= t; rp[31-j] ^= t;
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}
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rp[16] ^= ALPHA_TO[mod255(feedback + RS_poly[15])];
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}
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/* Shift 32 byte RS register one position down */
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for(i=0;i<31;i++)
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rp[i] = rp[i+1];
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/* Handle highest order coefficient, which is unity */
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if(feedback != A0){
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rp[31] = ALPHA_TO[feedback];
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} else {
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rp[31] = 0;
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}
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scramble_and_encode(c);
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m_encoded_bytes++;
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}
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/* This function should be called 64 times after the 256 data bytes
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* have been passed to update_encoder. Each call scrambles, encodes and
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* interleaves one byte of Reed-Solomon parity.
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*/
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//void CCodecAO40::encode_parity(void) {
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void encode_parity(void) {
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unsigned char c;
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c = m_RS_block[m_encoded_bytes & 1][(m_encoded_bytes-256)>>1];
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scramble_and_encode(c);
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if(++m_encoded_bytes == 320){
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/* Tail off the convolutional encoder (flush) */
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encode_and_interleave(0,6);
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}
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}
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//void CCodecAO40::descramble_to_rsblocks(unsigned char viterbi_decoded[NBITS_OUT], char rsblocks[RSBLOCKS][NN])
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void descramble_to_rsblocks(unsigned char viterbi_decoded[NBITS_OUT], char rsblocks[RSBLOCKS][NN])
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{
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/* interleave into Reed Solomon codeblocks */
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memset(rsblocks,0,RSBLOCKS*NN); /* Zero rsblocks array */
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int di = 0;
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int si = 0;
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for(int col=RSPAD;col<NN;col++)
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{
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for(int row=0;row<RSBLOCKS;row++)
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{
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rsblocks[row][col] = viterbi_decoded[di++] ^ Scrambler[si++]; /* Remove scrambling */
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}
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}
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}
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/* -------- */
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/* Decoder */
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/* -------- */
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/* ------------------- */
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/* There are two RS decoders, processing 128 bytes each.
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*
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* If both RS decoders are SUCCESSFUL
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* On exit:
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* rs_failures = 0
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* rserrs[x] = number of errors corrected; range 0 to 16 (x= 0 or 1)
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* Data output is in array RSdecdata[256].
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*
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* If an RS decoder FAILS
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* On exit:
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* rs_failures = 1 or 2 (i.e. != 0)
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* rserrs[x] contains -1
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* Data output should not be used.
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*/
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//int CCodecAO40::decode(unsigned char vitdecdata[NBITS_OUT], unsigned char *decoded_data)
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int decode(unsigned char vitdecdata[NBITS_OUT], unsigned char *decoded_data)
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{
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int row, col, row_errors, total_errors;
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char rsblocks[RSBLOCKS][NN];
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descramble_to_rsblocks(vitdecdata, rsblocks);
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/* Run RS-decoder(s) */
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row_errors = total_errors = 0;
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|
for(row=0; row<RSBLOCKS && row_errors!= -1; row++)
|
|
{
|
|
// decode row, returns -1 on failure or number of corrected errors
|
|
row_errors = decode_rs_8(rsblocks[row],NULL,0);
|
|
total_errors += row_errors;
|
|
}
|
|
|
|
if(row_errors != -1)
|
|
{
|
|
/* if frame decoded OK, deinterleave data from RS codeword(s) */
|
|
int j = 0;
|
|
for(col=RSPAD;col<KK;col++)
|
|
{
|
|
for(row=0;row<RSBLOCKS;row++)
|
|
{
|
|
decoded_data[j++] = rsblocks[row][col];
|
|
}
|
|
}
|
|
}
|
|
else
|
|
{
|
|
total_errors = -1;
|
|
}
|
|
|
|
return total_errors;
|
|
}
|
|
|
|
/* Compairs raw input symbols to current buffer of encoded symbols and counts the errors */
|
|
//int CCodecAO40::count_errors(unsigned char *raw_symbols)
|
|
int count_errors(unsigned char *raw_symbols)
|
|
{
|
|
int error_count = 0;
|
|
for(int i=0;i<SYMPBLOCK;i++)
|
|
{
|
|
if ( m_encoded[i] != (raw_symbols[i]>>7) )
|
|
{
|
|
error_count++ ;
|
|
}
|
|
}
|
|
return error_count;
|
|
}
|