/* Reed-Solomon decoder * Copyright 2002 Phil Karn, KA9Q * May be used under the terms of the GNU Lesser General Public License (LGPL) */#ifdef DEBUG#include <stdio.h>#endif#include <string.h>#define NULL ((void *)0)#define	min(a,b)	((a) < (b) ? (a) : (b))#ifdef FIXED#include "fixed.h"#elif defined(BIGSYM)#include "int.h"#else#include "char.h"#endifint DECODE_RS(#ifdef FIXEDdata_t *data, int *eras_pos, int no_eras,int pad){#elsevoid *p,data_t *data, int *eras_pos, int no_eras){  struct rs *rs = (struct rs *)p;#endif  int deg_lambda, el, deg_omega;  int i, j, r,k;  data_t u,q,tmp,num1,num2,den,discr_r;  data_t lambda[NROOTS+1], s[NROOTS];	/* Err+Eras Locator poly					 * and syndrome poly */  data_t b[NROOTS+1], t[NROOTS+1], omega[NROOTS+1];  data_t root[NROOTS], reg[NROOTS+1], loc[NROOTS];  int syn_error, count;#ifdef FIXED  /* Check pad parameter for validity */  if(pad < 0 || pad >= NN)    return -1;#endif  /* form the syndromes; i.e., evaluate data(x) at roots of g(x) */  for(i=0;i<NROOTS;i++)    s[i] = data[0];  for(j=1;j<NN-PAD;j++){    for(i=0;i<NROOTS;i++){      if(s[i] == 0){	s[i] = data[j];      } else {	s[i] = data[j] ^ ALPHA_TO[MODNN(INDEX_OF[s[i]] + (FCR+i)*PRIM)];      }    }  }  /* Convert syndromes to index form, checking for nonzero condition */  syn_error = 0;  for(i=0;i<NROOTS;i++){    syn_error |= s[i];    s[i] = INDEX_OF[s[i]];  }  if (!syn_error) {    /* if syndrome is zero, data[] is a codeword and there are no     * errors to correct. So return data[] unmodified     */    count = 0;    goto finish;  }  memset(&lambda[1],0,NROOTS*sizeof(lambda[0]));  lambda[0] = 1;  if (no_eras > 0) {    /* Init lambda to be the erasure locator polynomial */    lambda[1] = ALPHA_TO[MODNN(PRIM*(NN-1-eras_pos[0]))];    for (i = 1; i < no_eras; i++) {      u = MODNN(PRIM*(NN-1-eras_pos[i]));      for (j = i+1; j > 0; j--) {	tmp = INDEX_OF[lambda[j - 1]];	if(tmp != A0)	  lambda[j] ^= ALPHA_TO[MODNN(u + tmp)];      }    }#if DEBUG >= 1    /* Test code that verifies the erasure locator polynomial just constructed       Needed only for decoder debugging. */        /* find roots of the erasure location polynomial */    for(i=1;i<=no_eras;i++)      reg[i] = INDEX_OF[lambda[i]];    count = 0;    for (i = 1,k=IPRIM-1; i <= NN; i++,k = MODNN(k+IPRIM)) {      q = 1;      for (j = 1; j <= no_eras; j++)	if (reg[j] != A0) {	  reg[j] = MODNN(reg[j] + j);	  q ^= ALPHA_TO[reg[j]];	}      if (q != 0)	continue;      /* store root and error location number indices */      root[count] = i;      loc[count] = k;      count++;    }    if (count != no_eras) {      printf("count = %d no_eras = %d\n lambda(x) is WRONG\n",count,no_eras);      count = -1;      goto finish;    }#if DEBUG >= 2    printf("\n Erasure positions as determined by roots of Eras Loc Poly:\n");    for (i = 0; i < count; i++)      printf("%d ", loc[i]);    printf("\n");#endif#endif  }  for(i=0;i<NROOTS+1;i++)    b[i] = INDEX_OF[lambda[i]];    /*   * Begin Berlekamp-Massey algorithm to determine error+erasure   * locator polynomial   */  r = no_eras;  el = no_eras;  while (++r <= NROOTS) {	/* r is the step number */    /* Compute discrepancy at the r-th step in poly-form */    discr_r = 0;    for (i = 0; i < r; i++){      if ((lambda[i] != 0) && (s[r-i-1] != A0)) {	discr_r ^= ALPHA_TO[MODNN(INDEX_OF[lambda[i]] + s[r-i-1])];      }    }    discr_r = INDEX_OF[discr_r];	/* Index form */    if (discr_r == A0) {      /* 2 lines below: B(x) <-- x*B(x) */      memmove(&b[1],b,NROOTS*sizeof(b[0]));      b[0] = A0;    } else {      /* 7 lines below: T(x) <-- lambda(x) - discr_r*x*b(x) */      t[0] = lambda[0];      for (i = 0 ; i < NROOTS; i++) {	if(b[i] != A0)	  t[i+1] = lambda[i+1] ^ ALPHA_TO[MODNN(discr_r + b[i])];	else	  t[i+1] = lambda[i+1];      }      if (2 * el <= r + no_eras - 1) {	el = r + no_eras - el;	/*	 * 2 lines below: B(x) <-- inv(discr_r) *	 * lambda(x)	 */	for (i = 0; i <= NROOTS; i++)	  b[i] = (lambda[i] == 0) ? A0 : MODNN(INDEX_OF[lambda[i]] - discr_r + NN);      } else {	/* 2 lines below: B(x) <-- x*B(x) */	memmove(&b[1],b,NROOTS*sizeof(b[0]));	b[0] = A0;      }      memcpy(lambda,t,(NROOTS+1)*sizeof(t[0]));    }  }  /* Convert lambda to index form and compute deg(lambda(x)) */  deg_lambda = 0;  for(i=0;i<NROOTS+1;i++){    lambda[i] = INDEX_OF[lambda[i]];    if(lambda[i] != A0)      deg_lambda = i;  }  /* Find roots of the error+erasure locator polynomial by Chien search */  memcpy(&reg[1],&lambda[1],NROOTS*sizeof(reg[0]));  count = 0;		/* Number of roots of lambda(x) */  for (i = 1,k=IPRIM-1; i <= NN; i++,k = MODNN(k+IPRIM)) {    q = 1; /* lambda[0] is always 0 */    for (j = deg_lambda; j > 0; j--){      if (reg[j] != A0) {	reg[j] = MODNN(reg[j] + j);	q ^= ALPHA_TO[reg[j]];      }    }    if (q != 0)      continue; /* Not a root */    /* store root (index-form) and error location number */#if DEBUG>=2    printf("count %d root %d loc %d\n",count,i,k);#endif    root[count] = i;    loc[count] = k;    /* If we've already found max possible roots,     * abort the search to save time     */    if(++count == deg_lambda)      break;  }  if (deg_lambda != count) {    /*     * deg(lambda) unequal to number of roots => uncorrectable     * error detected     */    count = -1;    goto finish;  }  /*   * Compute err+eras evaluator poly omega(x) = s(x)*lambda(x) (modulo   * x**NROOTS). in index form. Also find deg(omega).   */  deg_omega = deg_lambda-1;  for (i = 0; i <= deg_omega;i++){    tmp = 0;    for(j=i;j >= 0; j--){      if ((s[i - j] != A0) && (lambda[j] != A0))	tmp ^= ALPHA_TO[MODNN(s[i - j] + lambda[j])];    }    omega[i] = INDEX_OF[tmp];  }  /*   * Compute error values in poly-form. num1 = omega(inv(X(l))), num2 =   * inv(X(l))**(FCR-1) and den = lambda_pr(inv(X(l))) all in poly-form   */  for (j = count-1; j >=0; j--) {    num1 = 0;    for (i = deg_omega; i >= 0; i--) {      if (omega[i] != A0)	num1  ^= ALPHA_TO[MODNN(omega[i] + i * root[j])];    }    num2 = ALPHA_TO[MODNN(root[j] * (FCR - 1) + NN)];    den = 0;        /* lambda[i+1] for i even is the formal derivative lambda_pr of lambda[i] */    for (i = min(deg_lambda,NROOTS-1) & ~1; i >= 0; i -=2) {      if(lambda[i+1] != A0)	den ^= ALPHA_TO[MODNN(lambda[i+1] + i * root[j])];    }#if DEBUG >= 1    if (den == 0) {      printf("\n ERROR: denominator = 0\n");      count = -1;      goto finish;    }#endif    /* Apply error to data */    if (num1 != 0 && loc[j] >= PAD) {      data[loc[j]-PAD] ^= ALPHA_TO[MODNN(INDEX_OF[num1] + INDEX_OF[num2] + NN - INDEX_OF[den])];    }  } finish:  if(eras_pos != NULL){    for(i=0;i<count;i++)      eras_pos[i] = loc[i];  }  return count;}