viterbi39_port.c

FEC Optimized viterbi code

/* K=9 r=1/3 Viterbi decoder in portable C
 * Copyright Aug 2006, Phil Karn, KA9Q
 * May be used under the terms of the GNU Lesser General Public License (LGPL)
 */
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include "fec.h"

typedef union { unsigned int w[256]; } metric_t;
typedef union { unsigned long w[8];} decision_t;

static union { unsigned char c[128]; } Branchtab39[3];
static int Init = 0;

/* State info for instance of Viterbi decoder */
struct v39 {
  metric_t metrics1; /* path metric buffer 1 */
  metric_t metrics2; /* path metric buffer 2 */
  decision_t *dp;          /* Pointer to current decision */
  metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */
  decision_t *decisions;   /* Beginning of decisions for block */
};

/* Initialize Viterbi decoder for start of new frame */
int init_viterbi39_port(void *p,int starting_state){
  struct v39 *vp = p;
  int i;

  if(p == NULL)
    return -1;
  for(i=0;i<256;i++)
    vp->metrics1.w[i] = 63;

  vp->old_metrics = &vp->metrics1;
  vp->new_metrics = &vp->metrics2;
  vp->dp = vp->decisions;
  vp->old_metrics->w[starting_state & 255] = 0; /* Bias known start state */
  return 0;
}

void set_viterbi39_polynomial_port(int polys[3]){
  int state;

  for(state=0;state < 128;state++){
    Branchtab39[0].c[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0;
    Branchtab39[1].c[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0;
    Branchtab39[2].c[state] = (polys[2] < 0) ^ parity((2*state) & abs(polys[2])) ? 255 : 0;
  }
  Init++;
}

/* Create a new instance of a Viterbi decoder */
void *create_viterbi39_port(int len){
  struct v39 *vp;

  if(!Init){
    int polys[3] = {V39POLYA,V39POLYB,V39POLYC};
    set_viterbi39_polynomial_port(polys);
  }
  if((vp = (struct v39 *)malloc(sizeof(struct v39))) == NULL)
    return NULL;

  if((vp->decisions = (decision_t *)malloc((len+8)*sizeof(decision_t))) == NULL){
    free(vp);
    return NULL;
  }
  init_viterbi39_port(vp,0);

  return vp;
}


/* Viterbi chainback */
int chainback_viterbi39_port(
      void *p,
      unsigned char *data, /* Decoded output data */
      unsigned int nbits, /* Number of data bits */
      unsigned int endstate){ /* Terminal encoder state */
  struct v39 *vp = p;
  decision_t *d;

  if(p == NULL)
    return -1;

  d = vp->decisions;
  /* Make room beyond the end of the encoder register so we can
   * accumulate a full byte of decoded data
   */
  endstate %= 256;

  /* The store into data[] only needs to be done every 8 bits.
   * But this avoids a conditional branch, and the writes will
   * combine in the cache anyway
   */
  d += 8; /* Look past tail */
  while(nbits-- != 0){
    int k;
    
    k = (d[nbits].w[(endstate)/32] >> (endstate%32)) & 1;
    data[nbits>>3] = endstate = (endstate >> 1) | (k << 7);
  }
  return 0;
}


/* Delete instance of a Viterbi decoder */
void delete_viterbi39_port(void *p){
  struct v39 *vp = p;

  if(vp != NULL){
    free(vp->decisions);
    free(vp);
  }
}

/* C-language butterfly */
#define BFLY(i) {\
unsigned int metric,m0,m1,decision;\
    metric = (Branchtab39[0].c[i] ^ sym0) + (Branchtab39[1].c[i] ^ sym1) + \
     (Branchtab39[2].c[i] ^ sym2);\
    m0 = vp->old_metrics->w[i] + metric;\
    m1 = vp->old_metrics->w[i+128] + (765 - metric);\
    decision = (signed int)(m0-m1) > 0;\
    vp->new_metrics->w[2*i] = decision ? m1 : m0;\
    d->w[i/16] |= decision << ((2*i)&31);\
    m0 -= (metric+metric-765);\
    m1 += (metric+metric-765);\
    decision = (signed int)(m0-m1) > 0;\
    vp->new_metrics->w[2*i+1] = decision ? m1 : m0;\
    d->w[i/16] |= decision << ((2*i+1)&31);\
}

/* Update decoder with a block of demodulated symbols
 * Note that nbits is the number of decoded data bits, not the number
 * of symbols!
 */

int update_viterbi39_blk_port(void *p,unsigned char *syms,int nbits){
  struct v39 *vp = p;
  decision_t *d;

  if(p == NULL)
    return -1;
  
  d = (decision_t *)vp->dp;
  while(nbits--){
    void *tmp;
    unsigned char sym0,sym1,sym2;
    int i;

    for(i=0;i<8;i++)
      d->w[i] = 0;
    sym0 = *syms++;
    sym1 = *syms++;
    sym2 = *syms++;

    for(i=0;i<128;i++)
      BFLY(i);

    d++;
    tmp = vp->old_metrics;
    vp->old_metrics = vp->new_metrics;
    vp->new_metrics = tmp;
  }  
  vp->dp = d;
  return 0;
}