srsRAN_4G/srslte/lib/fec/viterbi37_port.c

/* Adapted Phil Karn's r=1/3 k=9 viterbi decoder to r=1/3 k=7
 * 
 * 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 <stdint.h>

#include <memory.h>
#include "viterbi37.h"
#include "parity.h"
#include <limits.h>

//#define DEBUG

typedef union {
  uint32_t w[64];
} metric_t;
typedef union {
  unsigned long w[2];
} decision_t;

static union {
  uint8_t c[32];
} Branchtab37[3];

/* State info for instance of Viterbi decoder */
struct v37 {
  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_viterbi37_port(void *p, int starting_state) {
  struct v37 *vp = p;
  uint32_t i;

  if (p == NULL)
    return -1;
  for (i = 0; i < 64; i++)
    vp->metrics1.w[i] = 63;

  vp->old_metrics = &vp->metrics1;
  vp->new_metrics = &vp->metrics2;
  vp->dp = vp->decisions;
  if (starting_state != -1) {
    vp->old_metrics->w[starting_state & 255] = 0; /* Bias known start state */
  }
  return 0;
}

void set_viterbi37_polynomial_port(int polys[3]) {
  int state;

  for (state = 0; state < 32; state++) {
    Branchtab37[0].c[state] =
        (polys[0] < 0) ^ parity((2 * state) & abs(polys[0])) ? 255 : 0;
    Branchtab37[1].c[state] =
        (polys[1] < 0) ^ parity((2 * state) & abs(polys[1])) ? 255 : 0;
    Branchtab37[2].c[state] =
        (polys[2] < 0) ^ parity((2 * state) & abs(polys[2])) ? 255 : 0;
  }
}

/* Create a new instance of a Viterbi decoder */
void *create_viterbi37_port(int polys[3], uint32_t len) {
  struct v37 *vp;

  set_viterbi37_polynomial_port(polys);

  if ((vp = (struct v37 *) malloc(sizeof(struct v37))) == NULL)
    return NULL ;

  if ((vp->decisions = (decision_t *) malloc((len + 6) * sizeof(decision_t)))
      == NULL) {
    free(vp);
    return NULL ;
  }

  return vp;
}

/* Viterbi chainback */
int chainback_viterbi37_port(void *p, uint8_t *data, /* Decoded output data */
    uint32_t nbits, /* Number of data bits */
    uint32_t endstate) { /* Terminal encoder state */
  struct v37 *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 %= 64;
  endstate <<= 2;

  /* 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 += 6; /* Look past tail */
  while (nbits-- != 0) {
    int k;

    k = (d[nbits].w[(endstate >> 2) / 32] >> ((endstate >> 2) % 32)) & 1;
    endstate = (endstate >> 1) | (k << 7);
    data[nbits] = k;
#ifdef DEBUG
 //   printf("endstate=%3d, k=%d, w[0]=%d, w[1]=%d\n", endstate, k, d[nbits].w[0]&1, d[nbits].w[1]&1);
#endif
  }
  return 0;
}

/* Delete instance of a Viterbi decoder */
void delete_viterbi37_port(void *p) {
  struct v37 *vp = p;

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

/* C-language butterfly */
#define BFLY(i) {\
uint32_t metric,m0,m1,decision;\
    metric = (Branchtab37[0].c[i] ^ sym0) + (Branchtab37[1].c[i] ^ sym1) + \
     (Branchtab37[2].c[i] ^ sym2);\
    m0 = vp->old_metrics->w[i] + metric;\
    m1 = vp->old_metrics->w[i+32] + (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_viterbi37_blk_port(void *p, uint8_t *syms, uint32_t nbits, uint32_t *best_state) {
  struct v37 *vp = p;
  decision_t *d;

  if (p == NULL)
    return -1;
  uint32_t k=0;
  d = (decision_t *) vp->dp;
  
#ifdef DEBUG
  printf("[");
#endif
  
  while (nbits--) {
    void *tmp;
    uint8_t sym0, sym1, sym2;
    uint32_t i;

    d->w[0] = d->w[1] = 0;

    sym0 = *syms++;
    sym1 = *syms++;
    sym2 = *syms++;

    k++;
    for (i = 0; i < 32; i++)
      BFLY(i);
    
#ifdef DEBUG
    uint32_t wmin=UINT_MAX;
    int minstate = 0; 
    for (int j=0;j<64;j++) {
      if (vp->new_metrics->w[j] <= wmin) {
        wmin = vp->new_metrics->w[j];
        minstate = j; 
      }
    }

    printf("%3d, ", minstate);
#endif
    
    d++;
    tmp = vp->old_metrics;
    vp->old_metrics = vp->new_metrics;
    vp->new_metrics = tmp;
  }
  if (best_state) {
    uint32_t i, bst=0;
    uint32_t minmetric=UINT_MAX;
    for (i=0;i<64;i++) {
      if (vp->old_metrics->w[i] <= minmetric) {
        bst = i;
        minmetric = vp->old_metrics->w[i];
      }
    }
    *best_state = bst;
  }
  vp->dp = d;

#ifdef DEBUG
  printf("];\n");
#endif

  return 0;
}