1/* K=9 r=1/3 Viterbi decoder in portable C
2 * Copyright Aug 2006, Phil Karn, KA9Q
3 * May be used under the terms of the GNU Lesser General Public License (LGPL)
4 */
5#include <stdio.h>
6#include <stdlib.h>
7#include <memory.h>
8#include "fec.h"
9
10typedef union { unsigned int w[256]; } metric_t;
11typedef union { unsigned long w[8];} decision_t;
12
13static union { unsigned char c[128]; } Branchtab39[3];
14static int Init = 0;
15
16/* State info for instance of Viterbi decoder */
17struct v39 {
18  metric_t metrics1; /* path metric buffer 1 */
19  metric_t metrics2; /* path metric buffer 2 */
20  decision_t *dp;          /* Pointer to current decision */
21  metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */
22  decision_t *decisions;   /* Beginning of decisions for block */
23};
24
25/* Initialize Viterbi decoder for start of new frame */
26int init_viterbi39_port(void *p,int starting_state){
27  struct v39 *vp = p;
28  int i;
29
30  if(p == NULL)
31    return -1;
32  for(i=0;i<256;i++)
33    vp->metrics1.w[i] = 63;
34
35  vp->old_metrics = &vp->metrics1;
36  vp->new_metrics = &vp->metrics2;
37  vp->dp = vp->decisions;
38  vp->old_metrics->w[starting_state & 255] = 0; /* Bias known start state */
39  return 0;
40}
41
42void set_viterbi39_polynomial_port(int polys[3]){
43  int state;
44
45  for(state=0;state < 128;state++){
46    Branchtab39[0].c[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0;
47    Branchtab39[1].c[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0;
48    Branchtab39[2].c[state] = (polys[2] < 0) ^ parity((2*state) & abs(polys[2])) ? 255 : 0;
49  }
50  Init++;
51}
52
53/* Create a new instance of a Viterbi decoder */
54void *create_viterbi39_port(int len){
55  struct v39 *vp;
56
57  if(!Init){
58    int polys[3] = {V39POLYA,V39POLYB,V39POLYC};
59    set_viterbi39_polynomial_port(polys);
60  }
61  if((vp = (struct v39 *)malloc(sizeof(struct v39))) == NULL)
62    return NULL;
63
64  if((vp->decisions = (decision_t *)malloc((len+8)*sizeof(decision_t))) == NULL){
65    free(vp);
66    return NULL;
67  }
68  init_viterbi39_port(vp,0);
69
70  return vp;
71}
72
73
74/* Viterbi chainback */
75int chainback_viterbi39_port(
76      void *p,
77      unsigned char *data, /* Decoded output data */
78      unsigned int nbits, /* Number of data bits */
79      unsigned int endstate){ /* Terminal encoder state */
80  struct v39 *vp = p;
81  decision_t *d;
82
83  if(p == NULL)
84    return -1;
85
86  d = vp->decisions;
87  /* Make room beyond the end of the encoder register so we can
88   * accumulate a full byte of decoded data
89   */
90  endstate %= 256;
91
92  /* The store into data[] only needs to be done every 8 bits.
93   * But this avoids a conditional branch, and the writes will
94   * combine in the cache anyway
95   */
96  d += 8; /* Look past tail */
97  while(nbits-- != 0){
98    int k;
99
100    k = (d[nbits].w[(endstate)/32] >> (endstate%32)) & 1;
101    data[nbits>>3] = endstate = (endstate >> 1) | (k << 7);
102  }
103  return 0;
104}
105
106
107/* Delete instance of a Viterbi decoder */
108void delete_viterbi39_port(void *p){
109  struct v39 *vp = p;
110
111  if(vp != NULL){
112    free(vp->decisions);
113    free(vp);
114  }
115}
116
117/* C-language butterfly */
118#define BFLY(i) {\
119unsigned int metric,m0,m1,decision;\
120    metric = (Branchtab39[0].c[i] ^ sym0) + (Branchtab39[1].c[i] ^ sym1) + \
121     (Branchtab39[2].c[i] ^ sym2);\
122    m0 = vp->old_metrics->w[i] + metric;\
123    m1 = vp->old_metrics->w[i+128] + (765 - metric);\
124    decision = (signed int)(m0-m1) > 0;\
125    vp->new_metrics->w[2*i] = decision ? m1 : m0;\
126    d->w[i/16] |= decision << ((2*i)&31);\
127    m0 -= (metric+metric-765);\
128    m1 += (metric+metric-765);\
129    decision = (signed int)(m0-m1) > 0;\
130    vp->new_metrics->w[2*i+1] = decision ? m1 : m0;\
131    d->w[i/16] |= decision << ((2*i+1)&31);\
132}
133
134/* Update decoder with a block of demodulated symbols
135 * Note that nbits is the number of decoded data bits, not the number
136 * of symbols!
137 */
138
139int update_viterbi39_blk_port(void *p,unsigned char *syms,int nbits){
140  struct v39 *vp = p;
141  decision_t *d;
142
143  if(p == NULL)
144    return -1;
145
146  d = (decision_t *)vp->dp;
147  while(nbits--){
148    void *tmp;
149    unsigned char sym0,sym1,sym2;
150    int i;
151
152    for(i=0;i<8;i++)
153      d->w[i] = 0;
154    sym0 = *syms++;
155    sym1 = *syms++;
156    sym2 = *syms++;
157
158    for(i=0;i<128;i++)
159      BFLY(i);
160
161    d++;
162    tmp = vp->old_metrics;
163    vp->old_metrics = vp->new_metrics;
164    vp->new_metrics = tmp;
165  }
166  vp->dp = d;
167  return 0;
168}
169