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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 
10 typedef union { unsigned int w[256]; } metric_t;
11 typedef union { unsigned long w[8];} decision_t;
12 
13 static union { unsigned char c[128]; } Branchtab39[3];
14 static int Init = 0;
15 
16 /* State info for instance of Viterbi decoder */
17 struct 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 */
init_viterbi39_port(void * p,int starting_state)26 int 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 
set_viterbi39_polynomial_port(int polys[3])42 void 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 */
create_viterbi39_port(int len)54 void *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 */
chainback_viterbi39_port(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)75 int 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 */
delete_viterbi39_port(void * p)108 void 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) {\
119 unsigned 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 
update_viterbi39_blk_port(void * p,unsigned char * syms,int nbits)139 int 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