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