1 /* K=9 r=1/2 Viterbi decoder in portable C
2 * Copyright Feb 2004, 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]; } Branchtab29[2];
14 static int Init = 0;
15
16 /* State info for instance of Viterbi decoder */
17 struct v29 {
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_viterbi29_port(void * p,int starting_state)26 int init_viterbi29_port(void *p,int starting_state){
27 struct v29 *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_viterbi29_polynomial_port(int polys[2])42 void set_viterbi29_polynomial_port(int polys[2]){
43 int state;
44
45 for(state=0;state < 128;state++){
46 Branchtab29[0].c[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0;
47 Branchtab29[1].c[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0;
48 }
49 Init++;
50 }
51
52
53 /* Create a new instance of a Viterbi decoder */
create_viterbi29_port(int len)54 void *create_viterbi29_port(int len){
55 struct v29 *vp;
56
57 if(!Init){
58 int polys[2] = {V29POLYA,V29POLYB};
59 set_viterbi29_polynomial_port(polys);
60 }
61 if((vp = (struct v29 *)malloc(sizeof(struct v29))) == 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_viterbi29_port(vp,0);
69
70 return vp;
71 }
72
73
74 /* Viterbi chainback */
chainback_viterbi29_port(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)75 int chainback_viterbi29_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 v29 *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_viterbi29_port(void * p)108 void delete_viterbi29_port(void *p){
109 struct v29 *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 = (Branchtab29[0].c[i] ^ sym0) + (Branchtab29[1].c[i] ^ sym1);\
121 m0 = vp->old_metrics->w[i] + metric;\
122 m1 = vp->old_metrics->w[i+128] + (510 - metric);\
123 decision = (signed int)(m0-m1) > 0;\
124 vp->new_metrics->w[2*i] = decision ? m1 : m0;\
125 d->w[i/16] |= decision << ((2*i)&31);\
126 m0 -= (metric+metric-510);\
127 m1 += (metric+metric-510);\
128 decision = (signed int)(m0-m1) > 0;\
129 vp->new_metrics->w[2*i+1] = decision ? m1 : m0;\
130 d->w[i/16] |= decision << ((2*i+1)&31);\
131 }
132
133 /* Update decoder with a block of demodulated symbols
134 * Note that nbits is the number of decoded data bits, not the number
135 * of symbols!
136 */
137
update_viterbi29_blk_port(void * p,unsigned char * syms,int nbits)138 int update_viterbi29_blk_port(void *p,unsigned char *syms,int nbits){
139 struct v29 *vp = p;
140 decision_t *d;
141
142 if(p == NULL)
143 return -1;
144
145 d = (decision_t *)vp->dp;
146 while(nbits--){
147 void *tmp;
148 unsigned char sym0,sym1;
149 int i;
150
151 for(i=0;i<8;i++)
152 d->w[i] = 0;
153 sym0 = *syms++;
154 sym1 = *syms++;
155
156 for(i=0;i<128;i++)
157 BFLY(i);
158
159 d++;
160 tmp = vp->old_metrics;
161 vp->old_metrics = vp->new_metrics;
162 vp->new_metrics = tmp;
163 }
164 vp->dp = d;
165 return 0;
166 }
167