1 /* K=15 r=1/6 Viterbi decoder in portable C
2  * Copyright Mar 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 <limits.h>
9 #include "fec.h"
10 
11 typedef union { unsigned long w[512]; unsigned char c[2048];} decision_t;
12 typedef union { unsigned long w[16384]; } metric_t;
13 
14 static union branchtab615 { unsigned long w[8192]; } Branchtab615[6] __attribute__ ((aligned(16)));
15 static int Init = 0;
16 
17 /* State info for instance of Viterbi decoder */
18 struct v615 {
19   metric_t metrics1; /* path metric buffer 1 */
20   metric_t metrics2; /* path metric buffer 2 */
21   decision_t *dp;          /* Pointer to current decision */
22   metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */
23   decision_t *decisions;   /* Beginning of decisions for block */
24 };
25 
26 /* Create a new instance of a Viterbi decoder */
create_viterbi615_port(int len)27 void *create_viterbi615_port(int len){
28   struct v615 *vp;
29 
30   if(!Init){
31     int polys[6] = { V615POLYA,V615POLYB,V615POLYC,V615POLYD,V615POLYE,V615POLYF };
32     set_viterbi615_polynomial_port(polys);
33   }
34   if((vp = (struct v615 *)malloc(sizeof(struct v615))) == NULL)
35     return NULL;
36   if((vp->decisions = malloc((len+14)*sizeof(decision_t))) == NULL){
37     free(vp);
38     return NULL;
39   }
40   init_viterbi615(vp,0);
41   return vp;
42 }
43 
set_viterbi615_polynomial_port(int polys[6])44 void set_viterbi615_polynomial_port(int polys[6]){
45   int state;
46   int i;
47 
48   for(state=0;state < 8192;state++){
49     for(i=0;i<6;i++)
50       Branchtab615[i].w[state] = (polys[i] < 0) ^ parity((2*state) & abs(polys[i])) ? 255 : 0;
51   }
52   Init++;
53 }
54 
55 /* Initialize Viterbi decoder for start of new frame */
init_viterbi615_port(void * p,int starting_state)56 int init_viterbi615_port(void *p,int starting_state){
57   struct v615 *vp = p;
58   int i;
59 
60   if(p == NULL)
61     return -1;
62   for(i=0;i<16384;i++)
63     vp->metrics1.w[i] = 1000;
64 
65   vp->old_metrics = &vp->metrics1;
66   vp->new_metrics = &vp->metrics2;
67   vp->dp = vp->decisions;
68   vp->old_metrics->w[starting_state & 16383] = 0; /* Bias known start state */
69   return 0;
70 }
71 
72 /* Viterbi chainback */
chainback_viterbi615_port(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)73 int chainback_viterbi615_port(
74       void *p,
75       unsigned char *data, /* Decoded output data */
76       unsigned int nbits, /* Number of data bits */
77       unsigned int endstate){ /* Terminal encoder state */
78   struct v615 *vp = p;
79   decision_t *d;
80 
81   if(p == NULL)
82     return -1;
83   d = (decision_t *)vp->decisions;
84   endstate %= 16384;
85 
86   /* The store into data[] only needs to be done every 8 bits.
87    * But this avoids a conditional branch, and the writes will
88    * combine in the cache anyway
89    */
90   d += 14; /* Look past tail */
91   while(nbits-- != 0){
92     int k;
93 
94     k = (d[nbits].c[endstate/8] >> (endstate%8)) & 1;
95     endstate = (k << 13) | (endstate >> 1);
96     data[nbits>>3] = endstate >> 6;
97   }
98   return 0;
99 }
100 
101 /* Delete instance of a Viterbi decoder */
delete_viterbi615_port(void * p)102 void delete_viterbi615_port(void *p){
103   struct v615 *vp = p;
104 
105   if(vp != NULL){
106     free(vp->decisions);
107     free(vp);
108   }
109 }
110 
111 /* C-language butterfly */
112 #define BFLY(i) {\
113 unsigned long metric,m0,m1,m2,m3,decision0,decision1;\
114     metric = ((Branchtab615[0].w[i] ^ syms[0]) + (Branchtab615[1].w[i] ^ syms[1])\
115 	      +(Branchtab615[2].w[i] ^ syms[2]) + (Branchtab615[3].w[i] ^ syms[3])\
116 	      +(Branchtab615[4].w[i] ^ syms[4]) + (Branchtab615[5].w[i] ^ syms[5]));\
117     m0 = vp->old_metrics->w[i] + metric;\
118     m1 = vp->old_metrics->w[i+8192] + (1530 - metric);\
119     m2 = vp->old_metrics->w[i] + (1530-metric);\
120     m3 = vp->old_metrics->w[i+8192] + metric;\
121     decision0 = (signed long)(m0-m1) >= 0;\
122     decision1 = (signed long)(m2-m3) >= 0;\
123     vp->new_metrics->w[2*i] = decision0 ? m1 : m0;\
124     vp->new_metrics->w[2*i+1] = decision1 ? m3 : m2;\
125     d->c[i/4] |= ((decision0|(decision1<<1)) << ((2*i)&7));\
126 }
127 /* Update decoder with a block of demodulated symbols
128  * Note that nbits is the number of decoded data bits, not the number
129  * of symbols!
130  */
131 
update_viterbi615_blk_port(void * p,unsigned char * syms,int nbits)132 int update_viterbi615_blk_port(void *p,unsigned char *syms,int nbits){
133   struct v615 *vp = p;
134   void *tmp;
135   decision_t *d;
136   int i;
137 
138   if(p == NULL)
139     return -1;
140   d = (decision_t *)vp->dp;
141   while(nbits--){
142     memset(d,0,sizeof(decision_t));
143     for(i=0;i<8192;i++)
144       BFLY(i);
145 
146     syms += 6;
147     d++;
148     /* Swap pointers to old and new metrics */
149     tmp = vp->old_metrics;
150     vp->old_metrics = vp->new_metrics;
151     vp->new_metrics = tmp;
152   }
153   vp->dp = d;
154   return 0;
155 }
156 
157