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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