240 lines
6.8 KiB
Plaintext
240 lines
6.8 KiB
Plaintext
/*
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This file is part of wsprd.
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File name: fano.c
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Description: Soft decision Fano sequential decoder for K=32 r=1/2
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convolutional code.
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Copyright 1994, Phil Karn, KA9Q
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Minor modifications by Joe Taylor, K1JT
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*/
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#define LL 1 // Select Layland-Lushbaugh code
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#include <stdio.h>
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#include <stdlib.h>
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#include <math.h>
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#include "fano.h"
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struct node {
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unsigned long encstate; // Encoder state of next node
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long gamma; // Cumulative metric to this node
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int metrics[4]; // Metrics indexed by all possible tx syms
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int tm[2]; // Sorted metrics for current hypotheses
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int i; // Current branch being tested
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};
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// Convolutional coding polynomials. All are rate 1/2, K=32
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#ifdef NASA_STANDARD
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/* "NASA standard" code by Massey & Costello
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* Nonsystematic, quick look-in, dmin=11, dfree=23
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* used on Pioneer 10-12, Helios A,B
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*/
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#define POLY1 0xbbef6bb7
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#define POLY2 0xbbef6bb5
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#endif
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#ifdef MJ
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/* Massey-Johannesson code
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* Nonsystematic, quick look-in, dmin=13, dfree>=23
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* Purported to be more computationally efficient than Massey-Costello
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*/
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#define POLY1 0xb840a20f
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#define POLY2 0xb840a20d
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#endif
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#ifdef LL
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/* Layland-Lushbaugh code
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* Nonsystematic, non-quick look-in, dmin=?, dfree=?
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*/
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#define POLY1 0xf2d05351
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#define POLY2 0xe4613c47
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#endif
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/* Convolutionally encode a packet. The input data bytes are read
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* high bit first and the encoded packet is written into 'symbols',
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* one symbol per byte. The first symbol is generated from POLY1,
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* the second from POLY2.
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*
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* Storing only one symbol per byte uses more space, but it is faster
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* and easier than trying to pack them more compactly.
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*/
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int encode(
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unsigned char *symbols, // Output buffer, 2*nbytes*8
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unsigned char *data, // Input buffer, nbytes
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unsigned int nbytes) // Number of bytes in data
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{
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unsigned long encstate;
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int sym;
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int i;
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encstate = 0;
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while(nbytes-- != 0) {
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for(i=7;i>=0;i--) {
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encstate = (encstate << 1) | ((*data >> i) & 1);
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ENCODE(sym,encstate);
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*symbols++ = sym >> 1;
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*symbols++ = sym & 1;
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}
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data++;
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}
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return 0;
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}
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/* Decode packet with the Fano algorithm.
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* Return 0 on success, -1 on timeout
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*/
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int fano(
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unsigned int *metric, // Final path metric (returned value)
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unsigned int *cycles, // Cycle count (returned value)
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unsigned int *maxnp, // Progress before timeout (returned value)
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unsigned char *data, // Decoded output data
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unsigned char *symbols, // Raw deinterleaved input symbols
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unsigned int nbits, // Number of output bits
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int mettab[2][256], // Metric table, [sent sym][rx symbol]
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int delta, // Threshold adjust parameter
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unsigned int maxcycles) // Decoding timeout in cycles per bit
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{
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struct node *nodes; // First node
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struct node *np; // Current node
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struct node *lastnode; // Last node
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struct node *tail; // First node of tail
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int t; // Threshold
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int m0,m1;
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int ngamma;
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unsigned int lsym;
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unsigned int i;
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if((nodes = (struct node *)malloc((nbits+1)*sizeof(struct node))) == NULL) {
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printf("malloc failed\n");
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return 0;
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}
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lastnode = &nodes[nbits-1];
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tail = &nodes[nbits-31];
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*maxnp = 0;
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/* Compute all possible branch metrics for each symbol pair
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* This is the only place we actually look at the raw input symbols
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*/
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for(np=nodes;np <= lastnode;np++) {
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np->metrics[0] = mettab[0][symbols[0]] + mettab[0][symbols[1]];
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np->metrics[1] = mettab[0][symbols[0]] + mettab[1][symbols[1]];
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np->metrics[2] = mettab[1][symbols[0]] + mettab[0][symbols[1]];
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np->metrics[3] = mettab[1][symbols[0]] + mettab[1][symbols[1]];
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symbols += 2;
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}
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np = nodes;
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np->encstate = 0;
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// Compute and sort branch metrics from root node */
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ENCODE(lsym,np->encstate); // 0-branch (LSB is 0)
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m0 = np->metrics[lsym];
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/* Now do the 1-branch. To save another ENCODE call here and
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* inside the loop, we assume that both polynomials are odd,
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* providing complementary pairs of branch symbols.
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* This code should be modified if a systematic code were used.
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*/
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m1 = np->metrics[3^lsym];
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if(m0 > m1) {
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np->tm[0] = m0; // 0-branch has better metric
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np->tm[1] = m1;
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} else {
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np->tm[0] = m1; // 1-branch is better
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np->tm[1] = m0;
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np->encstate++; // Set low bit
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}
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np->i = 0; // Start with best branch
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maxcycles *= nbits;
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np->gamma = t = 0;
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// Start the Fano decoder
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for(i=1;i <= maxcycles;i++) {
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if((int)(np-nodes) > (int)*maxnp) *maxnp=(int)(np-nodes);
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#ifdef debug
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printf("k=%ld, g=%ld, t=%d, m[%d]=%d, maxnp=%d, encstate=%lx\n",
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np-nodes,np->gamma,t,np->i,np->tm[np->i],*maxnp,np->encstate);
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#endif
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// Look forward */
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ngamma = np->gamma + np->tm[np->i];
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if(ngamma >= t) {
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if(np->gamma < t + delta) { // Node is acceptable
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/* First time we've visited this node;
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* Tighten threshold.
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*
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* This loop could be replaced with
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* t += delta * ((ngamma - t)/delta);
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* but the multiply and divide are slower.
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*/
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while(ngamma >= t + delta) t += delta;
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}
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np[1].gamma = ngamma; // Move forward
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np[1].encstate = np->encstate << 1;
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if( ++np == (lastnode+1) ) {
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break; // Done!
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}
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/* Compute and sort metrics, starting with the
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* zero branch
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*/
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ENCODE(lsym,np->encstate);
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if(np >= tail) {
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/* The tail must be all zeroes, so don't
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* bother computing the 1-branches here.
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*/
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np->tm[0] = np->metrics[lsym];
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} else {
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m0 = np->metrics[lsym];
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m1 = np->metrics[3^lsym];
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if(m0 > m1) {
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np->tm[0] = m0; // 0-branch is better
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np->tm[1] = m1;
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} else {
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np->tm[0] = m1; // 1-branch is better
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np->tm[1] = m0;
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np->encstate++; // Set low bit
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}
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}
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np->i = 0; // Start with best branch
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continue;
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}
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// Threshold violated, can't go forward
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for(;;) { // Look backward
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if(np == nodes || np[-1].gamma < t) {
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/* Can't back up either.
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* Relax threshold and and look
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* forward again to better branch.
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*/
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t -= delta;
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if(np->i != 0) {
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np->i = 0;
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np->encstate ^= 1;
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}
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break;
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}
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// Back up
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if(--np < tail && np->i != 1) {
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np->i++; // Search next best branch
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np->encstate ^= 1;
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break;
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} // else keep looking back
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}
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}
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*metric = np->gamma; // Return the final path metric
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// Copy decoded data to user's buffer
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nbits >>= 3;
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np = &nodes[7];
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while(nbits-- != 0) {
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*data++ = np->encstate;
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np += 8;
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}
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*cycles = i+1;
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free(nodes);
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if(i >= maxcycles) return -1; // Decoder timed out
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return 0; // Successful completion
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}
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