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