vitfilt27.c

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/* Viterbi decoder for K=7 rate=1/2 convolutional code
 * continuous traceback version
 * Copyright 1996 Phil Karn, KA9Q
 *
 * This version of the Viterbi decoder reads a continous stream of
 * 8-bit soft decision samples from standard input in offset-binary
 * form, i.e., a 255 sample is the strongest possible "1" symbol and a
 * 0 is the strongest possible "0" symbol. 128 is an erasure (unknown).
 *
 * The decoded output is written to stdout in big-endian form (the first
 * decoded bit appears in the high order bit of the first output byte).
 *
 * The metric table is fixed, and no attempt is made (yet) to find proper
 * symbol synchronization. These are likely future enhancements.
 */
#include <stdio.h>
#include <limits.h>
#include "viterbi27.h"

/* This parameter sizes the path memory in bits, which is organized as a
 * circular buffer through which we periodically "trace back" to
 * produce the decoded data. PATHMEM must be greater than
 * MERGEDIST+TRACECHUNK, and for efficiency it should also be a power of 2.
 * Don't make it *too* large, or it will spill out of the CPU's on-chip cache
 * and decrease performance. Each bit of path memory costs 8 bytes for the
 * K=7 code.
 */
#define PATHMEM		128

/* In theory, a Viterbi decoder is true maximum likelihood only if
 * the path memory is as long as the entire message and a single traceback
 * is made from the terminal state (usually zero) after the entire message
 * is received.
 *
 * In practice, performance is essentially optimum as long as decoding
 * decisions are deferred by at least 4-5 constraint lengths (28-35 bits
 * for K=7) from the most recently received symbols. MERGEDIST sets this
 * parameter. We give ourselves some margin here in case the code is
 * punctured (which slows merging) and also to let us start each traceback
 * from an arbitrary current state instead of taking the time to find the
 * path with the highest current metric.
 */
#define	MERGEDIST	64	/* Distance to trace back before decoding */

/* Since each traceback is costly (thanks to the overhead of having to
 * go back MERGEDIST bits before we produce our first decoded bit) we'd like
 * to decode as many bits as possible per traceback at the expense of
 * increased decoding delay. TRACECHUNK sets how many bits to
 * decode on each traceback. Since output is produced in 8-bit bytes,
 * TRACECHUNK MUST be a multiple of 8.
 */
#define	TRACECHUNK	64	/* How many bits to decode on each traceback */

/* The path metrics need to be periodicially adjusted downward
 * to prevent an integer overflow that could cause the signed comparisons
 * in the butterfly macros to fail.
 *
 * It's possible to code the comparisons to work in modulo fashion, e.g.,
 * as 'if((a-b) > 0)' rather than 'if(a >b)'. A good optimizer would generate
 * code like 'cmp a,b;js foo' for this, but GCC doesn't.
 *
 * This constant should be larger than the maximum path metric spread.
 * Experimentally this seems to be 2040, which is probably related to the
 * free distance of the code (10) and the symbol metric scale (0-255).
 */
#define	RENORMALIZE	10000

#if (TRACECHUNK + MERGEDIST > PATHMEM)
#error "TRACECHUNK + MERGEDIST > PATHMEM"
#endif

#if ((TRACECHUNK % 8) != 0)
#error "TRACECHUNK not multiple of 8"
#endif

static void traceback(unsigned long paths[],unsigned int pi);
static void flush(unsigned long paths[],unsigned int pi);

/* Need some options here: user settable metric table, verbosity options, etc */
main(int argc,char *argv[])
{
	unsigned int bitcnt = 0;
	int beststate,i;
	long cmetric[64],nmetric[64];
	unsigned long paths[2*PATHMEM];
	register unsigned long dec;
	int mets[4];
	unsigned int pi = 0,first=1;
	unsigned char symbols[2];
	int mettab[2][256];
	
	/* Initialize metric table (make this an option)
	 * This table assumes a symbol of 0 is the
	 * strongest possible '0', and a symbol
	 * of 255 is the strongest possible '1'. A symbol
	 * of 128 is an erasure
	 */
	for(i=0;i<256;i++){
		mettab[0][i] = 128 - i;
		mettab[1][255-i] = 127 - i;
	}
	cmetric[0] = 0;
	for(i=1;i<64;i++)
		cmetric[i] = -99999;

	/* Main loop -- read input symbols and run ACS butterflies,
	 * periodically tracing back to produce decoded output data.
	 * The loop is unrolled to process two bits per iteration.
	 */
	for(;;){
		/* Renormalize metrics to prevent overflow */
		if(cmetric[0] > (LONG_MAX - RENORMALIZE)){
			for(i=0;i<64;i++)
				cmetric[i] -= LONG_MAX;
		} else if(cmetric[0] < LONG_MIN+RENORMALIZE){
			for(i=0;i<64;i++)
				cmetric[i] += LONG_MAX;
		}
		/* Read input symbol pair and compute branch metrics */
		symbols[0] = getchar();
		symbols[1] = getchar();
		if(feof(stdin))
			break;
		mets[0] = mettab[0][symbols[0]] + mettab[0][symbols[1]];
		mets[1] = mettab[0][symbols[0]] + mettab[1][symbols[1]];
		mets[3] = mettab[1][symbols[0]] + mettab[1][symbols[1]];
		mets[2] = mettab[1][symbols[0]] + mettab[0][symbols[1]];

		/* On even numbered bits, the butterflies read from cmetrics[]
		 * and write to nmetrics[]. On odd numbered bits, the reverse
		 * is done
		 */
		dec = 0;
		BUTTERFLY(0,0);
		BUTTERFLY(6,0);
		BUTTERFLY(8,0);
		BUTTERFLY(14,0);
		BUTTERFLY(2,3);
		BUTTERFLY(4,3);
		BUTTERFLY(10,3);
		BUTTERFLY(12,3);
		BUTTERFLY(1,1);
		BUTTERFLY(7,1);
		BUTTERFLY(9,1);
		BUTTERFLY(15,1);
		BUTTERFLY(3,2);
		BUTTERFLY(5,2);
		BUTTERFLY(11,2);
		BUTTERFLY(13,2);
		paths[2*pi] = dec;
		dec = 0;
		BUTTERFLY(19,0);
		BUTTERFLY(21,0);
		BUTTERFLY(27,0);
		BUTTERFLY(29,0);
		BUTTERFLY(17,3);
		BUTTERFLY(23,3);
		BUTTERFLY(25,3);
		BUTTERFLY(31,3);
		BUTTERFLY(18,1);
		BUTTERFLY(20,1);
		BUTTERFLY(26,1);
		BUTTERFLY(28,1);
		BUTTERFLY(16,2);
		BUTTERFLY(22,2);
		BUTTERFLY(24,2);
		BUTTERFLY(30,2);
		paths[2*pi+1] = dec;
		pi++;

		/* Read input symbol pair and compute branch metrics */
		symbols[0] = getchar();
		symbols[1] = getchar();
		if(feof(stdin))
			break;
		mets[0] = mettab[0][symbols[0]] + mettab[0][symbols[1]];
		mets[1] = mettab[0][symbols[0]] + mettab[1][symbols[1]];
		mets[3] = mettab[1][symbols[0]] + mettab[1][symbols[1]];
		mets[2] = mettab[1][symbols[0]] + mettab[0][symbols[1]];

		dec = 0;
		BUTTERFLY2(0,0);
		BUTTERFLY2(6,0);
		BUTTERFLY2(8,0);
		BUTTERFLY2(14,0);
		BUTTERFLY2(2,3);
		BUTTERFLY2(4,3);
		BUTTERFLY2(10,3);
		BUTTERFLY2(12,3);
		BUTTERFLY2(1,1);
		BUTTERFLY2(7,1);
		BUTTERFLY2(9,1);
		BUTTERFLY2(15,1);
		BUTTERFLY2(3,2);
		BUTTERFLY2(5,2);
		BUTTERFLY2(11,2);
		BUTTERFLY2(13,2);
		paths[2*pi] = dec;
		dec = 0;
		BUTTERFLY2(19,0);
		BUTTERFLY2(21,0);
		BUTTERFLY2(27,0);
		BUTTERFLY2(29,0);
		BUTTERFLY2(17,3);
		BUTTERFLY2(23,3);
		BUTTERFLY2(25,3);
		BUTTERFLY2(31,3);
		BUTTERFLY2(18,1);
		BUTTERFLY2(20,1);
		BUTTERFLY2(26,1);
		BUTTERFLY2(28,1);
		BUTTERFLY2(16,2);
		BUTTERFLY2(22,2);
		BUTTERFLY2(24,2);
		BUTTERFLY2(30,2);
		paths[2*pi+1] = dec;
		pi = (pi + 1) % PATHMEM;
		if((pi % TRACECHUNK) == 0){
			if(!first)
				traceback(paths,pi);
			first = 0;
		}
	}
	flush(paths,pi);
}

/* Periodic traceback to produce decoded data */
static void
traceback(unsigned long paths[],unsigned int pi)
{
	int beststate,i,j;
	unsigned char data[TRACECHUNK/8];

	/* Start on an arbitrary path and trace it back until it's almost
	 * certain we've merged onto the best path
	 */
	beststate = 0;	/* arbitrary */
	pi = (pi - 1) % PATHMEM;	/* Undo last increment of pi */
	for(i=0;i < MERGEDIST-6;i++){
		if(paths[2*pi + (beststate >> 5)] & (1 << (beststate & 31))){
			beststate |= 64;	/* 2^(K-1) */
		}
		beststate >>= 1;
		pi = (pi - 1) % PATHMEM;
	}
	/* bestpath is now the encoder state on the best path, MERGEDIST
	 * bits back. We continue to chain back until we accumulate
	 * TRACECHUNK bits of decoded data
	 */
	for(j=sizeof(data)-1;j >= 0;j--){
		data[j] = 0;
		for(i=0;i<8;i++){
			if(paths[2*pi + (beststate >> 5)] & (1 << (beststate & 31))){
				beststate |= 64;	/* 2^(K-1) */
				data[j] |= 1 << i;
			}
			beststate >>= 1;
			pi = (pi - 1) % PATHMEM;
		}
	}
	fwrite(data,1,sizeof(data),stdout);
}

/* Final traceback at end of trellis, assuming sender tailed to zero after an
 * integral number of data bytes. Trailing bits are dropped.
 */
static void
flush(unsigned long paths[],unsigned int pi)
{
	int beststate,i,j,n,off;
	unsigned char data[(MERGEDIST+TRACECHUNK)/8];

	beststate = 0;	/* Assume encoder tailing to 0 state */
	n = (MERGEDIST-6 + (pi % TRACECHUNK)) / 8;
	off = (MERGEDIST-6 + (pi % TRACECHUNK)) % 8;	/* ignored partial byte */
	pi = (pi - off - 1) % PATHMEM;
	for(j=n-1;j >= 0;j--){
		data[j] = 0;
		for(i=0;i<8;i++){
			if(paths[2*pi + (beststate >> 5)] & (1 << (beststate & 31))){
				beststate |= 64;	/* 2^(K-1) */
				data[j] |= 1 << i;
			}
			beststate >>= 1;
			pi = (pi - 1) % PATHMEM;
		}
	}
	fwrite(data,1,n,stdout);
}