descramble.h

/* descramble.h */

// Functions for processing the radio-header:
// FECdecoder

// Viterbi decoder using Traceback method. 

// Original Source was written by Sho Tamaoki and Tom Wada
// See http://www.lsi.ie.u-ryukyu.ac.jp/~sho/midterm/
//  Modified by Satoshi Yasuda 7m3tjz/ad6gz
// Modified by Jonathan Nayor, G4KLX (C) 2009

// Converted from C++ to C by Kristoff Bonne, ON1ARF

// (C) 2011 Jonathan Naylor G4KLX

/*
 *	This program is free software; you can redistribute it and/or modify
 *	it under the terms of the GNU General Public License as published by
 *	the Free Software Foundation; version 2 of the License.
 *
 *	This program is distributed in the hope that it will be useful,
 *	but WITHOUT ANY WARRANTY; without even the implied warranty of
 *	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *	GNU General Public License for more details.
 */
 
// This code was originally written by JOnathan Naylor, G4KLX, as part
// of the "pcrepeatercontroller" project
// More info:
// http://groups.yahoo.com/group/pcrepeatercontroller

// Changes:
// Convert C++ to C

// Version 20111106: initial release
#include <stdint.h>
#include <string.h>

// function traceBack
int traceBack (int * out, int * m_pathMemory0, int * m_pathMemory1, int * m_pathMemory2, int * m_pathMemory3) {
	enum FEC_STATE { S0, S1, S2, S3 } state;
	int loop;
	int length=0;

	state=S0;

	for (loop=329; loop >= 0; loop--, length++) {
		switch (state) {
			case S0: // if state S0
				if (m_pathMemory0[loop]) {
					state = S2; // lower path
				} else {
					state = S0; // upper path
				} // end else - if
				out[loop]=0;
				break;
			case S1: // if state S1
				if (m_pathMemory1[loop]) {
					state = S2; // lower path
				} else {
					state = S0; // upper path
				} // end else - if
				out[loop]=1;
				break;
			case S2: // if state S2
				if (m_pathMemory2[loop]) {
					state = S3; // lower path
				} else {
					state = S1; // upper path
				} // end else - if
				out[loop]=0;
				break;
			case S3: // if state S3
				if (m_pathMemory3[loop]) {
					state = S3; // lower path
				} else {
					state = S1; // upper path
				} // end else - if
				out[loop]=1;
				break;
		} // end switch
	} // end for

    return(length);
} // end function

// function viterbiDecode
void viterbiDecode (int n, int *data, int *m_pathMemory0, int *m_pathMemory1, int *m_pathMemory2, int *m_pathMemory3, int *m_pathMetric) {
	int tempMetric[4];
	int metric[8];
	int loop;

	int m1;
	int m2;

	metric[0]=(data[1]^0)+(data[0]^0);
	metric[1]=(data[1]^1)+(data[0]^1);
	metric[2]=(data[1]^1)+(data[0]^0);
	metric[3]=(data[1]^0)+(data[0]^1);
	metric[4]=(data[1]^1)+(data[0]^1);
	metric[5]=(data[1]^0)+(data[0]^0);
	metric[6]=(data[1]^0)+(data[0]^1);
	metric[7]=(data[1]^1)+(data[0]^0);

	// Pres. state = S0, Prev. state = S0 & S2
	m1=metric[0]+m_pathMetric[0];
	m2=metric[4]+m_pathMetric[2];
	if (m1<m2) {
		m_pathMemory0[n]=0;
		tempMetric[0]=m1;
	} else {
		m_pathMemory0[n]=1;
		tempMetric[0]=m2;
	} // end else - if

	// Pres. state = S1, Prev. state = S0 & S2
	m1=metric[1]+m_pathMetric[0];
	m2=metric[5]+m_pathMetric[2];
	if (m1<m2) {
		m_pathMemory1[n]=0;
		tempMetric[1]=m1;
	} else {
		m_pathMemory1[n]=1;
		tempMetric[1]=m2;
	} // end else - if

	// Pres. state = S2, Prev. state = S2 & S3
	m1=metric[2]+m_pathMetric[1];
	m2=metric[6]+m_pathMetric[3];
	if (m1<m2) {
		m_pathMemory2[n]=0;
		tempMetric[2]=m1;
	} else {
		m_pathMemory2[n]=1;
		tempMetric[2]=m2;
	}

	// Pres. state = S3, Prev. state = S1 & S3
	m1=metric[3]+m_pathMetric[1];
	m2=metric[7]+m_pathMetric[3];
	if (m1 < m2) {
		m_pathMemory3[n]=0;
		tempMetric[3]=m1;
	} else {
		m_pathMemory3[n]=1;
		tempMetric[3]=m2;
	} // end else - if

	for (loop=0;loop<4;loop++) {
		m_pathMetric[loop]=tempMetric[loop];
	} // end for

} // end function ViterbiDecode

// function FECdecoder 
// returns outlen
int FECdecoder (int * in, int * out) {
    int outLen;

    int m_pathMemory0[330];
    int m_pathMemory1[330];
    int m_pathMemory2[330];
    int m_pathMemory3[330];
    int m_pathMetric[4];

    int loop,loop2;

    int n=0;

    memset(m_pathMemory0,0,330*sizeof(int));
    memset(m_pathMemory1,0,330*sizeof(int));
    memset(m_pathMemory2,0,330*sizeof(int));
    memset(m_pathMemory3,0,330*sizeof(int));

    for (loop=0;loop<4;loop++) {
	    m_pathMetric[loop]=0;
    } // end for

    for (loop2=0;loop2<660;loop2+=2, n++) {
	    int data[2];

	    if (in[loop2]) {
		    data[1]=1;
	    } else {
		    data[1]=0;
	    } // end else - if

	    if (in[loop2+1]) {
		    data[0]=1;
	    } else {
		    data[0]=0;
	    } // end else - if

	    viterbiDecode(n, data, m_pathMemory0, m_pathMemory1, m_pathMemory2, m_pathMemory3, m_pathMetric);
    } // end for
    outLen=traceBack(out, m_pathMemory0, m_pathMemory1, m_pathMemory2, m_pathMemory3);

    // Swap endian-ness
    // code removed (done converting bits into octets), done in main program

    //for (loop=0;loop<330;loop+=8) {
    //	int temp;
    //	temp=out[loop];out[loop]=out[loop+7];out[loop+7]=temp;
    //	temp=out[loop+1];out[loop+1]=out[loop+6];out[loop+6]=temp;
    //	temp=out[loop+2];out[loop+2]=out[loop+5];out[loop+5]=temp;
    //	temp=out[loop+3];out[loop+3]=out[loop+4];out[loop+4]=temp;
    //}

    return(outLen);
} // end function FECdecoder