M17_Implementations/SP5WWP/m17-packet/viterbi.c

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include "viterbi.h"

#define K			5               //constraint length
#define NUM_STATES	(1 << (K - 1))  //number of states

//vars
uint32_t prevMetrics[NUM_STATES];
uint32_t currMetrics[NUM_STATES];

uint32_t prevMetricsData[NUM_STATES];
uint32_t currMetricsData[NUM_STATES];

uint16_t history[244];

/**
 * Decode unpunctured convolutionally encoded data.
 *
 * @param out: destination array where decoded data is written.
 * @param in: input data.
 * @param len: input length in bits
 * @return number of bit errors corrected.
 */
uint32_t decode(uint8_t* out, const uint16_t* in, uint16_t len)
{
    if(len > 244*2)
		fprintf(stderr, "Input size exceeds max history\n");

    reset();

    size_t pos = 0;
    for(size_t i = 0; i < len; i += 2)
    {
        uint16_t s0 = in[i];
        uint16_t s1 = in[i + 1];

        decodeBit(s0, s1, pos);
        pos++;
    }

    return chainback(out, pos, len/2);
}

/**
 * Decode punctured convolutionally encoded data.
 *
 * @param out: destination array where decoded data is written.
 * @param in: input data.
 * @param punct: puncturing matrix.
 * @param in_len: input data length.
 * @param p_len: puncturing matrix length (entries).
 * @return number of bit errors corrected.
 */
uint32_t decodePunctured(uint8_t* out, const uint16_t* in, const uint8_t* punct, const uint16_t in_len, const uint16_t p_len)
{
    if(in_len > 244*2)
		fprintf(stderr, "Input size exceeds max history\n");

	uint16_t umsg[244*2];           //unpunctured message
	uint8_t p=0;		            //puncturer matrix entry
	uint16_t u=0;		            //bits count - unpunctured message
    uint16_t i=0;                   //bits read from the input message

	while(i<in_len)
	{
		if(punct[p])
		{
			umsg[u]=in[i];
			i++;
		}
		else
		{
			umsg[u]=0x7FFF;
		}

		u++;
		p++;
		p%=p_len;
	}

    return decode(out, umsg, u) - (u-in_len)*0x7FFF;
}

/**
 * Decode one bit and update trellis.
 *
 * @param s0: cost of the first symbol.
 * @param s1: cost of the second symbol.
 * @param pos: bit position in history.
 */
void decodeBit(uint16_t s0, uint16_t s1, size_t pos)
{
    static const uint16_t COST_TABLE_0[] = {0, 0, 0, 0, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF};
    static const uint16_t COST_TABLE_1[] = {0, 0xFFFF, 0xFFFF, 0, 0, 0xFFFF, 0xFFFF, 0};

    for(uint8_t i = 0; i < NUM_STATES/2; i++)
    {
        uint32_t metric = q_AbsDiff(COST_TABLE_0[i], s0)
                        + q_AbsDiff(COST_TABLE_1[i], s1);


        uint32_t m0 = prevMetrics[i] + metric;
        uint32_t m1 = prevMetrics[i + NUM_STATES/2] + (0x1FFFE - metric);

        uint32_t m2 = prevMetrics[i] + (0x1FFFE - metric);
        uint32_t m3 = prevMetrics[i + NUM_STATES/2] + metric;

        uint8_t i0 = 2 * i;
        uint8_t i1 = i0 + 1;

        if(m0 >= m1)
        {
            history[pos]|=(1<<i0);
            currMetrics[i0] = m1;
        }
        else
        {
            history[pos]&=~(1<<i0);
            currMetrics[i0] = m0;
        }

        if(m2 >= m3)
        {
            history[pos]|=(1<<i1);
            currMetrics[i1] = m3;
        }
        else
        {
            history[pos]&=~(1<<i1);
            currMetrics[i1] = m2;
        }
    }

    //swap
    uint32_t tmp[NUM_STATES];
    for(uint8_t i=0; i<NUM_STATES; i++)
    {
    	tmp[i]=currMetrics[i];
	}
	for(uint8_t i=0; i<NUM_STATES; i++)
    {
    	currMetrics[i]=prevMetrics[i];
    	prevMetrics[i]=tmp[i];
	}
}

/**
 * History chainback to obtain final byte array.
 *
 * @param out: destination byte array for decoded data.
 * @param pos: starting position for the chainback.
 * @param len: length of the output in bits.
 * @return minimum Viterbi cost at the end of the decode sequence.
 */
uint32_t chainback(uint8_t* out, size_t pos, uint16_t len)
{
    uint8_t state = 0;
    size_t bitPos = len+4;

    memset(out, 0, (len-1)/8+1);

    while(pos > 0)
    {
        bitPos--;
        pos--;
        uint16_t bit = history[pos]&((1<<(state>>4)));
        state >>= 1;
        if(bit)
        {
        	state |= 0x80;
        	out[bitPos/8]|=1<<(7-(bitPos%8));
		}
    }

    uint32_t cost = prevMetrics[0];

    for(size_t i = 0; i < NUM_STATES; i++)
    {
        uint32_t m = prevMetrics[i];
        if(m < cost) cost = m;
    }

    return cost;
}

/**
 * Utility function to compute the absolute value of a difference between
 * two fixed-point values.
 *
 * @param v1: first value
 * @param v2: second value
 * @return abs(v1-v2)
 */
uint16_t q_AbsDiff(const uint16_t v1, const uint16_t v2)
{
    if(v2 > v1) return v2 - v1;
    return v1 - v2;
}

/**
 * Reset the decoder state.
 *
 */
void reset(void)
{
	memset((uint8_t*)history, 0, 2*244);
	memset((uint8_t*)currMetrics, 0, 4*NUM_STATES);
    memset((uint8_t*)prevMetrics, 0, 4*NUM_STATES);
    memset((uint8_t*)currMetricsData, 0, 4*NUM_STATES);
    memset((uint8_t*)prevMetricsData, 0, 4*NUM_STATES);
}