raw data compression feature

[yzliao]

two methods:

1. lossless compression
2. avoid bit waste -- append data by bits

[yzliao]

sample code

#if DATA_COMPRESSION_ENABLED
                compress_data(samples_sorted_bytes, uncomp_data_len, comp_samples_sorted_bytes,&comp_data_len);
                if(comp_data_len==0){
                    sf_tdma_disable_tx(); //should we access this via app_connection?
                    leds_off(LEDS_GREEN);
                }
                else{
                    leds_on(LEDS_GREEN);
                    sf_tdma_enable_tx();
                    app_conn_send(comp_samples_sorted_bytes,sizeof(uint8_t)*comp_data_len);
                }

void compress_data(const uint8_t *uncomp_data, uint8_t data_len, const uint8_t *comp_data, uint8_t *comp_data_len)
{

#ifdef I2C_SENSOR
    int acc_y, acc_z;
    acc_y = (uncomp_data[2]<<8)+uncomp_data[3];
    acc_z = (uncomp_data[4]<<8)+uncomp_data[5];
    if(abs_value(acc_y)>=abs_value(acc_z)){
        *comp_data_len= data_len;
        copy_byte_array(uncomp_data,comp_data,data_len);
    }
    else{
        *comp_data_len = 0;
    }
#endif

#ifdef ADC_SENSOR
    *comp_data_len= data_len;
    copy_byte_array(uncomp_data,comp_data,data_len);
#endif

}

void copy_byte_array(uint8_t * from, uint8_t * to, const uint8_t len)
{
    int i;
    for(i=0;i<len;i++)
        *(to+i)=*(from+i);
}