File content as of revision 16:c3c6b13c3c42:

#include "AlohaPacket.h"

uint8_t crc8(const uint8_t *data, int len)
{
    unsigned int crc = 0;
    
    for (int j = len; j; j--, data++)
    {
        crc ^= (*data << 8);
        for (int i = 8; i; i--)
        {
            if (crc & 0x8000)
            {
                crc ^= (0x1070 << 3);
            }
            crc <<= 1;
        }
    }
    
    return (uint8_t)(crc >> 8);
}

bool dissectAlohaPacket(uint8_t *input, HeaderStruct *header, DataStruct *data)
{
    // assume user has already prepare memory in *in
    
    // get header
    if (header)
    {
        header->fid = input[0] >> 4; // higher four bits
        header->no = input[0] & 0x0f; // lower four bits
    }
    
    // get data
    if (data)
    {
        data->pd0 = input[1];
        data->pd1 = input[2];
    }
    
    // check crc
    if (header && data)
    {
        return input[3] == crc8(input, 3);
    }
    else
    {
        return false;
    }
}

void createAlohaPacket(uint8_t *output, HeaderStruct *header, DataStruct *data)
{
    // assume user has already allocated memory for output
    
    // create header
    if (header)
    {
        output[0] = header->no;
        output[0] |= header->fid << 4;
    }
    
    // fit data
    if (data)
    {
        output[1] = data->pd0;
        output[2] = data->pd1;
    }
    
    // calculate CRC
    if (header && data)
    {
        output[3] = crc8(output, 3);
    }
}