an_packet_protocol.cpp

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/****************************************************************/
/*                                                              */
/*          Advanced Navigation Packet Protocol Library         */
/*          C Language Dynamic Spatial SDK, Version 2.3         */
/*   Copyright 2013, Xavier Orr, Advanced Navigation Pty Ltd    */
/*                                                              */
/****************************************************************/
/*
 * Copyright (C) 2013 Advanced Navigation Pty Ltd
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 */

#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include "an_packet_protocol.h"

/*
 * Function to calculate the CRC16 of data
 * CRC16-CCITT
 * Initial value = 0xFFFF
 * Polynomial = x^16 + x^12 + x^5 + x^0
 */
uint16_t calculate_crc16(const void *data, uint16_t length)
{
        uint8_t *bytes = (uint8_t *) data;
        uint16_t crc = 0xFFFF, i;
        for (i = 0; i < length; i++)
        {
                crc = (uint16_t)((crc << 8) ^ crc16_table[(crc >> 8) ^ bytes[i]]);
        }
        return crc;
}

/*
 * Function to calculate a 4 byte LRC
 */
uint8_t calculate_header_lrc(uint8_t *data)
{
        return ((data[0] + data[1] + data[2] + data[3]) ^ 0xFF) + 1;
}

/*
 * Function to dynamically allocate an an_packet
 */
an_packet_t *an_packet_allocate(uint8_t length, uint8_t id)
{
        an_packet_t *an_packet = (an_packet_t *)malloc(sizeof(an_packet_t) + length * sizeof(uint8_t));
        if (an_packet != NULL)
        {
                an_packet->id = id;
                an_packet->length = length;
        }
        return an_packet;
}

/*
 * Function to free an an_packet
 */
void an_packet_free(an_packet_t **an_packet)
{
        free(*an_packet);
        *an_packet = NULL;
}

/*
 * Initialise the decoder
 */
void an_decoder_initialise(an_decoder_t *an_decoder)
{
        an_decoder->buffer_length = 0;
        an_decoder->crc_errors = 0;
}

/*
 * Function to decode an_packets from raw data
 * Returns a pointer to the packet decoded or NULL if no packet was decoded
 */
an_packet_t *an_packet_decode(an_decoder_t *an_decoder)
{
        uint16_t decode_iterator = 0;
        an_packet_t *an_packet = NULL;
        uint8_t header_lrc, id, length;
        uint16_t crc;

        while (decode_iterator + AN_PACKET_HEADER_SIZE <= an_decoder->buffer_length)
        {
                header_lrc = an_decoder->buffer[decode_iterator++];
                if (header_lrc == calculate_header_lrc(&an_decoder->buffer[decode_iterator]))
                {
                        id = an_decoder->buffer[decode_iterator++];
                        length = an_decoder->buffer[decode_iterator++];
                        crc = an_decoder->buffer[decode_iterator++];
                        crc |= an_decoder->buffer[decode_iterator++] << 8;

                        if (decode_iterator + length > an_decoder->buffer_length)
                        {
                                decode_iterator -= AN_PACKET_HEADER_SIZE;
                                break;
                        }

                        if (crc == calculate_crc16(&an_decoder->buffer[decode_iterator], length))
                        {
                                an_packet = an_packet_allocate(length, id);
                                if (an_packet != NULL)
                                {
                                        memcpy(an_packet->header, &an_decoder->buffer[decode_iterator - AN_PACKET_HEADER_SIZE], AN_PACKET_HEADER_SIZE * sizeof(uint8_t));
                                        memcpy(an_packet->data, &an_decoder->buffer[decode_iterator], length * sizeof(uint8_t));
                                }
                                decode_iterator += length;
                                break;
                        }
                        else
                        {
                                decode_iterator -= (AN_PACKET_HEADER_SIZE - 1);
                                an_decoder->crc_errors++;
                        }
                }
        }
        if (decode_iterator < an_decoder->buffer_length)
        {
                if (decode_iterator > 0)
                {
                        memmove(&an_decoder->buffer[0], &an_decoder->buffer[decode_iterator], (an_decoder->buffer_length - decode_iterator) * sizeof(uint8_t));
                        an_decoder->buffer_length -= decode_iterator;
                }
        }
        else an_decoder->buffer_length = 0;

        return an_packet;
}

/*
 * Function to encode an an_packet
 */
void an_packet_encode(an_packet_t *an_packet)
{
        uint16_t crc;
        an_packet->header[1] = an_packet->id;
        an_packet->header[2] = an_packet->length;
        crc = calculate_crc16(an_packet->data, an_packet->length);
        memcpy(&an_packet->header[3], &crc, sizeof(uint16_t));
        an_packet->header[0] = calculate_header_lrc(&an_packet->header[1]);
}