protocol.c

/**
 * Proof-of-concept for establishing design bassis for an extremely compact
 * data transfer protocol for wireless data transmission
 *
 * The idea is to have a protocol that is flexible in that arbitrary data types
 * can be passed without the wasted space that would be caused by a struct-based
 * approach that would need to reserve more space than required for a given message
 *
 * This example uses a byte for each data point to determine its type. This seems
 * wasteful to use up a full extra byte for each data type, however, as seen with
 * Node 5 in the example below, the end goal is to have abstract types associated
 * with sensor data. Thus we have a type-space of 255 possible sensor types which
 * will carry semantic weight as well as deeper indication of how to parse out the
 * subsequent bytes
 *
 * The general messaging format will look like:
 *
 * NODE_ID MSG_ID MSG_COUNT DATA_TYPE DATA DATA_TYPE DATA ... etc
 *
 * e.g. Air quality sample data point #100 from node 3 might look like:
 *
 * 3 100 1 AIR_QUALITY 20 1234567
 *
 * where AIR_QAULITY is a constant in the type space, 20 is some meaningful value
 * for the specified type and 1234567 is a bogus timestamp. The protocol would
 * know to parse out the IDs, and the number of records, and then parse the
 * remainder of the message according to the type and data of each record
 *
 * Or another example which does not require a timestamp:
 *
 * 3 101 BATTERY_LEVEL 39
 *
 * Where the BATTERY_LEVEL data type is protocol-determined to be divided by 10
 *
 * In reality, there will be multiple nodes each with multiple data points. So,
 * for example:
 *
 * 2 1 3 BAT 39 GPS 3 42.04507 -87.68770 DUST 20 12345 3 1 1 BAT 42
 *
 * Is a messages containing Node 2 data message #1, with battery, GPS, and
 * air quality records (with the latter having a timestamp) and Node 3 having
 * only a battery value to deliver
 *
 * The following example does some sanity checking on standard numerica data
 * types, then includes a sensor node data set for Node 5
 *
 **/
#include <stdio.h>
#include <stdlib.h>

#define CHAR 1
#define INT8 2
#define UINT8 3
#define INT16 4
#define UINT16 5

#define BATTERY 6
#define GPS 7
#define AIR_PARTICULATE 8


/** Utilities for traversing the byte message */

uint8_t next_8_bit(uint8_t* data, int len, int* index, uint8_t* val)
{
    *val = data[*index];
    *index += 1;
    if (*index > len) {
        printf("\nBAD BYTE SEQUENCE\n");
        return 0;
    }
    return 1;
}

uint8_t next_16_bit(uint8_t* data, int len, int* index, uint16_t* val)
{
    if (*index > len-2) {
        printf("\nBAD BYTE SEQUENCE: Not enough data for 16-bit value\n");
        return 0;
    }
    *val = (data[*index] << 8) | (data[*index+1] & 0xff);
    *index += 2;
    return 1;
}

uint8_t next_32_bit(uint8_t* data, int len, int* index, uint32_t* val)
{
    if (*index > len-4) {
        printf("\nBAD BYTE SEQUENCE: Not enough data for 32-bit value\n");
        return 0;
    }
    *val = (data[*index] << 24)
         | (data[*index+1] << 16)
         | (data[*index+2] << 8)
         | (data[*index+3] & 0xff);
    *index += 4;
    return 1;
}

void printval8(uint8_t val, uint8_t data_type)
{
    if (data_type == CHAR) {
        printf("%c ", (char)val);
    } else if (data_type == INT8) {
        printf("%d ", (int8_t)val);
    } else if (data_type == UINT8) {
        printf("%d ", val);
    }
}

void printval16(uint16_t val, uint8_t data_type)
{
    if (data_type == INT16) {
        printf("%d ", (int16_t)val);
    } else if (data_type == UINT16) {
        printf("%d ", val);
    }
}

/**
 * Floating points are not supported. Instead, types will be protocol defined
 * to divide by a power of 10 according to required precision for that specific
 * data type
 */

/**
 * This example byte sequence will result in human-readable print of:
 *
 * NODE ID: 3; MSG ID: 1; MSG COUNT: 8; MESSAGES:
 * ! ~ A Z a z 0 255
 *
 * NODE ID: 4; MSG ID: 1; MSG COUNT: 7; MESSAGES:
 * -128 127 65535 0 32767 -32768 0
 *
 * NODE ID: 5; MSG ID: 1; MSG COUNT: 3; MESSAGES:
 * BAT: 3.9; SATS: 3; LAT: 42.04507; LON: -87.68770; DUST: 20; TIMESTAMP: 12345;
 *
 **/
int main(int argc, char** argv)
{
    int32_t latitude = 4204507;
    int32_t longitude = -8768770;
    uint8_t example[] = {

        // Node 3, some 8-bit testing
        3, 1, 8,
        CHAR, '!',
        CHAR, '~',
        CHAR, 'A',
        CHAR, 'Z',
        CHAR, 'a',
        CHAR, 'z',

        UINT8, 0,
        UINT8, 255,

        // Node 4, 8 and 16-bit testing
        4, 1, 7,

        INT8, -128,
        INT8, 127,

        UINT16, 65535 >> 8, 65535 & 0xff,
        UINT16, 0 >> 8, 0 & 0xff,

        INT16, 32767 >> 8, 32767 & 0xff,
        INT16, -32768 >> 8, -32768 & 0xff,
        INT16, 0 >> 8, 0 & 0xff,

        // Node 5 starts to look like real sensor data
        5, 1, 3,

        BATTERY, 39,
        GPS, 3,
        latitude >> 24, latitude >> 16,
        latitude >> 8, latitude & 0xff,

        longitude >> 24, longitude >> 16,
        longitude >> 8, longitude & 0xff,

        AIR_PARTICULATE, 20, 12345 >> 8, 12345 & 0xff
    };
    int len = sizeof(example);

    uint8_t node_id;
    uint8_t msg_id;
    uint8_t record_count;
    uint8_t data_type;
    uint8_t val8;
    uint16_t val16;
    uint32_t val32;
    for (int i=0; i+4<sizeof(example);) { // min 5 bytes for a node message
        node_id = example[i++];
        msg_id = example[i++];
        record_count = example[i++];
        printf("NODE ID: %d; MSG ID: %d; MSG COUNT: %d; MESSAGES:\n",
            node_id, msg_id, record_count);
        for (int j=0; j<record_count; j++) {
            data_type = example[i++];
            switch(data_type) {
                /* 8-bit type testing */
                case CHAR :
                case INT8 :
                case UINT8 :
                    if (!next_8_bit(example, len, &i, &val8)) break;
                    printval8(val8, data_type);
                    break;
                /* 16-bit type testing */
                case INT16 :
                case UINT16 :
                    if (!next_16_bit(example, len, &i, &val16)) break;
                    printval16(val16, data_type);
                    break;
                /* abstract types */
                case BATTERY :
                    if (!next_8_bit(example, len, &i, &val8)) break;
                    printf("BAT: %2.1f; ", val8 / 10.0);
                    break;
                case GPS :
                    if (!next_8_bit(example, len, &i, &val8)) break;
                    printf("SATS: %d; ", val8);
                    if (!next_32_bit(example, len, &i, &val32)) break;
                    printf("LAT: %8.5f; ", (int32_t)val32 / 100000.0);
                    if (!next_32_bit(example, len, &i, &val32)) break;
                    printf("LON: %8.5f; ", (int32_t)val32 / 100000.0);
                    break;
                case AIR_PARTICULATE :
                    if (!next_8_bit(example, len, &i, &val8)) break;
                    printf("DUST: %d; ", val8);
                    if (!next_16_bit(example, len, &i, &val16)) break;
                    printf("TIMESTAMP: %d; ", val16);
                    break;
            }
        }
        printf("\n\n");
    }
}