RTCM3_Parser.cpp

/*
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
/*
  RTCMv3 parser, used to support moving baseline RTK mode between two
  GPS modules
*/

#include <string.h>
#include <AP_Math/AP_Math.h>
#include "RTCM3_Parser.h"

// reset state
void RTCM3_Parser::reset(void)
{
    pkt_len = 0;
    pkt_bytes = 0;
    found_len = 0;
}

// clear previous packet
void RTCM3_Parser::clear_packet(void)
{
    if (found_len == 0) {
        return;
    }
    // clear previous packet
    if (pkt_bytes > found_len) {
        memmove(&pkt[0], &pkt[found_len], pkt_bytes-found_len);
        pkt_bytes -= found_len;
    } else {
        pkt_bytes = 0;
    }
    found_len = 0;
    pkt_len = 0;
}

// return length of found packet
uint16_t RTCM3_Parser::get_len(const uint8_t *&bytes) const
{
    if (found_len > 0) {
        bytes = &pkt[0];
    }
    return found_len;
}

// return ID of found packet
uint16_t RTCM3_Parser::get_id(void) const
{
    if (found_len == 0) {
        return 0;
    }
    return (pkt[3]<<8 | pkt[4]) >> 4;
}

// look for preamble to try to resync
void RTCM3_Parser::resync(void)
{
    const uint8_t *p = (const uint8_t *)memchr(&pkt[1], RTCMv3_PREAMBLE, pkt_bytes-1);
    if (p != nullptr) {
        const uint16_t idx = p - &pkt[0];
        memmove(&pkt[0], p, pkt_bytes - idx);
        pkt_bytes -= idx;
        if (pkt_bytes >= 3) {
            pkt_len = (pkt[1]<<8 | pkt[2]) & 0x3ff;
        } else {
            pkt_len = 0;
        }
    } else {
        reset();
    }
}

// parse packet
bool RTCM3_Parser::parse(void)
{
    const uint8_t *parity = &pkt[pkt_len+3];
    uint32_t crc1 = (parity[0] << 16) | (parity[1] << 8) | parity[2];
    uint32_t crc2 = crc_crc24(pkt, pkt_len+3);
    if (crc1 != crc2) {
        resync();
        return false;
    }

    // we got a good packet
    found_len = pkt_len+6;
    return true;
}

// read in one byte, return true if a full packet is available
bool RTCM3_Parser::read(uint8_t byte)
{
    clear_packet();

    if (pkt_bytes > 0 && pkt[0] != RTCMv3_PREAMBLE) {
        resync();
    }

    if (pkt_bytes == 0 && byte != RTCMv3_PREAMBLE) {
        // discard
        return false;
    }

    if (pkt_bytes >= sizeof(pkt)) {
        // too long, resync
        resync();
    }

    pkt[pkt_bytes++] = byte;

    if (pkt_len == 0 && pkt_bytes >= 3) {
        pkt_len = (pkt[1]<<8 | pkt[2]) & 0x3ff;
        if (pkt_len == 0) {
            resync();
        }
    }

    if (pkt_len != 0 && pkt_bytes >= pkt_len + 6) {
        // got header, packet body and parity
        return parse();
    }

    // need more bytes
    return false;
}

#ifdef RTCM_MAIN_TEST
/*
  parsing test, taking a raw file captured from UART to u-blox F9
 */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, const char *argv[])
{
    const char *fname = argv[1];
    int fd = ::open(fname, O_RDONLY);
    if (fd == -1) {
        perror(fname);
        ::exit(1);
    }
    RTCM3_Parser parser {};
    uint8_t b;
    while (::read(fd, &b, 1) == 1) {
        if (parser.read(b)) {
            const uint8_t *bytes;
            printf("packet len %u ID %u\n", parser.get_len(bytes), parser.get_id());
        }
    }
    return 0;
}
#endif