Program Listing for File lms_buffer.h

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/*
 * lms_buffer.h
 *
 *  Author: Mike Purvis <mpurvis@clearpathrobotics.com>
 *  Author: Shravan S Rai <shravansomashekara.rai@gmail.com>
 ***************************************************************************
 *   This library is free software; you can redistribute it and/or         *
 *   modify it under the terms of the GNU Lesser General Public            *
 *   License as published by the Free Software Foundation; either          *
 *   version 2.1 of the License, or (at your option) any later version.    *
 *                                                                         *
 *   This library 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     *
 *   Lesser General Public License for more details.                       *
 *                                                                         *
 *   You should have received a copy of the GNU Lesser General Public      *
 *   License along with this library; if not, write to the Free Software   *
 *   Foundation, Inc., 59 Temple Place,                                    *
 *   Suite 330, Boston, MA  02111-1307  USA                                *
 *                                                                         *
 ***************************************************************************/

#ifndef LMS1XX_LMS_BUFFER_H_
#define LMS1XX_LMS_BUFFER_H_

#include <stdint.h>
#include <string.h>
#include <unistd.h>


#define LMS_BUFFER_SIZE 50000
#define LMS_STX 0x02
#define LMS_ETX 0x03


class LMSBuffer
{
public:
  LMSBuffer() : total_length_(0), end_of_first_message_(0)
  {
  }

  void readFrom(int fd)
  {
    int ret = read(fd, buffer_ + total_length_, sizeof(buffer_) - total_length_);

    if (ret > 0)
    {
      total_length_ += ret;
      //printf("Read %d bytes from fd, total length is %d.", ret, total_length_);
    }
    else
    {
      //printf("Buffer read() returned error.");
    }
  }

  char* getNextBuffer()
  {
    if (total_length_ == 0)
    {
      // Buffer is empty, no scan data present.
      //printf("Empty buffer, nothing to return.");
      return NULL;
    }

    // The objective is to have a message starting at the start of the buffer, so if that's not
    // the case, then we look for a start-of-message character and shift the buffer back, discarding
    // any characters in the middle.
    char* start_of_message = (char*)memchr(buffer_, LMS_STX, total_length_);
    if (start_of_message == NULL)
    {
      // None found, buffer reset.
      //printf("No STX found, dropping %d bytes from buffer.", total_length_);
      total_length_ = 0;
    }
    else if (buffer_ != start_of_message)
    {
      // Start of message found, ahead of the start of buffer. Therefore shift the buffer back.
      //printf("Shifting buffer, dropping %ld bytes, %ld bytes remain.",
      //        (start_of_message - buffer_), total_length_ - (start_of_message - buffer_));
      shiftBuffer(start_of_message);
    }

    // Now look for the end of message character.
    end_of_first_message_ = (char*)memchr(buffer_, LMS_ETX, total_length_);
    if (end_of_first_message_ == NULL)
    {
      // No end of message found, therefore no message to parse and return.
      //printf("No ETX found, nothing to return.");
      return NULL;
    }

    // Null-terminate buffer.
    *end_of_first_message_ = 0;
    return buffer_;
  }

  void popLastBuffer()
  {
    if (end_of_first_message_)
    {
      shiftBuffer(end_of_first_message_ + 1);
      end_of_first_message_ = NULL;
    }
  }

private:
  void shiftBuffer(char* new_start)
  {
    // Shift back anything remaining in the buffer.
    uint16_t remaining_length = total_length_ - (new_start - buffer_);

    if (remaining_length > 0)
    {
      memmove(buffer_, new_start, remaining_length);
    }
    total_length_ = remaining_length;
  }

  char buffer_[LMS_BUFFER_SIZE];
  uint16_t total_length_;

  char* end_of_first_message_;
};

#endif  // LMS1XX_LMS_BUFFER_H_