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int HardwareSerial::availableForWrite(void)
{
  tx_buffer_index_t head;
  tx_buffer_index_t tail;

  TX_BUFFER_ATOMIC {
    head = _tx_buffer_head;
    tail = _tx_buffer_tail;
  }
  if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
  return tail - head - 1;
}

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int HardwareSerial::availableForWrite(void)
{

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  tx_buffer_index_t head;
  tx_buffer_index_t tail;
 
  TX_BUFFER_ATOMIC {
    head = _tx_buffer_head;
    tail = _tx_buffer_tail;
  }
  if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
  return tail - head - 1;
}

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/*
  Copyright (c) 2019 garretlab.
  Added source code explanation by garretlab.
*/

/*
  HardwareSerial.cpp - Hardware serial library for Wiring
  Copyright (c) 2006 Nicholas Zambetti.  All right reserved.

  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., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

  Modified 23 November 2006 by David A. Mellis
  Modified 28 September 2010 by Mark Sproul
  Modified 14 August 2012 by Alarus
  Modified 3 December 2013 by Matthijs Kooijman
*/