mavlink_helpers.h

#ifndef  _MAVLINK_HELPERS_H_
#define  _MAVLINK_HELPERS_H_

#include "string.h"
#include "checksum.h"
#include "mavlink_types.h"
#include "mavlink_conversions.h"

#ifndef MAVLINK_HELPER
#define MAVLINK_HELPER
#endif

/*
 * Internal function to give access to the channel status for each channel
 */
#ifndef MAVLINK_GET_CHANNEL_STATUS
MAVLINK_HELPER mavlink_status_t* mavlink_get_channel_status(uint8_t chan)
{
#ifdef MAVLINK_EXTERNAL_RX_STATUS
	// No m_mavlink_status array defined in function,
	// has to be defined externally
#else
	static mavlink_status_t m_mavlink_status[MAVLINK_COMM_NUM_BUFFERS];
#endif
	return &m_mavlink_status[chan];
}
#endif

/*
 * Internal function to give access to the channel buffer for each channel
 */
#ifndef MAVLINK_GET_CHANNEL_BUFFER
MAVLINK_HELPER mavlink_message_t* mavlink_get_channel_buffer(uint8_t chan)
{
	
#ifdef MAVLINK_EXTERNAL_RX_BUFFER
	// No m_mavlink_buffer array defined in function,
	// has to be defined externally
#else
	static mavlink_message_t m_mavlink_buffer[MAVLINK_COMM_NUM_BUFFERS];
#endif
	return &m_mavlink_buffer[chan];
}
#endif

/**
 * @brief Reset the status of a channel.
 */
MAVLINK_HELPER void mavlink_reset_channel_status(uint8_t chan)
{
	mavlink_status_t *status = mavlink_get_channel_status(chan);
	status->parse_state = MAVLINK_PARSE_STATE_IDLE;
}

/**
 * @brief Finalize a MAVLink message with channel assignment
 *
 * This function calculates the checksum and sets length and aircraft id correctly.
 * It assumes that the message id and the payload are already correctly set. This function
 * can also be used if the message header has already been written before (as in mavlink_msg_xxx_pack
 * instead of mavlink_msg_xxx_pack_headerless), it just introduces little extra overhead.
 *
 * @param msg Message to finalize
 * @param system_id Id of the sending (this) system, 1-127
 * @param length Message length
 */
#if MAVLINK_CRC_EXTRA
MAVLINK_HELPER uint16_t mavlink_finalize_message_buffer(mavlink_message_t* msg, uint8_t system_id, uint8_t component_id,
                                                        mavlink_status_t* status, uint8_t min_length, uint8_t length, uint8_t crc_extra)
#else
MAVLINK_HELPER uint16_t mavlink_finalize_message_buffer(mavlink_message_t* msg, uint8_t system_id, uint8_t component_id,
                                                        mavlink_status_t* status, uint8_t length)
#endif
{
	// This is only used for the v2 protocol and we silence it here
	(void)min_length;
	// This code part is the same for all messages;
	msg->magic = MAVLINK_STX;
	msg->len = length;
	msg->sysid = system_id;
	msg->compid = component_id;
    msg->seq = status->current_tx_seq;
    status->current_tx_seq = status->current_tx_seq+1;
	msg->checksum = crc_calculate(((const uint8_t*)(msg)) + 3, MAVLINK_CORE_HEADER_LEN);
	crc_accumulate_buffer(&msg->checksum, _MAV_PAYLOAD(msg), msg->len);
#if MAVLINK_CRC_EXTRA
	crc_accumulate(crc_extra, &msg->checksum);
#endif
	mavlink_ck_a(msg) = (uint8_t)(msg->checksum & 0xFF);
	mavlink_ck_b(msg) = (uint8_t)(msg->checksum >> 8);

	return length + MAVLINK_NUM_NON_PAYLOAD_BYTES;
}

/**
 * @brief Finalize a MAVLink message with channel assignment
 *
 * This function calculates the checksum and sets length and aircraft id correctly.
 * It assumes that the message id and the payload are already correctly set. This function
 * can also be used if the message header has already been written before (as in mavlink_msg_xxx_pack
 * instead of mavlink_msg_xxx_pack_headerless), it just introduces little extra overhead.
 *
 * @param msg Message to finalize
 * @param system_id Id of the sending (this) system, 1-127
 * @param length Message length
 */
#if MAVLINK_CRC_EXTRA
MAVLINK_HELPER uint16_t mavlink_finalize_message_chan(mavlink_message_t* msg, uint8_t system_id, uint8_t component_id, 
						      uint8_t chan, uint8_t min_length, uint8_t length, uint8_t crc_extra)
#else
MAVLINK_HELPER uint16_t mavlink_finalize_message_chan(mavlink_message_t* msg, uint8_t system_id, uint8_t component_id, 
						      uint8_t chan, uint8_t length)
#endif
{
    // This is only used for the v2 protocol and we silence it here
	(void)min_length;
	// This code part is the same for all messages;
	msg->magic = MAVLINK_STX;
	msg->len = length;
	msg->sysid = system_id;
	msg->compid = component_id;
	// One sequence number per channel
	msg->seq = mavlink_get_channel_status(chan)->current_tx_seq;
	mavlink_get_channel_status(chan)->current_tx_seq = mavlink_get_channel_status(chan)->current_tx_seq+1;
	msg->checksum = crc_calculate(((const uint8_t*)(msg)) + 3, MAVLINK_CORE_HEADER_LEN);
	crc_accumulate_buffer(&msg->checksum, _MAV_PAYLOAD(msg), msg->len);
#if MAVLINK_CRC_EXTRA
	crc_accumulate(crc_extra, &msg->checksum);
#endif
	mavlink_ck_a(msg) = (uint8_t)(msg->checksum & 0xFF);
	mavlink_ck_b(msg) = (uint8_t)(msg->checksum >> 8);

	return length + MAVLINK_NUM_NON_PAYLOAD_BYTES;
}


/**
 * @brief Finalize a MAVLink message with MAVLINK_COMM_0 as default channel
 */
#if MAVLINK_CRC_EXTRA
MAVLINK_HELPER uint16_t mavlink_finalize_message(mavlink_message_t* msg, uint8_t system_id, uint8_t component_id, 
						 uint8_t min_length, uint8_t length, uint8_t crc_extra)
{
    return mavlink_finalize_message_chan(msg, system_id, component_id, MAVLINK_COMM_0, min_length, length, crc_extra);
}
#else
MAVLINK_HELPER uint16_t mavlink_finalize_message(mavlink_message_t* msg, uint8_t system_id, uint8_t component_id, 
						 uint8_t length)
{
	return mavlink_finalize_message_chan(msg, system_id, component_id, MAVLINK_COMM_0, length);
}
#endif

#ifdef MAVLINK_USE_CONVENIENCE_FUNCTIONS
MAVLINK_HELPER void _mavlink_send_uart(mavlink_channel_t chan, const char *buf, uint16_t len);

/**
 * @brief Finalize a MAVLink message with channel assignment and send
 */
#if MAVLINK_CRC_EXTRA
MAVLINK_HELPER void _mav_finalize_message_chan_send(mavlink_channel_t chan, uint8_t msgid, const char *packet, 
						    uint8_t min_length, uint8_t length, uint8_t crc_extra)
#else
MAVLINK_HELPER void _mav_finalize_message_chan_send(mavlink_channel_t chan, uint8_t msgid, const char *packet, uint8_t length)
#endif
{
	uint16_t checksum;
	uint8_t buf[MAVLINK_NUM_HEADER_BYTES];
	uint8_t ck[2];
	mavlink_status_t *status = mavlink_get_channel_status(chan);
	buf[0] = MAVLINK_STX;
	buf[1] = length;
	buf[2] = status->current_tx_seq;
	buf[3] = mavlink_system.sysid;
	buf[4] = mavlink_system.compid;
	buf[5] = msgid;
	status->current_tx_seq++;
	checksum = crc_calculate((const uint8_t*)&buf[1], MAVLINK_CORE_HEADER_LEN);
	crc_accumulate_buffer(&checksum, packet, length);
#if MAVLINK_CRC_EXTRA
	crc_accumulate(crc_extra, &checksum);
#endif
	ck[0] = (uint8_t)(checksum & 0xFF);
	ck[1] = (uint8_t)(checksum >> 8);

	MAVLINK_START_UART_SEND(chan, MAVLINK_NUM_NON_PAYLOAD_BYTES + (uint16_t)length);
	_mavlink_send_uart(chan, (const char *)buf, MAVLINK_NUM_HEADER_BYTES);
	_mavlink_send_uart(chan, packet, length);
	_mavlink_send_uart(chan, (const char *)ck, 2);
	MAVLINK_END_UART_SEND(chan, MAVLINK_NUM_NON_PAYLOAD_BYTES + (uint16_t)length);
}

/**
 * @brief re-send a message over a uart channel
 * this is more stack efficient than re-marshalling the message
 */
MAVLINK_HELPER void _mavlink_resend_uart(mavlink_channel_t chan, const mavlink_message_t *msg)
{
	uint8_t ck[2];

	ck[0] = (uint8_t)(msg->checksum & 0xFF);
	ck[1] = (uint8_t)(msg->checksum >> 8);
	// XXX use the right sequence here

	MAVLINK_START_UART_SEND(chan, MAVLINK_NUM_NON_PAYLOAD_BYTES + msg->len);
	_mavlink_send_uart(chan, (const char *)&msg->magic, MAVLINK_NUM_HEADER_BYTES);
	_mavlink_send_uart(chan, _MAV_PAYLOAD(msg), msg->len);
	_mavlink_send_uart(chan, (const char *)ck, 2);
	MAVLINK_END_UART_SEND(chan, MAVLINK_NUM_NON_PAYLOAD_BYTES + msg->len);
}
#endif // MAVLINK_USE_CONVENIENCE_FUNCTIONS

/**
 * @brief Pack a message to send it over a serial byte stream
 */
MAVLINK_HELPER uint16_t mavlink_msg_to_send_buffer(uint8_t *buffer, const mavlink_message_t *msg)
{
	memcpy(buffer, (const uint8_t *)&msg->magic, MAVLINK_NUM_HEADER_BYTES + (uint16_t)msg->len);

	uint8_t *ck = buffer + (MAVLINK_NUM_HEADER_BYTES + (uint16_t)msg->len);

	ck[0] = (uint8_t)(msg->checksum & 0xFF);
	ck[1] = (uint8_t)(msg->checksum >> 8);

	return MAVLINK_NUM_NON_PAYLOAD_BYTES + (uint16_t)msg->len;
}

union __mavlink_bitfield {
	uint8_t uint8;
	int8_t int8;
	uint16_t uint16;
	int16_t int16;
	uint32_t uint32;
	int32_t int32;
};


MAVLINK_HELPER void mavlink_start_checksum(mavlink_message_t* msg)
{
	crc_init(&msg->checksum);
}

MAVLINK_HELPER void mavlink_update_checksum(mavlink_message_t* msg, uint8_t c)
{
	crc_accumulate(c, &msg->checksum);
}

/**
 * This is a variant of mavlink_frame_char() but with caller supplied
 * parsing buffers. It is useful when you want to create a MAVLink
 * parser in a library that doesn't use any global variables
 *
 * @param rxmsg    parsing message buffer
 * @param status   parsing status buffer 
 * @param c        The char to parse
 *
 * @param r_message NULL if no message could be decoded, otherwise the message data
 * @param r_mavlink_status if a message was decoded, this is filled with the channel's stats
 * @return 0 if no message could be decoded, 1 on good message and CRC, 2 on bad CRC
 */
MAVLINK_HELPER uint8_t mavlink_frame_char_buffer(mavlink_message_t* rxmsg, 
                                                 mavlink_status_t* status,
                                                 uint8_t c, 
                                                 mavlink_message_t* r_message, 
                                                 mavlink_status_t* r_mavlink_status)
{
        /*
	  default message crc function. You can override this per-system to
	  put this data in a different memory segment
	*/
#if MAVLINK_CRC_EXTRA
#ifndef MAVLINK_MESSAGE_CRC
	static const uint8_t mavlink_message_crcs[256] = MAVLINK_MESSAGE_CRCS;
#define MAVLINK_MESSAGE_CRC(msgid) mavlink_message_crcs[msgid]
#endif
#endif

	/* Enable this option to check the length of each message.
	   This allows invalid messages to be caught much sooner. Use if the transmission
	   medium is prone to missing (or extra) characters (e.g. a radio that fades in
	   and out). Only use if the channel will only contain messages types listed in
	   the headers.
	*/
#ifdef MAVLINK_CHECK_MESSAGE_LENGTH
#ifndef MAVLINK_MESSAGE_LENGTH
	static const uint8_t mavlink_message_lengths[256] = MAVLINK_MESSAGE_LENGTHS;
#define MAVLINK_MESSAGE_LENGTH(msgid) mavlink_message_lengths[msgid]
#endif
#endif

	status->msg_received = MAVLINK_FRAMING_INCOMPLETE;

	switch (status->parse_state)
	{
	case MAVLINK_PARSE_STATE_UNINIT:
	case MAVLINK_PARSE_STATE_IDLE:
		if (c == MAVLINK_STX)
		{
			status->parse_state = MAVLINK_PARSE_STATE_GOT_STX;
			rxmsg->len = 0;
			rxmsg->magic = c;
			mavlink_start_checksum(rxmsg);
		}
		break;

	case MAVLINK_PARSE_STATE_GOT_STX:
			if (status->msg_received 
/* Support shorter buffers than the
   default maximum packet size */
#if (MAVLINK_MAX_PAYLOAD_LEN < 255)
				|| c > MAVLINK_MAX_PAYLOAD_LEN
#endif
				)
		{
			status->buffer_overrun++;
			status->parse_error++;
			status->msg_received = 0;
			status->parse_state = MAVLINK_PARSE_STATE_IDLE;
		}
		else
		{
			// NOT counting STX, LENGTH, SEQ, SYSID, COMPID, MSGID, CRC1 and CRC2
			rxmsg->len = c;
			status->packet_idx = 0;
			mavlink_update_checksum(rxmsg, c);
			status->parse_state = MAVLINK_PARSE_STATE_GOT_LENGTH;
		}
		break;

	case MAVLINK_PARSE_STATE_GOT_LENGTH:
		rxmsg->seq = c;
		mavlink_update_checksum(rxmsg, c);
		status->parse_state = MAVLINK_PARSE_STATE_GOT_SEQ;
		break;

	case MAVLINK_PARSE_STATE_GOT_SEQ:
		rxmsg->sysid = c;
		mavlink_update_checksum(rxmsg, c);
		status->parse_state = MAVLINK_PARSE_STATE_GOT_SYSID;
		break;

	case MAVLINK_PARSE_STATE_GOT_SYSID:
		rxmsg->compid = c;
		mavlink_update_checksum(rxmsg, c);
		status->parse_state = MAVLINK_PARSE_STATE_GOT_COMPID;
		break;

	case MAVLINK_PARSE_STATE_GOT_COMPID:
#ifdef MAVLINK_CHECK_MESSAGE_LENGTH
	        if (rxmsg->len != MAVLINK_MESSAGE_LENGTH(c))
		{
			status->parse_error++;
			status->parse_state = MAVLINK_PARSE_STATE_IDLE;
			break;
	    }
#endif
		rxmsg->msgid = c;
		mavlink_update_checksum(rxmsg, c);
		if (rxmsg->len == 0)
		{
			status->parse_state = MAVLINK_PARSE_STATE_GOT_PAYLOAD;
		}
		else
		{
			status->parse_state = MAVLINK_PARSE_STATE_GOT_MSGID;
		}
		break;

	case MAVLINK_PARSE_STATE_GOT_MSGID:
		_MAV_PAYLOAD_NON_CONST(rxmsg)[status->packet_idx++] = (char)c;
		mavlink_update_checksum(rxmsg, c);
		if (status->packet_idx == rxmsg->len)
		{
			status->parse_state = MAVLINK_PARSE_STATE_GOT_PAYLOAD;
		}
		break;

	case MAVLINK_PARSE_STATE_GOT_PAYLOAD:
#if MAVLINK_CRC_EXTRA
		mavlink_update_checksum(rxmsg, MAVLINK_MESSAGE_CRC(rxmsg->msgid));
#endif
		if (c != (rxmsg->checksum & 0xFF)) {
			status->parse_state = MAVLINK_PARSE_STATE_GOT_BAD_CRC1;
		} else {
			status->parse_state = MAVLINK_PARSE_STATE_GOT_CRC1;
		}
                _MAV_PAYLOAD_NON_CONST(rxmsg)[status->packet_idx] = (char)c;
		break;

	case MAVLINK_PARSE_STATE_GOT_CRC1:
	case MAVLINK_PARSE_STATE_GOT_BAD_CRC1:
		if (status->parse_state == MAVLINK_PARSE_STATE_GOT_BAD_CRC1 || c != (rxmsg->checksum >> 8)) {
			// got a bad CRC message
			status->msg_received = MAVLINK_FRAMING_BAD_CRC;
		} else {
			// Successfully got message
			status->msg_received = MAVLINK_FRAMING_OK;
                }
                status->parse_state = MAVLINK_PARSE_STATE_IDLE;
                _MAV_PAYLOAD_NON_CONST(rxmsg)[status->packet_idx+1] = (char)c;
                memcpy(r_message, rxmsg, sizeof(mavlink_message_t));
		break;
	}

	// If a message has been successfully decoded, check index
	if (status->msg_received == MAVLINK_FRAMING_OK)
	{
		//while(status->current_seq != rxmsg->seq)
		//{
		//	status->packet_rx_drop_count++;
		//               status->current_seq++;
		//}
		status->current_rx_seq = rxmsg->seq;
		// Initial condition: If no packet has been received so far, drop count is undefined
		if (status->packet_rx_success_count == 0) status->packet_rx_drop_count = 0;
		// Count this packet as received
		status->packet_rx_success_count++;
	}

	r_message->len = rxmsg->len; // Provide visibility on how far we are into current msg
	r_mavlink_status->parse_state = status->parse_state;
	r_mavlink_status->packet_idx = status->packet_idx;
	r_mavlink_status->current_rx_seq = status->current_rx_seq+1;
	r_mavlink_status->packet_rx_success_count = status->packet_rx_success_count;
	r_mavlink_status->packet_rx_drop_count = status->parse_error;
	status->parse_error = 0;

	if (status->msg_received == MAVLINK_FRAMING_BAD_CRC) {
		/*
		  the CRC came out wrong. We now need to overwrite the
		  msg CRC with the one on the wire so that if the
		  caller decides to forward the message anyway that
		  mavlink_msg_to_send_buffer() won't overwrite the
		  checksum
		 */
		r_message->checksum = _MAV_PAYLOAD(rxmsg)[status->packet_idx] | (_MAV_PAYLOAD(rxmsg)[status->packet_idx+1]<<8);
	}

	return status->msg_received;
}

/**
 * This is a convenience function which handles the complete MAVLink parsing.
 * the function will parse one byte at a time and return the complete packet once
 * it could be successfully decoded. This function will return 0, 1 or
 * 2 (MAVLINK_FRAMING_INCOMPLETE, MAVLINK_FRAMING_OK or MAVLINK_FRAMING_BAD_CRC)
 *
 * Messages are parsed into an internal buffer (one for each channel). When a complete
 * message is received it is copies into *r_message and the channel's status is
 * copied into *r_mavlink_status.
 *
 * @param chan     ID of the channel to be parsed.
 *                 A channel is not a physical message channel like a serial port, but a logical partition of
 *                 the communication streams. COMM_NB is the limit for the number of channels
 *                 on MCU (e.g. ARM7), while COMM_NB_HIGH is the limit for the number of channels in Linux/Windows
 * @param c        The char to parse
 *
 * @param r_message NULL if no message could be decoded, the message data else
 * @param r_mavlink_status if a message was decoded, this is filled with the channel's stats
 * @return 0 if no message could be decoded, 1 on good message and CRC, 2 on bad CRC
 *
 * A typical use scenario of this function call is:
 *
 * @code
 * #include <mavlink.h>
 *
 * mavlink_status_t status;
 * mavlink_message_t msg;
 * int chan = 0;
 *
 *
 * while(serial.bytesAvailable > 0)
 * {
 *   uint8_t byte = serial.getNextByte();
 *   if (mavlink_frame_char(chan, byte, &msg, &status) != MAVLINK_FRAMING_INCOMPLETE)
 *     {
 *     printf("Received message with ID %d, sequence: %d from component %d of system %d", msg.msgid, msg.seq, msg.compid, msg.sysid);
 *     }
 * }
 *
 *
 * @endcode
 */
MAVLINK_HELPER uint8_t mavlink_frame_char(uint8_t chan, uint8_t c, mavlink_message_t* r_message, mavlink_status_t* r_mavlink_status)
{
	return mavlink_frame_char_buffer(mavlink_get_channel_buffer(chan),
					 mavlink_get_channel_status(chan),
					 c,
					 r_message,
					 r_mavlink_status);
}


/**
 * This is a convenience function which handles the complete MAVLink parsing.
 * the function will parse one byte at a time and return the complete packet once
 * it could be successfully decoded. This function will return 0 or 1.
 *
 * Messages are parsed into an internal buffer (one for each channel). When a complete
 * message is received it is copies into *r_message and the channel's status is
 * copied into *r_mavlink_status.
 *
 * @param chan     ID of the channel to be parsed.
 *                 A channel is not a physical message channel like a serial port, but a logical partition of
 *                 the communication streams. COMM_NB is the limit for the number of channels
 *                 on MCU (e.g. ARM7), while COMM_NB_HIGH is the limit for the number of channels in Linux/Windows
 * @param c        The char to parse
 *
 * @param r_message NULL if no message could be decoded, otherwise the message data.
 * @param r_mavlink_status if a message was decoded, this is filled with the channel's stats
 * @return 0 if no message could be decoded or bad CRC, 1 on good message and CRC
 *
 * A typical use scenario of this function call is:
 *
 * @code
 * #include <mavlink.h>
 *
 * mavlink_status_t status;
 * mavlink_message_t msg;
 * int chan = 0;
 *
 *
 * while(serial.bytesAvailable > 0)
 * {
 *   uint8_t byte = serial.getNextByte();
 *   if (mavlink_parse_char(chan, byte, &msg, &status))
 *     {
 *     printf("Received message with ID %d, sequence: %d from component %d of system %d", msg.msgid, msg.seq, msg.compid, msg.sysid);
 *     }
 * }
 *
 *
 * @endcode
 */
MAVLINK_HELPER uint8_t mavlink_parse_char(uint8_t chan, uint8_t c, mavlink_message_t* r_message, mavlink_status_t* r_mavlink_status)
{
    uint8_t msg_received = mavlink_frame_char(chan, c, r_message, r_mavlink_status);
    if (msg_received == MAVLINK_FRAMING_BAD_CRC) {
	    // we got a bad CRC. Treat as a parse failure
	    mavlink_message_t* rxmsg = mavlink_get_channel_buffer(chan);
	    mavlink_status_t* status = mavlink_get_channel_status(chan);
	    status->parse_error++;
	    status->msg_received = MAVLINK_FRAMING_INCOMPLETE;
	    status->parse_state = MAVLINK_PARSE_STATE_IDLE;
	    if (c == MAVLINK_STX)
	    {
		    status->parse_state = MAVLINK_PARSE_STATE_GOT_STX;
		    rxmsg->len = 0;
		    mavlink_start_checksum(rxmsg);
	    }
	    return 0;
    }
    return msg_received;
}

/**
 * @brief Put a bitfield of length 1-32 bit into the buffer
 *
 * @param b the value to add, will be encoded in the bitfield
 * @param bits number of bits to use to encode b, e.g. 1 for boolean, 2, 3, etc.
 * @param packet_index the position in the packet (the index of the first byte to use)
 * @param bit_index the position in the byte (the index of the first bit to use)
 * @param buffer packet buffer to write into
 * @return new position of the last used byte in the buffer
 */
MAVLINK_HELPER uint8_t put_bitfield_n_by_index(int32_t b, uint8_t bits, uint8_t packet_index, uint8_t bit_index, uint8_t* r_bit_index, uint8_t* buffer)
{
	uint16_t bits_remain = bits;
	// Transform number into network order
	int32_t v;
	uint8_t i_bit_index, i_byte_index, curr_bits_n;
#if MAVLINK_NEED_BYTE_SWAP
	union {
		int32_t i;
		uint8_t b[4];
	} bin, bout;
	bin.i = b;
	bout.b[0] = bin.b[3];
	bout.b[1] = bin.b[2];
	bout.b[2] = bin.b[1];
	bout.b[3] = bin.b[0];
	v = bout.i;
#else
	v = b;
#endif

	// buffer in
	// 01100000 01000000 00000000 11110001
	// buffer out
	// 11110001 00000000 01000000 01100000

	// Existing partly filled byte (four free slots)
	// 0111xxxx

	// Mask n free bits
	// 00001111 = 2^0 + 2^1 + 2^2 + 2^3 = 2^n - 1
	// = ((uint32_t)(1 << n)) - 1; // = 2^n - 1

	// Shift n bits into the right position
	// out = in >> n;

	// Mask and shift bytes
	i_bit_index = bit_index;
	i_byte_index = packet_index;
	if (bit_index > 0)
	{
		// If bits were available at start, they were available
		// in the byte before the current index
		i_byte_index--;
	}

	// While bits have not been packed yet
	while (bits_remain > 0)
	{
		// Bits still have to be packed
		// there can be more than 8 bits, so
		// we might have to pack them into more than one byte

		// First pack everything we can into the current 'open' byte
		//curr_bits_n = bits_remain << 3; // Equals  bits_remain mod 8
		//FIXME
		if (bits_remain <= (uint8_t)(8 - i_bit_index))
		{
			// Enough space
			curr_bits_n = (uint8_t)bits_remain;
		}
		else
		{
			curr_bits_n = (8 - i_bit_index);
		}
		
		// Pack these n bits into the current byte
		// Mask out whatever was at that position with ones (xxx11111)
		buffer[i_byte_index] &= (0xFF >> (8 - curr_bits_n));
		// Put content to this position, by masking out the non-used part
		buffer[i_byte_index] |= ((0x00 << curr_bits_n) & v);
		
		// Increment the bit index
		i_bit_index += curr_bits_n;

		// Now proceed to the next byte, if necessary
		bits_remain -= curr_bits_n;
		if (bits_remain > 0)
		{
			// Offer another 8 bits / one byte
			i_byte_index++;
			i_bit_index = 0;
		}
	}
	
	*r_bit_index = i_bit_index;
	// If a partly filled byte is present, mark this as consumed
	if (i_bit_index != 7) i_byte_index++;
	return i_byte_index - packet_index;
}

#ifdef MAVLINK_USE_CONVENIENCE_FUNCTIONS

// To make MAVLink work on your MCU, define comm_send_ch() if you wish
// to send 1 byte at a time, or MAVLINK_SEND_UART_BYTES() to send a
// whole packet at a time

/*

#include "mavlink_types.h"

void comm_send_ch(mavlink_channel_t chan, uint8_t ch)
{
    if (chan == MAVLINK_COMM_0)
    {
        uart0_transmit(ch);
    }
    if (chan == MAVLINK_COMM_1)
    {
    	uart1_transmit(ch);
    }
}
 */

MAVLINK_HELPER void _mavlink_send_uart(mavlink_channel_t chan, const char *buf, uint16_t len)
{
#ifdef MAVLINK_SEND_UART_BYTES
	/* this is the more efficient approach, if the platform
	   defines it */
	MAVLINK_SEND_UART_BYTES(chan, (const uint8_t *)buf, len);
#else
	/* fallback to one byte at a time */
	uint16_t i;
	for (i = 0; i < len; i++) {
		comm_send_ch(chan, (uint8_t)buf[i]);
	}
#endif
}
#endif // MAVLINK_USE_CONVENIENCE_FUNCTIONS

#endif /* _MAVLINK_HELPERS_H_ */