dynamixel2/

bus.rs

use serial2::SerialPort;
use std::path::Path;
use std::time::{Duration, Instant};

use crate::bytestuff;
use crate::checksum::calculate_checksum;
use crate::endian::{read_u16_le, read_u32_le, read_u8_le, write_u16_le};
use crate::{ReadError, TransferError, WriteError};

const HEADER_PREFIX: [u8; 4] = [0xFF, 0xFF, 0xFD, 0x00];
const HEADER_SIZE: usize = 8;
const STATUS_HEADER_SIZE: usize = 9;

/// Dynamixel Protocol 2 communication bus.
pub struct Bus<ReadBuffer, WriteBuffer> {
	/// The underlying stream (normally a serial port).
	serial_port: SerialPort,

	/// The baud rate of the serial port, if known.
	baud_rate: u32,

	/// The buffer for reading incoming messages.
	read_buffer: ReadBuffer,

	/// The total number of valid bytes in the read buffer.
	read_len: usize,

	/// The number of leading bytes in the read buffer that have already been used.
	used_bytes: usize,

	/// The buffer for outgoing messages.
	write_buffer: WriteBuffer,
}

impl<ReadBuffer, WriteBuffer> std::fmt::Debug for Bus<ReadBuffer, WriteBuffer> {
	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
		#[derive(Debug)]
		#[allow(dead_code)] // Dead code analysis ignores derive debug impls, but that is the whole point of this struct.
		enum Raw {
			#[cfg(unix)]
			Fd(std::os::unix::io::RawFd),
			#[cfg(windows)]
			Handle(std::os::windows::io::RawHandle),
		}

		#[cfg(unix)]
		let raw = {
			use std::os::unix::io::AsRawFd;
			Raw::Fd(self.serial_port.as_raw_fd())
		};
		#[cfg(windows)]
		let raw = {
			use std::os::windows::io::AsRawHandle;
			Raw::Handle(self.serial_port.as_raw_handle())
		};

		f.debug_struct("Bus")
			.field("serial_port", &raw)
			.field("baud_rate", &self.baud_rate)
			.finish_non_exhaustive()
	}
}

impl Bus<Vec<u8>, Vec<u8>> {
	/// Open a serial port with the given baud rate.
	///
	/// This will allocate a new read and write buffer of 128 bytes each.
	/// Use [`Self::open_with_buffers()`] if you want to use a custom buffers.
	pub fn open(path: impl AsRef<Path>, baud_rate: u32) -> std::io::Result<Self> {
		let port = SerialPort::open(path, baud_rate)?;
		Ok(Self::with_buffers_and_baud_rate(port, vec![0; 128], vec![0; 128], baud_rate))
	}

	/// Create a new bus for an open serial port.
	///
	/// The serial port must already be configured in raw mode with the correct baud rate,
	/// character size (8), parity (disabled) and stop bits (1).
	///
	/// This will allocate a new read and write buffer of 128 bytes each.
	/// Use [`Self::with_buffers()`] if you want to use a custom buffers.
	pub fn new(serial_port: SerialPort) -> Result<Self, crate::InitializeError> {
		Self::with_buffers(serial_port, vec![0; 128], vec![0; 128])
	}
}

impl<ReadBuffer, WriteBuffer> Bus<ReadBuffer, WriteBuffer>
where
	ReadBuffer: AsRef<[u8]> + AsMut<[u8]>,
	WriteBuffer: AsRef<[u8]> + AsMut<[u8]>,
{
	/// Open a serial port with the given baud rate.
	///
	/// This will allocate a new read and write buffer of 128 bytes each.
	pub fn open_with_buffers(
		path: impl AsRef<Path>,
		baud_rate: u32,
		read_buffer: ReadBuffer,
		write_buffer: WriteBuffer,
	) -> std::io::Result<Self> {
		let port = SerialPort::open(path, baud_rate)?;
		Ok(Self::with_buffers_and_baud_rate(port, read_buffer, write_buffer, baud_rate))
	}

	/// Create a new bus using pre-allocated buffers.
	///
	/// The serial port must already be configured in raw mode with the correct baud rate,
	/// character size (8), parity (disabled) and stop bits (1).
	pub fn with_buffers(serial_port: SerialPort, read_buffer: ReadBuffer, write_buffer: WriteBuffer) -> Result<Self, crate::InitializeError> {
		let baud_rate = serial_port.get_configuration()
			.map_err(crate::InitializeError::GetConfiguration)?
			.get_baud_rate()
			.map_err(crate::InitializeError::GetBaudRate)?;

		Ok(Self::with_buffers_and_baud_rate(serial_port, read_buffer, write_buffer, baud_rate))
	}

	/// Create a new bus using pre-allocated buffers.
	fn with_buffers_and_baud_rate(serial_port: SerialPort, read_buffer: ReadBuffer, mut write_buffer: WriteBuffer, baud_rate: u32) -> Self {
		// Pre-fill write buffer with the header prefix.
		// TODO: return Err instead of panicing.
		assert!(write_buffer.as_mut().len() >= HEADER_SIZE + 2);
		write_buffer.as_mut()[..4].copy_from_slice(&HEADER_PREFIX);

		Self {
			serial_port,
			baud_rate,
			read_buffer,
			read_len: 0,
			used_bytes: 0,
			write_buffer,
		}
	}

	/// Get a reference to the underlying [`SerialPort`].
	///
	/// Note that performing any read or write with the [`SerialPort`] bypasses the read/write buffer of the bus,
	/// and may disrupt the communication with the motors.
	/// In general, it should be safe to read and write to the bus manually in between instructions,
	/// if the response from the motors has already been received.
	pub fn serial_port(&self) -> &SerialPort {
		&self.serial_port
	}

	/// Consume this bus object to get ownership of the serial port.
	///
	/// This discards any data in internal the read buffer of the bus object.
	/// This is normally not a problem, since all data in the read buffer is also discarded when transmitting a new command.
	pub fn into_serial_port(self) -> SerialPort {
		self.serial_port
	}

	/// Get the baud rate of the bus.
	pub fn baud_rate(&self) -> u32 {
		self.baud_rate
	}

	/// Set the baud rate of the underlying serial port.
	pub fn set_baud_rate(&mut self, baud_rate: u32) -> Result<(), std::io::Error> {
		let mut settings = self.serial_port.get_configuration()?;
		settings.set_baud_rate(baud_rate)?;
		self.serial_port.set_configuration(&settings)?;
		self.baud_rate = baud_rate;
		Ok(())
	}

	/// Write a raw instruction to a stream, and read a single raw response.
	///
	/// This function also checks that the packet ID of the status response matches the one from the instruction.
	///
	/// This is not suitable for broadcast instructions.
	/// For broadcast instructions, each motor sends an individual response or no response is send at all.
	/// Instead, use [`Self::write_instruction`] and [`Self::read_status_response`].
	pub fn transfer_single<F>(
		&mut self,
		packet_id: u8,
		instruction_id: u8,
		parameter_count: usize,
		expected_response_parameters: u16,
		encode_parameters: F,
	) -> Result<StatusPacket<'_>, TransferError>
	where
		F: FnOnce(&mut [u8]),
	{
		self.write_instruction(packet_id, instruction_id, parameter_count, encode_parameters)?;
		let response = self.read_status_response(expected_response_parameters)?;
		crate::error::InvalidPacketId::check(response.packet_id(), packet_id).map_err(crate::ReadError::from)?;
		Ok(response)
	}

	/// Write an instruction message to the bus.
	pub fn write_instruction<F>(
		&mut self,
		packet_id: u8,
		instruction_id: u8,
		parameter_count: usize,
		encode_parameters: F,
	) -> Result<(), WriteError>
	where
		F: FnOnce(&mut [u8]),
	{
		let buffer = self.write_buffer.as_mut();

		// Check if the buffer can hold the unstuffed message.
		crate::error::BufferTooSmallError::check(HEADER_SIZE + parameter_count + 2, buffer.len())?;

		// Add the header, with a placeholder for the length field.
		buffer[4] = packet_id;
		buffer[5] = 0;
		buffer[6] = 0;
		buffer[7] = instruction_id;
		encode_parameters(&mut buffer[HEADER_SIZE..][..parameter_count]);

		// Perform bitstuffing on the body.
		// The header never needs stuffing.
		let stuffed_body_len = bytestuff::stuff_inplace(&mut buffer[HEADER_SIZE..], parameter_count)?;

		write_u16_le(&mut buffer[5..], stuffed_body_len as u16 + 3);

		// Add checksum.
		let checksum_index = HEADER_SIZE + stuffed_body_len;
		let checksum = calculate_checksum(0, &buffer[..checksum_index]);
		write_u16_le(&mut buffer[checksum_index..], checksum);

		// Throw away old data in the read buffer and the kernel read buffer.
		// We don't do this when reading a reply, because we might receive multiple replies for one instruction,
		// and read() can potentially read more than one reply per syscall.
		self.read_len = 0;
		self.used_bytes = 0;
		self.serial_port.discard_input_buffer().map_err(WriteError::DiscardBuffer)?;

		// Send message.
		let stuffed_message = &buffer[..checksum_index + 2];
		trace!("sending instruction: {:02X?}", stuffed_message);
		self.serial_port.write_all(stuffed_message).map_err(WriteError::Write)?;
		Ok(())
	}

	/// Read a raw status response from the bus with the given deadline.
	pub fn read_status_response_deadline(&mut self, deadline: Instant) -> Result<StatusPacket, ReadError> {
		// Check that the read buffer is large enough to hold atleast a status packet header.
		crate::error::BufferTooSmallError::check(STATUS_HEADER_SIZE, self.read_buffer.as_mut().len())?;

		let stuffed_message_len = loop {
			self.remove_garbage();

			// The call to remove_garbage() removes all leading bytes that don't match a status header.
			// So if there's enough bytes left, it's a status header.
			if self.read_len > STATUS_HEADER_SIZE {
				let read_buffer = &self.read_buffer.as_mut()[..self.read_len];
				let body_len = read_buffer[5] as usize + read_buffer[6] as usize * 256;
				let body_len = body_len - 2; // Length includes instruction and error fields, which is already included in STATUS_HEADER_SIZE too.

				// Check if the read buffer is large enough for the entire message.
				// We don't have to remove the read bytes, because `write_instruction()` already clears the read buffer.
				crate::error::BufferTooSmallError::check(STATUS_HEADER_SIZE + body_len, self.read_buffer.as_mut().len())?;

				if self.read_len >= STATUS_HEADER_SIZE + body_len {
					break STATUS_HEADER_SIZE + body_len;
				}
			}

			let timeout = match deadline.checked_duration_since(Instant::now()) {
				Some(x) => x,
				None => {
					trace!(
						"timeout reading status response, data in buffer: {:02X?}",
						&self.read_buffer.as_ref()[..self.read_len]
					);
					return Err(std::io::ErrorKind::TimedOut.into());
				},
			};

			// Try to read more data into the buffer.
			self.serial_port.set_read_timeout(timeout).ok();
			let new_data = self.serial_port.read(&mut self.read_buffer.as_mut()[self.read_len..])?;
			if new_data == 0 {
				continue;
			}

			self.read_len += new_data;
		};

		let buffer = self.read_buffer.as_mut();
		let parameters_end = stuffed_message_len - 2;
		trace!("read packet: {:02X?}", &buffer[..parameters_end]);

		let checksum_message = read_u16_le(&buffer[parameters_end..]);
		let checksum_computed = calculate_checksum(0, &buffer[..parameters_end]);
		if checksum_message != checksum_computed {
			self.consume_read_bytes(stuffed_message_len);
			return Err(crate::InvalidChecksum {
				message: checksum_message,
				computed: checksum_computed,
			}
			.into());
		}

		// Mark the whole message as "used_bytes", so that the next call to `remove_garbage()` removes it.
		self.used_bytes += stuffed_message_len;

		// Remove byte-stuffing from the parameters.
		let parameter_count = bytestuff::unstuff_inplace(&mut buffer[STATUS_HEADER_SIZE..parameters_end]);

		// Wrap the data in a `StatusPacket`.
		let response = StatusPacket {
			data: &self.read_buffer.as_ref()[..STATUS_HEADER_SIZE + parameter_count],
		};

		crate::InvalidInstruction::check(response.instruction_id(), crate::instructions::instruction_id::STATUS)?;
		crate::MotorError::check(response.error())?;
		Ok(response)
	}

	/// Read a raw status response with an automatically calculated timeout.
	///
	/// The read timeout is determined by the expected number of response parameters and the baud rate of the bus.
	pub fn read_status_response(&mut self, expected_parameters: u16) -> Result<StatusPacket, ReadError> {
		// Offical SDK adds a flat 34 milliseconds, so lets just mimick that.
		let message_size = STATUS_HEADER_SIZE as u32 + u32::from(expected_parameters) + 2;
		let timeout = message_transfer_time(message_size, self.baud_rate) + Duration::from_millis(34);
		self.read_status_response_deadline(Instant::now() + timeout)
	}
}

/// Calculate the required time to transfer a message of a given size.
///
/// The size must include any headers and footers of the message.
pub(crate) fn message_transfer_time(message_size: u32, baud_rate: u32) -> Duration {
	let baud_rate = u64::from(baud_rate);
	let bits = u64::from(message_size) * 10; // each byte is 1 start bit, 8 data bits and 1 stop bit.
	let secs = bits / baud_rate;
	let subsec_bits = bits % baud_rate;
	let nanos = (subsec_bits * 1_000_000_000).div_ceil(baud_rate);
	Duration::new(secs, nanos as u32)
}

impl<ReadBuffer, WriteBuffer> Bus<ReadBuffer, WriteBuffer>
where
	ReadBuffer: AsRef<[u8]> + AsMut<[u8]>,
	WriteBuffer: AsRef<[u8]> + AsMut<[u8]>,
{
	/// Remove leading garbage data from the read buffer.
	fn remove_garbage(&mut self) {
		let read_buffer = self.read_buffer.as_mut();
		let garbage_len = find_header(&read_buffer[..self.read_len][self.used_bytes..]);
		if garbage_len > 0 {
			debug!("skipping {} bytes of leading garbage.", garbage_len);
			trace!("skipped garbage: {:02X?}", &read_buffer[..garbage_len]);
		}
		self.consume_read_bytes(self.used_bytes + garbage_len);
		debug_assert_eq!(self.used_bytes, 0);
	}

	fn consume_read_bytes(&mut self, len: usize) {
		debug_assert!(len <= self.read_len);
		self.read_buffer.as_mut().copy_within(len..self.read_len, 0);
		// Decrease both used_bytes and read_len together.
		// Some consumed bytes may be garbage instead of used bytes though.
		// So we use `saturating_sub` for `used_bytes` to cap the result at 0.
		self.used_bytes = self.used_bytes.saturating_sub(len);
		self.read_len -= len;
	}
}

/// A status response that is currently in the read buffer of a bus.
///
/// When dropped, the response data is removed from the read buffer.
#[derive(Debug)]
pub struct StatusPacket<'a> {
	/// Message data (with byte-stuffing already undone).
	data: &'a [u8],
}

impl<'a> StatusPacket<'a> {
	/// Get the raw bytes of the message.
	///
	/// This includes the message header and the parameters.
	/// It does not include the CRC or byte-stuffing.
	pub fn as_bytes(&self) -> &[u8] {
		self.data
	}

	/// The packet ID of the response.
	pub fn packet_id(&self) -> u8 {
		self.as_bytes()[4]
	}

	/// The instruction ID of the response.
	pub fn instruction_id(&self) -> u8 {
		self.as_bytes()[7]
	}

	/// The error field of the response.
	pub fn error(&self) -> u8 {
		self.as_bytes()[8]
	}

	/// The error number of the status packet.
	///
	/// This is the lower 7 bits of the error field.
	pub fn error_number(&self) -> u8 {
		self.error() & !0x80
	}

	/// The alert bit from the error field of the response.
	///
	/// This is the 8th bit of the error field.
	///
	/// If this bit is set, you can normally check the "Hardware Error" register for more details.
	/// Consult the manual of your motor for more information.
	pub fn alert(&self) -> bool {
		self.error() & 0x80 != 0
	}

	/// The parameters of the response.
	pub fn parameters(&self) -> &[u8] {
		&self.data[STATUS_HEADER_SIZE..]
	}
}

/// Find the potential starting position of a header.
///
/// This will return the first possible position of the header prefix.
/// Note that if the buffer ends with a partial header prefix,
/// the start position of the partial header prefix is returned.
fn find_header(buffer: &[u8]) -> usize {
	for i in 0..buffer.len() {
		let possible_prefix = HEADER_PREFIX.len().min(buffer.len() - i);
		if buffer[i..].starts_with(&HEADER_PREFIX[..possible_prefix]) {
			return i;
		}
	}

	buffer.len()
}

/// A response from a motor.
///
/// Note that the `Eq` and `PartialEq` compare all fields of the struct,
/// including the `motor_id` and `alert`.
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct Response<T> {
	/// The motor that sent the response.
	pub motor_id: u8,

	/// The alert bit from the response message.
	///
	/// If this bit is set, you can normally check the "Hardware Error" register for more details.
	/// Consult the manual of your motor for more information.
	pub alert: bool,

	/// The data from the motor.
	pub data: T,
}

impl<'a> TryFrom<StatusPacket<'a>> for Response<()> {
	type Error = crate::InvalidParameterCount;

	fn try_from(status_packet: StatusPacket<'a>) -> Result<Self, Self::Error> {
		crate::InvalidParameterCount::check(status_packet.parameters().len(), 0)?;
		Ok(Self {
			motor_id: status_packet.packet_id(),
			alert: status_packet.alert(),
			data: (),
		})
	}
}

impl<'a, 'b> From<&'b StatusPacket<'a>> for Response<&'b [u8]> {
	fn from(status_packet: &'b StatusPacket<'a>) -> Self {
		Self {
			motor_id: status_packet.packet_id(),
			alert: status_packet.alert(),
			data: status_packet.parameters(),
		}
	}
}

impl<'a> From<StatusPacket<'a>> for Response<Vec<u8>> {
	fn from(status_packet: StatusPacket<'a>) -> Self {
		Self {
			motor_id: status_packet.packet_id(),
			alert: status_packet.alert(),
			data: status_packet.parameters().to_owned(),
		}
	}
}

impl<'a> TryFrom<StatusPacket<'a>> for Response<u8> {
	type Error = crate::InvalidParameterCount;

	fn try_from(status_packet: StatusPacket<'a>) -> Result<Self, Self::Error> {
		crate::InvalidParameterCount::check(status_packet.parameters().len(), 1)?;
		Ok(Self {
			motor_id: status_packet.packet_id(),
			alert: status_packet.alert(),
			data: read_u8_le(status_packet.parameters()),
		})
	}
}

impl<'a> TryFrom<StatusPacket<'a>> for Response<u16> {
	type Error = crate::InvalidParameterCount;

	fn try_from(status_packet: StatusPacket<'a>) -> Result<Self, Self::Error> {
		crate::InvalidParameterCount::check(status_packet.parameters().len(), 2)?;
		Ok(Self {
			motor_id: status_packet.packet_id(),
			alert: status_packet.alert(),
			data: read_u16_le(status_packet.parameters()),
		})
	}
}

impl<'a> TryFrom<StatusPacket<'a>> for Response<u32> {
	type Error = crate::InvalidParameterCount;

	fn try_from(status_packet: StatusPacket<'a>) -> Result<Self, Self::Error> {
		crate::InvalidParameterCount::check(status_packet.parameters().len(), 4)?;
		Ok(Self {
			motor_id: status_packet.packet_id(),
			alert: status_packet.alert(),
			data: read_u32_le(status_packet.parameters()),
		})
	}
}

#[cfg(test)]
mod test {
	use super::*;
	use assert2::assert;

	#[test]
	fn test_find_garbage_end() {
		assert!(find_header(&[0xFF]) == 0);
		assert!(find_header(&[0xFF, 0xFF]) == 0);
		assert!(find_header(&[0xFF, 0xFF, 0xFD]) == 0);
		assert!(find_header(&[0xFF, 0xFF, 0xFD, 0x00]) == 0);
		assert!(find_header(&[0xFF, 0xFF, 0xFD, 0x00, 9]) == 0);

		assert!(find_header(&[0, 1, 2, 3, 4, 0xFF]) == 5);
		assert!(find_header(&[0, 1, 2, 3, 4, 0xFF, 0xFF]) == 5);
		assert!(find_header(&[0, 1, 2, 3, 4, 0xFF, 0xFF, 0xFD]) == 5);
		assert!(find_header(&[0, 1, 2, 3, 4, 0xFF, 0xFF, 0xFD, 0x00]) == 5);
		assert!(find_header(&[0, 1, 2, 3, 4, 0xFF, 0xFF, 0xFD, 0x00, 9]) == 5);

		assert!(find_header(&[0xFF, 1]) == 2);
		assert!(find_header(&[0, 1, 2, 3, 4, 0xFF, 6]) == 7);
	}

	#[test]
	fn test_message_transfer_time() {
		// Try a bunch of values to ensure we dealt with overflow correctly.
		assert!(message_transfer_time(100, 1_000) == Duration::from_secs(1));
		assert!(message_transfer_time(1_000, 10_000) == Duration::from_secs(1));
		assert!(message_transfer_time(1_000, 1_000_000) == Duration::from_millis(10));
		assert!(message_transfer_time(1_000, 10_000_000) == Duration::from_millis(1));
		assert!(message_transfer_time(1_000, 100_000_000) == Duration::from_micros(100));
		assert!(message_transfer_time(1_000, 1_000_000_000) == Duration::from_micros(10));
		assert!(message_transfer_time(1_000, 2_000_000_000) == Duration::from_micros(5));
		assert!(message_transfer_time(1_000, 4_000_000_000) == Duration::from_nanos(2500));
		assert!(message_transfer_time(10_000, 4_000_000_000) == Duration::from_micros(25));
		assert!(message_transfer_time(1_000_000, 4_000_000_000) == Duration::from_micros(2500));
		assert!(message_transfer_time(10_000_000, 4_000_000_000) == Duration::from_millis(25));
		assert!(message_transfer_time(100_000_000, 4_000_000_000) == Duration::from_millis(250));
		assert!(message_transfer_time(1_000_000_000, 4_000_000_000) == Duration::from_millis(2500));
		assert!(message_transfer_time(2_000_000_000, 4_000_000_000) == Duration::from_secs(5));
		assert!(message_transfer_time(4_000_000_000, 4_000_000_000) == Duration::from_secs(10));
		assert!(message_transfer_time(4_000_000_000, 2_000_000_000) == Duration::from_secs(20));
		assert!(message_transfer_time(4_000_000_000, 1_000_000_000) == Duration::from_secs(40));
		assert!(message_transfer_time(4_000_000_000, 100_000_000) == Duration::from_secs(400));
		assert!(message_transfer_time(4_000_000_000, 10_000_000) == Duration::from_secs(4_000));
		assert!(message_transfer_time(4_000_000_000, 1_000_000) == Duration::from_secs(40_000));
		assert!(message_transfer_time(4_000_000_000, 100_000) == Duration::from_secs(400_000));
		assert!(message_transfer_time(4_000_000_000, 10_000) == Duration::from_secs(4_000_000));
		assert!(message_transfer_time(4_000_000_000, 1_000) == Duration::from_secs(40_000_000));
		assert!(message_transfer_time(4_000_000_000, 100) == Duration::from_secs(400_000_000));
		assert!(message_transfer_time(4_000_000_000, 10) == Duration::from_secs(4_000_000_000));
		assert!(message_transfer_time(4_000_000_000, 1) == Duration::from_secs(40_000_000_000));

		assert!(message_transfer_time(43, 1) == Duration::from_secs(430));
		assert!(message_transfer_time(43, 10) == Duration::from_secs(43));
		assert!(message_transfer_time(43, 2) == Duration::from_secs(215));
		assert!(message_transfer_time(43, 20) == Duration::from_millis(21_500));
		assert!(message_transfer_time(43, 200) == Duration::from_millis(2_150));
		assert!(message_transfer_time(43, 2_000_000) == Duration::from_micros(215));
		assert!(message_transfer_time(43, 2_000_000_000) == Duration::from_nanos(215));
		assert!(message_transfer_time(43, 4_000_000_000) == Duration::from_nanos(108)); // rounded up
		assert!(message_transfer_time(3, 4_000_000_000) == Duration::from_nanos(8)); // rounded up
		assert!(message_transfer_time(5, 4_000_000_000) == Duration::from_nanos(13)); // rounded up

		let lots = u32::MAX - 1; // Use MAX - 1 because MAX is not cleanly divisible by 2.
		assert!(message_transfer_time(lots, 1) == Duration::from_secs(u64::from(lots) * 10));
		assert!(message_transfer_time(lots, lots) == Duration::from_secs(10));
		assert!(message_transfer_time(lots / 2, lots) == Duration::from_secs(5));
		assert!(message_transfer_time(lots, lots / 2) == Duration::from_secs(20));
	}
}