over/serial/com.py

#! /usr/bin/env python3
# encoding: utf-8

import time
import serial

from . import et

prefix = et.prefix

'''
===== OverTalk protocol spec =====
==== Transport layer ====

frame = [ 0x2a | destination (1) | source (1) | payload length (1) | payload (n) | checksum (1) ]

Any time an 0x2a is encountered the parser should flush its state and begin reading a new frame.
Bytes 0x2a, 0x2b, 0x11 and 0x13 are prefixed with 0x2b and inverted (XORed with 0xff) to avoid collisions.

Frames with destinations unknown to an OverShell are sent to the default interface, or dropped if
that's the interface they came from.

==== Command layer ====
Payload consists of one or more commands:

get_command = [ 0x00 | register (1) | target register (1) ]
set_command = [ 0x01 | register (1) | length (1) | value (n) ]

With a get_command the sender requests the receiver to read its own 'register' and issue
a set_command that sets the sender's 'target register' to that value.

A set_command does what is says on the box.
'''

class Transport:
	'''
	OverTalk Transport layer implementation.
	
	Reads data from multiple interfaces and either router frames or receives them.
	A received frame is then made available via a public attribute.
	
	Interfaces are objects that implement methods read() (returns one byte of data as int), write(buffer, len),
	and waiting property (contains number of bytes waiting).
	'''
	def __init__(self, my_id, interfaces, def_route):
		'''
		@param my_id OverTalk address of this Transport instance
		@param interfaces a dict of interfaces keyed by their IDs
		@param def_route ID of default interface for sending
		'''
		
		assert my_id not in interfaces
		assert def_route in interfaces
		
		self._print = et.Output('over.com.Transport', default_suffix='\n', timestamp=True)
		
		self.my_id = my_id
		self.def_route = def_route
		self.interfaces = interfaces
		
		self.destination_unknown = 0
		self.incoming = None
		
		self._print('<Cg>on-line<C/>')
	
	def update(self):
		self.incoming = None
		
		for interface_id, interface in self.interfaces.items():
			if interface.waiting:
				interface.last_data_received = time.time()
				byte = interface.read()
				
				if byte == 0x2a:
					interface.rxbuffer = []
					interface.reading_escape = False
					interface.reading_frame = True
				
				if byte == 0x2b:
					interface.reading_escape = True
					continue
				
				if interface.reading_escape:
					byte ^= 0xff
					interface.reading_escape = False
				
				if interface.reading_frame:
					interface.rxbuffer.append(byte)
			
			if self.verify_frame(interface):
				self.process_frame(interface_id, interface.rxbuffer)
				interface.rxbuffer = []
	
	def verify_frame(self, interface):
		buffer_length = len(interface.rxbuffer)
		
		if buffer_length >= 4:
			# at this point we can read frame's declared size
			if buffer_length >= interface.rxbuffer[3] + 5: # header (4) + payload + checksum (1)
				# a frame has been read, huzzah!
				interface.reading_frame = False
				
				# checksum
				if sum(interface.rxbuffer[:-1]) % 0x100 == interface.rxbuffer[-1]:
					return True
				
				else:
					interface.malformed_frames += 1
					self._print('broken frame received: <Cr>%s<C/>' %(interface.rxbuffer))
					interface.rxbuffer = []
		
		return False
	
	def escape_frame(self, frame):
		assert frame[0] == 0x2a
		
		frame_escaped = [0x2a]
		
		for byte in frame[1:]:
			if byte in (0x2a, 0x2b, 0x11, 0x13):
				frame_escaped.append(0x2b)
				byte ^= 0xff
			
			frame_escaped.append(byte)
		
		return frame_escaped
	
	def process_frame(self, source_interface_id, frame):
		payload = frame[4:-1]
		destination = frame[1]
		
		if destination == self.my_id:
			self.incoming = (frame[2], payload)
		else:
			if destination in self.interfaces:
				self._print('routing frame to [%d]' %(destination))
				self.interfaces[destination].write(self.escape_frame(frame))
			else:
				if source_interface_id == self.def_route:
					self.destination_unknown += 1
					self._print('unknown destination <Cr>%d<C/> for frame: <Cy>%s<C/>' %(destination,
						repr(frame)), prefix.fail)
				else:
					self._print('routing frame to default route [%d]' %(self.def_route))
					self.interfaces[self.def_route].write(self.escape_frame(frame))
	
	def send_data(self, destination, data):
		frame = [0x2a, destination, self.my_id, len(data)] + list(data)
		frame.append(sum(frame) % 0x100)
		frame = self.escape_frame(frame)
		
		if destination in self.interfaces:
			s = self.interfaces[destination]
		else:
			s = self.interfaces[self.def_route]
		
		s.write(frame)

class TTL_Interface:
	def __init__(self, interface='/dev/ttyUSB0', baudrate=57600):
		try:
			self.s = serial.Serial(interface, baudrate, timeout=1)
		except serial.serialutil.SerialException:
			self.s = None
		
		self.rxbuffer = []
		self.reading_escape = False
		self.reading_frame = False
		
		self.malformed_frames = 0
		self.last_data_received = 0
	
	@property
	def waiting(self):
		if self.s:
			return self.s.inWaiting()
		else:
			return 0
	
	def read(self):
		if self.s:
			return ord(self.s.read())
		else:
			return 0
	
	def write(self, data):
		print('Sending:', ''.join([hex(x)[2:].zfill(2) for x in data]))
		
		if self.s:
			self.s.write(bytes(data))