#!/usr/bin/python
# Act as a server for a BBC Micro requesting bits of disk images.
# Adam Sampson <ats@offog.org>

import sys, serial, time

def read_line(ser):
	s = ""
	while 1:
		c = ser.read(1)
		if c == "\r" or c == "\n":
			break
		s += c
	return s

def checksum(data):
	csum = 0
	for c in data:
		csum += ord(c)
	csum = csum & 0xFFFF
	return csum

def main():
	ser = serial.Serial("/dev/ttyS0", 19200, rtscts = 1)

	filename = None
	while 1:
		print "Waiting for request..."

		# Discard until *, in case we get out of sync
		while 1:
			c = ser.read(1)
			if c == "*":
				break

		while 1:
			c = ser.read(1).lower()
			if c != "*" and c != "":
				break
		print "Got command: " + c
		s = read_line(ser)
		print "Got arg: " + s

		# *nfilename - change filename
		if c == "n":
			print "Filename: " + s
			filename = s
			try:
				f = open(filename, "r")
				f.close()
				ser.write("y\n")
			except IOError:
				ser.write("n\n")

		# *rstart,count - read count bytes starting at start
		elif c == "r":
			(start, count) = map(int, s.split(","))
			print "Sending %d bytes from %d" % (count, start)

			try:
				f = open(filename, "r")
				f.seek(start)
				data = f.read(count)
				f.close()
			except IOError:
				data = None

			if data is None:
				ser.write("n\n")
			else:
				if len(data) < count:
					print "Short read -- padding with zeroes"
					data += "\0" * (count - len(data))

				sum = checksum(data)
				ser.write("y%d\n" % sum)
				time.sleep(0.1)
				ser.write(data)

		else:
			ser.write("n\n")

if __name__ == "__main__":
	main()