# NextBus prediction class.  For each route/stop, NextBus server is polled
# automatically at regular intervals.  Front-end app just needs to init
# this with stop data, which can be found using the routefinder.py script.

import threading
import time
import urllib
from xml.dom.minidom import parseString

class predict:
	interval  = 120 # Default polling interval = 2 minutes
	initSleep = 0   # Stagger polling threads to avoid load spikes

	# predict object initializer.  1 parameter, a 4-element tuple:
	# First element is agengy tag (e.g. 'actransit')
	# Second is line tag (e.g. '210')
	# Third is stop tag (e.g. '0702630')
	# Fourth is direction -- not a tag, this element is human-readable
	# and editable (e.g. 'Union Landing') -- for UI purposes you may
	# want to keep this short.  The other elements MUST be kept
	# verbatim as displayed by the routefinder.py script.
	# Each predict object spawns its own thread and will perform
	# periodic server queries in the background, which can then be
	# read via the predictions[] list (est. arrivals, in seconds).
	def __init__(self, data):
		self.data          = data
		self.predictions   = []
		self.lastQueryTime = time.time()
		t                  = threading.Thread(target=self.thread)
		t.daemon           = True
		t.start()

	# Periodically get predictions from server ---------------------------
	def thread(self):
		initSleep          = predict.initSleep
		predict.initSleep += 5   # Thread staggering may
		time.sleep(initSleep)    # drift over time, no problem
		while True:
			dom = predict.req('predictions' +
			  '&a=' + self.data[0] +   # Agency
			  '&r=' + self.data[1] +   # Route
			  '&s=' + self.data[2])    # Stop
			if dom is None: return     # Connection error
			self.lastQueryTime = time.time()
			predictions = dom.getElementsByTagName('prediction')
			newList     = []
			for p in predictions:      # Build new prediction list
				newList.append(
				  int(p.getAttribute('seconds')))
			self.predictions = newList # Replace current list
			time.sleep(predict.interval)

	# Open URL, send request, read & parse XML response ------------------
	@staticmethod
	def req(cmd):
		xml = None
		try:
			connection = urllib.urlopen(
			  'http://webservices.nextbus.com'  +
			  '/service/publicXMLFeed?command=' + cmd)
			raw = connection.read()
			connection.close()
			xml = parseString(raw)
		finally:
			return xml

	# Set polling interval (seconds) -------------------------------------
	@staticmethod
	def setInterval(i):
		interval = i