Simple Redis powered chat server for hyper-local chat.

comm.py

import redis
import simplejson as json
import logging
import settings
import math

log = logging.getLogger(__name__)

#
# The following formulas are adapted from the Aviation Formulary
# http://williams.best.vwh.net/avform.htm
#

nauticalMilePerLat = 60.00721
nauticalMilePerLongitude = 60.10793
rad = math.pi / 180.0
milesPerNauticalMile = 1.15078

def calcDistance(lat1, lon1, lat2, lon2):                      
    """
    Caclulate distance between two lat lons in NM
    """
    yDistance = (lat2 - lat1) * nauticalMilePerLat
    xDistance = (math.cos(lat1 * rad) + math.cos(lat2 * rad)) * (lon2 - lon1) * (nauticalMilePerLongitude / 2)

    distance = math.sqrt( yDistance**2 + xDistance**2 )

    return distance * milesPerNauticalMile


def broadcast(channel, latitude, longitude, message):
    """Broadcasts a message"""
    r = redis.Redis()
    message = {"latitude": latitude,
               "longitude": longitude,
               "message": message}
    r.publish(channel, json.dumps(message))


def listener(channel, latitude, longitude, radius):
    """Listens for incoming messages

   channel
       The Redis channel to listen on
   lat
       The latitude of the listener
   long
       The longitude of the listener
   radius
       The listening radius"""
    r = redis.Redis()

    r.subscribe(channel)

    for packet in r.listen():
        log.debug("Got: %s" % (packet, ))

        if packet.get("type") == "message":
            data = packet.get('data')
            try:
                # decode the message
                message = json.loads(data)
                # Check the lat/long of the message against the listener
                # yield the message if it is within the radius
                distance = calcDistance(latitude, longitude,
                                        message['latitude'],
                                        message['longitude'])
                if distance <= radius:
                    log.debug("Message is within %s mi of listener" \
                                  % (distance, ))
                    yield message
                else:
                    log.debug("Message is outside %s mi of listener" \
                                  % (distance, ))
            except Exception, e:
                log.exception(e)
                yield ("error", e)