import random
import threading

csprng = random.SystemRandom()

sessions_lock = threading.Lock()
max_sessions = 1024
sessions = {}

def new_session(userid):
	"""Creates a new session and returns its ID"""
	with sessions_lock:
		while True:
			sessionid = csprng.randrange(2 << 256)

			# Check that the ID is unique
			if sessionid not in sessions:
				break

		# Do we have too many sessions?
		# TODO: Implement the limit per-user
		if len(sessions) >= max_sessions:
			# Yes, drop a random one
			# TODO: Have some LRU thing here
			delete_id = random.choice(list(sessions.keys()))
			del sessions[delete_id]

		sessions[sessionid] = userid

	return sessionid

def get_userid(sessionid):
	"""Returns the user associated with session ID
	Returns None if no user was found"""
	with sessions_lock:
		if sessionid in sessions:
			return sessions[sessionid]

		return None