from datetime import datetime, timedelta

from database import QueueItem


class WorkQueue(object):
  def __init__(self, queue_name):
    self.queue_name = queue_name

  def put(self, message, available_after=0):
    """
    Put an item, if it shouldn't be processed for some number of seconds,
    specify that amount as available_after.
    """

    params = {
      'queue_name': self.queue_name,
      'body': message,
    }

    if available_after:
      available_date = datetime.now() + timedelta(seconds=available_after)
      params['available_after'] = available_date

    QueueItem.create(**params)

  def get(self, processing_time=300):
    """
    Get an available item and mark it as unavailable for the default of five
    minutes.
    """
    now = datetime.now()
    available_or_expired = ((QueueItem.available == True) |
                            (QueueItem.processing_expires <= now))

    # TODO the query and the update should be atomic, but for now we only
    # have one worker.
    avail = QueueItem.select().where(QueueItem.queue_name == self.queue_name,
                                     QueueItem.available_after <= now,
                                     available_or_expired)

    found = list(avail.limit(1).order_by(QueueItem.available_after))

    if found:
      item = found[0]
      item.available = False
      item.processing_expires = now + timedelta(seconds=processing_time)
      item.save()

      return item

    return None

  def complete(self, completed_item):
    completed_item.delete_instance()


image_diff_queue = WorkQueue('imagediff')