simo

def encodebookmark(created, key, direction):
  from urllib import quote_plus
  timestamp = time.mktime(created.timetuple())+1e-6*created.microsecond
  return quote_plus(base64.b64encode("%f|%s|%s" % (timestamp, key, direction)))

def decodebookmark(b64bookmark):
  from urllib import unquote_plus
  timestamp, key, direction = base64.b64decode(
    unquote_plus(b64bookmark)).split('|')
  created = datetime.datetime.fromtimestamp(float(timestamp))
  return created, key, direction

class PagingModel:
  PAGESIZE = 10

  @staticmethod
  def page(bm=None):
    PAGESIZE = PagingModel.PAGESIZE
    prev, next = None, None

    if bm:
      modified, key, direction = decodebookmark(bm)
      key = db.Key(key)
      if direction == 'next':
        query = Issue().all()
        query.filter('modified =', modified)
        query.filter('__key__ >=', key)
        query.order('__key__')
        issues = query.fetch(PAGESIZE+1)
        if len(issues) < PAGESIZE+1:
          query = Issue().all()
          query.filter('modified >', modified)
          query.order('modified')
          query.order('__key__')
          needed = (PAGESIZE+1) - len(issues)
          issues.extend(query.fetch(needed))
        if len(issues) > PAGESIZE:
          next_modified = issues[-1].modified
          next_key = issues[-1].key()
          next = encodebookmark(next_modified, next_key, 'next')
          issues = issues[:-1]

        query = Issue().all()
        query.filter('__key__ <', key)
        query.order('-__key__')
        query.order('-modified')
        previssue = query.fetch(1)
        if previssue:
          prev_modified = previssue[0].modified
          prev_key = previssue[0].key()
          prev = encodebookmark(prev_modified, prev_key, 'prev')
        return (issues, next, prev)
      else: # if direction == 'next'
        query = Issue().all()
        query.filter('modified =', modified)
        query.filter('__key__ <=', key)
        query.order('-__key__')
        issues = query.fetch(PAGESIZE+1)
        issues.reverse()
        if len(issues) < PAGESIZE+1:
          query = Issue().all()
          query.filter('modified <', modified)
          query.order('-modified')
          query.order('-__key__')
          needed = (PAGESIZE+1) - len(issues)
          extras = query.fetch(needed)
          extras.reverse()
          extras.extend(issues)
          issues = extras
        if len(issues) > PAGESIZE:
          prev_modified = issues[0].modified
          prev_key = issues[0].key()
          prev = encodebookmark(prev_modified, prev_key, 'prev')
          issues = issues[1:]

        query = Issue().all()
        query.filter('__key__ >', key)
        query.order('__key__')
        query.order('modified')
        nextissue = query.fetch(1)
        if nextissue:
          next_modified = nextissue[0].modified
          next_key = nextissue[0].key()
          next = encodebookmark(next_modified, next_key, 'next')
        return (issues, next, prev)
    else:
      query = Issue().all()
      query.order('modified')
      query.order('__key__')
      issues = query.fetch(PAGESIZE+1)
      if len(issues) > PAGESIZE:
        modified = issues[-1].modified
        key = issues[-1].key()
        next = encodebookmark(modified, key, 'next')
        issues = issues[:-1]
      return (issues, next, None)