Get user statistics for selected groups of items

    time:
      time in format '%Y-%m-%d_%H:%M:%S' used for practicing
    user:
      identifier of the user (only for stuff users)
    username:
      username of user (only for users with public profile)
    filters:                -- use this or body
      json as in BODY
    mastered:
      use model to compute number of mastered items - can be slowed
    language:
      language of the items

    BODY
      json in following format:
      {
        "#identifier": []         -- custom identifier (str) and filter
        ...
      }

def user_stats(request):
    """
    Get user statistics for selected groups of items

    time:
      time in format '%Y-%m-%d_%H:%M:%S' used for practicing
    user:
      identifier of the user (only for stuff users)
    username:
      username of user (only for users with public profile)
    filters:                -- use this or body
      json as in BODY
    mastered:
      use model to compute number of mastered items - can be slowed
    language:
      language of the items

    BODY
      json in following format:
      {
        "#identifier": []         -- custom identifier (str) and filter
        ...
      }
    """
    timer('user_stats')
    response = {}
    data = None
    if request.method == "POST":
        data = json.loads(request.body.decode("utf-8"))["filters"]
    if "filters" in request.GET:
        data = load_query_json(request.GET, "filters")
    if data is None:
        return render_json(request, {}, template='models_user_stats.html', help_text=user_stats.__doc__)
    environment = get_environment()
    if is_time_overridden(request):
        environment.shift_time(get_time(request))
    user_id = get_user_id(request)
    language = get_language(request)
    filter_names, filter_filters = list(zip(*sorted(data.items())))
    reachable_leaves = Item.objects.filter_all_reachable_leaves_many(filter_filters, language)
    all_leaves = sorted(list(set(flatten(reachable_leaves))))
    answers = environment.number_of_answers_more_items(all_leaves, user_id)
    correct_answers = environment.number_of_correct_answers_more_items(all_leaves, user_id)
    if request.GET.get("mastered"):
        timer('user_stats_mastered')
        mastery_threshold = get_mastery_trashold()
        predictions = Item.objects.predict_for_overview(environment, user_id, all_leaves)
        mastered = dict(list(zip(all_leaves, [p >= mastery_threshold for p in predictions])))
        LOGGER.debug("user_stats - getting predictions for items took %s seconds", (timer('user_stats_mastered')))
    for identifier, items in zip(filter_names, reachable_leaves):
        if len(items) == 0:
            response[identifier] = {
                "filter": data[identifier],
                "number_of_items": 0,
            }
        else:
            response[identifier] = {
                "filter": data[identifier],
                "number_of_items": len(items),
                "number_of_practiced_items": sum(answers[i] > 0 for i in items),
                "number_of_answers": sum(answers[i] for i in items),
                "number_of_correct_answers": sum(correct_answers[i] for i in items),
            }
            if request.GET.get("mastered"):
                response[identifier]["number_of_mastered_items"]= sum(mastered[i] for i in items)
    return render_json(request, response, template='models_user_stats.html', help_text=user_stats.__doc__)