Find active users

# Table: Users
#
# +-------------+----------+
# | Column Name | Type     |
# +-------------+----------+
# | user_id     | int      |
# | item        | varchar  |
# | created_at  | datetime |
# | amount      | int      |
# +-------------+----------+
# This table may contain duplicate records.
# Each row includes the user ID, the purchased item, the date of purchase, and the purchase amount.
#
# Write a solution to identify active users. An active user is a user that has made a second purchase within 7 days of any other of their purchases.
#
# For example, if the ending date is May 31, 2023. So any date between May 31, 2023, and June 7, 2023 (inclusive) would be considered "within 7 days" of May 31, 2023.
#
# Return a list of user_id which denotes the list of active users in any order.
#
# The result format is in the following example.
#
# Example 1:
# Input:
# Users table:
# +---------+-------------------+------------+--------+
# | user_id | item              | created_at | amount |
# +---------+-------------------+------------+--------+
# | 5       | Smart Crock Pot   | 2021-09-18 | 698882 |
# | 6       | Smart Lock        | 2021-09-14 | 11487  |
# | 6       | Smart Thermostat  | 2021-09-10 | 674762 |
# | 8       | Smart Light Strip | 2021-09-29 | 630773 |
# | 4       | Smart Cat Feeder  | 2021-09-02 | 693545 |
# | 4       | Smart Bed         | 2021-09-13 | 170249 |
# +---------+-------------------+------------+--------+
# Output:
# +---------+
# | user_id |
# +---------+
# | 6       |
# +---------+
# Explanation:
# - User with user_id 5 has only one transaction, so he is not an active user.
# - User with user_id 6 has two transaction his first transaction was on 2021-09-10 and second transation was on 2021-09-14. The distance between the first and second transactions date is <= 7 days. So he is an active user.
# - User with user_id 8 has only one transaction, so he is not an active user.
# - User with user_id 4 has two transaction his first transaction was on 2021-09-02 and second transation was on 2021-09-13. The distance between the first and second transactions date is > 7 days. So he is not an active user.

import pandas as pd


def find_active_users(users: pd.DataFrame) -> pd.DataFrame:
  users["created_at"] = pd.to_datetime(users["created_at"])
  users = users.sort_values(["user_id", "created_at"])
  users["prev_date"] = users.groupby("user_id")["created_at"].shift(1)
  users["diff"] = (users["created_at"] - users["prev_date"]).dt.days
  active = users[users["diff"].between(0, 7)]["user_id"].unique()
  return pd.DataFrame({"user_id": sorted(active)})