Customers with maximum number of transactions on consecutive days

# Table: Transactions
# 
# +------------------+------+
# | Column Name      | Type |
# +------------------+------+
# | transaction_id   | int  |
# | customer_id      | int  |
# | transaction_date | date |
# | amount           | int  |
# +------------------+------+
# transaction_id is the column with unique values of this table.
# Each row contains information about transactions that includes unique (customer_id, transaction_date) along with the corresponding customer_id and amount.
# 
# Write a solution to find all customer_id who made the maximum number of transactions on consecutive days.
# 
# Return all customer_id with the maximum number of consecutive transactions. Order the result table by customer_id in ascending order.
# 
# The result format is in the following example.
#
# Example 1:
# Input:
# Transactions table:
# +----------------+-------------+------------------+--------+
# | transaction_id | customer_id | transaction_date | amount |
# +----------------+-------------+------------------+--------+
# | 1              | 101         | 2023-05-01       | 100    |
# | 2              | 101         | 2023-05-02       | 150    |
# | 3              | 101         | 2023-05-03       | 200    |
# | 4              | 102         | 2023-05-01       | 50     |
# | 5              | 102         | 2023-05-03       | 100    |
# | 6              | 102         | 2023-05-04       | 200    |
# | 7              | 105         | 2023-05-01       | 100    |
# | 8              | 105         | 2023-05-02       | 150    |
# | 9              | 105         | 2023-05-03       | 200    |
# +----------------+-------------+------------------+--------+
# Output:
# +-------------+
# | customer_id |
# +-------------+
# | 101         |
# | 105         |
# +-------------+
# Explanation:
# - customer_id 101 has a total of 3 transactions, and all of them are consecutive.
# - customer_id 102 has a total of 3 transactions, but only 2 of them are consecutive.
# - customer_id 105 has a total of 3 transactions, and all of them are consecutive.
# In total, the highest number of consecutive transactions is 3, achieved by customer_id 101 and 105. The customer_id are sorted in ascending order.

import pandas as pd

def find_customers(transactions: pd.DataFrame) -> pd.DataFrame:
  transactions['transaction_date'] = pd.to_datetime(transactions['transaction_date'])
  transactions = transactions.sort_values(['customer_id', 'transaction_date'])
  
  # Assign group: for each customer, consecutive dates belong to same group
  transactions['day_rank'] = transactions.groupby('customer_id')['transaction_date'].rank(method='dense')
  transactions['group'] = transactions['transaction_date'] - pd.to_timedelta(transactions['day_rank'], unit='D')
  
  # Count consecutive days per group
  consecutive = transactions.groupby(['customer_id', 'group']).size().reset_index(name='consecutive_days')
  
  # Find overall max consecutive days
  max_consecutive = consecutive['consecutive_days'].max()
  
  # Return one row per qualifying group (one customer_id per group that hits the max)
  result = consecutive[consecutive['consecutive_days'] == max_consecutive][['customer_id']]
  return result.sort_values('customer_id').reset_index(drop=True)