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Mass Transit Billing

Code style: black

Overview

A billing system for a mass transit system which consists of a network of train stations, each of which belongs to a pricing zone.

Each time a user enters (IN) or exits (OUT) a station it is recorded. The system is provided data about the user_id, direction (IN/OUT of the station), the station, and the time of the entry/exit (in UTC) for all the journeys in a given period. The data is sorted by timestamp, but not necessarily by users.

Billing System

There is a £2 base fee for all journeys, and additional costs based on the entry and exit zones.

Zone In / Out Additional Cost
1 £0.80
2-3 £0.50
4-5 £0.30
6+ £0.10

Example 1

zone 1 -> zone 1 = 2.00 + 0.80 + 0.80 = £3.60

Example 2

zone 6 -> zone 4 = 2.00 + 0.10 + 0.30 = £2.40

For any erroneous journeys where an IN or OUT is missing a £5 fee is used as the total journey price. It should be assumed that all valid journeys are completed before midnight (i.e. all valid journeys will have an IN and an OUT on the same day).

There is also a daily cap of £15, and a monthly cap of £100. Caps include all journey costs and fees, once a given cap is reached the customer pays no extra for the given day or month.

Assumptions

  • The program and tests are run as explained in the Usage section.
  • CSV files are provided for the station zone map and journey data.
  • The CSV files are encoded in UTF-8.
  • The CSV files contain valid data in the correct format, and there are no missing values.
  • Zone names will be formatted as integers starting from 1.
  • Stations will only be in one zone.
  • There are no restrictions on customers tapping in or out of stations outside of those stated in the specification.
  • Pricing of the zones will remain fixed for the foreseeable future.
  • Any future changes to a station's zone will not be backdated, so refunds or additional charges will not be required.
  • There will be future extensions to the system, so the program is designed to be easily extended.

Usage

This program was written and tested using Python 3.11.3 on macOS Ventura 13.3.1.

Running the Program

No external libraries are required to run the program.

To run the program, use the following command from the src directory:

python mass_transit_billing.py <zone_map_file_path> <journey_map_file_path> <output_file_path>

For example:

python mass_transit_billing.py ../resources/zone_map.csv ../resources/journey_data.csv ../resources/output.csv

Running the Tests

The tests are written using the pytest library. Ensure that you have the correct version installed by running the following command from the root directory:

pip install -r requirements.txt

To run the tests, use the following command from the tests directory:

pytest

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