Splitwise

Setup

Dependencies

java: openjdk version "1.8.0_265"
maven: 3.6.3

Building and Running

Clone the repo to your local machine.
Go to the top level of the repository.
Run the following command for building the jar file:

 >> mvn clean install -DskipTests

Run the following command to start the application:

>> java -jar target/splitwise-0.0.1-SNAPSHOT.jar

Else you can also use the following command to start the application:

>> mvn spring-boot:run

The server will be listening on port number 8080 by default.

Postman Collection

The postman collection for interacting with the server can be found at: postman collection

Database

H2 database is used currently. It is configured to act as an in-memory database by default. You can view the tables and schema by visiting the following url after the application has started:

http://localhost:8080/h2-ui

The details to connect with the H2 database through the H2 UI are (some of them maybe filled by default):

Saved Settings: Generic H2 (Embedded)
Setting Name: Generic H2 (Embedded)
Driver Class: org.h2.Driver
JDBC URL: jdbc:h2:mem:splitwisedb
User Name: athul
Password: password

Scripts

A script is run during startup that populates the database with some users for easy testing. link

Design

Data Model

Features

Functional Features

create a split among a set of users
- splitting the amount equally
- splitting the total amount by amounts specified
get details of a split
settle outstanding amount
see overview of the users splits and settlements
see the total balance amounts the user has to pay to or get from other users

Non-Functional Features

Settling the amount between a set of people who have borrowed money is one of the central features of Splitwise. This is an NP-hard problem, hence the best we can come up with is a greedy solution. The greedy algorithm is as follows:

1. Get all the amount paid by each member of the group of users and the total amount owed.
2. Subtract the amount paid by the amount owed to get the amount to be settled for each user.
3. From the list of users find the maximum debiter (maxDeb) and the maximum creditor (maxCred).The maximum creditor
has the maximum amount to settle and the maximum debitor has the minimum amount to settle which
were calculated in the last step.
4. Pay as much amount as possible from maxDeb to maxCred till one of them becomes settled. This is the greedy choice.
(i.e., settled amount becomes zero) (This will result in the creation of a bill that has to be settled in the code).
5. Repeat steps 3 and 4 till all the amounts are settled.

Other Features

In splitwise the payment outlives the split that was used for created it. A payment entity has a life on its own without the split. For example, if we create a split and settle it and then delete the split, the payment won't be deleted. It will still be present.
BigDecimal is used throughout the code to represent amounts to minimize floating point and rounding errors.
Serializable is used extensively to resolve des/ser issues.
An Interceptor is used which intercepts all exceptions, prints a log and returns an error response. Therefore, we do not have to print a log when an exception occurs.
Amount credited to the user is treated positive throughout the code.
In Splitwise, the balances that are settled with a user are not showed. Only the ones that are lent or owed is showed.
Creation times and updation times should be sent by the clients to facilitate offline operations by the client.
Even if owes in balance shows as settled.

Future Scope

Edit Split
Delete Split
Send Reminders
Pay using payment gateway
Multicurrency support
Audit support to track changes

athultr1997 / splitwise