Application, represents the cash application which can load inputs and execute actions.
BankAccount, contains the information of a single bank account and basic deposit/withdraw actions to the user.
Bank, the bank entity which holds all the bank accounts and the total balance. All the deposit/withdraw actions go to bank and then acknowledged by bank account.
Command, represent a single action. e.g. deposit, withdraw, balance. way. All commands have been implemented the ICommand interface with Run(bank *Bank) method, so they can be processed by the application in a standard

The steps below shows how the application works.

When we start this application it will initialise a bank and the cash application instance.
All the actions recorded in a input file will be parsed and transferred to command
The Cash application will take the commands as input, process them and print out some basic information.

Assumptions & Trade offs

In order to start implementing this project, I have made a few assumptions below:

Only one seesion for any user at any single moment, which means there will no dirty reads/writes for a user
All the actions will be loaded through a input text file and will be processed following the sequence order, so it means the application currently can only run one command at a time.
I use Australian Dollar($) as the currency type, use int to represent cent as the currency unit.

What would I do more if I have more time

There are two things I want to do most in terms of building a proper cash app if I have more time.

Make it to be a CLI and support multiple users login and run actions at the same time.
Add some basic locks for depositing and withdrawing operations at both user and bank level.

How to run

docker-compose

Please make sure you have docker and docker-compose installed

Then just simply run the commands below

# One command to run application
docker-compose up

# If you want to run another failure case with withdrawing more money than the user has
docker-compose run cash go run main.go no_enough_money.txt

# Run tests
docker-compose run cash go test ./... -cover

Or you can try the classic way.

The classic Way

Similar, you should have go installed before we start

# Install all dependencies
go mod download

# Run application
go run main.go input.txt

# Run Tests
go test ./... -cover

Run the application against custom inputs

You can also build your custom input and define as many commands as you want.

There is a example below with single bank account.

OPEN_ACCOUNT Tom
DEPOSIT Tom 520.30
WITHDRAW Tom 340.00
BALANCE Tom
TOTAL_BALANCE

Then test it

docker-compose run cash go run main.go yourfile