Account Balance Manager: StripePay Backend

Imagine you are creating the backend for an app called StripePay (the next popular peer to peer payment method) that allows Stripe users to send each other money and transfer money to and from their bank accounts. Stripe users make thousands of transactions every day, so we need functionality that handles balance changes, transactions, and balance reads.

Please read the entire prompt before you start coding. Not all the test cases will pass unless all the constraints are met.

We will implement the core of the backend: users sending money to each other. However, our networks can be a bit unreliable, so sometimes we get transactions out of order; therefore, we must execute each command in chronological order. Additionally, StripePay users send each other money and transfer money to and from their connected bank accounts.

Given a list of commands for an account balance API, you will have to parse each command and return a comma-separated string of the command results in sequential order of the commands received, but they must be executed in chronological order. There can be at most 1 command per second. Some of the supported commands are:

1. INIT:

• Usage: INIT,name,balance,bank_1,bank_2,...,bank_n
  • This is the starting balance of name's Stripe account. Anything that follows the balance is the banks that they use.
  • Banks will never share the same name as a user.
  • INITs will not have a response value.

2. POST:

• Usage: POST,timestamp,sender,receiver,amount
  • This means that sender sent amount to receiver on timestamp.
    • If this causes the sender's account balance to become negative, return FAILURE.
    • If the sender is a bank, then this is treated as a deposit to the receiver's Stripe account. If the receiver doesn't have the bank as one of their banks, return FAILURE. Otherwise, return SUCCESS.
    • If the receiver is a bank, this is treated as a withdrawal from the sender's Stripe account. If the sender doesn't have the bank as one of their banks, return FAILURE. Otherwise, return SUCCESS.
    • If both the sender and receiver are banks, return FAILURE.
    • Example: POST,169800812,Bob,Alice,50 means that Bob sent Alice $50 on timestamp 169800812.
    • If either the sender or receiver does not have an account (i.e. was not included in an INIT command), return FAILURE.

3. GET:

• Usage: GET,timestamp,name
  • Return the Stripe account balance for user name on timestamp.
    • For example, GET,2,Alice returns Alice's Stripe account balance on timestamp 2.
    • If the name specified was not created by an INIT command, return FAILURE.

The function's input will be a list of commands (string array), and the output should be a comma-separated string of all the responses from the commands, in sequential order of the commands received in the input. You can assume that the timestamps and commands will always be properly formatted.