This is an interview solution for a test of a company @ Oct 2020. The question is originally in a PDF, converted it into markdown format for github viewing and portability.

• All the requirements are unit tested in BillUsageSystem_test.py
• The solution is in GetPossibleCustomerIdsForOutstandingAmount.py.
  • This is to fulfill the question requirement of creating a function to be used as an API.

We have a system that issues usage bills to our customers every month.

When customers pay their outstanding usage bill, they will usually update us via email with transfer receipt. The finance department will then mark the bill as paid.

However, customers will sometimes fail to notify us. When our finance department checks the account statement, they will notice payments from unknown parties. Finance will then manually check against outstanding bills and try to determine possible payors.

As we issue thousands of bills per month, this manual process is naturally untenable. Finance has requested for a function that can return list of possible customers when an amount is keyed-in.

Notes:

• When a bill is created, the PaidAmount is 0 and PaidDate is undefined (null).
• When a bill is fully paid for, the PaidDate is set, and the PaidAmount shall be the same as BillAmount.
• It is possible for a Bill to be partially paid. In this scenario, the PaidAmount shall indicate the partially paid amount, but the PaidDate shall remain undefined.
• Therefore, outstanding amount = BillAmount - PaidAmount
• A customer may have zero or multiple outstanding bill at any time.

Please write a function, in your programming language of choice, to solve the following.

IEnumerab1e<int> GetPossibleCustomerIdsForOutstandingAmount(
    List<Bi11> outstandingBi11s,
    decimal amountToMatch)
{
    // ...
}

Your function will be given input of a list of outstanding bills, and the outstanding amount to match. Fully paid for bills will not be included in outstandingBills. Your function must return zero, one or multiple customer IDs that may have one or more bills with sum of outstanding amount matching the value specified by amountToMatch.