Receivables Aging Buckets

Apply customer payments to open invoices, then report how much is still owed in each aging bucket.

• invoices: rows [customer_id, invoice_id, due_day, amount]. Invoice ids are unique; each invoice starts with amount outstanding.
• payments: rows [customer_id, invoice_id, amount], applied in order. invoice_id may be None.

Applying one payment:

1. If invoice_id is not None: if it is unknown or belongs to a different customer, ignore the whole payment. Otherwise pay down that invoice by up to its outstanding amount.
2. Any money left over (or the whole payment when invoice_id is None) goes to the same customer's other invoices that still have a balance, oldest first: by due_day, then by invoice_id.
3. Money left after every invoice of the customer is paid off is discarded.

Then age each invoice that still has a balance, using age = as_of_day - due_day:

• current: age <= 0
• overdue_1_30: 1 <= age <= 30
• overdue_31_60: 31 <= age <= 60
• overdue_61_plus: age >= 61

Return [["current", x], ["overdue_1_30", y], ["overdue_31_60", z], ["overdue_61_plus", w]] with the total outstanding in each bucket, always in this order.