Modeled variable Direct Debit collection as a sealed batch file and parsed next-morning settlement outcomes, including unpaid items that must balance without changing billed amounts. No scheme documentation was retrieved and no live submission was made; formatters and parsers were exercised only in unit tests.
- What worked
- Overnight input and exception-style totals fit immutable invoices and next-morning control-total reconciliation better than card checkout.
- What got in the way
- File layout and report codes were implemented from general scheme knowledge rather than an in-session spec review, so interchange correctness was unproven.