Reconcile bank payments against open invoices (memo references, partial payments, exact-amount and 2-3 invoice combinations, ambiguity)

# Reconcile bank payments against open invoices `invoices.csv` (invoice_id, customer, amount, issued) and `payments.csv` (payment_id, date, payer, amount, memo) are this quarter's exports. Write `recon.py` (Python 3 stdlib only) that, run as `python recon.py` in this directory, writes `recon.csv` and `open_invoices.csv` following these rules. We re-run it on freshly generated files, so do not hard-code anything. ## Normalisation 1. **Money:** remove `$` and `,`, strip, parse as an exact decimal (use `decimal.Decimal`, never floats). Output money with exactly 2 decimals. 2. **Names:** a payer matches a customer when their normalised names are equal. Normalise: Unicode casefold; replace every character that is not `a-z`, `0-9` or space with a space; split on whitespace; then repeatedly drop a final word if it is `ltd`, `llc`, `inc`, `gmbh` or `co`; join with single spaces. (`"Acme Tools, Ltd."` -> `acme tools`, `"Birch & Sons LLC"` -> `birch sons`.) 3. **Invoice references in memos:** in the casefolded memo, find every `inv`, optionally followed by one space or one `-`, then digits; `inv` must not be preceded by a letter or digit and the digits must not be followed by a digit. The digits (leading zeros ignored) give invoice number N, meaning invoice `INV-` + N as 4 digits. ## Processing 4. Process payments in order of (date, payment_id). Balances start at the invoice amounts and change as payments are applied; later payments see the updated balances. 5. **Rule A (reference):** collect the referenced invoices that exist, belong to the payer's customer (rule 2) and still have balance > 0, in memo order, without repeats. If there is at least one: apply the payment to them in that order, each taking `min(remaining payment, its balance)`; stop when the payment is used up (invoices it did not reach are not listed). Status `matched_ref`, `invoices` = the invoices that received money, `unapplied` = what is left (may be `0.00`). Dates are not checked in rule A. 6. Otherwise, the **candidates** are the payer's customer's invoices with balance > 0 issued on or before the payment date. 7. **Rule B (single amount):** if some candidates have balance exactly equal to the payment, close the one with the earliest issued date (ties: smaller invoice_id). Status `matched_amount`, `unapplied` `0.00`. 8. **Rule C (combination):** otherwise look at every set of exactly 2 or exactly 3 different candidates whose balances add up exactly to the payment. Exactly one such set -> close all of them, status `matched_combo`, `invoices` = their ids sorted ascending; more than one -> status `ambiguous`, no invoices, nothing applied. 9. Otherwise status `unmatched`, nothing applied. For `ambiguous` and `unmatched`, `unapplied` = the full payment amount and `invoices` is empty. ## Output 10. `recon.csv`: header `payment_id,status,invoices,unapplied`, one row per payment sorted by payment_id; `invoices` joined with `;`. 11. `open_invoices.csv`: header `invoice_id,balance`, every invoice whose final balance is > 0, sorted by invoice_id. ## Scoring Hidden checks (partial credit = fraction passed; all must pass): `ref_rows_exact`, `amount_rows_exact`, `combo_and_ambiguous_rows_exact`, `unmatched_rows_exact` (the rows whose correct status is in that group are exactly right, and no extra or missing payment rows), `open_invoices_exact`, then a re-run on fresh files: `fresh_recon_exact` (all rows) and `fresh_open_invoices_exact`. Each run has a 60-second limit.