Merchant Fraud Flags from Events

Decide which merchants to flag for fraud after processing a stream of charge and dispute events.

Events, in order:

• ["CHARGE", charge_id, merchant_id, mcc, code]: a charge for merchant_id. mcc is the merchant's category; a merchant always uses the same mcc. The charge is fraudulent when code is in fraud_codes. Charge ids are unique.
• ["DISPUTE", charge_id]: the charge was disputed, so remove it from its merchant's counts entirely (from the total and, if it was fraudulent, from the fraudulent count). Disputes for unknown charge ids, or for charges already disputed, are ignored. A dispute always comes after its charge.

After all events, evaluate each merchant that still has at least one undisputed charge, using t = thresholds[mcc]:

• If t < 1, it is a ratio threshold: flag when fraudulent / total >= t.
• If t >= 1, it is a count threshold: flag when fraudulent >= t.

Return the flagged merchant ids sorted lexicographically.