Coupon Validation Rules

Specify coupon validation that cannot be bypassed by the client. The server decides the discount at charge time. COUPON FIELDS Code, percent or fixed amount, currency if fixed, starts and ends, max redemptions, per-customer limit, minimum subtotal, included or excluded products, first-time customers only, stackable or not. DELIVER 1) The validation order: exists, not expired, redemption counts, customer eligibility, cart eligibility, then compute. Each failure has a code the UI can translate, not a stack trace. 2) The compute function: integer minor units, a maximum so a coupon cannot make the total negative, and the line items it applies to. 3) The race on max redemptions: increment in the same transaction as the order, with a constraint, so the last use cannot be taken twice. 4) What the client sends: the code, not the discounted total. The server recomputes. Show the bug if we trust a total from the browser. 5) Tests for expired, wrong currency, excluded product, and a stacked pair when stacking is off. Do not log the list of all valid codes in a public error message.