# Coupon and Promotion Codes

## Objective

Apply discounts predictably instead of turning checkout into a loophole.

Redeemable codes with eligibility rules, redemption limits, expiry, defined stacking behaviour, and an accurate discounted total before purchase.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Validate every code on the server at redemption time against plan, customer, region, currency and remaining redemptions. A code checked only when it was typed can be redeemed after it expires.
2. Decide and enforce one stacking policy: codes combine, or the best one wins, or only one applies. Ambiguity here is how a plan ends up free.
3. Normalise codes on entry — trim and case-fold — so that a code copied out of an email works.
4. Show the actual discounted amount, the discount duration, and what the price becomes afterwards, before the user confirms. A percentage with no end date is not a price.
5. Do NOT enumerate valid codes through the validation endpoint. Rate-limit redemption attempts and keep the failure message the same shape for invalid and ineligible codes.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- A first-time-customer code applied by a returning customer must be refused, including when they return under a second account with the same payment method.
- Two concurrent redemptions of a single-use code must not both succeed. Claim the redemption atomically.
- A percentage discount on one currency's plan is not the same as a fixed-amount discount converted between currencies. Refuse currency mismatches rather than converting.
- A code applied to a subscription that later changes plan may no longer be eligible. Define whether it survives the change and apply that consistently.
- Expiry means expiry of eligibility to redeem, not of the discount already granted. Keep those two clocks separate.
- Discounts interact with tax and proration. The displayed total must come from the same source that produces the invoice, not a local subtraction.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Every code is validated server-side at the moment of redemption.
- [ ] The stacking policy is defined in one place and cannot be bypassed by applying codes in a different order.
- [ ] Codes are matched case-insensitively with surrounding whitespace ignored.
- [ ] Single-use and limited-redemption codes cannot be over-redeemed under concurrent requests.
- [ ] The discounted total, discount duration, and post-discount price are shown before purchase.
- [ ] Invalid and ineligible codes produce indistinguishable responses and are rate-limited.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.