# Currency Input

## Objective

Let people enter money naturally without rounding surprises.

A money field that keeps currency selection separate from the amount and stores exact values, not floats.

## 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. Store money as integer minor units or a fixed-precision decimal. Never a floating-point number — 0.1 + 0.2 is not 0.3, and money bugs found in production are always expensive.
2. Keep currency selection a distinct value from the amount. An amount without a currency is meaningless, and a currency inferred from the user's locale will be wrong for anyone working across borders.
3. Handle currencies whose minor unit is not two digits: JPY and KRW have none, BHD and KWD have three. A hardcoded ×100 is a real defect for real users.
4. Parse the separators the user's locale actually uses — 1.234,56 and 1,234.56 are the same amount. Accept both rather than rejecting one.
5. Decide and document how negative amounts are entered and shown, including the parenthesis convention in accounting contexts.
6. Caret behaviour, paste handling, and separator insertion are owned by Input Masks; extend that rather than writing a second masking implementation. Read-only formatting of stored amounts is owned by Locale-Aware Numbers and Dates.

## 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

- Changing currency after typing an amount must not silently convert it — clarify whether the number stays or resets.
- More decimal places than the currency allows must be handled by an explicit rounding rule, not truncated at random.
- Very large amounts must not lose precision at the boundaries of the storage type.
- An empty field and a zero amount are different; do not collapse them.
- Amounts displayed after storage must match what was typed, to the minor unit.
- The numeric keyboard on mobile must offer the decimal separator the locale actually uses.

## 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

- [ ] Amounts are stored as minor units or fixed-precision decimals, never floats.
- [ ] Currency is an explicit field, never inferred from locale alone.
- [ ] Zero-decimal and three-decimal currencies round-trip correctly.
- [ ] Both comma and full-stop decimal conventions parse to the same value.
- [ ] Negative and zero amounts have defined, documented behaviour.
- [ ] A stored amount redisplays exactly as it was entered.
- [ ] 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.