# Duplicate File Detection

## Objective

Stop storing the same file over and over.

Content-hash deduplication of uploads within an ownership boundary, so identical bytes are stored once but tracked per record.

## 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. Hash the stored bytes on the server with a strong content hash, streaming rather than loading the whole file into memory.
2. Never trust a filename, a size, or a client-supplied checksum as evidence of sameness. A client that can assert a hash can claim someone else's file.
3. Deduplicate only within an explicit boundary — the tenant, the workspace, or the single owner. Sharing bytes across tenants leaks the fact that another tenant holds an identical file.
4. Keep metadata separate from storage: filename, caption, uploader, timestamps, and permissions belong to each reference, not to the shared object.
5. Reference-count the stored object and only delete the bytes when the last reference goes.
6. Do NOT tell an uploader 'this file already exists' when the existing copy belongs to someone they cannot see. Deduplicate silently instead.

## 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

- Two uploads of the same file arriving at once must not both create the object — make the write conditional or unique-indexed.
- A hash collision check that compares only hashes is fine for a modern hash; a check that compares only file size is not.
- Deleting one record's copy must not remove the bytes another record still points at.
- Files that are transformed after upload (compressed, watermarked) change hash — decide whether you dedupe the original, the derivative, or both.
- Retention and legal-hold rules apply per reference; one tenant's deletion request cannot be satisfied by leaving shared bytes in place unless the boundary makes that lawful.
- Encrypted-at-rest storage with per-tenant keys makes cross-record deduplication impossible; check before assuming it works.
- Re-uploading a file the user deleted must work normally, not resurrect the old record's metadata.

## 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

- [ ] Hashes are computed server-side from the stored bytes, streamed.
- [ ] Client-supplied hashes are never trusted for deduplication decisions.
- [ ] Deduplication never crosses the defined ownership boundary.
- [ ] Each reference keeps its own filename, metadata, and permissions.
- [ ] Stored bytes are removed only when the last reference is deleted.
- [ ] Concurrent identical uploads produce exactly one stored object.
- [ ] 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.