# Audio Waveform Scrubber

## Objective

Show the shape of a track so listeners can see the quiet and loud parts and click to them.

A waveform rendered from precomputed amplitude data, doubling as the seek control on the audio player.

## 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. Compute the amplitude data on the server when the file is first stored, and serve it as a small dataset the page can draw. Do not decode the audio in the browser to build the waveform.
2. Replace the plain scrub bar in the app's existing Audio Player with this surface rather than adding a second seek control beside it.
3. Draw played and unplayed regions distinctly, and keep the playhead position exact under click, drag, and keyboard seeking on the same surface.
4. Reserve the waveform's height and width before the data arrives and show a placeholder in that space, so the player does not resize when it loads.
5. Do not make the waveform the only way to seek. Keep the elapsed and total time readable and keyboard seeking available, because a waveform is unusable to anyone who cannot see it.

## 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

- Decoding audio in the browser to draw a waveform blocks the main thread and downloads the whole file before playback can start. Precompute once at upload and reuse the result for every listener.
- A canvas drawn at layout size looks blurry on high-density screens and stretches on resize. Redraw at the device pixel ratio, and redraw again when the container width changes rather than scaling the existing bitmap.
- Waveform data arrives after the player does. Show a neutral placeholder of the right dimensions in the meantime and keep the player fully usable, rather than blocking playback on a decorative asset.
- Distinguishing played from unplayed by colour alone fails for colour-blind listeners and in high-contrast mode. Vary opacity, height, or fill as well.
- Click, drag, and keyboard must all seek the same surface. Dragging must not select surrounding text, and releasing outside the waveform must still resolve to a sensible position rather than leaving the player stuck in a drag.
- A file with no computed waveform, or one still being processed, must fall back to the plain scrub bar rather than showing an empty box.
- Very long recordings compress thousands of samples into a few hundred columns. Fix the number of columns to the drawn width so the data volume does not grow with duration.
- The waveform needs an accessible role and value so a screen reader announces the position as a slider, not as an unlabelled image.

## 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

- [ ] Waveform data is computed server-side at upload and served as a compact dataset.
- [ ] The waveform is the player's seek control, not a second one alongside it.
- [ ] Click, drag, and keyboard seeking all work on the same surface and land on the same position.
- [ ] Played and unplayed regions are distinguishable without relying on colour.
- [ ] The waveform redraws sharply on resize and on high-density displays.
- [ ] A placeholder of the correct dimensions shows while data loads, with no layout shift.
- [ ] Files with no waveform data fall back to the plain scrub bar and remain playable.
- [ ] 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.