# Concise Default View — design spec

## Problem

The index page shows every book grouped by status. As the library grows, this becomes
too much information at a glance. The user needs a focused default showing only what's
active right now, with easy paths to browse the rest.

## Design

The index view takes an optional `filter` query parameter. With no filter, it shows a
concise default: only books in-flight (READING) and the 5 most recently added WANT
books. All other books are accessible via filter links. With a filter (`all`, `read`,
`want`), it shows the full matching subset.

## Behaviour

### Default / no filter (concise view)

- **READING group**: all READING books (unchanged from current).
- **WANT group**: sorted by `:ADDED:` date descending, limited to 5.
- **READ / ABANDONED**: replaced by summary links — e.g. "Read · 3" as a clickable
  link to `/index?filter=read`.
- **Filter nav bar** shown at the top: links to `All books (N)`, `Want list`,
  `Read (N)`.

### `?filter=all`

Full index, all groups, no truncation — identical to today's default.

### `?filter=read`

Only the READ group. Single group in the listing.

### `?filter=want`

Only the WANT group. Single group in the listing.

### Error handling

- An unrecognised/empty filter value is treated as `nil` (concise default).
- A WANT book with no `:ADDED:` property sorts before dated entries (treated as
  oldest).
- Fewer than 5 WANT books → all are shown, no padding or placeholder rows.

## Implementation

### Model: `spine-index-model`

Signature changes from:
```
spine-index-model (books &optional selected-id)
```
to:
```
spine-index-model (books &optional filter selected-id)
```

When `filter` is nil, after building the group alist:
- Sort the `:want` group entries by `added` descending (ISO date string; nils sort
  first = oldest).
- Truncate to 5 entries.
- Replace `:read` and `:abandoned` groups with a `:summary-groups` list. Each
  entry is a plist matching the existing model convention:
  ```
  (:label "read" :count 3 :href "/index?filter=read")
  ```
  The template renders these as clickable `.row` links.
  When `filter` is non-nil, `:summary-groups` is absent.
When `filter` is non-nil, return only the matching group (or all groups for `"all"`).

### Handler: `defservlet index`

Read the `filter` key from the request query alist. Pass it to `spine-index-model`.
Pass `current_filter` in the template context for conditional rendering.

### Template: `index.mustache`

- **Filter bar** (above groups): shown when `current_filter` is nil (links to expanded
  views) or truthy (link back to concise view).
- **Summary groups**: new `{{#summary_groups}}` block renders a simple `.row` link per
  suppressed group (READ, ABANDONED).
- Existing group rendering unchanged — still handles full listing when `filter=all`
  or single-group views.

### Sorting

Within the WANT group for the concise view: sort by `added` field descending. The
`added` field is already part of each book's model entry. Nil dates sort before dated
entries (oldest).

## Out of scope

- Pagination for large WANT lists (5 is a design choice; if the user wants more, the
  "All books" link is there).
- Remembering the user's last selected filter across requests (stateless is simpler).
- Keyboard-driven filter switching (plain links for now).