# segmented history

a room's history is sections of events with durable gaps between them. the sdk
event cache keeps the sections, gap identity, boundaries and each gap's cursor,
encrypted at rest, across room close and restart. macltok shows gaps inline and
fills them on request.

## opening a room

opening loads retained history and its gaps, then starts a recent load in the
background, so opening never waits on the network. the recent load fetches up
to four pages of 50 events from a fresh server position. if it reaches the
newest retained event the sections join. if it doesn't, a gap is left between
the fresh window and what was retained.

`Event::RecentHistoryChanged` reports it, one event per edge.

- `Loading` as it starts
- `Loaded`, `Cancelled` or `Failed(message)` as it settles
- `Idle` when the room closes, whatever was happening

`Failed` stays until `HistoryRepairCommand::RetryRecent`, and closing the room
turns it to `Idle` like anything else. a failed load on a room timeline with no
events yet falls back to paginating `EngineConfig::initial_history_limit`
events, so the room isn't left empty.

## filling a gap

`HistoryRepairCommand::FillGap { room_id, gap_id }` walks one gap backwards, up
to four pages of 50, each page committing its events and the gap's new cursor
together. `Event::HistoryRepairChanged` carries a `HistoryRepairReport`, once
with phase `Bridging` as the fill starts and once more as it settles.

- `Ready`, continuity `Verified`. the older section was reached and the gap is gone
- `Paused` with a stop. `Budget` after four pages, `CursorStalled` when the
  server repeats a cursor, `HistoryUnavailable` (continuity `AccessibleStart`)
  when accessible history ends first, `Cancelled`
- `Paused` with a `failure`, and the gap row goes `Failed`

a gap that is already unavailable answers at once and counts no page. one that
has already joined is gone, so filling it answers `EventNotFound`.

## one operation per room

a room runs one history operation at a time. `RetryRecent` joins a recent load
that is already running. everything else that overlaps, a fill during a recent
load, a recent load during a fill, a second fill, answers `RoomBusy`.

`HistoryRepairCommand::Cancel` stops whatever is running. its own answer is
`Ok`, the cancelled command answers `Interrupted`, and the published outcome is
`Cancelled`, never a failure. pages already committed stay.

## the timeline

gaps appear in `TimelineState.items` as `TimelineItem::HistoryGap`, between the
sections they separate. a row keeps its id while its lifecycle changes, and is
removed once the gap joins. joined gaps never appear.

history lands as `TimelineChanged` diffs, never a `TimelineReset`. they are
incremental unless the cache's loaded window changed under the operation, after
a restart or a shrink, when the sdk sends the room as one `Reset` diff. before
answering, the room fences on `Timeline::flush_history`, which applies what the
cache already queued without replacing the timeline, so receipts, the read
marker and row ids survive. a failed or cancelled operation that committed
pages is fenced the same way.

a command's answer comes after its outcome is published, and after the mirror
has applied it when one is attached.

## wire shapes

serde enums are externally tagged.

```json
{ "HistoryRepair": { "FillGap": { "room_id": "!room:example.org", "gap_id": "opaque" } } }
{ "HistoryRepair": { "RetryRecent": { "room_id": "!room:example.org" } } }
{ "HistoryRepair": { "Cancel": { "room_id": "!room:example.org" } } }
```

a gap row.

```json
{
  "HistoryGap": {
    "id": "opaque",
    "older_boundary": "$older:example.org",
    "newer_boundary": "$newer:example.org",
    "lifecycle": "Idle",
    "failure": null
  }
}
```

lifecycle is `Idle`, `Paused`, `Failed` or `Unavailable`. `failure` is set with
`Failed`.

a settled fill.

```json
{
  "room_id": "!room:example.org",
  "gap_id": "opaque",
  "pages": 2,
  "events": 63,
  "phase": "Ready",
  "continuity": "Verified",
  "stop": null,
  "failure": null
}
```

the mirror keeps `State.recent_history`, a room without an entry being idle,
and `State.repairs`, the last fill report per room. closing a room drops both.