# T12 — Trip rename / delete / merge

**Phase** 4 · **Depends on** T10, T11 · **Status** Done

## Goal

Let rides be renamed, deleted, and merged. Done when a ride can be given a meaningful
name, junk test rides can be removed, and two trips can be combined into one with correct
recomputed aggregates.

## Context

Merge exists as a **safety valve**. It was originally promised to rescue a ride wrongly
split by the migration heuristic; that heuristic is gone now that v1 data is being
discarded, but merge keeps earning its place:

- A ride accidentally stopped and restarted mid-route
- Test rides that should be one logical trip
- Any future automatic splitting (v3+) needs an undo

Delete is the mundane but most-used of the three — a session of testing leaves a dozen
30-second trips cluttering the list.

`Trip.name` is already nullable in the T02 schema, with the UI deriving a date label when
it is null. CASCADE foreign keys mean delete is a single statement.

## State after T08–T11

`TripDetailViewModel` already has `rename(name)` and `delete(onDeleted)`;
`TripsViewModel` has `delete(tripId)`. The repository has `renameTrip` (which already
collapses blank to null) and `deleteTrip`. What is missing is the **merge** operation and
all of the UI: selection mode on the list, the rename affordance on detail, and the
confirmation dialogs. `ConfirmDialog` exists in `ui/components/Stats.kt`.

## Design

### Rename

Inline edit on the detail header. Empty input clears back to null (date-derived label)
rather than storing an empty string — those two states must not diverge.

### Delete

From both the list (swipe or overflow) and detail. Confirmation stating what is lost.
CASCADE removes segments and points. After deleting from detail, navigate back.

### Merge

Selected from the trips list: long-press to enter selection mode, pick exactly two, then
Merge.

Semantics:
- The **earlier** trip (by `startedAt`) is the survivor
- The later trip's segments are re-parented to the survivor
- **Segments are never joined** — the boundary between the two rides becomes a segment
  boundary, exactly like a pause. This is correct: the rider genuinely was not recording
  in between, and joining them would draw a false straight line across the gap
- `endedAt` becomes the later trip's `endedAt`
- The now-empty later trip row is deleted
- Aggregates are **recomputed from scratch** with `RideStatistics.compute()`, not summed
- The survivor keeps its name if set; otherwise inherits the later trip's

Merge runs in a single `@Transaction` — a partial merge would leave orphaned segments.

Merging is not offered while either trip is active.

## Implementation

1. `TripRepository.renameTrip(id, name: String?)`
2. `TripRepository.deleteTrip(id)` — CASCADE
3. `TripRepository.mergeTrips(survivorId, absorbedId)` in one transaction:
   re-parent segments → update `endedAt` → delete absorbed row → recompute aggregates
4. Selection mode in `TripsScreen`, enabled at exactly two selections
5. Confirmation dialogs for delete and merge; merge states the resulting distance
6. Inline rename on the detail header

## Acceptance criteria

- [x] Rename persists; clearing reverts to the date label
- [x] Delete removes the trip and every point (verified by row count)
- [x] Merge re-parents all segments and deletes the absorbed row
- [x] Merged aggregates equal a fresh computation over the combined points
- [x] Merged trip shows a segment boundary at the join, not a joined line
- [x] Merge unavailable unless exactly two completed trips are selected
- [x] Merge is atomic — an interrupted merge leaves no orphans

## Tests

Instrumented:
- Delete cascades: zero segments, zero points remain
- Merge: segment count is the sum of both; point count is the sum; no orphans
- Merged aggregates match `RideStatistics.compute()` over combined points
- Merge of trips out of chronological order still picks the earlier as survivor
- Rename to empty stores null, not `""`

Compose UI: selection mode enables Merge only at two selections. **Not written** — the
UI-test debt from T08–T11 covers this too; tracked in T18.

## Risks / gotchas

- **Do not sum aggregates on merge.** Distance is not additive across the join — there is
  a real gap. Recompute.
- **Chronological survivor.** Selection order is not ride order; sort by `startedAt`.
- **Deleting the active trip** must go through the service's discard path (T06), not a
  raw delete, or the service keeps writing into a deleted trip.

## Out of scope

Splitting a trip; bulk delete; undo.