# Battle Viewer UX

Phase 27 ships a dedicated viewer for battles (`/games/<id>/battle/<battleId>`).
Bombings stay where they were in Phase 23 — a static table in the
Reports view (`section-bombings.svelte`). The two domains are
deliberately not mixed in any visual surface or click target.

## Data shape

The `BattleViewer` component (`lib/battle-player/battle-viewer.svelte`)
is logically isolated. It accepts a `BattleReport` matching
`pkg/model/report/battle.go`. The fields it uses:

- `id`, `planet`, `planetName` — header + the central-planet glyph.
- `races: { [raceId]: raceUUID }` — race index space used by the
  protocol's `a` / `d` fields.
- `ships: { [groupKey]: BattleReportGroup }` — ship-group rosters
  with `race` name, `className`, initial `num`, end-state `numLeft`,
  and the `inBattle` flag. Observer groups (`inBattle: false`) are
  never drawn.
- `protocol: BattleActionReport[]` — flat list of shots. Each carries
  attacker `(a, sa)`, defender `(d, sd)`, and `x` (destroyed?).

The component asks `timeline.ts.buildFrames(report)` to expand the
protocol into `protocol.length + 1` frames; frame 0 is the initial
state and frame `N` reflects state after action `protocol[N-1]`. The
race index per ship group is derived from the protocol itself —
every in-battle group appears at least once as attacker or defender,
and the engine never crosses these wires.

## Radial scene

The scene (`lib/battle-player/battle-scene.svelte`, SVG) places the
planet at the centre and arrays the still-active races on an outer
ring at equal angular spacing. Each race anchor hosts a *cloud* of
class circles arranged on a Vogel sunflower spiral biased toward the
planet (the cluster anchor is pushed inward by a quarter step so the
rank-0 node — the heaviest group by NumberLeft — sits closest to the
planet, and the spiral fans the rest behind it). When a race is
wiped out, it drops out of the active list and the survivors are
re-spaced on the next frame.

Each class circle is one *bucket* keyed by `(race, className)`:
tech-variants of the same class collapse into one node so the scene
stays readable when a race fields a dozen tech levels of the same
hull. The per-bucket label `<className>:<numLeft>` sums NumberLeft
across the underlying groups; per-tech detail is available in the
Reports view (Foreign Ship Classes / My Ship Types).

Bucket order inside a cluster is **locked at battle start** by the
initial ship count (`num` summed across tech variants, descending).
As ships die during playback only the label number changes — every
bucket keeps its slot in the Vogel spiral, so the user does not see
the cluster reshuffle when a class empties. Vogel positions are
then reassigned per rank by their inward distance toward the
planet, so the rank-0 bucket (the largest at battle start) always
sits at the most-inward spiral slot.

Circle radius scales with per-ship FullMass (Empty + Carrying via
the per-ship `LoadQuantity`). The viewer resolves a
`(race, className) → ShipClassRef` lookup from the surrounding
`GameReport.localShipClass` + `otherShipClass` tables and runs it
through the existing wasm bridge to `pkg/calc/ship.go`
(`emptyMass` + `carryingMass` + `fullMass`). The radius is then
`MIN_RADIUS + (MAX_RADIUS − MIN_RADIUS) × sqrt(mass / maxMassInBattle)`
clamped to `[6, 24]` pixels — per-battle normalisation, so the
heaviest ship in any given battle renders at the cap. Unknown class
or invalid params fall back to MAX_RADIUS so the bucket stays
visible.

The current frame's shot is drawn as a thin line from the attacker's
class circle to the defender's class circle. Colour:

- red (`#ee3344`) when the action's `x === true` (the defender
  ship was destroyed),
- green (`#44dd66`) otherwise.

Each frame redraws the line in isolation, so continuous playback
produces the "shot-shot-shot" pulse the user wanted.

## Playback controls

`lib/battle-player/playback-controls.svelte` ships the full set:

| Control       | Effect                                     |
| ------------- | ------------------------------------------ |
| ⏮ rewind      | Stop, jump to frame 0                      |
| ◀︎ step back  | Stop, frame ← frame − 1                    |
| ▶︎ / ⏸ play   | Toggle continuous playback                 |
| ▶︎▶︎ step fwd | Stop, frame ← frame + 1                    |
| 1x / 2x / 4x  | Speed switch: 400 / 200 / 100 ms per frame |

When the timeline is at its end and the user hits play, the frame
counter wraps to 0 and continues. Step buttons disable themselves at
their boundary.

## Accessibility

Below the scene the viewer renders a static `<ol>` text protocol —
one line per action, formatted from `BattleReportGroup.race` and
`BattleReportGroup.className`. The line for the current frame is
highlighted so a non-visual reader can follow along by scrolling
the log instead of watching the SVG. The list is always present
and never hidden, satisfying the original Phase 27 acceptance "the
same data is accessible as a static text log".

Each log row is also a `<button>`: a click or Enter/Space jumps
playback to that shot (pauses and seeks). The list auto-scrolls
the current row into view as the timeline advances, so the user
does not have to chase the highlight on long battles.

## Playback details

On play, the shot line + the defender circle's colour flash gate
on a per-frame timer that blinks them off during the last 10 % of
the frame's duration. Two consecutive shots from the same attacker
on the same defender therefore look like two distinct pulses
rather than one continuous line. On pause the line and flash
stay drawn so the user can study the current shot.

## Phantom destroys

Legacy emitters (the `dg` engine format that feeds the synthetic-
report path) occasionally log more `Destroyed` lines against a
ship-group bucket than the bucket's initial population — the
emitter keeps recording hits past the moment the group emptied.
`buildFrames` clamps each per-group remaining count at zero and
only decrements race totals on a real shrink, so a race stays on
the scene until its actual ships are gone. The phantom shots still
draw a line during the frame they belong to; only the running
counters are protected.

## Height fit

The viewer is pinned to the viewport: `.active-view` uses
`calc(100dvh − 80px)` so the in-game-shell header + optional
HistoryBanner do not push the scene below the fold. Inside the
viewer, the scene grows (`flex: 1`) and the log shrinks to a
30 dvh ceiling with its own internal scroll, so the page itself
never scrolls vertically. The 80 px allowance maps to the current
Header + HistoryBanner total on desktop; mobile breakpoints reuse
the same calc because dvh tracks the dynamic viewport.

## Map markers

`map/battle-markers.ts` emits two marker kinds per
current-turn report. Both are wired into the binding's
`hitLookup` so a click goes through the existing hit-test plumbing.

### Battle marker — yellow cross

For every `report.battles[i]` whose `planet` resolves to a visible
planet, the marker emits two `LinePrim` lines through the opposite
corners of the square circumscribed around the planet circle. The
result is an X-shaped cross overlaid on the planet glyph.

The stroke width is computed by `battleMarkerStrokeWidth(shots)`:
1 shot → 1 px, 100 shots → 5 px, linearly interpolated in between
(`width = 1 + (shots − 1) × 4 / 99`, clamped). A click on either
line navigates to `/games/<id>/battle/<battleId>?turn=<turn>`.

### Bombing marker — colored ring

For every `report.bombings[i]`, the marker emits a single
stroke-only `CirclePrim` slightly larger than the planet circle.
Colour:

- yellow (`#FFD400`) when `wiped: false`,
- red (`#FF3030`) when `wiped: true`.

A click on the ring navigates to `/games/<id>/report#report-bombings`
and scrolls the matching `report-bombing-row` (by `data-planet`)
into view. Bombing markers never open the Battle Viewer — the two
domains stay separate.

## Data source

The Battle Viewer page (`lib/active-view/battle.svelte`) calls
`api/battle-fetch.ts.fetchBattle(gameId, turn, battleId)`. The
loader has two modes:

- **Synthetic** — when `gameId` carries the
  `synthetic-` prefix, the lookup is served from
  `api/synthetic-battle.ts`. Vitest unit tests and Playwright e2e
  tests register fixture battles via `registerSyntheticBattle`
  before mounting the route.
- **Production** — otherwise the loader issues
  `GET /api/v1/user/games/{gameId}/battles/{turn}/{battleId}`
  against the backend gateway route added in
  `backend/internal/server/handlers_user_games.go.Battle`. The
  gateway forwards verbatim to the engine's
  `GET /api/v1/battle/:turn/:uuid`.

## TODOs

- Push event `game.battle.new` + toast → viewer link (deferred —
  needs an engine emit-side change).
- Richer ship-class visuals derived from the class's mass,
  armament, shields. Current MVP uses a small circle plus
  `<class>:<numLeft>` label.
- Animated transitions when a race drops out and the survivors
  re-distribute. Current implementation hard-jumps on the next
  frame.