# Ship Class Calculator — UX

The ship-class designer and calculator are fused into one sidebar
tool (`lib/sidebar/calculator-tab.svelte`). The standalone designer
view/route was replaced by this combined tool. All numeric math lives in
`pkg/calc` and is reached through the `Core` WASM bridge; the calculator
holds input state and orchestrates, it never computes.

## Modes

- **Calculator** (`ship`): the full tool — design area, derived results,
  planet build, goal-seek.
- **Modernization**: reuses the design area and shows per-block and
  total `BlockUpgradeCost` from the current tech to an editable target
  tech. The design-area component is extracted
  (`lib/calculator/ship-design-area.svelte`) so the future ship-group
  upgrade flow can reuse it.

The `path` mode from the original plan was dropped (MVP path-finding is
brute force); reach circles on the map replace it. `bombing` is folded
in as a per-ship result rather than a separate mode.

## Areas

1. **Ship Class design area** — five blocks (drive, armament, weapons,
   shields, cargo) and four tech levels (drive, weapons, shields,
   cargo). Tech defaults to the player's current tech and shows a lock
   icon once overridden; clicking it resets to the default.
2. **Calculator area** — derived results: empty/loaded mass, empty/
   loaded speed, attack, defence, bombing (per ship), cargo capacity.
   A load toggle (empty / full / custom) sets the cargo load (in cargo
   units) that the loaded-column results use. At **full** the toggle
   shows the ship's cargo capacity; a **custom** load over that capacity
   is flagged as an error. With a zero cargo block there is no hold, so
   the load is pinned to empty and the toggle is disabled.
3. **Planet area** — when an own planet is selected on the map, shows
   its MAT (overridable) and the single-turn build rate (ships per turn,
   turns per ship). The realistic multi-turn forecast with CAP/COL
   supply is planned (see ../ROADMAP.md).

## Locks and goal-seek

Two distinct lock semantics share one icon (a closed padlock; it only
appears once a value is pinned, click to release):

- **Override locks** on inputs that have a default — the four techs and
  the planet MAT. Editing one overrides the default; the lock resets it.
  Any number may be overridden at once.
- **Goal-seek locks** on derived results. Pinning a result back-solves
  the single input it claims, which then renders read-only (computed):

  | result        | claims        |
  | ------------- | ------------- |
  | attack        | weapons block |
  | defence       | shields block |
  | empty speed   | drive block   |
  | loaded speed  | drive block   |
  | empty mass    | cargo block   |
  | loaded mass   | cargo load    |

  Only **one** result may be locked at a time (the others' lock
  affordances disable with a tooltip). An unreachable target — e.g. a
  speed at or above the stripped-hull ceiling `20 × driveTech`, or a
  solved block that fails the value rules — leaves the locked cell in a
  red error state and does not apply. Inverse solving lives in
  `pkg/calc/solve.go`; the bisection for defence → shields is the only
  non-analytic case. Locking a speed is disabled when the drive block is
  zero (a deliberately immobile ship has no speed to back-solve).

## Validation and display

Every numeric input is validated independently and an offending one gets
a red border and a hover/tap tooltip with the reason: no value may be
negative, the five blocks follow the engine value rules
(`pkg/calc/validator.go`, surfaced per-field by
`shipClassFieldErrors`), and a custom load may not exceed cargo capacity.

Every displayed number — the derived results and the goal-seek
back-solved input — is rounded **up** to three decimals through the
shared `pkg/calc/number.go.Ceil3` (bridged as `core.ceil3`), so a value
is never shown lower than it is (a speed of 5.0003 reads 5.001). The
engine keeps its own round-to-nearest `util.Fixed*`; `Ceil3` is a
display-only helper that lives in `pkg/calc` so the UI and Go share one
implementation.

## Create / load / delete

The name field is a combobox over the player's existing classes. Picking
an existing class loads it as a template (so you can tweak and Create a
new one); Create is disabled while the name is invalid or duplicate
(reusing `lib/util/ship-class-validation.ts`). When a saved class is
loaded, a Delete affordance appears. Create / Delete reuse the existing
`createShipClass` / `removeShipClass` order-draft flow, so the optimistic
overlay reflects the change immediately. Ship classes are immutable after
creation (per `game/rules.txt`), so there is no edit — only Create-new
and Delete.

## Reach circles

When an own planet is selected in calculator mode, the calculator
publishes the planet origin and the design's loaded speed to a shared
store (`lib/calculator/reach.svelte`). The map view
(`lib/active-view/map.svelte`) reads it and draws 1–3 thin concentric
reach circles (`map/reach-circles.ts`) for 1/2/3 turns. The ring count
shrinks as speed grows: a ring is dropped once the previous one reaches
the torus wrap-midpoint (half the shorter side) or the no-wrap map edge
(farthest corner). The circles clear when the selection clears or the
design is invalid.

## State preservation and history

Calculator inputs are component-local state. The sidebar keeps the tab
mounted while the player navigates between active views, so inputs
persist across view switches per the global state-preservation rule
(`ui/docs/navigation.md`). Tech levels track the rendered report, so in
history mode the calculator computes against the viewed snapshot's tech.

The ship-classes table and the view/bottom menus open the calculator via
a shared request store (`lib/calculator/load-request.svelte`): the
in-game layout flips the sidebar to the calculator tab and the
calculator loads the requested class (or starts a fresh design).

## Layout and mobile

Everything stacks vertically to fit the 18 rem sidebar; the design and
result rows use compact two-column (ship/tech, empty/loaded) grids. On
mobile the sidebar is the existing bottom-sheet/overlay; the calc bottom
tab opens it.

## Runtime note

The new bridge functions are only present after `make wasm` rebuilds
`ui/frontend/static/core.wasm` (needs TinyGo). Vitest injects a fake
`Core` (`tests/fake-core.ts`) mirroring `pkg/calc`, so unit/component
tests do not need the rebuild; the Playwright suite and the live app do.
