# Half-cell day/night preference marks — Design

**Date:** 2026-04-22
**Status:** approved, ready to implement

## Problem

In the original Excel schedule, each day cell was split horizontally
(left = night, right = day). Users marked preferences/obstacles by
filling the appropriate half with a color before shift planning began.

In the current app, each day = one cell. Users can only express
day-vs-night preferences via cell comments (small blue triangle), which
is not visual at a glance and therefore "annoying."

Multiple users have requested restoring the Excel behavior.

## Decisions

1. **Coexist with shifts** — preference marks stay visible after a shift
   is assigned (so we can see whether we honored or violated the
   request).
2. **Free color from palette** — same flexibility as the existing
   whole-cell color picker; semantics are user-defined.
3. **Horizontal split: left = night, right = day** — matches the
   original Excel.
4. **Visual when shift is assigned: corner triangles** — full shift
   color fills the cell; small ~8×8 px triangles in the bottom corners
   indicate the marks (bottom-left = night, bottom-right = day).
5. **Toggle in context menu** — segmented control `Celá | Noční | Denní`
   above the color palette selects which field the next color click
   updates.

## Data model

`CellValue` (in `types.ts`) gains two optional fields:

```ts
interface CellValue {
  v?: string
  color?: string        // existing — full-cell background
  dayColor?: string     // NEW — right half / day mark
  nightColor?: string   // NEW — left half / night mark
}
```

The change is purely additive. Existing JSON files load unchanged.

## Rendering rules

| State | Look |
|-------|------|
| empty + no marks | empty (current) |
| `dayColor` and/or `nightColor` set, no shift | left half = `nightColor`, right half = `dayColor` (each independent; missing half = transparent or `color` fallback) |
| shift assigned, no marks | full shift color (current) |
| shift assigned + marks | full shift color + 8 px triangles: bottom-left = night, bottom-right = day |

Priority for picking renderer:

1. If `dayColor || nightColor` → split-half mode
2. Else if `color` → full-cell color (current behavior)
3. Else → empty

## Interaction

Right-click cell → existing context menu, with new segmented toggle
above the color palette:

```
Barva:
  [ Celá ] [ Noční ] [ Denní ]    ← default: Celá
  ● ● ● ● ● ● ● [Bez barvy]
```

- Toggle selects target field (`color`, `nightColor`, `dayColor`)
- Color click writes only to the selected target
- "Bez barvy" clears only the selected target
- Three fields are independent

## Drag-and-drop

Half marks are tied to the **day**, not the shift. When a user drags a
shift cell:

- Only `v` (and `color` of the shift) moves to the destination
- `dayColor` / `nightColor` stay on both source and destination days

## Compare / diff view

Differences in `dayColor` / `nightColor` count as cell changes — render
the same orange "modified" border as for value changes.

## Filters

- "Color only" filter (rainbow dot) — also hides cells whose only
  content is `dayColor` / `nightColor` (no `v`)
- Hidden value filter (e.g. hide "A") — hides the cell entirely
  including marks (marks are auxiliary)

## Scope

Half-mark editing is available on:

- ✅ Person cells (Pohotovost TKB / Pohotovost IT)
- ✅ Info rows (TKB / SAZLT / Metro / D8)
- ❌ PMS columns (month-end summary, doesn't make sense)

## Read-only

Existing `isReadOnly` guard handles viewer mode automatically. Viewers
see marks but cannot edit.

## Out of scope (phase 1)

- **Excel export/import** — original template has no clean
  representation; round-trip would lose marks. Documented as a known
  limitation; revisit if users request it.
- **PDF export** — will render half marks using same rules (no extra
  work expected).

## Files to change

1. `web/src/types.ts` — extend `CellValue`
2. `web/src/useScheduleState.ts` — add `setCellHalfColor(personId,
   dayIdx, target, color)` reducer + undo support
3. `web/src/ScheduleTable.tsx` — render split halves / corner triangles
   in `PersonRow` and `renderInfoRowCells`
4. `web/src/ContextMenu.tsx` — add segmented toggle, route color clicks
   to selected target
5. `web/src/cellColors.ts` — no change

Estimate: ~150 LoC across 4 files plus minimal CSS.