# Architecture

Calendula is a single-activity Jetpack Compose app layered strictly on top
of Android's calendar provider. This document is the orientation tour: the
principles, the layers, and the three pipelines that are not obvious from
the package list (recurring writes, save conflicts, reminder delivery).

## Principles

1. **`CalendarContract` is the single source of truth.** No app database,
   no caching layer, no sync code. Reads query the provider; writes go
   straight back to it. Sync is DAVx5's / Google's / the system's job.
2. **Observer-driven UI.** A `ContentObserver` on the provider triggers
   re-queries; every screen recomposes from fresh provider state. After a
   write, nothing is patched by hand — the provider notifies, the views
   refresh. This also covers external changes (sync) for free.
3. **JVM-first testing.** Everything between the UI and the
   `ContentResolver` is shaped so it runs as a plain JUnit 5 test: pure
   domain logic, cursor-free mappers, a `FakeCalendarDataSource` for
   repository tests. Instrumented tests are a last resort.
4. **No network.** The app declares no `INTERNET` permission. Anything that
   would need one is an explicit, documented product decision first
   (see the roadmap's idea backlog).

## Layers

```mermaid
flowchart TD
    subgraph UI ["ui/ — Compose screens + ViewModels"]
        Screens["Month / Week / Day\nDetail / Edit / Settings\nPermission + Reminder onboarding"]
    end
    subgraph Data ["data/"]
        Repo["CalendarRepository\n(interface + impl, Flow-based, io-dispatched)"]
        DS["CalendarDataSource\n(interface + AndroidCalendarDataSource)"]
        Prefs["SettingsPrefs / CalendarPrefs\n(DataStore)"]
        Rem["reminders/\nReminderAlertStore + ReminderNotifier"]
    end
    Provider[("CalendarContract\n(system calendar provider)")]

    Screens --> Repo
    Screens --> Prefs
    Repo --> DS
    DS --> Provider
    Provider -. "ContentObserver tick" .-> Repo
    Provider -. "EVENT_REMINDER broadcast" .-> Rem
    Rem --> Provider
```

- **`domain/`** — pure Kotlin, no Android imports: models
  (`EventInstance`, `EventDetail`, `CalendarSource`, …), the `EventForm`
  with validation, `SimpleRecurrence` (RRULE parse/render for the picker),
  and `EditSnapshot` (conflict detection). All JVM-tested.
- **`data/calendar/`** — the provider seam. `AndroidCalendarDataSource`
  owns every `ContentResolver` call; cursor parsing lives in mappers
  (`InstanceMapper`, `EventDetailMapper`, `CalendarMapper`) that read
  through a `ColumnReader` abstraction so tests feed them plain maps.
  `EventWriteMapper` builds dirty-checked update value sets. `TimeBridge`
  converts provider epoch millis ↔ `kotlin.time.Instant`.
- **`data/reminders/`** — the notification pipeline (see below). Kept out
  of `data/calendar/` because the receiver needs neither the repository
  nor its flows.
- **`data/prefs/`** — DataStore-backed settings (theme, week start, form
  field defaults, reminders toggle) and small state (last-used calendar).
- **`ui/`** — one package per screen, each with Screen + ViewModel +
  UiState. Shared pieces in `ui/common/` (OptionCard — the app's only
  sanctioned selection-dialog style —, recurrence humanizer, FAB column,
  drawer, transitions).

## Navigation

There is no navigation library. `MainActivity` hosts `RootScreen`, which
gates on the calendar permission and the one-time reminder onboarding, then
shows `CalendarHost`. `CalendarHost` holds the active view (month/week/day)
plus overlay state for detail, edit, and settings — full-screen overlays
driven by `AnimatedVisibility` with a *held-key* pattern: the last shown
key stays alive through the slide-out so content never flashes empty.
A tapped reminder notification routes through `MainActivity` (`singleTop` +
`onNewIntent`) as an external detail key that `CalendarHost` consumes
exactly like an event tap.

## Recurring writes

The provider's invariants drive the design (learned the hard way, verified
on-device — see plan 03):

- Recurring rows carry `RRULE` + `DURATION` (no `DTEND`); one-off rows
  carry `DTEND`.
- *Only this event* → insert a **modified-occurrence exception** via
  `CONTENT_EXCEPTION_URI` (the provider clones the series row, so empty
  optionals are written as explicit NULLs).
- *This and following* → **series split**: insert the new event first (if
  that fails the original is untouched), then truncate the original's
  RRULE with `UNTIL`.
- Truncation updates must send the **complete time-column set**
  (`DTSTART`/`DURATION`/`RRULE`/`ALL_DAY`/`EVENT_TIMEZONE`) — the provider
  regenerates cached instances only from the values carried by the update
  itself; an RRULE-only update leaves stale instances behind.
- `UNTIL` is written as the local end of the previous day expressed in
  UTC, so zones ahead of UTC can't leak an extra occurrence.
- All-day events are normalised to UTC midnights with an exclusive end.

## Save conflicts

No locking. `openForEdit` keeps an `EditSnapshot` — the prefilled form
*plus the raw Events-row times* (the form derives its times from the tapped
occurrence, so a remotely moved event would otherwise be invisible to it).
Right before writing, the event is re-read and snapshots compared: a
mismatch parks the save in an overwrite/discard dialog; a vanished event
informs and closes. Overwrite still writes only dirty fields, so external
changes to untouched fields survive either way. Fields the form cannot
write (attendees, status, reminder methods) are excluded so sync noise
can't fake a conflict.

## Reminder delivery

The provider schedules reminder alarms (for `METHOD_ALERT` rows only) and
broadcasts `EVENT_REMINDER` — but posts no notification; a calendar app
must (the Etar model):

```mermaid
sequenceDiagram
    participant P as CalendarProvider
    participant R as EventReminderReceiver
    participant S as ReminderAlertStore
    participant N as ReminderNotifier
    P->>R: EVENT_REMINDER broadcast (manifest receiver, exported)
    R->>S: dueAlerts(now) — CalendarAlerts: SCHEDULED, alarmTime ≤ now
    S-->>R: due alerts
    R->>N: post(alert) — one notification per alert, tag = alert id
    R->>S: markFired(ids) — best effort, needs WRITE_CALENDAR
```

Posting happens before marking: a crash in between re-posts silently (same
tag + `setOnlyAlertOnce`) rather than losing a reminder. Swiped
notifications never return because `FIRED` rows are never re-queried.
Deliberately absent until real devices prove it necessary: own alarm
scheduling, `BOOT_COMPLETED`, snooze/dismiss actions, battery-exemption
prompts.

## Testing

JUnit 5 + Truth + Turbine on the JVM. The seams that make it work:
`CalendarDataSource` is faked (`FakeCalendarDataSource` records writes),
mappers parse `ColumnReader`/plain maps instead of cursors, domain logic
(recurrence, validation, snapshots, write-value building) is pure. CI
(Gitea Actions) runs `lint test assembleDebug` on every push; release tags
additionally build, sign, and publish to the self-hosted F-Droid repo.