nocal/docs/ARCHITECTURE.md

# Architecture

Nocal is split into four layers. Dependencies point inward, keeping remote sync
and terminal rendering replaceable.

```text
src/main.cpp
   ├── tui/              ANSI rendering, input decoding, terminal lifecycle
   ├── storage/          iCalendar file adapter (later: SQLite cache)
   └── domain/           Event, Calendar, civil dates, month layout and queries

future sync workers
   ├── caldav/           Nextcloud and generic CalDAV
   ├── google/           Google Calendar REST/OAuth
   └── graph/            Microsoft Graph/OAuth
            └──────────> domain/ through a SyncProvider interface
```

## Domain

The domain layer uses C++20 `<chrono>` civil dates and time points. It knows
nothing about escape sequences, files, OAuth, or HTTP. Month layout always
produces 42 cells, starting on Monday, which keeps redraw geometry stable.

Events retain a stable UID, title, half-open start/end interval, all-day flag,
optional descriptive fields, and calendar identity. Queries define overlap as
`event.start < range.end && event.end > range.start`; this matters for events
crossing midnight.

Recurring events remain one domain object. Occurrence queries expand only the
requested time window and return values that point back to their source series,
preserving series-level storage and mutation semantics. The supported rule
surface is deliberately bounded to daily/weekly/monthly/yearly frequency,
interval, count or until, weekly `BYDAY`, monthly `BYMONTHDAY`, yearly
`BYMONTH`, and `EXDATE`. Invalid metadata retains at most the base event rather
than crashing a render.

Search matching is a pure domain predicate over normalized event text; the TUI
owns result presentation and occurrence identity. Recurring searches expand a
finite window of five years on either side of the selected date so an unbounded
rule cannot turn an interactive query into unbounded work.

Calendar visibility is session presentation state. The controller derives its
calendar list from event calendar identifiers and applies the same domain
predicate to month rendering, agenda queries, focus traversal, and search.
Hidden events remain in the canonical collection passed to persistence, and
focus identities are always derived from that complete collection so filtering
cannot renumber events with duplicate or missing UIDs.

The agenda is a TUI projection over domain occurrence queries, not a second
calendar model. It requests exactly one 42-day window at a time so recurrence
expansion remains finite, preserves occurrence identity when returning to the
month grid, and keeps its window and selection as non-persistent controller
state. Calendar and search overlays may be opened above it, but mutations stay
in the month workflow and continue to operate on the canonical event
collection.

Timed events retain their source basis: UTC, floating process-local time, or an
IANA `TZID`. C++20 time-zone conversion keeps zoned recurrences at their civil
wall time across daylight-saving transitions. The month grid displays instants
in the user's local zone; the reader also names an explicit source zone.

## Storage

Version 0.1 uses a user-owned iCalendar file, defaulting to
`$XDG_DATA_HOME/nocal/calendar.ics` or `~/.local/share/nocal/calendar.ics`.
The adapter unfolds content lines before parsing and escapes text on output.
On POSIX systems, writes are serialized with an advisory lock, staged in a
same-directory private temporary file, flushed, and atomically renamed over the
destination. A load retains an immutable snapshot of the exact source bytes.
Before saving, the writer compares that snapshot with the destination while it
holds the same lock used for replacement, then checks again after staging and
immediately before commit. This catches same-size and same-timestamp changes
and fully serializes cooperating Nocal writers. The lock is advisory: an
unrelated program can ignore it, so no portable filesystem API can provide a
true compare-and-swap against every possible external writer.

Before an existing destination is replaced, its exact bytes are atomically
written to the adjacent `.bak` file. Explicit backup restoration uses the same
lock, source-revision check, private staging, and atomic replacement path; it
does not consume the backup. The TUI mutates a copyable in-memory model and
rolls it back if persistence fails. Its bounded undo/redo history contains only
mutations that crossed the persistence boundary successfully.

Explicit CLI import and export reuse the storage parser, safety classification,
validation, and atomic persistence boundary; these workflows do not depend on
the TUI and perform no network access. Import loads and validates both source
and target before staging a target replacement. UIDs in both files must be
non-empty and unique, every interval must be valid, and every component in both
files must be safe to round-trip. Matching UID plus identical fields is an
idempotent duplicate; matching UID with different fields is a collision that
aborts the whole operation. A committed merge writes the exact old target to
`.bak` before atomic replacement. If every source event is already present
exactly, import performs no write and does not alter the backup.

Export accepts only a valid, safely round-trippable source, serializes a
canonical calendar through private staging, and atomically creates a destination
that must not already exist. Destination refusal is part of the
commit boundary, so export never replaces an existing path. Import, export,
demo, non-interactive frame printing, and backup restoration are distinct
top-level application modes; import and export cannot be selected together or
combined with any of the other modes.

The current writer deliberately refuses to mutate an existing file when the
loader encounters information it cannot round-trip, including recurrence
overrides, `RDATE`, custom `VTIMEZONE` definitions, alarms, attendees, unknown
properties, or malformed components. Supported recurrence rules, exclusions,
and system IANA time-zone identifiers round-trip semantically. Browsing remains
available for unsupported files. This conservative boundary is more important
than partial editing because a successful-looking edit must not erase unrelated
calendar data.

Provider synchronization will not write directly into this UI file. It will use
a transaction-capable local cache and preserve remote ETags/sync tokens in a
provider metadata table.

The planned cache schema has `calendars`, `events`, `event_instances`, and
`sync_state`. Raw provider payloads are retained alongside normalized fields so
an older Nocal cannot destroy fields it does not understand.

## Terminal backend

The initial backend depends only on POSIX `termios`, `poll`, and ANSI/ECMA-48.
It uses the alternate screen, hides the cursor while rendering, restores all
terminal state through RAII, and repaints from a complete frame buffer. A resize
signal only sets a flag; terminal size and rendering are handled safely in the
main loop.

Colors are semantic ANSI slots and default background, intentionally delegating
actual RGB values to the terminal theme. This is both simpler and more native
than shipping an application theme that fights the desktop palette.

## Sync roadmap

All providers implement the same conceptual operations: authenticate, list
calendars, perform an incremental pull, push a local mutation, and resolve a
conflict. CalDAV uses sync tokens/ETags, Google uses page/sync tokens, and
Microsoft uses Graph delta links. Secrets belong in the Freedesktop Secret
Service, never in the calendar database or command line.

Conflict resolution is deterministic: unchanged side wins; otherwise retain
both versions and mark a conflict for the user. Network work happens outside the
render loop and publishes immutable snapshots to it.