nocal/docs/ARCHITECTURE.md

Architecture

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

src/main.cpp
   ├── tui/              ANSI rendering, input decoding, terminal lifecycle
   ├── storage/          iCalendar file adapter and provider-neutral 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 an application-selected weekday (Monday by default), which keeps redraw geometry stable. The selected week start rotates the header, grid spill cells, and the finite range queried for that grid. It is presentation input only: agenda windows, recurrence rules, and storage remain independent of it.

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, compatible DATE or DATE-TIME RDATE, and EXDATE. An occurrence set merges DTSTART, rule expansion, and additional RDATE starts, deduplicates by start, and then subtracts exclusions. Mutations continue to address the source series as a whole. 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 periods or values incompatible with DTSTART's time basis or TZID, custom VTIMEZONE definitions, alarms, attendees, unknown properties, or malformed components. Supported recurrence rules, compatible additional starts, 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 does not write directly into this UI file. Its prerequisite durable boundary is a provider-neutral SQLite database in WAL mode. The database is a private local file, and schema changes run as versioned, all-or-nothing migrations so a failed upgrade cannot leave a partially migrated cache.

The cache separates provider records from normalized projections. It retains raw provider payloads alongside normalized fields so an older Nocal cannot destroy fields it does not understand. Inactive calendars and tombstoned events remain recorded rather than disappearing from history. Normalized event instances are materialized for immutable UI snapshots; provider payload details do not cross into rendering or the domain model.

Incremental progress is scoped by provider account, calendar, and synchronization window. A provider's opaque cursor is committed in the same transaction as each pulled page, preventing a crash from advancing the cursor beyond durable data. Transactional outbox and conflict records are reserved in the boundary for future local writes and deterministic conflict handling. This cache does not itself implement networking, authentication, or Microsoft Graph synchronization. OAuth tokens and other secrets are explicitly excluded from SQLite and belong in the Freedesktop Secret Service.

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.

HTTP and OAuth boundary

Provider code depends on generic request/response and injected transport interfaces rather than a concrete networking library. Deterministic fakes make success, protocol failure, transport failure, cancellation, and malformed input testable without network access. The generic layer performs no automatic retries: only a provider knows whether an operation is idempotent and how to interpret service-specific throttling or retry guidance.

Microsoft authentication is designed for a public desktop client. It uses the system browser and authorization-code flow, a fresh PKCE verifier and S256 challenge, and a loopback redirect. The Entra application registration is multitenant and selects the intended organizational and personal Microsoft account audience. The initial read-only Graph scope is the delegated Calendars.Read permission. It normally permits user consent, while tenant consent policy remains authoritative and may require administrator approval or block consent. A desktop binary distributed as open source is not a confidential client and must not embed or depend on a client secret.

The generic boundary deliberately contains no concrete networking or desktop code. The Linux adapters below supply the transport, browser launch, and loopback listener. Token JSON parsing, Secret Service storage, Graph requests, and usable account setup remain later slices. Read-only Microsoft Graph calendar discovery and delta synchronization follows; delta links remain opaque and are persisted only through the calendar/window transaction described above. Remote writes and conflict resolution come later.

Concrete Linux adapters

The concrete adapter phase provides a libcurl transport that keeps certificate and host verification enabled, bounds response bodies and connection/operation timeouts, and disables automatic redirects. It implements one request attempt only: neither the adapter nor the generic transport retries. Cleartext HTTP is accepted solely for explicit loopback tests; provider traffic uses HTTPS.

The browser adapter launches xdg-open directly without a shell, so an authorization URL is never interpreted as shell syntax. The callback receiver binds IPv4 127.0.0.1 on a dynamically assigned port, advertises that port with the localhost host and fixed /nocal/oauth/callback path, and handles one bounded callback before closing. Bounded header/input sizes and a deadline keep a local peer from holding the authorization process indefinitely.

The Entra registration must configure Nocal as a Mobile and desktop public client with http://localhost/nocal/oauth/callback. Microsoft ignores the port when matching a localhost redirect, which permits the runtime-selected port. Using an HTTP redirect with the literal 127.0.0.1 host instead requires editing the application manifest, and IPv6 loopback redirect addresses are currently unsupported. The advertised localhost URL therefore remains distinct from the IPv4-only listener binding.

Concrete token JSON parsing, Secret Service storage, Graph requests, account UI, and usable account setup are still absent. libsecret's synchronous calls may block; the next adapter isolates them behind an off-render-thread boundary rather than calling them from the TUI loop. Read-only Graph discovery and delta follow token parsing and secret persistence.