# lectio Liturgical Calendar Engine — Design (Sub-project #1)

**Status:** design, awaiting review
**Date:** 2026-07-27
**Supersedes scraping for calendar computation; part of the self-contained-calendar epic.**

## Goal

Make lectio compute the liturgical day **offline, from first principles**, instead of
scraping niedziela.pl / missalemeum. This document specifies **sub-project #1: the
Ordinary Form (OF) calendar engine** — given a Gregorian date, produce the day's
liturgical identity (season, celebration(s), rank, colour, the observed celebration)
plus any *inline* proper readings. It also fixes the **data formats** and **package
architecture** that the whole epic builds on.

## Longevity mandate (global constraints)

Every decision below serves a 25–50 year lifespan. These are binding:

- **Language:** Go. The core engine uses **stdlib only, zero third-party dependencies.**
- **No network in the core.** The engine is pure and total: `(date, selection, layers) → day`.
- **Durable, hand-editable formats only:** **INI** (config + all calendar/celebration data),
  **TSV** (scripture corpora only), **JSON + iCalendar** (API output only, later phase).
  No TOML, no YAML.
- **Unix philosophy:** one concern per unit; the engine emits data, never renders; front-ends
  and API are thin, disposable consumers; compose via text.
- **Data is the crown jewel:** authoritative, owned, plain-text, outlives any code.

## Where #1 fits (the epic)

Four linked sub-projects, one-way dependency flow:

```
INPUTS →  DATA (embedded base calendar, corpora)   [#1 base, #3 lectionary]
          CONFIG (INI, app behavior)               [#1]
          CUSTOMIZATION (INI layer files, stacked)  [#2]
              │
CORE     →  LOGIC / ENGINE (pure Go, stdlib)         [#1 calendar, #3 readings]
              │  emits LiturgicalDay
CONSUMERS→  UI (cli/tui/web)   API (json/ical)        [migration, #4]
```

1. **Calendar engine** (this doc) — day identity. FOUNDATION.
2. **Customization** — load user INI layer files, ordered-stack merge (`use = …`), authoring CLI.
3. **Lectionary** — the temporal reading cycle (Sundays A/B/C, weekdays I/II) + full
   reading resolution.
4. **API** — expose calendars as JSON + iCal.

## Sub-project #1 scope

**In:**
- The pure **calendar engine**: Computus → temporal cycle → sanctoral → precedence → `LiturgicalDay`.
- The **embedded universal General Roman Calendar** data (the base layer).
- The **INI data formats**: config, and the celebration-record schema (incl. reading fields).
- The **layer-merge mechanism** in the engine (accepts an ordered `[]Layer`), unit-tested with
  synthetic layers; #1 ships only the embedded universal layer.
- **Config layer**: TOML→INI migration + calendar-selection keys.
- A **CLI surface** to compute and print a day: `lectio calendar [DATE]`.
- **Validation**: regression oracle + unit tests proving correctness before we trust it offline.

**Out (deferred, but interfaces designed so they slot in):**
- Loading user layer files + `use =` composition + `calendar new`/`calendar check` → **#2**.
- Temporal reading cycle (Sundays A/B/C, weekday cycle I/II) + reading-vs-ferial resolution → **#3**.
- JSON / iCal API → **#4**.
- Migrating the TUI/web daily view off the scraper (the scraper stays, quarantined, until then).
- The Extraordinary Form (1962) engine — same shape, later; `selection.form` reserves the axis.
- Copyrighted `bt` text — skipped; readings render from embedded public-domain corpora.

## Data formats

### Config — INI (`internal/config`, migrated from TOML)

Flat `key = value`, `[sections]` only where grouping helps. New section:

```ini
[calendar]
lectionary = new          ; new (OF) | old (EF, later)
epiphany   = fixed         ; fixed (Jan 6) | sunday   — national placement knob
ascension  = thursday      ; thursday | sunday
corpus_christi = thursday   ; thursday | sunday
```

Existing keys (ui_language, sigla_style, versions, display, web_*, width, pager, offline,
default_version, …) migrate 1:1 into INI. **Migration:** on startup, if `config.ini` is absent
but `config.toml` exists, auto-convert once (read the old TOML, write `config.ini`); thereafter
INI is authoritative. The TOML reader is retained only for this one-shot conversion and removed in
a later cleanup. (`use =`, the customization stack, is introduced in #2 — not in #1's schema.)

### Celebration records — INI sections (the base calendar AND, later, override layers)

One `[slug]` per celebration; `[slug/variant]` for extra Masses. Used by the embedded base
calendar now and by user override files in #2 — same format so overrides layer cleanly and
dioceses copy universal entries as templates.

```ini
[layer]                    ; layer header (metadata for the file)
id   = universal
name = General Roman Calendar
type = universal

[assumption]
date    = 08-15            ; fixed MM-DD, or a movable expression (see DateSpec)
rank    = solemnity        ; solemnity | feast | memorial | optional | (ferial is implicit)
class   = bvm              ; lord | bvm | saint  — drives precedence within a rank
colour  = white            ; white | red | green | violet | rose | black
name.en = Assumption of the Blessed Virgin Mary
name.pl = Wniebowzięcie Najświętszej Maryi Panny
name.la = In Assumptione Beatae Mariae Virginis
reading.first  = Ap 11,19; 12,1-6.10   ; proper readings — citations only (see Readings)
reading.psalm  = Ps 45,10-16
reading.second = 1 Kor 15,20-26
reading.gospel = Łk 1,39-56

[assumption/vigil]         ; a second Mass formulary
reading.first  = 1 Krn 15,3-4.15-16; 16,1-2
reading.gospel = Łk 11,27-28
```

Override operations (semantics defined here; **loading is #2**): a `[slug]` in a higher layer
**merges over** the same slug below (field-level; unlisted fields inherit); a new slug **adds**;
`suppress = true` removes; `date =` moves; `rank =`/`class =` re-rank. Movable-relative
sanctoral (e.g. a diocesan patron on a Sunday) use a DateSpec.

**DateSpec grammar** (minimal for #1): `MM-DD` (fixed); `easter±N`; `advent-sunday-N`;
`sunday-after MM-DD`; `christmas±N`. Most base sanctoral are `MM-DD`; the movable **temporal**
solemnities (Easter, Ascension, Pentecost, Trinity, Corpus Christi, Sacred Heart, Christ the King,
Baptism, Holy Family) are computed in `temporal.go`, not stored as data.

### Readings — citations, not text

`reading.<part>` values are **scripture citations** in the config sigla dialect. They are resolved
against the **embedded corpora** via the existing `bible.BookTable.ParseRef` → `bible.Verses`
machinery (the `--ref` path). Citations are references (not copyrightable); text renders from
public-domain Wujek/Vulgate/Douay/Greek. Parts: `first, psalm, second, acclamation, gospel`
(reusing the established PartID vocabulary). **#1 surfaces inline propers when present**; it does
not compute ferial/Sunday readings (that's #3). Note: romcal supplies the *calendar* but not
reading citations, so populating universal proper readings is incremental — major solemnities in
#1, the rest with the lectionary in #3.

### Corpora — TSV (unchanged)

Scripture stays TSV (`internal/bible`). Genuinely tabular, already embedded, already proven.

## The engine (LOGIC)

Pure, stdlib-only, total. Package `internal/calendar` holds both the vocabulary types and the
computation; `internal/caldata` embeds+parses the base data into `calendar.Layer`.

### Types (the boundary contract — UI/API depend only on these)

```
type Rank int        // ferial < optional < memorial < feast < solemnity
type Class int       // saint < bvm < lord   (tiebreak within a rank)
type Colour string   // white,red,green,violet,rose,black
type Season string   // advent,christmas,ordinary,lent,triduum,easter

type Reading struct { Part, Citation string }
type Mass    struct { Variant string; Readings []Reading }

type Celebration struct {
    Slug   string
    Name   map[string]string   // lang → name
    Rank   Rank
    Class  Class
    Colour Colour
    Date   DateSpec
    Masses []Mass              // proper readings, if any
    Layer  string             // provenance: which layer contributed it
}

type Layer struct {           // one calendar layer (base or, later, an override file)
    ID, Name, Type string
    Cels map[string]Celebration
}

type Selection struct {       // from config
    Form           string     // "new" (OF). "old" reserved for the EF engine.
    Epiphany       string     // "fixed" | "sunday"
    Ascension      string     // "thursday" | "sunday"
    CorpusChristi  string     // "thursday" | "sunday"
}

type LiturgicalDay struct {
    Date     time.Time
    Season   Season
    Week     int              // week-within-season (for the lectionary phase)
    Weekday  time.Weekday
    Observed Celebration      // the winner after precedence
    Others   []Celebration    // commemorations / optional memorials also available today
    Colour   Colour
    SundayCycle  string       // "A"|"B"|"C"  (computed; used by #3)
    WeekdayCycle string       // "I"|"II"    (computed; used by #3)
}

func Compute(date time.Time, sel Selection, layers []Layer) LiturgicalDay
```

### Algorithm (`Compute`)

1. **Merge** `layers` in order (later wins, field-level) → the resolved celebration set.
2. **Computus** (`computus.go`): Gregorian Easter via the Anonymous Gregorian algorithm
   (Meeus/Jones/Butcher). Deterministic, ~15 lines.
3. **Temporal** (`temporal.go`): from Easter + the Advent anchor (4th Sunday before Dec 25),
   derive the season, week, weekday, and the movable solemnities/feasts, honoring `Selection`'s
   placement knobs. Produce the day's *temporal* candidate with its own rank.
4. **Sanctoral** (`sanctoral.go`): resolve each celebration's `DateSpec` for this year; collect
   any landing on `date`.
5. **Precedence** (`precedence.go`): map every candidate (temporal + sanctoral) to its position in
   the **Table of Liturgical Days** (Universal Norms, 1969) using (Rank, Class, scope,
   season-context); pick the highest → `Observed`; the rest → `Others` (commemorations / optional).
   Apply **transfer of impeded solemnities** (e.g. Annunciation / St Joseph in Holy Week → after
   the Easter octave; a solemnity on a privileged Sunday → following Monday).
6. **Colour/season** from the observed celebration (falling back to the season colour).

`Compute` returns a fully-populated `LiturgicalDay`. It never errors (any valid Gregorian date
yields a day; pre-1970 applies modern rules retroactively — documented).

## Package layout (the six layers → Go packages)

| Concern | Package | Notes |
|---|---|---|
| shared INI reader | `internal/ini` **(new)** | tiny zero-dep reader/writer; used by config + caldata (+ overrides in #2) |
| **logic** | `internal/calendar` **(new)** | types + `Compute`; **stdlib only** |
| **data** | `internal/caldata` **(new)** | `//go:embed roman-calendar.ini`; parse → `calendar.Layer`; imports `calendar` for types |
| **config** | `internal/config` (modify) | TOML→INI migration; `[calendar]` selection keys |
| corpora | `internal/bible` (reuse) | TSV; citation resolution for inline propers |
| CLI surface | `internal/cli` (add) | `lectio calendar [DATE]` demo/validation command |
| (scraper) | `internal/liturgy`, `internal/tradlit` | **unchanged, quarantined**; still the default readings source until the migration |

Dependency arrows point one way: `caldata → calendar → (stdlib)`; `config → ini`; `cli → calendar,
caldata, config, bible`. No cycles. The pure `calendar` package is the 25–50 year artifact.

## Base calendar data (source)

Bootstrap `roman-calendar.ini` from **romcal's MIT-licensed** General Roman Calendar data
(name/rank/class/colour per celebration), **verify against the official General Roman Calendar**,
and commit the result as **our owned data** (romcal credited in `NOTICE`). The regression oracle
(below) catches conversion errors. Proper-reading citations are added incrementally (major
solemnities in #1).

## CLI surface for #1

```
lectio calendar [DATE]     # print the computed liturgical day for DATE (default: today)
                           #   season, observed celebration (name/rank/colour), commemorations,
                           #   and inline proper readings if the celebration has them
  --lectionary new|old     # override config (old = EF, not yet implemented → clear error)
```

This is the demo + manual-validation surface. The daily-readings view keeps using the scraper
until a later migration, so nothing user-facing breaks.

## Validation & testing

Proving the algorithm is the point of #1 — we must trust it before it replaces the scraper.

- **Unit tests:** known Gregorian Easter dates (published table, ~1990–2050); season boundaries
  (Advent I, Ash Wednesday, Pentecost, Christ the King, Baptism); fixed solemnities; a set of
  hand-picked collision/transfer cases (Annunciation in Holy Week; a memorial on a Lenten weekday
  becoming optional; a solemnity on a Sunday of Ordinary Time).
- **Merge tests:** synthetic layers exercising add / field-override / move / re-rank / suppress.
- **Regression oracle:** compare `Compute` output structurally (season, observed slug↔identity,
  rank, colour — ignoring wording/localization) against an authoritative implementation
  (**calapi.inadiutorium.cz** / calendarium-romanum) across **2020–2040**. A one-time fetch script
  (outside the test run — no network in tests) snapshots the oracle to
  `internal/calendar/testdata/oracle-2020-2040.json`; the test diffs against it.
- **Secondary check:** compare against the user's existing cached scraped `DayInfo` where present.

Target: zero structural diffs against the oracle over the range (documented, explained exceptions
only — e.g. genuine national-vs-universal differences, which the oracle config must match).

## Error handling

- `Compute` is total — no error path for valid dates.
- **Data load** (`caldata`): malformed embedded INI is a programmer error → fail fast at
  init/startup with a precise message; covered by tests so it never ships.
- **Config**: invalid INI → clear message, fall back to documented defaults where safe
  (never silently wrong).
- **Citations** in propers that fail to resolve → surfaced like the existing `--ref` "(no
  reference)" path, never a crash.

## Open questions / risks

- **Transfer rules** are the fiddliest part (impeded solemnities, Advent/Lent interactions). The
  oracle is the safety net; expect iteration here. (Confidence 8/10 overall; ~6/10 that the first
  cut of transfers is complete.)
- **Proper-reading data coverage** in #1 is partial by design (major feasts); full coverage lands
  with the lectionary in #3. User-visible readings from the engine are therefore limited until #3.
- **calapi/romcal national settings** must be matched to the universal base for a clean diff
  (their defaults may assume a nation).

## Non-goals

Restating for clarity: no reading *cycle* (#3), no user layer *loading* (#2), no API (#4), no EF
engine yet, no `bt`, no UI migration in #1. #1 delivers a **trustworthy, offline, pure-Go engine
that computes the universal OF day**, with the formats and interfaces the rest of the epic needs.