# OF Liturgical Calendar Engine Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** A pure, offline, stdlib-only Go engine that computes the Ordinary Form liturgical day (season, celebrations, rank, colour, the observed celebration, inline proper readings) from a Gregorian date, replacing the niedziela.pl/missalemeum scrape for calendar computation.

**Architecture:** A dependency-free `internal/calendar` package (types + Computus + temporal cycle + sanctoral date resolution + layer merge + Universal-Norms precedence + `Compute`) fed by an `internal/caldata` package that embeds the universal General Roman Calendar as INI records, plus a shared `internal/ini` reader and a config migration to INI. Correctness is proven by a regression oracle diffing against calapi/romcal over 2020–2040. Front-ends stay on the scraper; a new `lectio calendar [DATE]` command is the engine's demo/validation surface.

**Tech Stack:** Go 1.24, standard library only for the engine (`time`, `strings`, `strconv`, `embed`, `bufio`, `sort`, `fmt`). Module path `github.com/lukaszkasprzak/lectio`.

## Global Constraints

- **Engine purity:** `internal/calendar` and `internal/ini` import **only the Go standard library** — no third-party, no other internal packages. `internal/caldata` may import `internal/calendar` (for types) + `internal/ini` only.
- **No network in the core, ever.** `Compute` is total: every valid Gregorian date returns a `LiturgicalDay`; it never returns an error and never performs I/O.
- **Formats:** config = INI; celebration data = INI `[slug]` sections; scripture corpora stay TSV. No TOML/YAML in new code (the existing go-toml dependency is retained **only** for one-shot config migration).
- **Longevity:** boring stdlib, plain-text data, focused files, frequent commits. Data is owned plain text.
- **Precedence authority:** the *Universal Norms on the Liturgical Year and the Calendar* (1969) "Table of Liturgical Days" governs which celebration is observed.
- **Scope of this plan (from the spec):** universal OF calendar only. OUT: temporal reading cycle (Sundays A/B/C, weekday I/II), user override-file loading + `use =` stacking, JSON/iCal API, EF engine, `bt`, UI migration. The engine's `Compute(date, sel, layers []Layer)` signature accepts an ordered layer stack so #2 slots in, but this plan ships only the embedded universal layer.
- **Pre-1970 dates** apply the modern rules retroactively (documented, matches romcal).

---

## File Structure

- `internal/ini/ini.go` — shared zero-dep INI reader/writer. Parses ordered sections (incl. `[a/b]` subsections), `key = value`, dotted keys, `#`/`;` comments; a comma-list helper. One responsibility: INI ↔ ordered data.
- `internal/calendar/types.go` — the vocabulary types (`Rank`, `Class`, `Colour`, `Season`, `Reading`, `Mass`, `DateSpec`, `Celebration`, `RawCelebration`, `Layer`, `Selection`, `LiturgicalDay`) + parse helpers for enums.
- `internal/calendar/computus.go` — `Easter(year) time.Time`.
- `internal/calendar/datespec.go` — `resolveDate(spec DateSpec, year int, easter time.Time) (time.Time, bool)`.
- `internal/calendar/temporal.go` — `temporal(date time.Time, sel Selection) temporalDay` (season, week, weekday, temporal celebration + its precedence inputs, colour).
- `internal/calendar/merge.go` — `mergeLayers(layers []Layer) map[string]RawCelebration`.
- `internal/calendar/precedence.go` — `precedence(kind precedenceInput) int` (Table of Liturgical Days) + `pick(cands []candidate) (observed candidate, others []candidate)` + `transfer` of impeded solemnities.
- `internal/calendar/calendar.go` — `Compute(date time.Time, sel Selection, layers []Layer) LiturgicalDay`, `buildCelebration`, cycle helpers `sundayCycle`/`weekdayCycle`.
- `internal/caldata/caldata.go` — `//go:embed roman-calendar.ini`, `Universal() calendar.Layer`.
- `internal/caldata/roman-calendar.ini` — the owned universal sanctoral data (bootstrapped from romcal, verified).
- `internal/config/config.go` — modified: read/write INI, one-shot TOML→INI migration, `[calendar]` keys, `Selection()` accessor.
- `internal/config/config.ini` — new embedded INI seed (replaces `config.toml`).
- `internal/calendar/oracle_test.go` + `internal/calendar/testdata/oracle-2020-2040.json` — regression oracle.
- `scripts/build-oracle.sh` — one-time (network) snapshot generator; not run in tests.
- `internal/cli/calendar.go` — the `lectio calendar [DATE]` command.

Each `calendar/*.go` file is small and single-purpose; the package as a whole is the 25–50 year artifact.

---

## Task 1: Shared INI reader (`internal/ini`)

**Files:**
- Create: `internal/ini/ini.go`
- Test: `internal/ini/ini_test.go`

**Interfaces:**
- Produces: `type Section struct { Name string; Pairs []Pair }`; `type Pair struct { Key, Val string }`; `func Parse(data []byte) ([]Section, error)`; `func List(val string) []string` (splits a comma value, trims, drops empties).

- [ ] **Step 1: Write the failing test**

```go
package ini

import "testing"

func TestParseSectionsAndPairs(t *testing.T) {
	in := []byte(`
# a comment
lectionary = new      ; trailing comment

[assumption]
date = 08-15
name.pl = Wniebowzięcie   ; unicode ok
rank= solemnity

[assumption/vigil]
reading.gospel = Łk 11,27-28
`)
	secs, err := Parse(in)
	if err != nil {
		t.Fatal(err)
	}
	// top-level pairs land in a section with empty name, first.
	if secs[0].Name != "" || len(secs[0].Pairs) != 1 || secs[0].Pairs[0] != (Pair{"lectionary", "new"}) {
		t.Fatalf("top-level = %+v", secs[0])
	}
	if secs[1].Name != "assumption" || secs[1].Pairs[1] != (Pair{"name.pl", "Wniebowzięcie"}) {
		t.Fatalf("assumption = %+v", secs[1])
	}
	if secs[1].Pairs[2] != (Pair{"rank", "solemnity"}) {
		t.Fatalf("no-space key = %+v", secs[1].Pairs[2])
	}
	if secs[2].Name != "assumption/vigil" {
		t.Fatalf("subsection = %q", secs[2].Name)
	}
}

func TestList(t *testing.T) {
	got := List("bt, wuj ,, vul")
	want := []string{"bt", "wuj", "vul"}
	if len(got) != len(want) {
		t.Fatalf("List = %v", got)
	}
	for i := range want {
		if got[i] != want[i] {
			t.Fatalf("List[%d] = %q want %q", i, got[i], want[i])
		}
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/ini/`
Expected: FAIL (`undefined: Parse` / `undefined: List`).

- [ ] **Step 3: Write minimal implementation**

```go
// Package ini is a tiny dependency-free reader for the boring INI files lectio
// uses for config and calendar data: [sections] (including "a/b" subsections),
// "key = value" pairs (keys may be dotted, e.g. name.pl), and "#"/";" comments.
// It preserves order and never interprets values beyond trimming.
package ini

import (
	"bufio"
	"bytes"
	"fmt"
	"strings"
)

type Pair struct{ Key, Val string }

type Section struct {
	Name  string
	Pairs []Pair
}

// Parse reads INI bytes into ordered sections. Pairs before the first [section]
// go into a leading section with an empty Name.
func Parse(data []byte) ([]Section, error) {
	secs := []Section{{Name: ""}}
	sc := bufio.NewScanner(bytes.NewReader(data))
	sc.Buffer(make([]byte, 0, 64*1024), 1024*1024)
	line := 0
	for sc.Scan() {
		line++
		raw := stripComment(sc.Text())
		s := strings.TrimSpace(raw)
		if s == "" {
			continue
		}
		if strings.HasPrefix(s, "[") {
			if !strings.HasSuffix(s, "]") {
				return nil, fmt.Errorf("ini: line %d: unclosed section header %q", line, s)
			}
			name := strings.TrimSpace(s[1 : len(s)-1])
			secs = append(secs, Section{Name: name})
			continue
		}
		eq := strings.IndexByte(s, '=')
		if eq < 0 {
			return nil, fmt.Errorf("ini: line %d: expected key = value, got %q", line, s)
		}
		key := strings.TrimSpace(s[:eq])
		val := strings.TrimSpace(s[eq+1:])
		cur := &secs[len(secs)-1]
		cur.Pairs = append(cur.Pairs, Pair{key, val})
	}
	return secs, sc.Err()
}

// stripComment removes a trailing "#" or ";" comment (not inside the value's
// meaningful text — our values never contain unescaped # or ;).
func stripComment(line string) string {
	for i := 0; i < len(line); i++ {
		if line[i] == '#' || line[i] == ';' {
			return line[:i]
		}
	}
	return line
}

// List splits a comma-separated value, trimming spaces and dropping empties.
func List(val string) []string {
	var out []string
	for _, p := range strings.Split(val, ",") {
		if t := strings.TrimSpace(p); t != "" {
			out = append(out, t)
		}
	}
	return out
}
```

- [ ] **Step 4: Run test to verify it passes**

Run: `go test ./internal/ini/`
Expected: PASS.

- [ ] **Step 5: Commit**

```bash
git add internal/ini/
git commit -m "feat(ini): dependency-free INI reader for config and calendar data"
```

---

## Task 2: Calendar types + Computus (`internal/calendar`)

**Files:**
- Create: `internal/calendar/types.go`, `internal/calendar/computus.go`
- Test: `internal/calendar/computus_test.go`, `internal/calendar/types_test.go`

**Interfaces:**
- Produces the whole type vocabulary (below) and `func Easter(year int) time.Time` (UTC midnight, Gregorian Easter Sunday). `func ParseRank(string) Rank`, `func ParseClass(string) Class`, `func ParseColour(string) Colour` (each returns a zero value for unknown input).

- [ ] **Step 1: Write the failing Easter test**

```go
package calendar

import (
	"testing"
	"time"
)

func TestEasterKnownDates(t *testing.T) {
	// Published Gregorian Easter Sundays.
	cases := map[int]string{
		2020: "2020-04-12", 2021: "2021-04-04", 2022: "2022-04-17",
		2024: "2024-03-31", 2025: "2025-04-20", 2027: "2027-03-28",
		2038: "2038-04-25", 2000: "2000-04-23",
	}
	for y, want := range cases {
		got := Easter(y).Format("2006-01-02")
		if got != want {
			t.Errorf("Easter(%d) = %s, want %s", y, got, want)
		}
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/calendar/ -run TestEaster`
Expected: FAIL (`undefined: Easter`).

- [ ] **Step 3: Write the types**

```go
// Package calendar computes the Ordinary Form liturgical day for any Gregorian
// date, offline and from first principles. It imports only the standard library.
package calendar

import "time"

type Rank int

const (
	RankFerial   Rank = iota // ordinary weekday
	RankOptional             // optional memorial
	RankMemorial             // obligatory memorial
	RankFeast
	RankSolemnity
)

type Class int

const (
	ClassNone  Class = iota // temporal / ferial
	ClassSaint              // a saint
	ClassBVM                // the Blessed Virgin Mary
	ClassLord               // the Lord
)

type Colour string

const (
	White  Colour = "white"
	Red    Colour = "red"
	Green  Colour = "green"
	Violet Colour = "violet"
	Rose   Colour = "rose"
	Black  Colour = "black"
)

type Season string

const (
	Advent    Season = "advent"
	Christmas Season = "christmas"
	Lent      Season = "lent"
	Triduum   Season = "triduum"
	Easter_   Season = "easter" // trailing underscore: Easter is the func name
	Ordinary  Season = "ordinary"
)

type Reading struct{ Part, Citation string }

type Mass struct {
	Variant  string // "" = the day Mass; e.g. "vigil"
	Readings []Reading
}

// DateSpec is a celebration's raw date expression, resolved by resolveDate:
// "MM-DD" (fixed) | "easter±N" | "christmas±N" | "advent-sunday-N" |
// "sunday-after MM-DD".
type DateSpec string

// RawCelebration is a celebration's fields exactly as parsed from a layer,
// before typing/merging. Fields are raw INI values keyed by INI key
// (e.g. "rank", "name.pl"); Variants holds "[slug/variant]" sub-sections.
type RawCelebration struct {
	Fields   map[string]string
	Variants map[string]map[string]string
}

// Celebration is the typed, built form used in results.
type Celebration struct {
	Slug   string
	Name   map[string]string // lang -> name
	Rank   Rank
	Class  Class
	Colour Colour
	Date   DateSpec
	Masses []Mass
	Layer  string // provenance
}

// Layer is one calendar layer (the embedded universal base, or later an
// override file). Cels is keyed by slug; insertion order is not significant.
type Layer struct {
	ID, Name, Type string
	Cels           map[string]RawCelebration
}

// Selection carries the config choices that steer computation.
type Selection struct {
	Form          string // "new" (OF). "old" reserved.
	Epiphany      string // "fixed" | "sunday"
	Ascension     string // "thursday" | "sunday"
	CorpusChristi string // "thursday" | "sunday"
}

// DefaultSelection is the Universal Roman Calendar default.
func DefaultSelection() Selection {
	return Selection{Form: "new", Epiphany: "fixed", Ascension: "thursday", CorpusChristi: "thursday"}
}

type LiturgicalDay struct {
	Date         time.Time
	Season       Season
	Week         int
	Weekday      time.Weekday
	Observed     Celebration
	Others       []Celebration
	Colour       Colour
	SundayCycle  string // "A" | "B" | "C"
	WeekdayCycle string // "I" | "II"
}

func ParseRank(s string) Rank {
	switch s {
	case "solemnity":
		return RankSolemnity
	case "feast":
		return RankFeast
	case "memorial":
		return RankMemorial
	case "optional":
		return RankOptional
	default:
		return RankFerial
	}
}

func ParseClass(s string) Class {
	switch s {
	case "lord":
		return ClassLord
	case "bvm":
		return ClassBVM
	case "saint":
		return ClassSaint
	default:
		return ClassNone
	}
}

func ParseColour(s string) Colour {
	switch Colour(s) {
	case White, Red, Green, Violet, Rose, Black:
		return Colour(s)
	default:
		return ""
	}
}
```

- [ ] **Step 4: Write Computus**

```go
package calendar

import "time"

// Easter returns Gregorian Easter Sunday (UTC midnight) via the Anonymous
// Gregorian algorithm (Meeus/Jones/Butcher).
func Easter(year int) time.Time {
	a := year % 19
	b := year / 100
	c := year % 100
	d := b / 4
	e := b % 4
	f := (b + 8) / 25
	g := (b - f + 1) / 3
	h := (19*a + b - d - g + 15) % 30
	i := c / 4
	k := c % 4
	l := (32 + 2*e + 2*i - h - k) % 7
	m := (a + 11*h + 22*l) / 451
	month := (h + l - 7*m + 114) / 31
	day := ((h + l - 7*m + 114) % 31) + 1
	return time.Date(year, time.Month(month), day, 0, 0, 0, 0, time.UTC)
}
```

- [ ] **Step 5: Add the enum test**

```go
package calendar

import "testing"

func TestParseEnums(t *testing.T) {
	if ParseRank("feast") != RankFeast || ParseRank("nonsense") != RankFerial {
		t.Error("ParseRank")
	}
	if ParseClass("bvm") != ClassBVM || ParseClass("") != ClassNone {
		t.Error("ParseClass")
	}
	if ParseColour("violet") != Violet || ParseColour("chartreuse") != "" {
		t.Error("ParseColour")
	}
}
```

- [ ] **Step 6: Run tests to verify they pass**

Run: `go test ./internal/calendar/`
Expected: PASS.

- [ ] **Step 7: Commit**

```bash
git add internal/calendar/types.go internal/calendar/computus.go internal/calendar/computus_test.go internal/calendar/types_test.go
git commit -m "feat(calendar): core types + Gregorian Computus"
```

---

## Task 3: DateSpec resolution (`internal/calendar/datespec.go`)

**Files:**
- Create: `internal/calendar/datespec.go`
- Test: `internal/calendar/datespec_test.go`

**Interfaces:**
- Consumes: `Easter(year)` (Task 2), `DateSpec` (Task 2).
- Produces: `func resolveDate(spec DateSpec, year int, easter time.Time) (time.Time, bool)`. Returns the date (UTC midnight) the spec names in `year`, or `ok=false` if unparseable.

- [ ] **Step 1: Write the failing test**

```go
package calendar

import (
	"testing"
	"time"
)

func TestResolveDate(t *testing.T) {
	y := 2025
	e := Easter(y) // 2025-04-20
	check := func(spec, want string) {
		got, ok := resolveDate(DateSpec(spec), y, e)
		if !ok || got.Format("2006-01-02") != want {
			t.Errorf("resolveDate(%q) = %s ok=%v, want %s", spec, got.Format("2006-01-02"), ok, want)
		}
	}
	check("08-15", "2025-08-15")             // fixed
	check("easter+60", "2025-06-19")         // Corpus Christi (Thursday)
	check("easter-46", "2025-03-05")         // Ash Wednesday
	check("christmas+7", "2026-01-01")       // year rolls over
	check("sunday-after 01-06", "2025-01-12")
	if _, ok := resolveDate("garbage", y, e); ok {
		t.Error("garbage should not resolve")
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/calendar/ -run TestResolveDate`
Expected: FAIL (`undefined: resolveDate`).

- [ ] **Step 3: Write the implementation**

```go
package calendar

import (
	"strconv"
	"strings"
	"time"
)

func resolveDate(spec DateSpec, year int, easter time.Time) (time.Time, bool) {
	s := strings.TrimSpace(string(spec))
	switch {
	case len(s) == 5 && s[2] == '-': // MM-DD
		mo, err1 := strconv.Atoi(s[0:2])
		da, err2 := strconv.Atoi(s[3:5])
		if err1 != nil || err2 != nil {
			return time.Time{}, false
		}
		return time.Date(year, time.Month(mo), da, 0, 0, 0, 0, time.UTC), true
	case strings.HasPrefix(s, "easter"):
		return offset(easter, s[len("easter"):])
	case strings.HasPrefix(s, "christmas"):
		xmas := time.Date(year, 12, 25, 0, 0, 0, 0, time.UTC)
		return offset(xmas, s[len("christmas"):])
	case strings.HasPrefix(s, "sunday-after "):
		base, ok := resolveDate(DateSpec(strings.TrimPrefix(s, "sunday-after ")), year, easter)
		if !ok {
			return time.Time{}, false
		}
		return nextWeekday(base.AddDate(0, 0, 1), time.Sunday), true
	}
	return time.Time{}, false
}

// offset parses "±N" and adds N days to base.
func offset(base time.Time, pm string) (time.Time, bool) {
	if pm == "" {
		return base, true
	}
	n, err := strconv.Atoi(pm) // strconv.Atoi handles a leading '+' and '-'
	if err != nil {
		return time.Time{}, false
	}
	return base.AddDate(0, 0, n), true
}

// nextWeekday returns the first day >= from whose weekday is wd.
func nextWeekday(from time.Time, wd time.Weekday) time.Time {
	delta := (int(wd) - int(from.Weekday()) + 7) % 7
	return from.AddDate(0, 0, delta)
}
```

- [ ] **Step 4: Run test to verify it passes**

Run: `go test ./internal/calendar/ -run TestResolveDate`
Expected: PASS.

- [ ] **Step 5: Commit**

```bash
git add internal/calendar/datespec.go internal/calendar/datespec_test.go
git commit -m "feat(calendar): DateSpec resolution (fixed + easter/christmas offsets + sunday-after)"
```

---

## Task 4: Temporal cycle (`internal/calendar/temporal.go`)

**Files:**
- Create: `internal/calendar/temporal.go`
- Test: `internal/calendar/temporal_test.go`

**Interfaces:**
- Consumes: `Easter`, `Selection`, `Season`, `Colour`, `Rank`, `Class`, `nextWeekday` (Task 3).
- Produces: `type temporalDay struct { Season Season; Week int; Colour Colour; Cel Celebration; Rank Rank; Class Class; Privileged bool }` and `func temporal(date time.Time, sel Selection) temporalDay`. `Cel` is the temporal celebration (a Sunday, a ferial weekday, or a movable solemnity/feast) with a stable `Slug`; `Privileged` marks days no sanctoral may override (Sundays of Advent/Lent/Easter, Holy Week, Easter octave, Triduum). Also `func adventStart(year int) time.Time` (1st Sunday of Advent for the liturgical year *ending* Dec of `year`).

- [ ] **Step 1: Write the failing test**

```go
package calendar

import (
	"testing"
	"time"
)

func d(s string) time.Time {
	t, _ := time.Parse("2006-01-02", s)
	return t.UTC()
}

func TestTemporalSeasons(t *testing.T) {
	sel := DefaultSelection()
	cases := []struct {
		date   string
		season Season
	}{
		{"2025-12-01", Advent},    // Mon after Advent I (2025 Advent I = Nov 30)
		{"2025-12-25", Christmas}, // Christmas
		{"2025-03-05", Lent},      // Ash Wednesday 2025
		{"2025-04-20", Easter_},   // Easter Sunday 2025
		{"2025-07-15", Ordinary},  // deep Ordinary Time
	}
	for _, c := range cases {
		got := temporal(d(c.date), sel)
		if got.Season != c.season {
			t.Errorf("temporal(%s).Season = %s, want %s", c.date, got.Season, c.season)
		}
	}
}

func TestTemporalPrivilegedSunday(t *testing.T) {
	// 2nd Sunday of Advent 2025 (Dec 7) is privileged and violet.
	td := temporal(d("2025-12-07"), DefaultSelection())
	if !td.Privileged {
		t.Error("Sunday of Advent must be privileged")
	}
	if td.Colour != Violet {
		t.Errorf("Advent colour = %s want violet", td.Colour)
	}
}

func TestMovableSolemnities(t *testing.T) {
	// Corpus Christi 2025 (Thursday, easter+60) = 2025-06-19.
	td := temporal(d("2025-06-19"), DefaultSelection())
	if td.Cel.Slug != "corpus-christi" {
		t.Errorf("2025-06-19 temporal slug = %q want corpus-christi", td.Cel.Slug)
	}
	if td.Rank != RankSolemnity {
		t.Error("Corpus Christi must be a solemnity")
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/calendar/ -run 'TestTemporal|TestMovable'`
Expected: FAIL (`undefined: temporal`).

- [ ] **Step 3: Write the implementation**

The temporal cycle is derived entirely from Easter and the Advent anchor. This
implementation covers the seasons, Sundays/weekdays, and the movable
solemnities/feasts of the General Roman Calendar. Weeks are numbered per the
liturgical convention (Ordinary Time weeks bridge across Lent/Easter).

```go
package calendar

import "time"

type temporalDay struct {
	Season     Season
	Week       int
	Colour     Colour
	Cel        Celebration
	Rank       Rank
	Class      Class
	Privileged bool
}

// adventStart returns the First Sunday of Advent that opens the liturgical year
// containing Christmas of the given calendar year: the 4th Sunday before Dec 25.
func adventStart(year int) time.Time {
	xmas := time.Date(year, 12, 25, 0, 0, 0, 0, time.UTC)
	// Sunday on/before Christmas, then back 3 more Sundays.
	sun := xmas.AddDate(0, 0, -int(xmas.Weekday())) // Sunday <= Christmas (Sunday=0)
	return sun.AddDate(0, 0, -21)
}

func sameDay(a, b time.Time) bool {
	return a.Year() == b.Year() && a.YearDay() == b.YearDay()
}

func weeksBetween(from, to time.Time) int {
	return int(to.Sub(from).Hours()/24) / 7
}

// temporal computes the temporal (season/movable) identity of date.
func temporal(date time.Time, sel Selection) temporalDay {
	date = date.UTC().Truncate(24 * time.Hour)
	y := date.Year()
	easter := Easter(y)
	ashWed := easter.AddDate(0, 0, -46)
	pentecost := easter.AddDate(0, 0, 49)
	triduumStart := easter.AddDate(0, 0, -3) // Holy Thursday evening; treat Thu-Sat as Triduum
	palmSunday := easter.AddDate(0, 0, -7)
	adventThis := adventStart(y)
	adventPrev := adventStart(y - 1)
	christmasThis := time.Date(y, 12, 25, 0, 0, 0, 0, time.UTC)
	baptism := baptismOfLord(y, sel)

	// --- movable solemnities/feasts keyed to Easter (highest first) ---
	switch {
	case sameDay(date, easter):
		return sol(date, Easter_, "easter-sunday", White, true)
	case date.After(easter) && date.Before(easter.AddDate(0, 0, 7)):
		return octave(date, Easter_, "easter-octave", White, weeksBetween(easter, date))
	case sameDay(date, ascension(y, sel)):
		return sol(date, Easter_, "ascension", White, true)
	case sameDay(date, pentecost):
		return sol(date, Easter_, "pentecost", Red, true)
	case sameDay(date, easter.AddDate(0, 0, 56)): // Trinity, easter+56
		return sol(date, Ordinary, "trinity", White, false)
	case sameDay(date, corpusChristi(y, sel)):
		return sol(date, Ordinary, "corpus-christi", White, false)
	case sameDay(date, easter.AddDate(0, 0, 68)): // Sacred Heart, easter+68
		return sol(date, Ordinary, "sacred-heart", White, false)
	}

	// --- seasons by span ---
	switch {
	case !date.Before(triduumStart) && date.Before(easter):
		return sol(date, Triduum, "triduum", Red, true)
	case !date.Before(palmSunday) && date.Before(triduumStart): // Holy Week Mon–Wed + Palm Sunday
		return privWeek(date, Lent, "holy-week", Violet)
	case !date.Before(ashWed) && date.Before(palmSunday):
		return lent(date, ashWed)
	case (!date.Before(easter) && date.Before(pentecost.AddDate(0, 0, 1))):
		return eastertide(date, easter)
	case !date.Before(adventThis): // Advent of the new liturgical year (late this year)
		return advent(date, adventThis, sel)
	case date.Before(baptism.AddDate(0, 0, 1)): // Christmas season into early Jan (opened last year)
		return christmasSeason(date, adventPrev, christmasPrev(y), baptism, sel)
	case !date.Before(christmasThis): // Christmas Day..Dec 31 of this year
		return christmasSeason(date, adventThis, christmasThis, baptismOfLord(y+1, sel), sel)
	default:
		return ordinary(date, baptism, adventThis)
	}
}
```

Supporting helpers (same file):

```go
// small constructors keep temporal() readable.
func sol(date time.Time, s Season, slug string, col Colour, privileged bool) temporalDay {
	return temporalDay{Season: s, Colour: col, Rank: RankSolemnity, Class: ClassLord, Privileged: privileged,
		Cel: Celebration{Slug: slug, Rank: RankSolemnity, Class: ClassLord, Colour: col, Layer: "temporal"}}
}
func privWeek(date time.Time, s Season, slug string, col Colour) temporalDay {
	r := RankFerial
	priv := true
	if date.Weekday() == time.Sunday {
		r = RankSolemnity // Palm Sunday ranks with Sundays of Lent
	}
	return temporalDay{Season: s, Colour: col, Rank: r, Privileged: priv,
		Cel: Celebration{Slug: slug, Rank: r, Colour: col, Layer: "temporal"}}
}
func octave(date time.Time, s Season, slug string, col Colour, day int) temporalDay {
	return temporalDay{Season: s, Colour: col, Rank: RankSolemnity, Privileged: true, Week: day,
		Cel: Celebration{Slug: slug, Rank: RankSolemnity, Colour: col, Layer: "temporal"}}
}

// ascension: Thursday (easter+39) or, where transferred, the 7th Sunday of Easter (easter+42).
func ascension(y int, sel Selection) time.Time {
	e := Easter(y)
	if sel.Ascension == "sunday" {
		return e.AddDate(0, 0, 42)
	}
	return e.AddDate(0, 0, 39)
}
func corpusChristi(y int, sel Selection) time.Time {
	e := Easter(y)
	if sel.CorpusChristi == "sunday" {
		return e.AddDate(0, 0, 63)
	}
	return e.AddDate(0, 0, 60)
}
```

> **Implementer note:** `lent`, `eastertide`, `advent`, `christmasSeason`, `ordinary`,
> `baptismOfLord`, `christmasPrev`, `Weekday`, `(temporalDay).norm` are the remaining
> season builders. Each sets `Season`, `Week` (count of Sundays since the season start,
> or the Ordinary-Time week bridged across Lent/Easter), `Colour` (Advent/Lent violet,
> Gaudete/Laetare 3rd-Advent/4th-Lent Sunday rose, Christmas/Easter white, else green),
> a ferial or Sunday `Cel` with slug `"<season>-<week>-<weekday>"` / `"<season>-sunday-<week>"`,
> `Rank` (`RankSolemnity` for privileged Sundays, else `RankFerial`), and `Privileged`
> (true for Sundays of Advent/Lent/Easter). Baptism of the Lord = the Sunday after Epiphany
> (`sel.Epiphany=="fixed"` → Epiphany Jan 6, Baptism the following Sunday; `"sunday"` →
> Epiphany the Sunday between Jan 2–8, Baptism the next Sunday, or Monday Jan 9 when Epiphany
> falls on Jan 7/8). Write each builder with its own failing test first (Step 3 is TDD-internal:
> add a test per builder, run red, implement, run green) before wiring them into `temporal`.
> The season boundaries and week numbers are validated wholesale by the regression oracle
> (Task 10); use it to drive the fiddly cases rather than guessing.

- [ ] **Step 4: Run tests to verify they pass**

Run: `go test ./internal/calendar/ -run 'TestTemporal|TestMovable'`
Expected: PASS.

- [ ] **Step 5: Commit**

```bash
git add internal/calendar/temporal.go internal/calendar/temporal_test.go
git commit -m "feat(calendar): temporal cycle (seasons, Sundays, movable solemnities)"
```

---

## Task 5: Layer merge (`internal/calendar/merge.go`)

**Files:**
- Create: `internal/calendar/merge.go`
- Test: `internal/calendar/merge_test.go`

**Interfaces:**
- Consumes: `Layer`, `RawCelebration` (Task 2).
- Produces: `func mergeLayers(layers []Layer) map[string]RawCelebration`. Later layers override earlier field-by-field; `Fields["suppress"]=="true"` removes the slug; variant sub-maps merge per-variant.

- [ ] **Step 1: Write the failing test**

```go
package calendar

import "testing"

func raw(fields map[string]string) RawCelebration {
	return RawCelebration{Fields: fields, Variants: map[string]map[string]string{}}
}

func TestMergeLayers(t *testing.T) {
	base := Layer{ID: "universal", Cels: map[string]RawCelebration{
		"assumption": raw(map[string]string{"rank": "solemnity", "colour": "white", "name.en": "Assumption"}),
		"st-x":       raw(map[string]string{"rank": "optional", "date": "01-02"}),
		"drop-me":    raw(map[string]string{"rank": "memorial", "date": "03-03"}),
	}}
	local := Layer{ID: "krakow", Cels: map[string]RawCelebration{
		"st-x":       raw(map[string]string{"rank": "feast"}),           // elevate; keep date
		"drop-me":    raw(map[string]string{"suppress": "true"}),         // remove
		"st-new":     raw(map[string]string{"rank": "solemnity", "date": "05-08"}), // add
	}}
	got := mergeLayers([]Layer{base, local})

	if got["st-x"].Fields["rank"] != "feast" || got["st-x"].Fields["date"] != "01-02" {
		t.Errorf("st-x merge = %+v", got["st-x"].Fields)
	}
	if _, ok := got["drop-me"]; ok {
		t.Error("drop-me should be suppressed")
	}
	if got["st-new"].Fields["date"] != "05-08" {
		t.Error("st-new should be added")
	}
	if got["assumption"].Fields["name.en"] != "Assumption" {
		t.Error("untouched base entry lost")
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/calendar/ -run TestMergeLayers`
Expected: FAIL (`undefined: mergeLayers`).

- [ ] **Step 3: Write the implementation**

```go
package calendar

// mergeLayers folds an ordered layer stack into one raw celebration set. Later
// layers override earlier ones field-by-field; a slug with Fields["suppress"]
// == "true" is removed; variant sub-sections merge per variant.
func mergeLayers(layers []Layer) map[string]RawCelebration {
	out := map[string]RawCelebration{}
	for _, layer := range layers {
		for slug, rc := range layer.Cels {
			if rc.Fields["suppress"] == "true" {
				delete(out, slug)
				continue
			}
			cur, ok := out[slug]
			if !ok {
				cur = RawCelebration{Fields: map[string]string{}, Variants: map[string]map[string]string{}}
			} else {
				cur = cloneRaw(cur)
			}
			for k, v := range rc.Fields {
				cur.Fields[k] = v
			}
			for variant, fields := range rc.Variants {
				if cur.Variants[variant] == nil {
					cur.Variants[variant] = map[string]string{}
				}
				for k, v := range fields {
					cur.Variants[variant][k] = v
				}
			}
			cur.Fields["layer"] = layer.ID // provenance: last writer
			out[slug] = cur
		}
	}
	return out
}

func cloneRaw(rc RawCelebration) RawCelebration {
	nf := make(map[string]string, len(rc.Fields))
	for k, v := range rc.Fields {
		nf[k] = v
	}
	nv := make(map[string]map[string]string, len(rc.Variants))
	for variant, fields := range rc.Variants {
		m := make(map[string]string, len(fields))
		for k, v := range fields {
			m[k] = v
		}
		nv[variant] = m
	}
	return RawCelebration{Fields: nf, Variants: nv}
}
```

- [ ] **Step 4: Run test to verify it passes**

Run: `go test ./internal/calendar/ -run TestMergeLayers`
Expected: PASS.

- [ ] **Step 5: Commit**

```bash
git add internal/calendar/merge.go internal/calendar/merge_test.go
git commit -m "feat(calendar): ordered-stack layer merge (override/suppress/add)"
```

---

## Task 6: Precedence + transfer (`internal/calendar/precedence.go`)

**Files:**
- Create: `internal/calendar/precedence.go`
- Test: `internal/calendar/precedence_test.go`

**Interfaces:**
- Consumes: `Rank`, `Class`, `Season` (Task 2), `temporalDay` (Task 4).
- Produces: `type candidate struct { Cel Celebration; Temporal bool; Privileged bool; Season Season }`, `func precedence(c candidate) int` (Table of Liturgical Days — **lower = higher precedence**), and `func pick(cands []candidate) (observed candidate, others []candidate)`. Transfer of impeded solemnities is handled in `Compute` (Task 7) using `precedence`.

The Table of Liturgical Days (Universal Norms, 1969), encoded as bands:

| Band | Days |
|---|---|
| 1 | Triduum |
| 2 | Christmas, Epiphany, Ascension, Pentecost; Sundays of Advent/Lent/Easter; Ash Wednesday; Holy Week; Easter octave |
| 3 | Solemnities (Lord, BVM, saints) in the General Calendar; All Souls |
| 4 | Proper solemnities |
| 5 | Feasts of the Lord in the General Calendar |
| 6 | Sundays of Christmas and Ordinary Time |
| 7 | Feasts of the BVM/saints in the General Calendar |
| 8 | Proper feasts |
| 9 | Privileged ferias: Advent Dec 17–24, Christmas octave, Lenten ferias |
| 10 | Obligatory memorials, General Calendar |
| 11 | Proper obligatory memorials |
| 12 | Optional memorials |
| 13 | Ordinary ferias |

- [ ] **Step 1: Write the failing test**

```go
package calendar

import "testing"

func tempCand(rank Rank, class Class, priv bool, s Season) candidate {
	return candidate{Cel: Celebration{Rank: rank, Class: class}, Temporal: true, Privileged: priv, Season: s}
}
func saintCand(rank Rank, class Class) candidate {
	return candidate{Cel: Celebration{Rank: rank, Class: class}, Temporal: false, Season: Ordinary}
}

func TestPrecedenceBands(t *testing.T) {
	sundayLent := tempCand(RankSolemnity, ClassNone, true, Lent)
	genSolemnity := saintCand(RankSolemnity, ClassSaint)
	genFeast := saintCand(RankFeast, ClassSaint)
	genMemorial := saintCand(RankMemorial, ClassSaint)
	// A Sunday of Lent is band 2; a general solemnity is band 3 → the Sunday wins.
	if !(precedence(sundayLent) < precedence(genSolemnity)) {
		t.Error("Sunday of Lent (band 2) must outrank a general solemnity (band 3)")
	}
	if !(precedence(genSolemnity) < precedence(genFeast) && precedence(genFeast) < precedence(genMemorial)) {
		t.Error("solemnity > feast > memorial ordering broken")
	}
}

func TestPick(t *testing.T) {
	sunday := tempCand(RankSolemnity, ClassNone, true, Ordinary) // band 6
	memorial := saintCand(RankMemorial, ClassSaint)              // band 10
	obs, others := pick([]candidate{memorial, sunday})
	if obs.Cel.Rank != RankSolemnity || !obs.Temporal {
		t.Errorf("Sunday must win, got %+v", obs.Cel)
	}
	if len(others) != 1 || others[0].Cel.Rank != RankMemorial {
		t.Errorf("memorial should be a commemoration, got %+v", others)
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/calendar/ -run 'TestPrecedence|TestPick'`
Expected: FAIL (`undefined: candidate`).

- [ ] **Step 3: Write the implementation**

```go
package calendar

import "sort"

type candidate struct {
	Cel        Celebration
	Temporal   bool
	Privileged bool
	Season     Season
}

// precedence maps a candidate to its Table-of-Liturgical-Days band
// (lower = higher precedence). "Proper" (non-General-Calendar) celebrations use
// the layer provenance: a non-empty, non-"universal"/"temporal" layer is proper.
func precedence(c candidate) int {
	proper := c.Cel.Layer != "" && c.Cel.Layer != "universal" && c.Cel.Layer != "temporal"
	if c.Temporal {
		switch {
		case c.Season == Triduum:
			return 1
		case c.Privileged: // privileged Sundays, Holy Week, Easter octave, top solemnities of the Lord
			return 2
		case c.Cel.Rank == RankSolemnity: // Trinity/Corpus Christi/Sacred Heart etc.
			return 3
		case c.Cel.Rank == RankFeast:
			return 5
		case isSunday(c): // ordinary/Christmas Sundays
			return 6
		case c.privilegedFeria():
			return 9
		default:
			return 13
		}
	}
	switch c.Cel.Rank {
	case RankSolemnity:
		if proper {
			return 4
		}
		return 3
	case RankFeast:
		if proper {
			return 8
		}
		return 7
	case RankMemorial:
		if proper {
			return 11
		}
		return 10
	case RankOptional:
		return 12
	default:
		return 13
	}
}

func isSunday(c candidate) bool { return c.Cel.Slug != "" && c.Privileged == false && c.Cel.Rank == RankSolemnity }

// privilegedFeria is set by the temporal builder via Rank/slug; encoded on the
// candidate by Compute (Advent 17–24, Christmas octave, Lenten ferias).
func (c candidate) privilegedFeria() bool { return c.Cel.Slug == "privileged-feria" }

// pick returns the highest-precedence candidate as observed and the rest as
// others (commemorations / optional memorials), stably ordered by precedence.
func pick(cands []candidate) (candidate, []candidate) {
	sorted := make([]candidate, len(cands))
	copy(sorted, cands)
	sort.SliceStable(sorted, func(i, j int) bool { return precedence(sorted[i]) < precedence(sorted[j]) })
	return sorted[0], sorted[1:]
}
```

> **Implementer note:** the `isSunday`/`privilegedFeria` predicates above are a first
> cut wired to slug/flag conventions the temporal builder sets; refine them so bands 6 and 9
> match the oracle. Encoding proper-vs-general via layer provenance is exact for #1 (only the
> universal layer ships). **Transfer of impeded solemnities** (band-3/4 solemnity colliding
> with band-1/2) is applied in `Compute` (Task 7): when a solemnity is impeded, move it to the
> nearest following day not itself impeded. Drive band 6/9 and transfer edge cases with the
> oracle (Task 10).

- [ ] **Step 4: Run test to verify it passes**

Run: `go test ./internal/calendar/ -run 'TestPrecedence|TestPick'`
Expected: PASS.

- [ ] **Step 5: Commit**

```bash
git add internal/calendar/precedence.go internal/calendar/precedence_test.go
git commit -m "feat(calendar): Universal Norms precedence bands + pick"
```

---

## Task 7: Compute — build + wire + cycles (`internal/calendar/calendar.go`)

**Files:**
- Create: `internal/calendar/calendar.go`
- Test: `internal/calendar/calendar_test.go`

**Interfaces:**
- Consumes: everything from Tasks 2–6.
- Produces: `func Compute(date time.Time, sel Selection, layers []Layer) LiturgicalDay`; `func buildCelebration(slug string, rc RawCelebration) Celebration`; `func sundayCycle(liturgicalYearStart time.Time) string`; `func weekdayCycle(liturgicalYearStartYear int) string`.

- [ ] **Step 1: Write the failing test**

```go
package calendar

import (
	"testing"
)

func universalTest() Layer {
	return Layer{ID: "universal", Type: "universal", Cels: map[string]RawCelebration{
		"assumption": {Fields: map[string]string{
			"date": "08-15", "rank": "solemnity", "class": "bvm", "colour": "white",
			"name.en": "Assumption of the BVM", "reading.gospel": "Lk 1:39-56",
		}, Variants: map[string]map[string]string{}},
		" st-monday-optional ": {Fields: map[string]string{}, Variants: map[string]map[string]string{}},
	}}
}

func TestComputeSolemnityBeatsFeria(t *testing.T) {
	got := Compute(d("2025-08-15"), DefaultSelection(), []Layer{universalTest()})
	if got.Observed.Slug != "assumption" {
		t.Fatalf("2025-08-15 observed = %q want assumption", got.Observed.Slug)
	}
	if got.Colour != White || got.Observed.Rank != RankSolemnity {
		t.Errorf("assumption colour/rank wrong: %s / %v", got.Colour, got.Observed.Rank)
	}
	if len(got.Observed.Masses) == 0 || got.Observed.Masses[0].Readings[0].Part != "gospel" {
		t.Errorf("proper reading not surfaced: %+v", got.Observed.Masses)
	}
}

func TestComputeCycles(t *testing.T) {
	// Advent 2024 opens liturgical year 2025 → Sunday cycle C, weekday cycle I.
	got := Compute(d("2025-01-15"), DefaultSelection(), []Layer{universalTest()})
	if got.SundayCycle != "C" {
		t.Errorf("2024-25 Sunday cycle = %s want C", got.SundayCycle)
	}
	if got.WeekdayCycle != "I" {
		t.Errorf("2025 weekday cycle = %s want I", got.WeekdayCycle)
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/calendar/ -run TestCompute`
Expected: FAIL (`undefined: Compute`).

- [ ] **Step 3: Write the implementation**

```go
package calendar

import (
	"sort"
	"strings"
	"time"
)

func Compute(date time.Time, sel Selection, layers []Layer) LiturgicalDay {
	date = date.UTC().Truncate(24 * time.Hour)
	td := temporal(date, sel)
	merged := mergeLayers(layers)

	// candidates: the temporal day + every sanctoral celebration landing today
	// (after transfer of impeded solemnities).
	cands := []candidate{{Cel: td.Cel, Temporal: true, Privileged: td.Privileged, Season: td.Season}}
	for slug, rc := range merged {
		cel := buildCelebration(slug, rc)
		when, ok := celebrationDate(cel, date.Year(), sel)
		if !ok {
			continue
		}
		if td.Privileged && cel.Rank < RankSolemnity {
			continue // nothing below a solemnity survives a privileged day
		}
		effective := transferIfImpeded(cel, when, sel)
		if sameDay(effective, date) {
			cands = append(cands, candidate{Cel: cel, Temporal: false, Season: td.Season})
		}
	}

	obs, others := pick(cands)
	day := LiturgicalDay{
		Date: date, Season: td.Season, Week: td.Week, Weekday: date.Weekday(),
		Observed: obs.Cel, Colour: colourOf(obs, td),
		SundayCycle:  sundayCycle(liturgicalYearStart(date)),
		WeekdayCycle: weekdayCycle(liturgicalYearStart(date).Year() + 1),
	}
	for _, o := range others {
		day.Others = append(day.Others, o.Cel)
	}
	return day
}

func colourOf(obs candidate, td temporalDay) Colour {
	if obs.Cel.Colour != "" {
		return obs.Cel.Colour
	}
	return td.Colour
}

// celebrationDate resolves a sanctoral celebration's date for the year.
func celebrationDate(cel Celebration, year int, sel Selection) (time.Time, bool) {
	return resolveDate(cel.Date, year, Easter(year))
}

// transferIfImpeded moves a solemnity off a higher-precedence day to the next
// free day. Non-solemnities are never transferred (they yield instead).
func transferIfImpeded(cel Celebration, when time.Time, sel Selection) time.Time {
	if cel.Rank != RankSolemnity {
		return when
	}
	day := when
	for i := 0; i < 8; i++ {
		blocker := temporal(day, sel)
		blockerBand := precedence(candidate{Cel: blocker.Cel, Temporal: true, Privileged: blocker.Privileged, Season: blocker.Season})
		if blockerBand <= 2 { // impeded by Triduum/privileged day: push forward
			day = day.AddDate(0, 0, 1)
			continue
		}
		return day
	}
	return day
}

func buildCelebration(slug string, rc RawCelebration) Celebration {
	c := Celebration{Slug: strings.TrimSpace(slug), Name: map[string]string{}, Layer: rc.Fields["layer"]}
	for k, v := range rc.Fields {
		switch {
		case k == "rank":
			c.Rank = ParseRank(v)
		case k == "class":
			c.Class = ParseClass(v)
		case k == "colour":
			c.Colour = ParseColour(v)
		case k == "date":
			c.Date = DateSpec(v)
		case strings.HasPrefix(k, "name."):
			c.Name[strings.TrimPrefix(k, "name.")] = v
		}
	}
	if m := massFrom(rc.Fields); len(m.Readings) > 0 {
		c.Masses = append(c.Masses, m)
	}
	// deterministic variant order
	var variants []string
	for v := range rc.Variants {
		variants = append(variants, v)
	}
	sort.Strings(variants)
	for _, v := range variants {
		mm := massFrom(rc.Variants[v])
		mm.Variant = v
		c.Masses = append(c.Masses, mm)
	}
	return c
}

var readingParts = []string{"first", "psalm", "second", "acclamation", "gospel"}

func massFrom(fields map[string]string) Mass {
	var m Mass
	for _, part := range readingParts {
		if cit := fields["reading."+part]; cit != "" {
			m.Readings = append(m.Readings, Reading{Part: part, Citation: cit})
		}
	}
	return m
}

// liturgicalYearStart returns the First Sunday of Advent that opened the
// liturgical year containing date.
func liturgicalYearStart(date time.Time) time.Time {
	a := adventStart(date.Year())
	if date.Before(a) {
		return adventStart(date.Year() - 1)
	}
	return a
}

// sundayCycle: Advent-year %3 → C(0),A(1),B(2).
func sundayCycle(start time.Time) string {
	switch (start.Year() + 1) % 3 {
	case 0:
		return "C"
	case 1:
		return "A"
	default:
		return "B"
	}
}

// weekdayCycle: odd civil year of the (Jan–) part → I, even → II.
func weekdayCycle(year int) string {
	if year%2 == 1 {
		return "I"
	}
	return "II"
}
```

- [ ] **Step 4: Run test to verify it passes**

Run: `go test ./internal/calendar/`
Expected: PASS.

- [ ] **Step 5: Commit**

```bash
git add internal/calendar/calendar.go internal/calendar/calendar_test.go
git commit -m "feat(calendar): Compute — build celebrations, apply precedence + transfer, cycles"
```

---

## Task 8: Base data package (`internal/caldata`)

**Files:**
- Create: `internal/caldata/caldata.go`, `internal/caldata/roman-calendar.ini`
- Test: `internal/caldata/caldata_test.go`

**Interfaces:**
- Consumes: `ini.Parse` (Task 1), `calendar.Layer`/`RawCelebration` (Task 2).
- Produces: `func Universal() calendar.Layer` (parses the embedded INI into a Layer; panics only on a malformed *embedded* file, which tests catch).

- [ ] **Step 1: Seed a real (initial) data file**

Create `internal/caldata/roman-calendar.ini` with the `[layer]` header and an
initial, real set of General Roman Calendar sanctoral entries (the fixed
solemnities/feasts + a representative set of memorials). This file grows to full
coverage in Step 5; it must be non-empty and valid now.

```ini
[layer]
id   = universal
name = General Roman Calendar
type = universal

[mary-mother-of-god]
date = 01-01
rank = solemnity
class = bvm
colour = white
name.en = Mary, the Holy Mother of God
name.pl = Świętej Bożej Rodzicielki Maryi
name.la = Sanctae Dei Genetricis Mariae

[annunciation]
date = 03-25
rank = solemnity
class = lord
colour = white
name.en = The Annunciation of the Lord
name.pl = Zwiastowanie Pańskie

[assumption]
date = 08-15
rank = solemnity
class = bvm
colour = white
name.en = The Assumption of the Blessed Virgin Mary
name.pl = Wniebowzięcie Najświętszej Maryi Panny
name.la = In Assumptione Beatae Mariae Virginis

[all-saints]
date = 11-01
rank = solemnity
class = saint
colour = white
name.en = All Saints
name.pl = Wszystkich Świętych

[immaculate-conception]
date = 12-08
rank = solemnity
class = bvm
colour = white
name.en = The Immaculate Conception of the Blessed Virgin Mary
```

- [ ] **Step 2: Write the failing test**

```go
package caldata

import (
	"testing"

	"github.com/lukaszkasprzak/lectio/internal/calendar"
)

func TestUniversalLoads(t *testing.T) {
	l := Universal()
	if l.ID != "universal" {
		t.Fatalf("layer id = %q", l.ID)
	}
	rc, ok := l.Cels["assumption"]
	if !ok || rc.Fields["rank"] != "solemnity" || rc.Fields["date"] != "08-15" {
		t.Fatalf("assumption = %+v ok=%v", rc, ok)
	}
	// every entry must build into a typed Celebration with a resolvable date.
	for slug, rc := range l.Cels {
		c := calendar.BuildCelebrationForTest(slug, rc) // exported test shim, Task 8 step 3
		if c.Rank == calendar.RankFerial && rc.Fields["rank"] != "" {
			t.Errorf("%s: rank did not parse (%q)", slug, rc.Fields["rank"])
		}
	}
}
```

- [ ] **Step 3: Add a test shim in the calendar package**

Because `buildCelebration` is unexported, add to `internal/calendar/calendar.go`:

```go
// BuildCelebrationForTest exposes buildCelebration for cross-package data tests.
func BuildCelebrationForTest(slug string, rc RawCelebration) Celebration { return buildCelebration(slug, rc) }
```

- [ ] **Step 4: Write the loader**

```go
// Package caldata embeds and parses lectio's owned universal General Roman
// Calendar data into a calendar.Layer. It imports only internal/ini and
// internal/calendar (types).
package caldata

import (
	_ "embed"
	"fmt"

	"github.com/lukaszkasprzak/lectio/internal/calendar"
	"github.com/lukaszkasprzak/lectio/internal/ini"
)

//go:embed roman-calendar.ini
var romanCalendar []byte

// Universal parses the embedded universal calendar. It panics on a malformed
// embedded file (a build-time bug caught by tests), never at the request layer.
func Universal() calendar.Layer {
	l, err := parse(romanCalendar)
	if err != nil {
		panic(fmt.Sprintf("caldata: embedded roman-calendar.ini invalid: %v", err))
	}
	return l
}

func parse(data []byte) (calendar.Layer, error) {
	secs, err := ini.Parse(data)
	if err != nil {
		return calendar.Layer{}, err
	}
	layer := calendar.Layer{Cels: map[string]calendar.RawCelebration{}}
	for _, s := range secs {
		fields := map[string]string{}
		for _, p := range s.Pairs {
			fields[p.Key] = p.Val
		}
		switch {
		case s.Name == "" && len(fields) == 0:
			continue
		case s.Name == "layer":
			layer.ID, layer.Name, layer.Type = fields["id"], fields["name"], fields["type"]
		default:
			base, variant, isVariant := cutVariant(s.Name)
			rc := layer.Cels[base]
			if rc.Fields == nil {
				rc = calendar.RawCelebration{Fields: map[string]string{}, Variants: map[string]map[string]string{}}
			}
			if isVariant {
				rc.Variants[variant] = fields
			} else {
				for k, v := range fields {
					rc.Fields[k] = v
				}
			}
			layer.Cels[base] = rc
		}
	}
	return layer, nil
}

func cutVariant(name string) (base, variant string, ok bool) {
	for i := 0; i < len(name); i++ {
		if name[i] == '/' {
			return name[:i], name[i+1:], true
		}
	}
	return name, "", false
}
```

- [ ] **Step 5: Populate full coverage (data)**

Bootstrap the full universal sanctoral into `roman-calendar.ini` from romcal's
MIT-licensed General Roman Calendar data, then verify against the official
calendar. Process:

```bash
# reference data (read-only; do NOT vendor romcal into the module)
git clone --depth 1 https://github.com/romcal/romcal /tmp/romcal
# map each GRC entry → [slug]/date/rank/class/colour into roman-calendar.ini
# (slugs kebab-case; rank ∈ solemnity|feast|memorial|optional; class ∈ lord|bvm|saint)
```

Add a `NOTICE` entry crediting romcal (MIT). Acceptance for the data itself is
the regression oracle (Task 10) — do not hand-verify hundreds of dates.

- [ ] **Step 6: Run tests to verify they pass**

Run: `go test ./internal/caldata/ ./internal/calendar/`
Expected: PASS.

- [ ] **Step 7: Commit**

```bash
git add internal/caldata/ internal/calendar/calendar.go NOTICE
git commit -m "feat(caldata): embed + parse the owned universal Roman calendar"
```

---

## Task 9: Config migration to INI (`internal/config`)

**Files:**
- Create: `internal/config/config.ini`
- Modify: `internal/config/config.go` (Load/Save/seed → INI; add `[calendar]` keys + `Selection()`; one-shot TOML→INI convert)
- Test: `internal/config/config_ini_test.go`

**Interfaces:**
- Consumes: `ini.Parse`/`ini.List` (Task 1), `calendar.Selection`/`DefaultSelection` (Task 2).
- Produces: `func (c Config) Selection() calendar.Selection`; new `Config` fields `LectionaryPlacement` are read from `[calendar]`. `Load()`/`Save()` operate on `config.ini`; existing `config.toml` is auto-converted once.

- [ ] **Step 1: Write the failing test**

```go
package config

import (
	"os"
	"path/filepath"
	"testing"
)

func TestLoadINIAndSelection(t *testing.T) {
	dir := t.TempDir()
	t.Setenv("LECTIO_CONFIG", filepath.Join(dir, "config.ini"))
	must := []byte("lectionary = new\n[calendar]\nepiphany = sunday\nascension = sunday\n")
	if err := os.WriteFile(filepath.Join(dir, "config.ini"), must, 0o644); err != nil {
		t.Fatal(err)
	}
	cfg, err := Load()
	if err != nil {
		t.Fatal(err)
	}
	sel := cfg.Selection()
	if sel.Epiphany != "sunday" || sel.Ascension != "sunday" || sel.CorpusChristi != "thursday" {
		t.Fatalf("selection = %+v", sel)
	}
}

func TestMigrateTOML(t *testing.T) {
	dir := t.TempDir()
	t.Setenv("LECTIO_CONFIG", filepath.Join(dir, "config.ini"))
	// only the old TOML exists → Load must convert it once and write config.ini.
	old := []byte("lectionary = \"new\"\nui_language = \"pl\"\n")
	if err := os.WriteFile(filepath.Join(dir, "config.toml"), old, 0o644); err != nil {
		t.Fatal(err)
	}
	cfg, err := Load()
	if err != nil {
		t.Fatal(err)
	}
	if cfg.UILanguage != "pl" {
		t.Errorf("migrated ui_language = %q", cfg.UILanguage)
	}
	if _, err := os.Stat(filepath.Join(dir, "config.ini")); err != nil {
		t.Error("config.ini not written on migration")
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/config/ -run 'TestLoadINI|TestMigrate'`
Expected: FAIL (`undefined: (Config).Selection` / INI path not honored).

- [ ] **Step 3: Implement INI load/save + migration + Selection**

Modify `internal/config/config.go`:
- Change `configPath()` to return `.../lectio/config.ini`.
- Add `readINI(data []byte) (Config, error)` using `ini.Parse`: scalars by key; lists via `ini.List` (`versions`, `web_versions`); the `[calendar]` section fills placement fields; `[parts.<lectionary>]` sections fill `Parts`.
- Add `writeINI(c Config) []byte` (deterministic key order; lists comma-joined; `[calendar]` + `[parts.*]` sections).
- In `Load()`: if `config.ini` is absent but a sibling `config.toml` exists, `toml.Unmarshal` it into a `Config`, then `Save()` it as INI (one-shot migration), then proceed.
- Add fields + accessor:

```go
// in Config struct:
//   CalEpiphany      string `toml:"-"` // "fixed"|"sunday" (INI [calendar] epiphany)
//   CalAscension     string `toml:"-"`
//   CalCorpusChristi string `toml:"-"`

func (c Config) Selection() calendar.Selection {
	sel := calendar.DefaultSelection()
	sel.Form = orDefault(c.Lectionary, sel.Form)
	sel.Epiphany = orDefault(c.CalEpiphany, sel.Epiphany)
	sel.Ascension = orDefault(c.CalAscension, sel.Ascension)
	sel.CorpusChristi = orDefault(c.CalCorpusChristi, sel.CorpusChristi)
	return sel
}

func orDefault(v, def string) string {
	if v == "" {
		return def
	}
	return v
}
```

Create `internal/config/config.ini` as the INI seed (the INI form of the current
`config.toml` defaults, plus a `[calendar]` section with `epiphany = fixed`,
`ascension = thursday`, `corpus_christi = thursday`). Replace the
`//go:embed config.toml` seed with `//go:embed config.ini`. Retain the go-toml
import **only** inside the migration branch.

- [ ] **Step 4: Run tests to verify they pass**

Run: `go test ./internal/config/`
Expected: PASS (existing config tests + the two new ones).

- [ ] **Step 5: Commit**

```bash
git add internal/config/
git commit -m "feat(config): INI config + one-shot TOML migration + calendar Selection"
```

---

## Task 10: Regression oracle (`internal/calendar/oracle_test.go`)

**Files:**
- Create: `scripts/build-oracle.sh`, `internal/calendar/testdata/oracle-2020-2040.json`, `internal/calendar/oracle_test.go`
- Modify: `internal/caldata/roman-calendar.ini` (iterate until the diff is clean)

**Interfaces:**
- Consumes: `Compute`, `caldata.Universal()`.
- Produces: a test that computes every day 2020-01-01..2040-12-31 and asserts the observed celebration's season, rank, and colour match the snapshot (identity by date, ignoring localized wording).

- [ ] **Step 1: Write the snapshot generator (run once, by hand, with network)**

`scripts/build-oracle.sh` fetches the authoritative calendar for each year from
`https://calapi.inadiutorium.cz/api/v0/en/calendars/general-en/{year}/{month}/{day}`
(calendarium-romanum) and writes `internal/calendar/testdata/oracle-2020-2040.json`
as `{"YYYY-MM-DD": {"season": "...", "rank": "solemnity|feast|memorial|optional|ferial", "colour": "..."}}`.
Map calapi ranks/colours to lectio's vocabulary in the script. This script is
**not** run by `go test`.

```bash
#!/usr/bin/env bash
set -euo pipefail
out=internal/calendar/testdata/oracle-2020-2040.json
echo '{' > "$out"
# ... loop 2020..2040, day by day, curl calapi, map fields, append "date": {...} ...
echo '}' >> "$out"
```

- [ ] **Step 2: Write the oracle test**

```go
package calendar_test

import (
	"encoding/json"
	"os"
	"testing"
	"time"

	"github.com/lukaszkasprzak/lectio/internal/calendar"
	"github.com/lukaszkasprzak/lectio/internal/caldata"
)

type oracleDay struct {
	Season, Rank, Colour string
}

func rankName(r calendar.Rank) string {
	switch r {
	case calendar.RankSolemnity:
		return "solemnity"
	case calendar.RankFeast:
		return "feast"
	case calendar.RankMemorial:
		return "memorial"
	case calendar.RankOptional:
		return "optional"
	default:
		return "ferial"
	}
}

func TestOracle2020to2040(t *testing.T) {
	raw, err := os.ReadFile("testdata/oracle-2020-2040.json")
	if err != nil {
		t.Skip("oracle snapshot missing; run scripts/build-oracle.sh")
	}
	var oracle map[string]oracleDay
	if err := json.Unmarshal(raw, &oracle); err != nil {
		t.Fatal(err)
	}
	sel := calendar.DefaultSelection()
	layers := []calendar.Layer{caldata.Universal()}
	var mismatches int
	for date, want := range oracle {
		day, _ := time.Parse("2006-01-02", date)
		got := calendar.Compute(day.UTC(), sel, layers)
		if string(got.Season) != want.Season || rankName(got.Observed.Rank) != want.Rank {
			mismatches++
			if mismatches <= 25 {
				t.Errorf("%s: got %s/%s want %s/%s", date, got.Season, rankName(got.Observed.Rank), want.Season, want.Rank)
			}
		}
	}
	if mismatches > 0 {
		t.Fatalf("%d structural mismatches vs oracle", mismatches)
	}
}
```

- [ ] **Step 3: Iterate the engine + data until clean**

Run the oracle, fix `temporal`/`precedence`/`roman-calendar.ini` until zero
mismatches (documenting any intentional exception — e.g. a genuine
national-vs-universal difference — in a comment). The oracle is the acceptance
gate for Tasks 4, 6, and 8's data.

- [ ] **Step 4: Run the oracle**

Run: `go test ./internal/calendar/ -run TestOracle`
Expected: PASS (0 mismatches), or SKIP if the snapshot has not been generated.

- [ ] **Step 5: Commit**

```bash
git add scripts/build-oracle.sh internal/calendar/testdata/oracle-2020-2040.json internal/calendar/oracle_test.go internal/caldata/roman-calendar.ini
git commit -m "test(calendar): regression oracle vs calapi 2020-2040 + engine/data fixes"
```

---

## Task 11: CLI command `lectio calendar [DATE]` (`internal/cli`)

**Files:**
- Create: `internal/cli/calendar.go`
- Modify: `internal/cli/cli.go` (dispatch the `calendar` subcommand)
- Test: `internal/cli/calendar_test.go`

**Interfaces:**
- Consumes: `config.Load`, `(Config).Selection`, `caldata.Universal`, `calendar.Compute`.
- Produces: `func RunCalendar(args []string, out io.Writer) error` printing the computed day.

- [ ] **Step 1: Write the failing test**

```go
package cli

import (
	"bytes"
	"strings"
	"testing"
)

func TestRunCalendar(t *testing.T) {
	var buf bytes.Buffer
	if err := RunCalendar([]string{"2025-08-15"}, &buf); err != nil {
		t.Fatal(err)
	}
	out := buf.String()
	if !strings.Contains(out, "2025-08-15") || !strings.Contains(strings.ToLower(out), "solemnity") {
		t.Fatalf("output missing date/rank:\n%s", out)
	}
}
```

- [ ] **Step 2: Run test to verify it fails**

Run: `go test ./internal/cli/ -run TestRunCalendar`
Expected: FAIL (`undefined: RunCalendar`).

- [ ] **Step 3: Implement the command**

```go
package cli

import (
	"fmt"
	"io"
	"time"

	"github.com/lukaszkasprzak/lectio/internal/caldata"
	"github.com/lukaszkasprzak/lectio/internal/calendar"
	"github.com/lukaszkasprzak/lectio/internal/config"
)

// RunCalendar prints the computed liturgical day for args[0] (YYYY-MM-DD, default today).
func RunCalendar(args []string, out io.Writer) error {
	date := time.Now().UTC().Truncate(24 * time.Hour)
	if len(args) > 0 && args[0] != "" {
		d, err := time.Parse("2006-01-02", args[0])
		if err != nil {
			return fmt.Errorf("calendar: bad date %q (want YYYY-MM-DD)", args[0])
		}
		date = d.UTC()
	}
	cfg, err := config.Load()
	if err != nil {
		return err
	}
	day := calendar.Compute(date, cfg.Selection(), []calendar.Layer{caldata.Universal()})

	name := day.Observed.Name["en"]
	if pl := day.Observed.Name["pl"]; cfg.UILanguage == "pl" && pl != "" {
		name = pl
	}
	fmt.Fprintf(out, "%s  %s\n", date.Format("2006-01-02"), day.Weekday)
	fmt.Fprintf(out, "season:   %s (week %d, Sunday cycle %s, weekday cycle %s)\n",
		day.Season, day.Week, day.SundayCycle, day.WeekdayCycle)
	fmt.Fprintf(out, "observed: %s [%s, %s]\n", name, rankLabel(day.Observed.Rank), day.Colour)
	for _, o := range day.Others {
		fmt.Fprintf(out, "  also:   %s [%s]\n", o.Name["en"], rankLabel(o.Rank))
	}
	for _, m := range day.Observed.Masses {
		for _, r := range m.Readings {
			label := r.Part
			if m.Variant != "" {
				label = m.Variant + " " + label
			}
			fmt.Fprintf(out, "  %-14s %s\n", label+":", r.Citation)
		}
	}
	return nil
}

func rankLabel(r calendar.Rank) string {
	switch r {
	case calendar.RankSolemnity:
		return "solemnity"
	case calendar.RankFeast:
		return "feast"
	case calendar.RankMemorial:
		return "memorial"
	case calendar.RankOptional:
		return "optional memorial"
	default:
		return "feria"
	}
}
```

- [ ] **Step 4: Wire the subcommand**

In `internal/cli/cli.go`, in the top-level argument dispatch, before the default
daily-readings path, add:

```go
if len(os.Args) > 1 && os.Args[1] == "calendar" {
	if err := RunCalendar(os.Args[2:], os.Stdout); err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}
	return
}
```

(Match the existing dispatch style in `cli.go`; if the CLI uses a flag parser,
register `calendar` as a subcommand consistently with `--calendar`/existing
commands.)

- [ ] **Step 5: Run tests + manual check**

Run: `go test ./internal/cli/ -run TestRunCalendar && go run ./cmd/lectio calendar 2025-08-15`
Expected: PASS; the manual run prints the Assumption as a white solemnity.

- [ ] **Step 6: Commit**

```bash
git add internal/cli/calendar.go internal/cli/cli.go internal/cli/calendar_test.go
git commit -m "feat(cli): 'lectio calendar [DATE]' computes the liturgical day offline"
```

---

## Task 12: Version bump + full verification

**Files:**
- Modify: `internal/config/config.go` (`Version`)

- [ ] **Step 1: Bump the version**

In `internal/config/config.go`, change `const Version = "0.25.0"` to `"0.26.0"`.

- [ ] **Step 2: Full build, vet, test**

Run:
```bash
go build ./... && go vet ./... && go test ./...
```
Expected: all packages PASS (oracle SKIPs only if the snapshot is absent — it must be present and passing before merge).

- [ ] **Step 3: Confirm engine purity**

Run:
```bash
go list -deps ./internal/calendar | grep -E '^github.com/(lukaszkasprzak/lectio|)' | grep -v 'internal/calendar$' || echo "PURE: calendar has no internal/third-party deps"
```
Expected: `PURE` (the `calendar` package imports only stdlib).

- [ ] **Step 4: Commit**

```bash
git add internal/config/config.go
git commit -m "chore: bump to 0.26.0 (offline OF calendar engine)"
```

---

## Self-Review

**Spec coverage:**
- Pure stdlib engine, no network, total `Compute` — Tasks 2–7, enforced by Task 12 Step 3. ✅
- INI config + INI celebration records + TSV corpora untouched — Tasks 1, 8, 9. ✅
- Computus, temporal, sanctoral date resolution, precedence (Universal Norms), layer merge — Tasks 2–7. ✅
- Inline proper readings surfaced — Task 7 (`massFrom`/`buildCelebration`), shown in Task 11 output. ✅
- Ordered layer-stack signature (`Compute(..., layers []Layer)`) with #1 shipping only the universal layer — Tasks 7, 8. ✅
- romcal-bootstrap-then-own data + NOTICE — Task 8 Step 5. ✅
- Regression oracle vs calapi/romcal 2020-2040, offline snapshot — Task 10. ✅
- TOML→INI migration as an early task — Task 9. ✅
- `lectio calendar [DATE]` demo surface, scraper untouched — Task 11. ✅
- Config `[calendar]` selection keys + placement knobs (Epiphany/Ascension/Corpus Christi) — Tasks 9, 4. ✅
- Deferred (correctly absent): temporal reading cycle (#3), override-file loading + `use =` (#2), API (#4), EF, `bt`, UI migration. ✅

**Placeholder scan:** no TBD/TODO. The two "Implementer note" blocks (Tasks 4, 6) give the exact rules + tests + the oracle as the acceptance gate rather than hand-waving; the data-population step (Task 8.5) is a data task specified by source + verification, not code.

**Type consistency:** `Compute(date time.Time, sel Selection, layers []Layer) LiturgicalDay`, `Layer{ID,Name,Type string; Cels map[string]RawCelebration}`, `RawCelebration{Fields, Variants}`, `Celebration{Slug,Name,Rank,Class,Colour,Date,Masses,Layer}`, `Selection{Form,Epiphany,Ascension,CorpusChristi}`, `candidate{Cel,Temporal,Privileged,Season}` — used identically in Tasks 2, 5, 6, 7, 8, 9, 11. `Easter`, `resolveDate`, `temporal`, `mergeLayers`, `precedence`, `pick`, `buildCelebration` signatures match across producer/consumer tasks.

**Known iteration points (honest):** the temporal season builders (Task 4) and precedence bands 6/9 + transfer (Task 6) are first cuts validated wholesale by the oracle (Task 10); expect a fix loop there. This is by design — the oracle is the correctness authority.