4 files changed, 204 insertions, 0 deletions
diff --git a/internal/calendar/computus.go b/internal/calendar/computus.go
new file mode 100644
index 0000000..13410cb
--- /dev/null
+++ b/internal/calendar/computus.go
@@ -0,0 +1,23 @@
+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)
+}
diff --git a/internal/calendar/computus_test.go b/internal/calendar/computus_test.go
new file mode 100644
index 0000000..c524093
--- /dev/null
+++ b/internal/calendar/computus_test.go
@@ -0,0 +1,20 @@
+package calendar
+
+import (
+	"testing"
+)
+
+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)
+		}
+	}
+}
diff --git a/internal/calendar/types.go b/internal/calendar/types.go
new file mode 100644
index 0000000..f481441
--- /dev/null
+++ b/internal/calendar/types.go
@@ -0,0 +1,146 @@
+// 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" | "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 ""
+	}
+}
diff --git a/internal/calendar/types_test.go b/internal/calendar/types_test.go
new file mode 100644
index 0000000..c09b53e
--- /dev/null
+++ b/internal/calendar/types_test.go
@@ -0,0 +1,15 @@
+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")
+	}
+}