2 files changed, 107 insertions, 0 deletions
diff --git a/internal/calendar/precedence.go b/internal/calendar/precedence.go
new file mode 100644
index 0000000..b013b77
--- /dev/null
+++ b/internal/calendar/precedence.go
@@ -0,0 +1,71 @@
+package calendar
+
+import "sort"
+
+// candidate is one celebration competing for a given day: the temporal day, or
+// a sanctoral celebration landing on it. The flags come from the temporal
+// builder (Task 4) or are empty for sanctoral entries.
+type candidate struct {
+	Cel        Celebration
+	Temporal   bool
+	Privileged bool // band 1-2 temporal day
+	PrivFeria  bool // band 9 privileged feria
+	Sunday     bool // Ordinary/Christmas Sunday (band 6)
+	Season     Season
+}
+
+// precedence maps a candidate to its band in the Table of Liturgical Days
+// (Universal Norms, 1969) — lower = higher precedence. "Proper" celebrations
+// (a non-universal, non-temporal layer) sit one band below their General
+// Calendar counterpart.
+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:
+			return 2
+		case c.Cel.Rank == RankSolemnity: // Trinity, Corpus Christi, Sacred Heart, Christ the King
+			return 3
+		case c.Cel.Rank == RankFeast: // feasts of the Lord (Baptism, Holy Family)
+			return 5
+		case c.Sunday:
+			return 6
+		case c.PrivFeria:
+			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
+	}
+}
+
+// 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:]
+}
diff --git a/internal/calendar/precedence_test.go b/internal/calendar/precedence_test.go
new file mode 100644
index 0000000..6d9dc15
--- /dev/null
+++ b/internal/calendar/precedence_test.go
@@ -0,0 +1,36 @@
+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)
+	memorial := saintCand(RankMemorial, ClassSaint)
+	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)
+	}
+}