From 4c08e94dbcce8dc7757afba3001abe509bd2b6a8 Mon Sep 17 00:00:00 2001
From: Lukasz Kasprzak <lukas@labunix.xyz>
Date: Mon, 27 Jul 2026 12:19:17 +0200
Subject: feat(calendar): Universal Norms precedence bands + pick

---
 internal/calendar/precedence.go | 71 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100644 internal/calendar/precedence.go

(limited to 'internal/calendar/precedence.go')

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:]
+}
-- 
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