From ab71c67414666800f19fdd3f6515f15c90cb822e Mon Sep 17 00:00:00 2001
From: Lukasz Kasprzak <lukas@labunix.xyz>
Date: Thu, 23 Jul 2026 12:41:29 +0200
Subject: psalter: port psalm versification

---
 internal/psalter/psalter.go      | 46 ++++++++++++++++++++++++++++++++++++++++
 internal/psalter/psalter_test.go | 28 ++++++++++++++++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 internal/psalter/psalter.go
 create mode 100644 internal/psalter/psalter_test.go

(limited to 'internal')

diff --git a/internal/psalter/psalter.go b/internal/psalter/psalter.go
new file mode 100644
index 0000000..815f660
--- /dev/null
+++ b/internal/psalter/psalter.go
@@ -0,0 +1,46 @@
+// Package psalter bridges psalm versification between the Vulgate-family
+// versions and the Douay-Rheims source. See the Go port of psalm_versify.py.
+package psalter
+
+// DRBTitleFold maps a Hebrew psalm number to the number of title verses the DRB
+// source folds into verse 1 (1, or 2 for a historical superscription). Untitled
+// psalms are absent (k = 0). Copy every entry from psalm_versify.py DRB_TITLE_FOLD.
+var DRBTitleFold = map[int]int{
+	3: 1, 4: 1, 5: 1, 6: 1, 7: 1, 8: 1, 11: 1, 12: 1, 14: 1, 18: 1, 19: 1,
+	20: 1, 21: 1, 22: 1, 30: 1, 31: 1, 34: 1, 36: 1, 38: 1, 39: 1, 40: 1,
+	41: 1, 42: 1, 44: 1, 45: 1, 46: 1, 47: 1, 48: 1, 49: 1, 51: 2, 52: 2,
+	53: 1, 54: 2, 55: 1, 56: 1, 57: 1, 58: 1, 59: 1, 60: 2, 61: 1, 62: 1,
+	63: 1, 64: 1, 65: 1, 67: 1, 68: 1, 69: 1, 70: 1, 75: 1, 76: 1, 77: 1,
+	80: 1, 81: 1, 83: 1, 84: 1, 85: 1, 88: 1, 89: 1, 92: 1, 102: 1, 108: 1,
+	140: 1, 142: 1,
+}
+
+// DrbVerse returns the DRB-source verse number for a lectionary (BT) psalm verse.
+func DrbVerse(hebrewPsalm, lectionaryVerse int) int {
+	v := lectionaryVerse - DRBTitleFold[hebrewPsalm]
+	if v < 1 {
+		return 1
+	}
+	return v
+}
+
+// HebrewToVulgateChapter maps a Masoretic psalm number to its Vulgate chapter.
+func HebrewToVulgateChapter(h int) int {
+	switch {
+	case h <= 8 || h >= 148:
+		return h
+	case h >= 9 && h <= 10:
+		return 9
+	case h >= 11 && h <= 113:
+		return h - 1
+	case h >= 114 && h <= 115:
+		return 113
+	case h == 116:
+		return 114
+	case h >= 117 && h <= 146:
+		return h - 1
+	case h == 147:
+		return 146
+	}
+	return h
+}
diff --git a/internal/psalter/psalter_test.go b/internal/psalter/psalter_test.go
new file mode 100644
index 0000000..612bf1d
--- /dev/null
+++ b/internal/psalter/psalter_test.go
@@ -0,0 +1,28 @@
+package psalter
+
+import "testing"
+
+func TestDrbVerse(t *testing.T) {
+	cases := []struct{ psalm, v, want int }{
+		{34, 2, 1}, // Ps 34: k=1, title folded
+		{34, 3, 2},
+		{51, 3, 1}, // Ps 51: k=2 (two-line title)
+		{63, 2, 1}, // Ps 63: k=1
+		{1, 5, 5},  // untitled: identity
+		{34, 1, 1}, // clamp: never below 1
+	}
+	for _, c := range cases {
+		if got := DrbVerse(c.psalm, c.v); got != c.want {
+			t.Errorf("DrbVerse(%d,%d)=%d want %d", c.psalm, c.v, got, c.want)
+		}
+	}
+}
+
+func TestHebrewToVulgateChapter(t *testing.T) {
+	cases := []struct{ h, want int }{{8, 8}, {34, 33}, {9, 9}, {10, 9}, {116, 114}, {147, 146}, {150, 150}}
+	for _, c := range cases {
+		if got := HebrewToVulgateChapter(c.h); got != c.want {
+			t.Errorf("HebrewToVulgateChapter(%d)=%d want %d", c.h, got, c.want)
+		}
+	}
+}
-- 
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