3 files changed, 155 insertions, 10 deletions
diff --git a/internal/caldata/caldata.go b/internal/caldata/caldata.go
index 9272fbb..14b3985 100644
--- a/internal/caldata/caldata.go
+++ b/internal/caldata/caldata.go
@@ -16,8 +16,11 @@ import (
 //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.
+//go:embed tridentine-calendar.ini
+var tridentineCalendar []byte
+
+// Universal parses the embedded Ordinary Form universal calendar. It panics on a
+// malformed embedded file (a build-time bug caught by tests).
 func Universal() calendar.Layer {
 	l, err := ParseLayer(romanCalendar)
 	if err != nil {
@@ -26,6 +29,24 @@ func Universal() calendar.Layer {
 	return l
 }
 
+// Tridentine parses the embedded Extraordinary Form (1962) universal calendar.
+func Tridentine() calendar.Layer {
+	l, err := ParseLayer(tridentineCalendar)
+	if err != nil {
+		panic(fmt.Sprintf("caldata: embedded tridentine-calendar.ini invalid: %v", err))
+	}
+	return l
+}
+
+// Base returns the universal base layer for a form: the 1962 calendar for
+// "old" (EF), the General Roman Calendar otherwise (OF).
+func Base(form string) calendar.Layer {
+	if form == "old" {
+		return Tridentine()
+	}
+	return Universal()
+}
+
 // LoadLayer reads and parses a user calendar layer file. id (the filename stem)
 // is used as the layer's ID when the file's [layer] header omits one.
 func LoadLayer(path, id string) (calendar.Layer, error) {
@@ -43,12 +64,12 @@ func LoadLayer(path, id string) (calendar.Layer, error) {
 	return l, nil
 }
 
-// Stack returns the ordered layer stack for the engine: the universal base
-// first, then each user layer named in use (matched to "<dir>/<id>.ini"), in
-// order. A layer that fails to load is skipped and reported in the error slice
-// so a single bad file never breaks calendar computation.
-func Stack(dir string, use []string) ([]calendar.Layer, []error) {
-	layers := []calendar.Layer{Universal()}
+// Stack returns the ordered layer stack for the engine: the form's universal
+// base first, then each user layer named in use (matched to "<dir>/<id>.ini"),
+// in order. A layer that fails to load is skipped and reported in the error
+// slice so a single bad file never breaks calendar computation.
+func Stack(form, dir string, use []string) ([]calendar.Layer, []error) {
+	layers := []calendar.Layer{Base(form)}
 	var errs []error
 	for _, id := range use {
 		l, err := LoadLayer(filepath.Join(dir, id+".ini"), id)
diff --git a/internal/caldata/stack_test.go b/internal/caldata/stack_test.go
index b9d5bfb..3c1cf3f 100644
--- a/internal/caldata/stack_test.go
+++ b/internal/caldata/stack_test.go
@@ -13,7 +13,7 @@ func TestStack(t *testing.T) {
 	os.WriteFile(filepath.Join(dir, "krakow.ini"),
 		[]byte("[layer]\nid = krakow\ntype = diocesan\n\n[st-stanislaus]\ndate = 05-08\nrank = solemnity\nclass = saint\ncolour = red\nname.pl = Św. Stanisława\n"), 0o644)
 
-	layers, errs := Stack(dir, []string{"poland", "krakow"})
+	layers, errs := Stack("new", dir, []string{"poland", "krakow"})
 	if len(errs) != 0 {
 		t.Fatalf("unexpected errors: %v", errs)
 	}
@@ -32,7 +32,7 @@ func TestStackMissingLayerSkips(t *testing.T) {
 	dir := t.TempDir()
 	os.WriteFile(filepath.Join(dir, "good.ini"),
 		[]byte("[layer]\nid = good\n\n[x]\ndate = 01-02\nrank = memorial\nname.en = X\n"), 0o644)
-	layers, errs := Stack(dir, []string{"good", "missing"})
+	layers, errs := Stack("new", dir, []string{"good", "missing"})
 	if len(errs) != 1 {
 		t.Fatalf("want 1 error for the missing layer, got %v", errs)
 	}
@@ -54,3 +54,15 @@ func TestLoadLayerDefaultsIDToStem(t *testing.T) {
 		t.Errorf("ID = %q, want myparish (from stem)", l.ID)
 	}
 }
+
+func TestStackFormBase(t *testing.T) {
+	dir := t.TempDir()
+	efLayers, _ := Stack("old", dir, nil)
+	if efLayers[0].ID != "tridentine" {
+		t.Errorf("EF base = %q want tridentine", efLayers[0].ID)
+	}
+	ofLayers, _ := Stack("new", dir, nil)
+	if ofLayers[0].ID != "universal" {
+		t.Errorf("OF base = %q want universal", ofLayers[0].ID)
+	}
+}
diff --git a/internal/caldata/tridentine-calendar.ini b/internal/caldata/tridentine-calendar.ini
new file mode 100644
index 0000000..f3e9111
--- /dev/null
+++ b/internal/caldata/tridentine-calendar.ini
@@ -0,0 +1,112 @@
+; General Roman Calendar of 1962 (Extraordinary Form) — universal sanctoral.
+; The temporal cycle (seasons, Sundays, Easter/Christmas/Epiphany, Ascension,
+; Pentecost, Trinity, Corpus Christi, Sacred Heart, Christ the King) is computed
+; by internal/calendar (temporalEF) and is NOT listed here.
+; Ranks use the 1960 Code of Rubrics: class-1..class-4 + commemoration.
+; INITIAL coverage; ranks/membership refined against the missalemeum oracle.
+; Bootstrapped with reference to Divinum Officium / Missal1962 — see NOTICE.
+
+[layer]
+id   = tridentine
+name = General Roman Calendar of 1962
+type = universal
+
+; --- I class ---
+
+[immaculate-conception]
+date = 12-08
+rank = class-1
+colour = white
+name.en = The Immaculate Conception of the Blessed Virgin Mary
+name.la = In Conceptione Immaculata Beatae Mariae Virginis
+
+[st-joseph]
+date = 03-19
+rank = class-1
+colour = white
+name.en = Saint Joseph, Spouse of the Blessed Virgin Mary
+name.la = Sancti Joseph Sponsi Beatae Mariae Virginis
+
+[annunciation]
+date = 03-25
+rank = class-1
+colour = white
+name.en = The Annunciation of the Blessed Virgin Mary
+name.la = In Annuntiatione Beatae Mariae Virginis
+
+[nativity-john-baptist]
+date = 06-24
+rank = class-1
+colour = white
+name.en = The Nativity of Saint John the Baptist
+
+[peter-and-paul]
+date = 06-29
+rank = class-1
+colour = red
+name.en = Saints Peter and Paul, Apostles
+
+[assumption]
+date = 08-15
+rank = class-1
+colour = white
+name.en = The Assumption of the Blessed Virgin Mary
+name.la = In Assumptione Beatae Mariae Virginis
+
+[st-michael]
+date = 09-29
+rank = class-1
+colour = white
+name.en = The Dedication of Saint Michael the Archangel
+
+[all-saints]
+date = 11-01
+rank = class-1
+colour = white
+name.en = All Saints
+
+; --- II class ---
+
+[purification]
+date = 02-02
+rank = class-2
+colour = white
+name.en = The Purification of the Blessed Virgin Mary
+
+[st-lawrence]
+date = 08-10
+rank = class-2
+colour = red
+name.en = Saint Lawrence, Martyr
+
+[nativity-bvm]
+date = 09-08
+rank = class-2
+colour = white
+name.en = The Nativity of the Blessed Virgin Mary
+
+[exaltation-of-the-cross]
+date = 09-14
+rank = class-2
+colour = red
+name.en = The Exaltation of the Holy Cross
+
+[st-andrew]
+date = 11-30
+rank = class-2
+colour = red
+name.en = Saint Andrew, Apostle
+
+; --- III class ---
+
+[conversion-of-paul]
+date = 01-25
+rank = class-3
+colour = white
+name.en = The Conversion of Saint Paul the Apostle
+
+[st-lucy]
+date = 12-13
+rank = class-3
+colour = red
+name.en = Saint Lucy, Virgin and Martyr