Finish the D&D module cleanup

internal/core/source/source_test.go

@@ -334,6 +334,9 @@ func TestDocumentIndexSnapshotsIdentityAndUnitPositions(t *testing.T) {
 	doc.ID = "changed"
 	doc.Units[0].ID = 99
 
+	if documentID, ok := index.DocumentID(); !ok || documentID != "source-1" {
+		t.Fatalf("DocumentID() = %q, %t, want source-1, true", documentID, ok)
+	}
 	if position, ok := index.Position(30); !ok || position != 0 {
 		t.Fatalf("Position(30) = %d, %t, want 0, true", position, ok)
 	}
@@ -348,6 +351,9 @@ func TestDocumentIndexSnapshotsIdentityAndUnitPositions(t *testing.T) {
 
 func TestZeroDocumentIndexIsSafe(t *testing.T) {
 	var index DocumentIndex
+	if documentID, ok := index.DocumentID(); ok || documentID != "" {
+		t.Fatalf("DocumentID() = %q, %t, want empty, false", documentID, ok)
+	}
 	if position, ok := index.Position(1); ok || position != 0 {
 		t.Fatalf("Position(1) = %d, %t, want 0, false", position, ok)
 	}

internal/core/source/validation.go

@@ -31,6 +31,14 @@ func NewDocumentIndex(doc *SourceDocument) DocumentIndex {
 	}
 }
 
+// DocumentID returns the indexed document identity.
+func (i DocumentIndex) DocumentID() (string, bool) {
+	if !i.hasDocument {
+		return "", false
+	}
+	return i.documentID, true
+}
+
 // Position returns the indexed document position for unitID.
 func (i DocumentIndex) Position(unitID int) (int, bool) {
 	position, ok := i.positions[unitID]