Share D&D cited source traversal

internal/modules/dnd/shared/citations.go

@@ -0,0 +1,36 @@
+package shared
+
+import (
+	"fmt"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/notarius/internal/core/source"
+)
+
+// CitedText resolves cited source ranges in document order, including each
+// source unit once, and joins the resulting text with newlines.
+func CitedText(doc *source.SourceDocument, refs []source.SourceRef) (string, error) {
+	if doc == nil {
+		return "", fmt.Errorf("source document must not be nil")
+	}
+
+	included := make([]bool, len(doc.Units))
+	for _, ref := range refs {
+		if err := source.ValidateRef(doc, ref); err != nil {
+			return "", fmt.Errorf("resolve cited source range: %w", err)
+		}
+		start, _ := source.UnitIndex(doc, ref.StartUnitID)
+		end, _ := source.UnitIndex(doc, ref.EndUnitID)
+		for index := start; index <= end; index++ {
+			included[index] = true
+		}
+	}
+
+	parts := make([]string, 0, len(doc.Units))
+	for index, unit := range doc.Units {
+		if included[index] {
+			parts = append(parts, unit.Text)
+		}
+	}
+	return strings.Join(parts, "\n"), nil
+}