Fix D&D extraction issues and retire the completed audit

internal/core/source/source_test.go

@@ -249,6 +249,9 @@ func TestValidateRefValid(t *testing.T) {
 	if err := ValidateRef(doc, ref); err != nil {
 		t.Fatalf("ValidateRef() error = %v, want nil", err)
 	}
+	if err := NewDocumentIndex(doc).ValidateRef(ref); err != nil {
+		t.Fatalf("DocumentIndex.ValidateRef() error = %v, want nil", err)
+	}
 }
 
 func TestValidateRefRejectsMalformedReferences(t *testing.T) {
@@ -301,13 +304,56 @@ func TestValidateRefRejectsMalformedReferences(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			err := ValidateRef(validDocument(), tt.ref)
-
-			requireErrorFragments(t, err, tt.fragments...)
+			doc := validDocument()
+			validators := []struct {
+				name     string
+				validate func(SourceRef) error
+			}{
+				{name: "document", validate: func(ref SourceRef) error { return ValidateRef(doc, ref) }},
+				{name: "index", validate: NewDocumentIndex(doc).ValidateRef},
+			}
+			for _, validator := range validators {
+				t.Run(validator.name, func(t *testing.T) {
+					requireErrorFragments(t, validator.validate(tt.ref), tt.fragments...)
+				})
+			}
 		})
 	}
 }
 
+func TestDocumentIndexSnapshotsIdentityAndUnitPositions(t *testing.T) {
+	doc := &SourceDocument{
+		ID: "source-1",
+		Units: []SourceUnit{
+			{ID: 30},
+			{ID: 10},
+			{ID: 30},
+		},
+	}
+	index := NewDocumentIndex(doc)
+	doc.ID = "changed"
+	doc.Units[0].ID = 99
+
+	if position, ok := index.Position(30); !ok || position != 0 {
+		t.Fatalf("Position(30) = %d, %t, want 0, true", position, ok)
+	}
+	if position, ok := index.Position(10); !ok || position != 1 {
+		t.Fatalf("Position(10) = %d, %t, want 1, true", position, ok)
+	}
+	ref := SourceRef{SourceID: "source-1", StartUnitID: 30, EndUnitID: 10}
+	if err := index.ValidateRef(ref); err != nil {
+		t.Fatalf("ValidateRef() error = %v, want nil", err)
+	}
+}
+
+func TestZeroDocumentIndexIsSafe(t *testing.T) {
+	var index DocumentIndex
+	if position, ok := index.Position(1); ok || position != 0 {
+		t.Fatalf("Position(1) = %d, %t, want 0, false", position, ok)
+	}
+	requireErrorFragments(t, index.ValidateRef(SourceRef{}), "source document must not be nil")
+}
+
 func TestUnitIndex(t *testing.T) {
 	doc := validDocument()