Add canonical source unit provenance

internal/core/source/source_test.go

@@ -142,6 +142,86 @@ func TestValidateDocumentDuplicateUnitIDs(t *testing.T) {
 	}
 }
 
+func TestValidateDocumentUnitReferences(t *testing.T) {
+	tests := []struct {
+		name    string
+		mutate  func(*SourceDocument)
+		wantErr string
+	}{
+		{
+			name:    "missing",
+			mutate:  func(doc *SourceDocument) { doc.Units[0].Ref = SourceRef{} },
+			wantErr: "source unit[0].ref: source ref source_id must not be empty",
+		},
+		{
+			name:    "foreign source",
+			mutate:  func(doc *SourceDocument) { doc.Units[0].Ref.SourceID = "source-2" },
+			wantErr: "source unit[0].ref: source ref source_id \"source-2\" does not match document id \"source-1\"",
+		},
+		{
+			name: "non-self range",
+			mutate: func(doc *SourceDocument) {
+				doc.Units[0].Ref.StartUnitID = 2
+				doc.Units[0].Ref.EndUnitID = 2
+			},
+			wantErr: "source unit[0].ref must identify source unit id 1",
+		},
+		{
+			name: "reversed range",
+			mutate: func(doc *SourceDocument) {
+				doc.Units[0].Ref.StartUnitID = 2
+				doc.Units[0].Ref.EndUnitID = 1
+			},
+			wantErr: "source unit[0].ref: source ref start_unit_id 2 appears after end_unit_id 1",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			doc := validDocument()
+			tt.mutate(doc)
+
+			err := ValidateDocument(doc)
+			if err == nil {
+				t.Fatal("ValidateDocument() error = nil, want unit reference error")
+			}
+			if err.Error() != tt.wantErr {
+				t.Fatalf("ValidateDocument() error = %q, want %q", err.Error(), tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestDigestDocumentIsDeterministicAndIncludesUnitReference(t *testing.T) {
+	doc := validDocument()
+	doc.Metadata = map[string]any{"second": "value", "first": true}
+	first, err := DigestDocument(doc)
+	if err != nil {
+		t.Fatalf("DigestDocument() error = %v, want nil", err)
+	}
+
+	reordered := validDocument()
+	reordered.Metadata = map[string]any{"first": true, "second": "value"}
+	second, err := DigestDocument(reordered)
+	if err != nil {
+		t.Fatalf("DigestDocument(reordered) error = %v, want nil", err)
+	}
+	if first != second {
+		t.Fatalf("digests = %q and %q, want deterministic map ordering", first, second)
+	}
+
+	changed := validDocument()
+	changed.Metadata = map[string]any{"first": true, "second": "value"}
+	changed.Units[0].Ref.SourceID = "different-source"
+	changedDigest, err := DigestDocument(changed)
+	if err != nil {
+		t.Fatalf("DigestDocument(changed) error = %v, want nil", err)
+	}
+	if first == changedDigest {
+		t.Fatalf("digest = %q after reference change, want different digest", changedDigest)
+	}
+}
+
 func TestValidateRefValid(t *testing.T) {
 	doc := validDocument()
 	ref := SourceRef{
@@ -274,11 +354,13 @@ func validDocument() *SourceDocument {
 				ID:   1,
 				Kind: "paragraph",
 				Text: "First unit.",
+				Ref:  SourceRef{SourceID: "source-1", StartUnitID: 1, EndUnitID: 1},
 			},
 			{
 				ID:   2,
 				Kind: "paragraph",
 				Text: "Second unit.",
+				Ref:  SourceRef{SourceID: "source-1", StartUnitID: 2, EndUnitID: 2},
 			},
 		},
 	}