test(ltl): property-based NNF laws

Lock double-negation identity, Always/Eventually duality with bound
preservation, leaf pushdown, and not(always true) reaching Violated.

internal/ltl/nnf_test.go

@@ -0,0 +1,97 @@
+package ltl
+
+import (
+	"testing"
+	"testing/quick"
+	"time"
+)
+
+// leaf builds a small set of representative atomic formulas indexed by a seed.
+func leafFor(seed uint8) Formula {
+	switch seed % 3 {
+	case 0:
+		return Pure(true)
+	case 1:
+		return Pure(false)
+	default:
+		return ThunkNamed("p", func() bool { return true })
+	}
+}
+
+func TestNNF_DoubleNegationIsIdentity(t *testing.T) {
+	law := func(seed uint8) bool {
+		leaf := leafFor(seed)
+		doubled := nnf(Not(Not(leaf)))
+		direct := nnf(leaf)
+		return doubled.describe() == direct.describe()
+	}
+	if err := quick.Check(law, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestNNF_NotAlwaysIsEventuallyNot(t *testing.T) {
+	law := func(seed uint8) bool {
+		leaf := leafFor(seed)
+		negated := nnf(Not(Always(leaf)))
+		expected := nnf(Eventually(Not(leaf)))
+		return negated.describe() == expected.describe()
+	}
+	if err := quick.Check(law, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestNNF_NotEventuallyIsAlwaysNot(t *testing.T) {
+	law := func(seed uint8) bool {
+		leaf := leafFor(seed)
+		negated := nnf(Not(Eventually(leaf)))
+		expected := nnf(Always(Not(leaf)))
+		return negated.describe() == expected.describe()
+	}
+	if err := quick.Check(law, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestNNF_BoundedEventuallyDualKeepsBound(t *testing.T) {
+	negated := nnf(Not(EventuallyWithinSteps(Pure(true), 4)))
+	always, ok := negated.(AlwaysFormula)
+	if !ok {
+		t.Fatalf("expected AlwaysFormula, got %T", negated)
+	}
+	if !always.HasStepBound || always.StepBound != 4 {
+		t.Errorf("bound not preserved: %+v", always)
+	}
+}
+
+func TestNNF_PushesNotToThunkLeaf(t *testing.T) {
+	formula := nnf(Always(Not(Always(ThunkNamed("p", func() bool { return true })))))
+	always, ok := formula.(AlwaysFormula)
+	if !ok {
+		t.Fatalf("expected AlwaysFormula, got %T", formula)
+	}
+	eventually, ok := always.Inner.(EventuallyFormula)
+	if !ok {
+		t.Fatalf("expected inner EventuallyFormula, got %T", always.Inner)
+	}
+	not, ok := eventually.Inner.(NotFormula)
+	if !ok {
+		t.Fatalf("expected NotFormula leaf, got %T", eventually.Inner)
+	}
+	if _, ok := not.Inner.(ThunkFormula); !ok {
+		t.Errorf("expected Not to wrap a Thunk, got %T", not.Inner)
+	}
+}
+
+func TestNNF_NotAlwaysTrueViaEvaluatorReportsViolated(t *testing.T) {
+	evaluator := NewEvaluator(Always(Not(Always(ThunkNamed("p", func() bool { return true })))))
+	for index := range 4 {
+		if got := evaluator.ObserveAt(time.Unix(int64(index), 0)); got == VerdictViolated {
+			t.Fatalf("step %d latched violated prematurely", index)
+		}
+	}
+	if got := evaluator.Finalize(); got != VerdictViolated {
+		t.Errorf("Finalize = %v, want violated", got)
+	}
+}