const in eval too

eval now sets the constant to a const rather
than assuming the user has constant propagated
everything away.

logic/c.go

@@ -203,7 +203,9 @@ func (c *C) InPos(dst []int) []int {
 // `c` is embedded in a sequential circuit `s`, then
 // the inputs include the latches of `s`.
 func (c *C) Eval(vs []bool) {
-	for i := range c.nodes {
+	N := len(c.nodes)
+	vs[1] = true
+	for i := 2; i < N; i++ {
 		n := &c.nodes[i]
 		if n.a < 4 {
 			continue
@@ -224,7 +226,9 @@ func (c *C) Eval(vs []bool) {
 // Eval64 is like Eval but evaluates 64 different inputs in
 // parallel as the bits of a uint64.
 func (c *C) Eval64(vs []uint64) {
-	for i := range c.nodes {
+	N := len(c.nodes)
+	vs[1] = (1 << 63) - 1
+	for i := 2; i < N; i++ {
 		n := &c.nodes[i]
 		if n.a < 4 {
 			continue

logic/c_test.go

@@ -92,6 +92,9 @@ func TestEval(t *testing.T) {
 	if !vs[4] {
 		t.Errorf("bad and eval")
 	}
+	if !vs[1] {
+		t.Errorf("bad const eval")
+	}
 }
 
 var rnd = rand.New(rand.NewSource(1))