fix: fix OR computation in case one input is constant and other varia…

…ble (#1181)

* test: add regression test

* fix: or for constant input

frontend/cs/scs/api.go

@@ -351,9 +351,6 @@ func (builder *builder) Or(a, b frontend.Variable) frontend.Variable {
 		return 0
 	}
 
-	res := builder.newInternalVariable()
-	builder.MarkBoolean(res)
-
 	// if one input is constant, ensure we put it in b
 	if aConstant {
 		a, b = b, a
@@ -362,20 +359,14 @@ func (builder *builder) Or(a, b frontend.Variable) frontend.Variable {
 	}
 
 	if bConstant {
-		xa := a.(expr.Term)
-		// b = b - 1
-		qL := _b
-		qL = builder.cs.Sub(qL, builder.tOne)
-		qL = builder.cs.Mul(qL, xa.Coeff)
-		// a * (b-1) + res == 0
-		builder.addPlonkConstraint(sparseR1C{
-			xa: xa.VID,
-			xc: res.VID,
-			qL: qL,
-			qO: builder.tOne,
-		})
-		return res
+		if builder.cs.IsOne(_b) {
+			return 1
+		} else {
+			return a
+		}
 	}
+	res := builder.newInternalVariable()
+	builder.MarkBoolean(res)
 	xa := a.(expr.Term)
 	xb := b.(expr.Term)
 	// -a - b + ab + res == 0

frontend/cs/scs/api_test.go

@@ -257,3 +257,36 @@ func TestSubSameNoConstraint(t *testing.T) {
 		t.Fatal("expected 0 constraints")
 	}
 }
+
+type regressionOr struct {
+	A          frontend.Variable
+	constOr    int
+	constCheck int
+}
+
+func (c *regressionOr) Define(api frontend.API) error {
+	y := api.Or(c.A, c.constOr)
+	api.AssertIsEqual(y, c.constCheck)
+	return nil
+}
+
+func TestRegressionOr(t *testing.T) {
+	assert := test.NewAssert(t)
+	for _, tc := range []struct{ in, o, c int }{
+		{1, 1, 1}, {0, 1, 1},
+		{1, 0, 1}, {0, 0, 0},
+	} {
+		ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, &regressionOr{constOr: tc.o, constCheck: tc.c})
+		assert.NoError(err)
+		w, err := frontend.NewWitness(&regressionOr{
+			A: tc.in,
+		}, ecc.BN254.ScalarField())
+		if err != nil {
+			t.Error("compile", err)
+		}
+		_, err = ccs.Solve(w)
+		if err != nil {
+			t.Error("solve", err)
+		}
+	}
+}