Fix FR inverse function and add test case (#8)

* fix inverse()

* fix FR inverse method and add tc

fr.go

@@ -225,7 +225,8 @@ func (f FR) Inverse() *FR {
 	b := &FR{FRR2.Copy()}
 	c := bigZeroFR.Copy()
 
-	for u.Cmp(frOne) != 0 && v.Cmp(frOne) != 0 {
+	one := &FRRepr{1, 0, 0, 0}
+	for u.Cmp(one) != 0 && v.Cmp(one) != 0 {
 		for u.IsEven() {
 			u.Div2()
 			if b.n.IsEven() {
@@ -254,7 +255,7 @@ func (f FR) Inverse() *FR {
 			c.SubAssign(b)
 		}
 	}
-	if u.IsZero() {
+	if u.Equals(one) {
 		return b
 	}
 	return c

fr_test.go

@@ -0,0 +1,18 @@
+package bls
+
+import (
+	"crypto/rand"
+	"testing"
+)
+
+func TestFRInverse(t *testing.T) {
+	one := FRReprToFR(&FRRepr{1, 0, 0, 0})
+	for i := 0; i < 10; i++ {
+		newFR, _ := RandFR(rand.Reader)
+		inverse := newFR.Inverse()
+		newFR.MulAssign(inverse)
+		if !one.Equals(newFR) {
+			t.Errorf("Multiplication with inverse must be one.")
+		}
+	}
+}