Address code review

imagehash.go

@@ -13,6 +13,8 @@ import (
 	"io"
 )
 
+var errNoOther = errors.New("other should not be nil")
+
 // Kind describes the kinds of hash.
 type Kind int
 
@@ -55,7 +57,7 @@ func (h *ImageHash) Bits() int {
 // Distance method returns a distance between two hashes.
 func (h *ImageHash) Distance(other *ImageHash) (int, error) {
 	if other == nil {
-		return -1, errors.New("other should not be nil")
+		return -1, errNoOther
 	}
 	if h.GetKind() != other.GetKind() {
 		return -1, errors.New("Image hashes's kind should be identical")

imagehash_test.go

@@ -59,9 +59,8 @@ func TestNewImageHash(t *testing.T) {
 func TestNil(t *testing.T) {
 	hash := NewImageHash(0, AHash)
 	dis, err := hash.Distance(nil)
-	expectedError := errors.New("other should not be nil")
-	if err != nil && err.Error() != expectedError.Error() {
-		t.Errorf("Expected err %s, actual %s", expectedError, err)
+	if !errors.Is(err, errNoOther) {
+		t.Errorf("Expected err %s, actual %s", errNoOther, err)
 	}
 	if dis != -1 {
 		t.Errorf("Distance is expected as %d but got %d", -1, dis)