gh-63: Return error if other is nil (#64)

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.6.x, 1.7.x, 1.8.x, 1.9.x, 1.10.x, 1.11.x, 1.12.x, 1.13.x, 1.14.x, 1.15.x, 1.16.x, 1.17.x, 1.18.x, 1.19.x, 1.20.x]
+        go-version: [1.6.x, 1.7.x, 1.8.x, 1.9.x, 1.10.x, 1.11.x, 1.12.x, 1.13.x, 1.14.x, 1.15.x, 1.16.x, 1.17.x, 1.18.x, 1.19.x, 1.20.x, 1.21.x]
         platform: [ubuntu-latest]
     runs-on: ${{ matrix.platform }}
     steps:

.github/workflows/ci_gomodule.yml

@@ -4,7 +4,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.11.x, 1.12.x, 1.13.x, 1.14.x, 1.15.x, 1.16.x, 1.17.x, 1.18.x, 1.19.x, 1.20.x]
+        go-version: [1.11.x, 1.12.x, 1.13.x, 1.14.x, 1.15.x, 1.16.x, 1.17.x, 1.18.x, 1.19.x, 1.20.x, 1.21.x]
         platform: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps:

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
 
@@ -54,6 +56,9 @@ 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, errNoOther
+	}
 	if h.GetKind() != other.GetKind() {
 		return -1, errors.New("Image hashes's kind should be identical")
 	}

imagehash_test.go

@@ -56,6 +56,17 @@ func TestNewImageHash(t *testing.T) {
 	}
 }
 
+func TestNil(t *testing.T) {
+	hash := NewImageHash(0, AHash)
+	dis, err := hash.Distance(nil)
+	if 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)
+	}
+}
+
 func TestSerialization(t *testing.T) {
 	checkErr := func(err error) {
 		if err != nil {