fix: add flag for diff matcher and correct matching logic

cmd/command.go

@@ -91,6 +91,18 @@ func Command() *cobra.Command { //nolint:funlen
 		nil,
 		"exclude resources by substring contents")
 
+	// Define --diff flag.
+	mf.StringErrorMatcher(matcher.NewDiffMatcher,
+		"diff",
+		"",
+		"include resources which differ from those in a file")
+
+	// Define --not-diff flag.
+	mf.StringErrorMatcher(matcher.NewDiffMatcher,
+		"not-diff",
+		"",
+		"exclude resources which differ from those in a file")
+
 	// Define --exec flag.
 	mf.StringErrorMatcher(matcher.NewExecMatcher,
 		"exec",

matcher/matcher-diff.go

@@ -37,7 +37,8 @@ type diffMatcher struct {
 func (m diffMatcher) Matches(item resources.Resource) bool {
 	for _, original := range m.originals {
 		switch {
-		// Check if the resource under investigation is the counterpart of any original resources.
+		// Check if the resource under investigation is the counterpart of any
+		// original resources.
 		case item.GetAPIVersion() != original.GetAPIVersion():
 			continue
 		case item.GetKind() != original.GetKind():
@@ -46,11 +47,14 @@ func (m diffMatcher) Matches(item resources.Resource) bool {
 			continue
 		case item.GetNamespace() != original.GetNamespace():
 			continue
-		// Diff the resource under investigation against its original counterpart.
-		case cmp.Equal(item.Object, original.Object):
-			return false
 		}
+
+		// Diff the resource under investigation against its original
+		// counterpart.
+		return !cmp.Equal(item.Object, original.Object)
 	}
 
+	// The resource under investigation did not have an original counterpart,
+	// and has therefore differed.
 	return true
 }

matcher/matcher-diff_test.go

@@ -19,6 +19,7 @@ func TestDiffMatcher(t *testing.T) {
 			title:   "diff resources",
 			matcher: must(matcher.NewDiffMatcher("testdata/resources.txt")),
 			matches: []string{
+				"ConfigMap/my-configmap",
 				"Deployment/nginx-deployment",
 				"Pod/test-pod",
 			},

matcher/testdata/resources.txt

@@ -93,4 +93,4 @@ metadata:
   labels:
     app.kubernetes.io/component: auth
   name: my-configmap
-  namespace: custom-app
+  namespace: custom-app-other # <------------------------------- changed value