feat: add WithDefault for flag/priority

This makes it easier to resolve these fields.

types.go

@@ -55,6 +55,19 @@ const (
 	True    Flag = 1
 )
 
+func (f Flag) WithDefault(defaultValue bool) bool {
+	switch f {
+	case False:
+		return false
+	case Default:
+		return defaultValue
+	case True:
+		return true
+	default:
+		panic(fmt.Sprintf("invalid flag value %d", f))
+	}
+}
+
 func (f Flag) MarshalJSON() ([]byte, error) {
 	switch f {
 	case Default:
@@ -110,6 +123,23 @@ const (
 	Disabled        Priority = -1
 )
 
+// WithDefault resolves the priority with the given default.
+//
+// If `defaultPriority` is Default/0, this function will return 0.
+func (p Priority) WithDefault(defaultPriority Priority) (priority int64, enabled bool) {
+	switch p {
+	case Disabled:
+		return 0, false
+	case DefaultPriority:
+		if defaultPriority < 0 {
+			return 0, false
+		}
+		return int64(defaultPriority), true
+	default:
+		return int64(p), true
+	}
+}
+
 func (p Priority) MarshalJSON() ([]byte, error) {
 	// > 0 == Priority
 	if p > 0 {

types_test.go

@@ -91,6 +91,30 @@ func TestFlag(t *testing.T) {
 		t.Errorf("expected default flag to be %q, got %q", Default, defaultFlag)
 	}
 
+	if defaultFlag.WithDefault(true) != true {
+		t.Error("expected default & true to be true")
+	}
+
+	if defaultFlag.WithDefault(false) != false {
+		t.Error("expected default & false to be false")
+	}
+
+	if True.WithDefault(false) != true {
+		t.Error("default should only apply to default")
+	}
+
+	if False.WithDefault(true) != false {
+		t.Error("default should only apply to default")
+	}
+
+	if True.WithDefault(true) != true {
+		t.Error("true & true is true")
+	}
+
+	if False.WithDefault(true) != false {
+		t.Error("false & false is false")
+	}
+
 	for jsonStr, goValue := range map[string]Flag{
 		"null":  Default,
 		"true":  True,
@@ -135,6 +159,18 @@ func TestPriority(t *testing.T) {
 		t.Errorf("expected default priority to be %q, got %q", DefaultPriority, defaultPriority)
 	}
 
+	if _, ok := defaultPriority.WithDefault(Disabled); ok {
+		t.Error("should have been disabled")
+	}
+
+	if p, ok := defaultPriority.WithDefault(1); !ok || p != 1 {
+		t.Errorf("priority should have been 1, got %d", p)
+	}
+
+	if p, ok := defaultPriority.WithDefault(DefaultPriority); !ok || p != 0 {
+		t.Errorf("priority should have been 0, got %d", p)
+	}
+
 	for jsonStr, goValue := range map[string]Priority{
 		"null":  DefaultPriority,
 		"false": Disabled,