Replace combinatorial decorators with capability registry (BC) (#28565)

api/capable.go

@@ -0,0 +1,36 @@
+package api
+
+import "reflect"
+
+// Capable is implemented by decorated types that support dynamic capability lookup.
+// This allows O(n) decorator code instead of O(2^n) combinatorial struct generation.
+type Capable interface {
+	Capability(typ reflect.Type) (any, bool)
+}
+
+// Cap checks whether v implements interface T, either directly (via Go type assertion)
+// or through the Capable registry (for decorated types) and returns the interface same
+// as a Go type assertion would do.
+func Cap[T any](v any) (T, bool) {
+	// fast path: direct type assertion (works for non-decorated concrete types)
+	if t, ok := v.(T); ok {
+		return t, ok
+	}
+
+	// slow path: capability registry lookup (for decorated types)
+	if c, ok := v.(Capable); ok {
+		if impl, ok := c.Capability(reflect.TypeFor[T]()); ok {
+			return impl.(T), true
+		}
+	}
+
+	var zero T
+	return zero, false
+}
+
+// HasCap checks whether v implements interface T, either directly (via Go type assertion)
+// or through the Capable registry (for decorated types).
+func HasCap[T any](v any) bool {
+	_, ok := Cap[T](v)
+	return ok
+}

api/capable_test.go

@@ -0,0 +1,89 @@
+package api
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// mock decorated type with capability registry
+type decoratedMeter struct {
+	Meter
+	caps map[reflect.Type]any
+}
+
+func (d *decoratedMeter) Capability(typ reflect.Type) (any, bool) {
+	c, ok := d.caps[typ]
+	return c, ok
+}
+
+type testMeterEnergyImpl struct{}
+
+func (t *testMeterEnergyImpl) TotalEnergy() (float64, error) {
+	return 99.0, nil
+}
+
+type testMeterImpl struct{}
+
+func (t *testMeterImpl) CurrentPower() (float64, error) {
+	return 100.0, nil
+}
+
+func TestCap_DirectTypeAssertion(t *testing.T) {
+	// concrete type that directly implements MeterEnergy
+	impl := &testMeterEnergyImpl{}
+
+	me, ok := Cap[MeterEnergy](impl)
+	require.True(t, ok)
+
+	energy, err := me.TotalEnergy()
+	assert.NoError(t, err)
+	assert.Equal(t, 99.0, energy)
+}
+
+func TestCap_CapableRegistryLookup(t *testing.T) {
+	base := &testMeterImpl{}
+
+	decorated := &decoratedMeter{
+		Meter: base,
+		caps: map[reflect.Type]any{
+			reflect.TypeFor[MeterEnergy](): &testMeterEnergyImpl{},
+		},
+	}
+
+	// should find MeterEnergy via registry
+	me, ok := Cap[MeterEnergy](decorated)
+	require.True(t, ok)
+
+	energy, err := me.TotalEnergy()
+	assert.NoError(t, err)
+	assert.Equal(t, 99.0, energy)
+
+	// should NOT find PhaseCurrents (not registered)
+	_, ok = Cap[PhaseCurrents](decorated)
+	assert.False(t, ok)
+}
+
+func TestCap_NilValue(t *testing.T) {
+	_, ok := Cap[MeterEnergy](nil)
+	assert.False(t, ok)
+}
+
+func TestCap_DirectTakesPrecedence(t *testing.T) {
+	// type that both directly implements AND has registry
+	type directAndCapable struct {
+		testMeterEnergyImpl
+		caps map[reflect.Type]any //nolint:unused
+	}
+
+	v := &directAndCapable{}
+
+	me, ok := Cap[MeterEnergy](v)
+	require.True(t, ok)
+
+	energy, err := me.TotalEnergy()
+	assert.NoError(t, err)
+	assert.Equal(t, 99.0, energy)
+}