Don't persist Z2M state with "zero" values

When storing Z2M state, skip properties with default "zero" values. This
should skip devices and properties that were not updated via MQTT yet.

bridge.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"log"
 	"net/http"
+	"reflect"
 	"strings"
 	"sync"
 	"time"
@@ -196,6 +197,11 @@ func (br *Bridge) saveZ2MState() error {
 		devState := make(map[string]any)
 
 		for prop, mapping := range dev.Mappings {
+			// don't bother persisting property if it is "zero"
+			if reflect.ValueOf(mapping.Characteristic.Val).IsZero() {
+				continue
+			}
+
 			v, err := mapping.ToExposedValue(mapping.Characteristic.Val)
 			if err != nil {
 				return err
@@ -205,6 +211,7 @@ func (br *Bridge) saveZ2MState() error {
 		}
 
 		// serialize into JSON
+		if len(devState) > 0 {
 			jsonState, err := json.Marshal(devState)
 			if err != nil {
 				return err
@@ -212,6 +219,12 @@ func (br *Bridge) saveZ2MState() error {
 
 			devices[name] = jsonState
 		}
+	}
+
+	// return early if there was nothing to persist
+	if len(devices) == 0 {
+		return nil
+	}
 
 	allJson, err := json.Marshal(devices)
 	if err != nil {

bridge_test.go

@@ -31,6 +31,15 @@ const ContactSensorTemplate = `
 			}
 		}`
 
+func fileSize(path string) (int64, error) {
+	fi, err := os.Stat(path)
+	sz := int64(0)
+	if err == nil {
+		sz = fi.Size()
+	}
+	return sz, err
+}
+
 func TestBridgePersistState(t *testing.T) {
 	dir, err := os.MkdirTemp("", "hapz2m-bridge*")
 	if err != nil {
@@ -64,6 +73,27 @@ func TestBridgePersistState(t *testing.T) {
 		t.Errorf("can't persist state: %v", err)
 	}
 
+	storeFname := dir + string(os.PathSeparator) + Z2M_STATE_STORE
+
+	if sz, err := fileSize(storeFname); err == nil && sz != 0 {
+		t.Errorf("expecting defaults to not be persisted, but got size %d, err %v", sz, err)
+	}
+
+	// alter sensor states to non-defaults
+	for _, dev := range b.devices {
+		dev.Mappings["contact"].Characteristic.Val = 1
+	}
+
+	// save 2 devices, again
+	t.Logf("persisting state with non-defaults")
+	if err := b.saveZ2MState(); err != nil {
+		t.Errorf("can't persist state: %v", err)
+	}
+
+	if sz, err := fileSize(storeFname); err != nil || sz == 0 {
+		t.Errorf("expecting state to be persisted, but got size %d, err %v", sz, err)
+	}
+
 	// re-create with less devices
 	b2 := NewBridge(ctx, dir)
 	b2.AddDevicesFromJSON(fmt.Appendf(nil, "[%s]", s1))