perf(go): deduplicate unchanged mileage samples

go/vehicle-gateway/internal/stats/daily_metric_test.go

@@ -149,6 +149,59 @@ func TestWriterEnsuresSchemaAndUpsertsDailyMileage(t *testing.T) {
 	}
 }
 
+func TestWriterSkipsConsecutiveDuplicateMileageSamples(t *testing.T) {
+	exec := &recordingExec{}
+	writer := NewWriter(exec, time.FixedZone("Asia/Shanghai", 8*3600))
+	event := envelope.FrameEnvelope{
+		Protocol:    envelope.ProtocolJT808,
+		VIN:         "LNBVIN00000000001",
+		EventTimeMS: time.Date(2026, 7, 1, 9, 0, 0, 0, time.FixedZone("Asia/Shanghai", 8*3600)).UnixMilli(),
+		Fields: map[string]any{
+			envelope.FieldTotalMileageKM: 10241.2,
+		},
+	}
+
+	if err := writer.Append(context.Background(), event); err != nil {
+		t.Fatalf("first Append() error = %v", err)
+	}
+	event.ReceivedAtMS += 1000
+	if err := writer.Append(context.Background(), event); err != nil {
+		t.Fatalf("duplicate Append() error = %v", err)
+	}
+
+	if len(exec.calls) != 1 {
+		t.Fatalf("exec calls = %d, want 1", len(exec.calls))
+	}
+}
+
+func TestWriterKeepsOnlyLatestDateInMileageCache(t *testing.T) {
+	exec := &recordingExec{}
+	writer := NewWriter(exec, time.FixedZone("Asia/Shanghai", 8*3600))
+	event := envelope.FrameEnvelope{
+		Protocol: envelope.ProtocolJT808,
+		VIN:      "LNBVIN00000000001",
+		Fields: map[string]any{
+			envelope.FieldTotalMileageKM: 10241.2,
+		},
+	}
+
+	event.EventTimeMS = time.Date(2026, 7, 1, 23, 59, 0, 0, time.FixedZone("Asia/Shanghai", 8*3600)).UnixMilli()
+	if err := writer.Append(context.Background(), event); err != nil {
+		t.Fatalf("first Append() error = %v", err)
+	}
+	event.EventTimeMS = time.Date(2026, 7, 2, 0, 1, 0, 0, time.FixedZone("Asia/Shanghai", 8*3600)).UnixMilli()
+	if err := writer.Append(context.Background(), event); err != nil {
+		t.Fatalf("next-day Append() error = %v", err)
+	}
+
+	if len(exec.calls) != 2 {
+		t.Fatalf("exec calls = %d, want 2", len(exec.calls))
+	}
+	if len(writer.lastTotalMileage) != 1 {
+		t.Fatalf("cache entries = %d, want 1", len(writer.lastTotalMileage))
+	}
+}
+
 type execCall struct {
 	query string
 	args  []any