diff --git a/backend/Backend.h b/backend/Backend.h
index 6aca247..fc9a733 100644
--- a/backend/Backend.h
+++ b/backend/Backend.h
@@ -29,15 +29,15 @@
                                       uint32_t *num_requests);
   virtual std::tuple<int, size_t> GetClientLayers(
       DrmHwcTwo::HwcDisplay *display,
-      const std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map);
+      const std::vector<DrmHwcTwo::HwcLayer *> &layers);
   virtual bool IsClientLayer(DrmHwcTwo::HwcDisplay *display,
                              DrmHwcTwo::HwcLayer *layer);
 
  protected:
   bool HardwareSupportsLayerType(HWC2::Composition comp_type);
-  uint32_t CalcPixOps(const std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map,
+  uint32_t CalcPixOps(const std::vector<DrmHwcTwo::HwcLayer *> &layers,
                       size_t first_z, size_t size);
-  void MarkValidated(std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map,
+  void MarkValidated(std::vector<DrmHwcTwo::HwcLayer *> &layers,
                      size_t client_first_z, size_t client_size);
   std::tuple<int, int> GetExtraClientRange(
       DrmHwcTwo::HwcDisplay *display,