optimized display update region

Peripherals.hpp

@@ -20,11 +20,32 @@ namespace chocochip8 {
 
   class Display {
   public:
-    friend class Interpreter;
-    Display(): mpFramebuffer{std::make_unique<Framebuffer>()} {}
+    Display():
+      mpFramebuffer{std::make_unique<Framebuffer>()},
+      miTopDirtyScanline{0},
+      miBottomDirtyScanline{gcHeight - 1} {}
+
     virtual ~Display() = default;
+
+    virtual void clear() {
+      for(auto &scanline : *mpFramebuffer) {
+        scanline.reset();
+      }
+      miTopDirtyScanline = 0;
+      miBottomDirtyScanline = gcHeight - 1;
+    }
+
+    virtual Scanline& getModifyableScanline(size_t y) {
+      Scanline& res = mpFramebuffer->at(y);
+      miTopDirtyScanline = std::min(miTopDirtyScanline, y);
+      miBottomDirtyScanline = std::max(miBottomDirtyScanline, y);
+      return res;
+    }
+
   protected:
     std::unique_ptr<Framebuffer> mpFramebuffer;
+    mutable size_t miTopDirtyScanline;
+    mutable size_t miBottomDirtyScanline;
   };
 
   class Buzzer {