From 22526c1b90ca2b8de53475a9bb74558e4a4d8d70 Mon Sep 17 00:00:00 2001
From: Pablo Rodriguez <prodriguez@noreply.git.rodlop.net>
Date: Fri, 14 Mar 2025 17:35:31 -0400
Subject: [PATCH] display now only draws chipxels that have changed

---
 DisplaySDL.cpp | 72 +++++++++++++++++++++++++++++++++-----------------
 DisplaySDL.hpp |  4 ++-
 2 files changed, 51 insertions(+), 25 deletions(-)

diff --git a/DisplaySDL.cpp b/DisplaySDL.cpp
index 4aff847..5692ac1 100644
--- a/DisplaySDL.cpp
+++ b/DisplaySDL.cpp
@@ -1,10 +1,13 @@
 #include "DisplaySDL.hpp"
 #include "Peripherals.hpp"
 
+#include <SDL2/SDL_surface.h>
 #include <stdexcept>
 
 DisplaySDL::DisplaySDL(int w, int h, uint32_t fgColor, uint32_t bgColor):
-  mpFramebuffer{std::make_unique<Framebuffer>()} {
+  mpFramebuffer{std::make_unique<Framebuffer>()},
+  mpDisplayState{std::make_unique<Framebuffer>()},
+  mDoClear{true} {
   // Create SDL Window
   mpWindow = SDL_CreateWindow(
     "ChocoChip-8",
@@ -41,44 +44,65 @@ void DisplaySDL::clear() {
   for(auto &scanline : *mpFramebuffer) {
     scanline.reset();
   }
-  SDL_FillRect(SDL_GetWindowSurface(mpWindow), nullptr, mBgColor);
+  mDoClear = true;
 }
 
 int DisplaySDL::blit(const chocochip8::Scanline &spriteScanline, int y) {
   using chocochip8::Scanline;
   Scanline &targetScanline = mpFramebuffer->at(y);
-  Scanline bitsToSet = spriteScanline & ~targetScanline;
-  Scanline bitsToReset = spriteScanline & targetScanline;
+  bool collision = (spriteScanline & targetScanline).any();
   targetScanline ^= spriteScanline;
+  return collision;
+}
 
+void DisplaySDL::updateWindow(bool doWindowUpdate) const {
+  using chocochip8::Scanline;
   SDL_Surface *pSurface = SDL_GetWindowSurface(mpWindow);
-  SDL_Rect rect;
-  // Map framebuffer pixel to SDL surface rectangle
-  auto mapChipxelToPixel = [&rect, pSurface](int x, int y) {
-      // Note that the MSB of an scanline's bitset is the leftmost pixel.
-      x = (chocochip8::gcWidth - 1) - x;
-      int x1 = (      x * pSurface->w) / chocochip8::gcWidth;
-      int x2 = ((x + 1) * pSurface->w) / chocochip8::gcWidth;
+  if(mDoClear) {
+    for(auto &scanline : *mpDisplayState) {
+      scanline.reset();
+    }
+    SDL_FillRect(pSurface, NULL, mBgColor);
+    mDoClear = false;
+    doWindowUpdate = true;
+  }
+
+  for(int y = 0; y < mpFramebuffer->size(); y++) {
+    Scanline &rNewScanline = (*mpFramebuffer)[y];
+    Scanline &rOldScanline = (*mpDisplayState)[y];
+    if(rNewScanline == rOldScanline) {
+      // Skip scanlines that haven't changed since the last update
+      continue;
+    }
+
+    for(int x = 0; x < rNewScanline.size(); x++) {
+      bool isSet = rNewScanline._Unchecked_test(x);
+      bool wasSet = rOldScanline._Unchecked_test(x);
+      if(isSet == wasSet) {
+        continue;
+      }
+
+      // Map framebuffer pixel to SDL surface rectangle,
+      // note that the MSB of an scanline's bitset is the leftmost pixel.
+      int z = (chocochip8::gcWidth - 1) - x;
+      int x1 = (      z * pSurface->w) / chocochip8::gcWidth;
+      int x2 = ((z + 1) * pSurface->w) / chocochip8::gcWidth;
       int y1 = (      y * pSurface->h) / chocochip8::gcHeight;
       int y2 = ((y + 1) * pSurface->h) / chocochip8::gcHeight;
+      SDL_Rect rect;
       rect.x = x1;
       rect.y = y1;
       rect.w = x2 - x1;
       rect.h = y2 - y1;
-  };
 
-  for(int x = bitsToReset._Find_first(); x < bitsToReset.size(); x = bitsToReset._Find_next(x)) {
-    mapChipxelToPixel(x, y);
-    SDL_FillRect(pSurface, &rect, mBgColor);
-  }
-  for(int x = bitsToSet._Find_first(); x < bitsToSet.size(); x = bitsToSet._Find_next(x)) {
-    mapChipxelToPixel(x, y);
-    SDL_FillRect(pSurface, &rect, mFgColor);
+      Uint32 color = (isSet ? mFgColor : mBgColor);
+      SDL_FillRect(pSurface, &rect, color);
+      doWindowUpdate = true;
+    }
   }
 
-  return bitsToReset.any();
-}
-
-void DisplaySDL::updateWindow() const {
-  SDL_UpdateWindowSurface(mpWindow);
+  if(doWindowUpdate) {
+    *mpDisplayState = *mpFramebuffer;
+    SDL_UpdateWindowSurface(mpWindow);
+  }
 }
diff --git a/DisplaySDL.hpp b/DisplaySDL.hpp
index ad47384..9c7f366 100644
--- a/DisplaySDL.hpp
+++ b/DisplaySDL.hpp
@@ -13,12 +13,14 @@ public:
   ~DisplaySDL() override;
   void clear() override;
   int blit(const chocochip8::Scanline &scanline, int y) override;
-  void updateWindow() const;
+  void updateWindow(bool forceWindowUpdate = false) const;
 
 private:
   using Framebuffer = std::array<chocochip8::Scanline, chocochip8::gcHeight>;
   std::unique_ptr<Framebuffer> mpFramebuffer;
+  mutable std::unique_ptr<Framebuffer> mpDisplayState;
   SDL_Window *mpWindow;
   Uint32 mFgColor;
   Uint32 mBgColor;
+  mutable bool mDoClear;
 };