add comments and clear_framebuffer

Makefile

@@ -10,4 +10,7 @@ build: $(SRC)
 run: build
 	$(OUT)
 
+clean:
+	rm $(OUT)
+
 .PHONY: build run

src/main.c

@@ -10,22 +10,38 @@
 
 uint32_t framebuffer[WIDTH * HEIGHT];
 
+// alter a pixel at an imaginary x & y coordinates
 void put_pixel(unsigned int x, unsigned int y, uint32_t color) {
+	if (x < 0 || x >= WIDTH || y < 0 || y >= HEIGHT) {
+		fprintf(stderr, "CRITICAL: attempted buffer overflow in put_pixel\n"
+			"at x = %d, y = %d", x, y);
+		return;
+	}
+
 	framebuffer[WIDTH * y + x] = color;
 }
 
-int main() {
-	if (SDL_Init(SDL_INIT_VIDEO) != 0) {
-		fprintf(stderr, "SDL_Init failed! %s\n", SDL_GetError());
-		return EXIT_FAILURE;
+// fill the entire framebuffer with a given color
+void clear_framebuffer(uint32_t color) {
+	for (int i = 0; i < WIDTH * HEIGHT; i++) {
+		framebuffer[i] = color;
 	}
+}
 
+int main() {
 	SDL_Window *window;
 	SDL_Renderer *renderer;
 	SDL_Texture *texture;
 	SDL_Event event;
 	bool is_running = true;
 
+	const double target_frame = 1.0 / 60.0;
+
+	if (SDL_Init(SDL_INIT_VIDEO) != 0) {
+		fprintf(stderr, "SDL_Init failed! %s\n", SDL_GetError());
+		return EXIT_FAILURE;
+	}
+
 	window = SDL_CreateWindow(
 		"SDL Framebuffer", 0, 0,
 		WIDTH, HEIGHT, 0
@@ -57,24 +73,51 @@ int main() {
 		return EXIT_FAILURE;
 	}
 
-	for (int i = 0; i < 20; i++) {
-		for (int j = 0; j < 20; j++) {
-			put_pixel(i, j, 0x00FF00);
-		}
-	}
+	bool left = true;
 
 	while (is_running) {
+		uint64_t start = SDL_GetPerformanceCounter();
+
 		while (SDL_PollEvent(&event)) {
 			if (event.type == SDL_QUIT) {
 				is_running = false;
 			}
 		}
 
+		// draw alternating green squares
+		if (left) {
+			clear_framebuffer(0x000000);
+			for (int i = 0; i < 20; i++) {
+				for (int j = 0; j < 20; j++) {
+					put_pixel(i, j, 0x00FF00);
+				}
+			}
+			left = false;
+		} else {
+			clear_framebuffer(0x000000);
+			for (int i = 20; i < 40; i++) {
+				for (int j = 20; j < 40; j++) {
+					put_pixel(i, j, 0x00FFFF);
+				}
+			}
+			left = true;
+		}
+		SDL_Delay(100);
+
 		SDL_UpdateTexture(texture, NULL, framebuffer, WIDTH * sizeof(uint32_t));
 
 		SDL_RenderClear(renderer);
 		SDL_RenderCopy(renderer, texture, NULL, NULL);
 		SDL_RenderPresent(renderer);
+
+		uint64_t end = SDL_GetPerformanceCounter();
+
+		double elapsed = (double)(end - start) / (double)SDL_GetPerformanceFrequency();
+
+		// cap the framerate at '1.0/target_frame' fps
+		if (elapsed < target_frame) {
+			SDL_Delay((target_frame - elapsed) * 1000);
+		}
 	}
 
 	SDL_DestroyTexture(texture);