Simple implementation of polygons, you can create them in the init and draw them

Makefile

@@ -11,7 +11,7 @@ CXXFLAGS=-Wall -g -O2 $(PROFILING_CFLAGS) $(GTK_CFLAGS)
 LIBS=$(GTK_LIBS) -lm
 
 PROGS=balls
-OBJS=balls.o c_index.o game.o gravity.o spaceship.o main.o
+OBJS=balls.o c_index.o game.o gravity.o spaceship.o main.o polygons.o
 
 # dependencies (gcc -MM *.cc)
 balls.o: balls.cc game.h balls.h vec2d.h gravity.h
@@ -24,7 +24,7 @@ stats.o: stats.cc
 
 .PHONY: run
 run: balls
-	./balls
+	./balls n=0
 
 .PHONY: all
 all: $(PROGS)

polygons.cc

@@ -0,0 +1,31 @@
+#include "polygons.h"
+
+#include <iostream>
+
+polygon* polygons = nullptr;
+uint n_polygons = 1;
+
+void polygons_init_state() {
+    polygons = new polygon[n_polygons];
+
+    polygon poly =
+        polygon{{400, 400}, 0, {{0, 0}, {0, 100}, {100, 100}, {100, 0}}};
+    polygons[0] = poly;
+}
+
+void polygon::draw(cairo_t* cr) const {
+    polygon& poly = polygons[0];
+    vec2d& center = poly.center;
+    cairo_set_source_rgb(cr, 1, 1, 1);
+
+    cairo_move_to(cr, center.x, center.y);
+    for (auto& point : poly.points)
+        cairo_line_to(cr, center.x + point.x, center.y + point.y);
+    cairo_line_to(cr, center.x, center.y);
+    cairo_stroke(cr);
+}
+
+void polygons_draw(cairo_t* cr) {
+    for (const polygon* p = polygons; p != polygons + n_polygons; ++p)
+        p->draw(cr);
+}

polygons.h

@@ -0,0 +1,28 @@
+#ifndef POLYGONS_H_INCLUDED
+#define POLYGONS_H_INCLUDED
+
+// extern ball spaceship;
+
+#include "vec2d.h"
+
+#include <gtk/gtk.h>
+#include <vector>
+
+class polygon {
+  public:
+    vec2d center;
+    double angle;
+    std::vector<vec2d> points;
+
+    void draw(cairo_t* cr) const;
+};
+
+extern polygon* polygons;
+extern uint n_polygons;
+
+extern void polygons_init_state();
+extern void polygons_update_state();
+// extern void polygon_control (double dx, double dy);
+extern void polygons_draw(cairo_t* cr);
+
+#endif