circles, triangles and rotations for primitive shapes

helper.c

@@ -1,3 +1,5 @@
+#include <math.h>
+
 #include "helper.h"
 
 Matrix4x4 identity() {
@@ -11,6 +13,34 @@ Matrix4x4 identity() {
     return result;
 }
 
+void mult_4x4(Matrix4x4 *multiplicant, Matrix4x4 *multiplier) {
+  Matrix4x4 result = identity();
+
+  for (int i = 0; i < 4; i++) {
+    for (int j = 0; j < 4; j++) {
+      result.m[i][j] = 0;
+      for (int k = 0; k < 4; k++) {
+        result.m[i][j] += multiplicant->m[i][k] * multiplier->m[k][j];
+      }
+    }
+  }
+
+  *multiplicant = result;
+}
+
+// TODO: rotate around the center of the element
+// currently we rotate around the 0, 0 point of the element
+void apply_rotation(Matrix4x4 *model, float angle) {
+  Matrix4x4 ident = identity();
+
+  ident.m[0][0] = cos(angle);
+  ident.m[0][1] = sin(angle);
+  ident.m[1][0] = -sin(angle);
+  ident.m[1][1] = cos(angle);
+
+  mult_4x4(model, &ident);
+}
+
 Matrix4x4 orthographic_projection_2d(float left, float right, float bottom, float top) {
     Matrix4x4 result = identity();
 
@@ -41,3 +71,7 @@ float clampf(float value, float min, float max) {
 
     return value;
 }
+
+float to_radians(float degrees) {
+  return degrees * (M_PI / 180.f);
+}