Implement asteroid bouncing

game.c

@@ -261,6 +261,24 @@ static unsigned check_collisions(collision_t *out)
 	return count;
 }
 
+static void bounce_asteroids(entity_t *a, entity_t *b)
+{
+	const float ma = a->radius * a->radius;
+	const float mb = b->radius * b->radius;
+	const float m = ma + mb;
+
+	const vec2_t n = vec2_norm(vec2_sub(b->pos, a->pos));
+
+	const float va1 = vec2_dot(a->vel, n);
+	const float vb1 = vec2_dot(b->vel, n);
+
+	const float va2 = (va1 * (ma - mb) + 2 * mb * vb1) / m;
+	const float vb2 = (vb1 * (mb - ma) + 2 * ma * va1) / m;
+
+	a->vel = vec2_add(a->vel, vec2_scale(n, va2 - va1));
+	b->vel = vec2_add(b->vel, vec2_scale(n, vb2 - vb1));
+}
+
 static void handle_collisions(const collision_t *collisions, unsigned count)
 {
 	for (unsigned i = 0; i < count; ++i) {
@@ -276,7 +294,10 @@ static void handle_collisions(const collision_t *collisions, unsigned count)
 		if (a->tag == COLLISION_SHOT && b->tag == COLLISION_SHOT)
 			continue;
 
-		if (a->tag == COLLISION_ASTEROID && b->tag == COLLISION_ASTEROID) { }
+		if (a->tag == COLLISION_ASTEROID && b->tag == COLLISION_ASTEROID) {
+			bounce_asteroids(a, b);
+			continue;
+		}
 
 		if (a->tag == COLLISION_SHOT) {
 			a->dead = true;

maths.c

@@ -27,6 +27,20 @@ vec3_t vec2_extend(vec2_t v)
 	return (vec3_t) { v.x, v.y, 1 };
 }
 
+vec2_t vec2_norm(vec2_t v)
+{
+	const float l = vec2_len(v);
+	return (vec2_t) {
+		v.x / l,
+		v.y / l,
+	};
+}
+
+float vec2_dot(vec2_t v1, vec2_t v2)
+{
+	return v1.x * v2.x + v1.y * v2.y;
+}
+
 mat2_t mat2_rotation(float theta)
 {
 	return (mat2_t) {

maths.h

@@ -24,6 +24,8 @@ vec2_t vec2_add(vec2_t v1, vec2_t v2);
 vec2_t vec2_sub(vec2_t v1, vec2_t v2);
 vec2_t vec2_scale(vec2_t v, float s);
 vec3_t vec2_extend(vec2_t v);
+vec2_t vec2_norm(vec2_t v);
+float vec2_dot(vec2_t v1, vec2_t v2);
 
 mat2_t mat2_rotation(float theta);
 vec2_t mat2_mul_vec2(mat2_t m, vec2_t v);