- if(pixel_hit_in_square(*sq, x, y)) return true;
- if(l > 0 && pixel_hit_in_square(*(sq-1), x, y)) return true;
- if(t > 0 && pixel_hit_in_square(*(sq-gw), x, y)) return true;
- if(l > 0 && t > 0 && pixel_hit_in_square(*(sq-1-gw), x, y)) return true;
- return false;
+ if((ret = pixel_hit_in_square(*sq, x, y))) return ret;
+ if(l > 0 && (ret = pixel_hit_in_square(*(sq-1), x, y))) return ret;
+ if(t > 0 && (ret = pixel_hit_in_square(*(sq-gw), x, y))) return ret;
+ if(l > 0 && t > 0 && (ret = pixel_hit_in_square(*(sq-1-gw), x, y))) return ret;
+ return 0;
+}
+
+
+float
+sprite_mass(Sprite *s)
+{
+ if(s->type == SHIP) return s->area;
+ else if(s->type == ROCK) return 3 * s->area;
+ else return 0;
+}
+
+void
+bounce(Sprite *a, Sprite *b)
+{
+ float x, y, n;
+ float va, vb, vc;
+ float ma, mb;
+
+ // (x, y) is unit vector pointing from A's center to B's center.
+ x = (b->x + b->w / 2) - (a->x + a->w / 2);
+ y = (b->y + b->h / 2) - (a->y + a->h / 2);
+ n = sqrt(x*x + y*y); x /= n; y /= n;
+
+ // velocities along (x, y), or 0 if already moving away.
+ va = x*a->dx + y*a->dy;
+ vb = x*b->dx + y*b->dy;
+ if(vb-va > 0) return;
+
+ ma = sprite_mass(a); mb = sprite_mass(b);
+ vc = (va*ma + vb*mb) / (ma+mb);
+
+ a->dx += 2*x*(vc-va); a->dy += 2*y*(vc-va);
+ b->dx += 2*x*(vc-vb); b->dy += 2*y*(vc-vb);