shithub: musw

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use semi-implicit euler for bullet dynamics.

--- a/physics.c

+++ b/physics.c

@@ -136,6 +136,18 @@

 }

 static void

+euler1u(Universe *u, Particle *p, double t, double Δt)

+{

+	static Derivative ZD = {0};

+	Derivative d;

+

+	d = evalu(u, p, t, Δt, &ZD);

+

+	p->v = addpt2(p->v, mulpt2(d.dv, Δt));

+	p->p = addpt2(p->p, mulpt2(p->v, Δt));

+}

+

+static void

 rk4u(Universe *u, Particle *p, double t, double Δt, Point2 (*acc)(Universe*,Particle*,double))

 {

 	static Derivative ZD = {0};

@@ -163,6 +175,6 @@

 		rk4u(u, &u->ships[i], t, Δt, accelship);

 		for(j = 0; j < nelem(u->ships[i].rounds); j++)

 			if(u->ships[i].rounds[j].fired)

-				rk4u(u, &u->ships[i].rounds[j], t, Δt, accelbullet);

+				euler1u(u, &u->ships[i].rounds[j], t, Δt, accelbullet);

 	}

 }