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#include <u.h>
#include <libc.h>
#include <draw.h>
#include "dat.h"
#include "fns.h"
//enum { DYNTIME, RENTIME, NSTATS };
//Stats simstats[NSTATS];
/*
* Dynamics stepper
*
* Currently set to a basic spring-damper system.
*/
static double
accel(GameState *s, double t)
{
static double k = 15, b = 0.1;
USED(t);
return -k*s->x - b*s->v;
}
static Derivative
eval(GameState *s0, double t, double Δt, Derivative *d)
{
GameState s;
Derivative res;
s.x = s0->x + d->dx*Δt;
s.v = s0->v + d->dv*Δt;
res.dx = s.v;
res.dv = accel(&s, t+Δt);
return res;
}
/*
* Explicit Euler Integrator
*/
static void
euler0(GameState *s, double t, double Δt)
{
static Derivative ZD = {0,0};
Derivative d;
d = eval(s, t, Δt, &ZD);
s->x += d.dx*Δt;
s->v += d.dv*Δt;
}
/*
* Semi-implicit Euler Integrator
*/
static void
euler1(GameState *s, double t, double Δt)
{
static Derivative ZD = {0,0};
Derivative d;
d = eval(s, t, Δt, &ZD);
s->v += d.dv*Δt;
s->x += s->v*Δt;
}
/*
* RK4 Integrator
*/
static void
rk4(GameState *s, double t, double Δt)
{
static Derivative ZD = {0,0};
Derivative a, b, c, d;
double dxdt, dvdt;
a = eval(s, t, 0, &ZD);
b = eval(s, t, Δt/2, &a);
c = eval(s, t, Δt/2, &b);
d = eval(s, t, Δt, &c);
dxdt = 1.0/6 * (a.dx + 2*(b.dx + c.dx) + d.dx);
dvdt = 1.0/6 * (a.dv + 2*(b.dv + c.dv) + d.dv);
s->x += dxdt*Δt;
s->v += dvdt*Δt;
}
/*
* The Integrator
*/
void
integrate(GameState *s, double t, double Δt)
{
//euler0(s, t, Δt);
//euler1(s, t, Δt);
rk4(s, t, Δt);
}
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