From 1546beac329d017b86331d0acd2dd0dd86101506 Mon Sep 17 00:00:00 2001
From: rodri <rgl@antares-labs.eu>
Date: Fri, 12 Mar 2021 11:37:09 +0000
Subject: implemented fixed-time dynamics stepper, animated sprites and cleaned
 things up.

---
 physics.c | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 100 insertions(+)
 create mode 100644 physics.c

(limited to 'physics.c')

diff --git a/physics.c b/physics.c
new file mode 100644
index 0000000..5f45fd9
--- /dev/null
+++ b/physics.c
@@ -0,0 +1,100 @@
+#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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