Initial commit.

physics/particle.cpp

@@ -0,0 +1,37 @@
+// -*- C++ -*-
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "particle.h"
+
+using namespace mbostock;
+
+Particle::Particle() : inverseMass(1.f) {
+}
+
+Vector Particle::velocity() const {
+  /* Note: ignores force * (inverseMass * timeStep / 2). */
+  return (position - previousPosition) / ParticleSimulator::timeStep();
+}
+
+ParticleSimulator::ParticleSimulator()
+    : drag_(1.f) {
+}
+
+ParticleSimulator::ParticleSimulator(float drag) 
+    : drag_(drag) {
+}
+
+float ParticleSimulator::timeStep() {
+  return .003f; // TODO .005 and interpolate
+}
+
+void ParticleSimulator::step(Particle& p) const {
+  static const float timeStepSquared = timeStep() * timeStep();
+
+  Vector p0 = p.previousPosition;
+  p.previousPosition = p.position;
+  p.position += (p.position - p0) * drag_
+      + p.force * (p.inverseMass * timeStepSquared);
+}