Move source to src.

src/room_force.cpp

@@ -0,0 +1,20 @@
+// -*- C++ -*-
+
+#include "room_force.h"
+#include "physics/particle.h"
+#include "physics/vector.h"
+
+using namespace mbostock;
+
+ConstantRoomForce::ConstantRoomForce(const Vector& min, const Vector& max,
+                                     const Vector& f)
+    : box_(min, max), force_(f) {
+}
+
+const Shape& ConstantRoomForce::shape() const {
+  return box_;
+}
+
+Vector ConstantRoomForce::force(const Particle& p) {
+  return box_.contains(p.position) ? force_ : Vector::ZERO();
+}