1 files changed, 101 insertions, 0 deletions
diff --git a/gfx/layers/AxisPhysicsModel.cpp b/gfx/layers/AxisPhysicsModel.cpp
new file mode 100644
index 0000000000..dc0cba3110
--- /dev/null
+++ b/gfx/layers/AxisPhysicsModel.cpp
@@ -0,0 +1,101 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AxisPhysicsModel.h"
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * The simulation is advanced forward in time with a fixed time step to ensure
+ * that it remains deterministic given variable framerates.  To determine the
+ * position at any variable time, two samples are interpolated.
+ *
+ * kFixedtimestep is set to 120hz in order to ensure that every frame in a
+ * common 60hz refresh rate display will have at least one physics simulation
+ * sample.  More accuracy can be obtained by reducing kFixedTimestep to smaller
+ * intervals, such as 240hz or 1000hz, at the cost of more CPU cycles.  If
+ * kFixedTimestep is increased to much longer intervals, interpolation will
+ * become less effective at reducing temporal jitter and the simulation will
+ * lose accuracy.
+ */
+const double AxisPhysicsModel::kFixedTimestep = 1.0 / 120.0;  // 120hz
+
+/**
+ * Constructs an AxisPhysicsModel with initial values for state.
+ *
+ * @param aInitialPosition sets the initial position of the simulation,
+ *        in AppUnits.
+ * @param aInitialVelocity sets the initial velocity of the simulation,
+ *        in AppUnits / second.
+ */
+AxisPhysicsModel::AxisPhysicsModel(double aInitialPosition,
+                                   double aInitialVelocity)
+    : mProgress(1.0),
+      mPrevState(aInitialPosition, aInitialVelocity),
+      mNextState(aInitialPosition, aInitialVelocity) {}
+
+AxisPhysicsModel::~AxisPhysicsModel() = default;
+
+double AxisPhysicsModel::GetVelocity() const {
+  return LinearInterpolate(mPrevState.v, mNextState.v, mProgress);
+}
+
+double AxisPhysicsModel::GetPosition() const {
+  return LinearInterpolate(mPrevState.p, mNextState.p, mProgress);
+}
+
+void AxisPhysicsModel::SetVelocity(double aVelocity) {
+  mNextState.v = aVelocity;
+  mNextState.p = GetPosition();
+  mProgress = 1.0;
+}
+
+void AxisPhysicsModel::SetPosition(double aPosition) {
+  mNextState.v = GetVelocity();
+  mNextState.p = aPosition;
+  mProgress = 1.0;
+}
+
+void AxisPhysicsModel::Simulate(const TimeDuration& aDeltaTime) {
+  for (mProgress += aDeltaTime.ToSeconds() / kFixedTimestep; mProgress > 1.0;
+       mProgress -= 1.0) {
+    Integrate(kFixedTimestep);
+  }
+}
+
+void AxisPhysicsModel::Integrate(double aDeltaTime) {
+  mPrevState = mNextState;
+
+  // RK4 (Runge-Kutta method) Integration
+  // http://en.wikipedia.org/wiki/Runge%E2%80%93Kutta_methods
+  Derivative a = Evaluate(mNextState, 0.0, Derivative());
+  Derivative b = Evaluate(mNextState, aDeltaTime * 0.5, a);
+  Derivative c = Evaluate(mNextState, aDeltaTime * 0.5, b);
+  Derivative d = Evaluate(mNextState, aDeltaTime, c);
+
+  double dpdt = 1.0 / 6.0 * (a.dp + 2.0 * (b.dp + c.dp) + d.dp);
+  double dvdt = 1.0 / 6.0 * (a.dv + 2.0 * (b.dv + c.dv) + d.dv);
+
+  mNextState.p += dpdt * aDeltaTime;
+  mNextState.v += dvdt * aDeltaTime;
+}
+
+AxisPhysicsModel::Derivative AxisPhysicsModel::Evaluate(
+    const State& aInitState, double aDeltaTime, const Derivative& aDerivative) {
+  State state(aInitState.p + aDerivative.dp * aDeltaTime,
+              aInitState.v + aDerivative.dv * aDeltaTime);
+
+  return Derivative(state.v, Acceleration(state));
+}
+
+double AxisPhysicsModel::LinearInterpolate(double aV1, double aV2,
+                                           double aBlend) {
+  return aV1 * (1.0 - aBlend) + aV2 * aBlend;
+}
+
+}  // namespace layers
+}  // namespace mozilla