/* -*- 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()

{

}

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