/* -*- 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/. */

#ifndef mozilla_layers_DesktopFlingPhysics_h_

#define mozilla_layers_DesktopFlingPhysics_h_

#include "AsyncPanZoomController.h"

#include "Units.h"

#include "gfxPrefs.h"

#include "mozilla/Assertions.h"

#define FLING_PHYS_LOG(...)

// #define FLING_PHYS_LOG(...) printf_stderr("FLING: " __VA_ARGS__)

namespace mozilla {

namespace layers {

class DesktopFlingPhysics {

public:

  void Init(const ParentLayerPoint& aStartingVelocity, float aPLPPI /* unused */)

  {

    mVelocity = aStartingVelocity;

  }

  void Sample(const TimeDuration& aDelta,

              ParentLayerPoint* aOutVelocity,

              ParentLayerPoint* aOutOffset)

  {

    float friction = gfxPrefs::APZFlingFriction();

    float threshold = gfxPrefs::APZFlingStoppedThreshold();

    mVelocity = ParentLayerPoint(

        ApplyFrictionOrCancel(mVelocity.x, aDelta, friction, threshold),

        ApplyFrictionOrCancel(mVelocity.y, aDelta, friction, threshold));

    *aOutVelocity = mVelocity;

    *aOutOffset = mVelocity * aDelta.ToMilliseconds();

  }

private:

  /**

   * Applies friction to the given velocity and returns the result, or

   * returns zero if the velocity is too low.

   * |aVelocity| is the incoming velocity.

   * |aDelta| is the amount of time that has passed since the last time

   * friction was applied.

   * |aFriction| is the amount of friction to apply.

   * |aThreshold| is the velocity below which the fling is cancelled.

   */

  static float ApplyFrictionOrCancel(float aVelocity, const TimeDuration& aDelta,

                                     float aFriction, float aThreshold)

  {

    if (fabsf(aVelocity) <= aThreshold) {

      // If the velocity is very low, just set it to 0 and stop the fling,

      // otherwise we'll just asymptotically approach 0 and the user won't

      // actually see any changes.

      return 0.0f;

    }

    aVelocity *= pow(1.0f - aFriction, float(aDelta.ToMilliseconds()));

    return aVelocity;

  }

  ParentLayerPoint mVelocity;

};

} // namespace layers

} // namespace mozilla

#endif // mozilla_layers_DesktopFlingPhysics_h_