/*
 * Copyright 2016 Google Inc. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "cobalt/renderer/smoothed_value.h"

#include <algorithm>

namespace cobalt {
namespace renderer {

SmoothedValue::SmoothedValue(base::TimeDelta time_to_converge)
      : time_to_converge_(time_to_converge), previous_derivative_(0) {
  DCHECK(base::TimeDelta() < time_to_converge_);
}

void SmoothedValue::SetTarget(double target, const base::TimeTicks& time) {
  // Determine the current derivative and value.
  double current_derivative = GetCurrentDerivative(time);
  base::optional<double> current_value;
  if (target_) {
    current_value = GetValueAtTime(time);
  }

  // Set the previous derivative and value to the current derivative and value.
  previous_derivative_ = current_derivative;
  previous_value_ = current_value;

  target_ = target;
  target_set_time_ = time;
}

void SmoothedValue::SnapToTarget() {
  previous_value_ = base::nullopt;
  previous_derivative_ = 0;
}

double SmoothedValue::GetValueAtTime(const base::TimeTicks& time) const {
  if (!previous_value_) {
    // If only one target has ever been set, simply return it.
    return *target_;
  }

  // Compute the current value based off of a cubic bezier curve.
  double t = SmoothedValue::t(time);
  double one_minus_t = 1 - t;
  double P0 = SmoothedValue::P0();
  double P1 = SmoothedValue::P1();
  double P2 = SmoothedValue::P2();
  double P3 = SmoothedValue::P3();

  return one_minus_t * one_minus_t * one_minus_t * P0 +
         3 * one_minus_t * one_minus_t * t * P1 + 3 * one_minus_t * t * t * P2 +
         t * t * t * P3;
}

double SmoothedValue::t(const base::TimeTicks& time) const {
  DCHECK(target_) << "SetTarget() must have been called previously.";

  base::TimeDelta time_diff = time - target_set_time_;

  double t = time_diff.InMillisecondsF() / time_to_converge_.InMillisecondsF();

  DCHECK_LE(0, t);

  return std::max(std::min(t, 1.0), 0.0);
}

double SmoothedValue::P1() const {
  // See comments in header for why P1() is calculated this way.
  return *previous_value_ + previous_derivative_ / 3.0f;
}

double SmoothedValue::P2() const {
  // See comments in header for why P2() is calculated this way.
  return P3();
}

double SmoothedValue::GetCurrentDerivative(const base::TimeTicks& time) const {
  if (!previous_value_) {
    // If only one target has ever been set, return 0 as our derivative.
    return 0;
  }

  double t = SmoothedValue::t(time);
  double one_minus_t = 1 - t;
  double P0 = SmoothedValue::P0();
  double P1 = SmoothedValue::P1();
  double P2 = SmoothedValue::P2();
  double P3 = SmoothedValue::P3();

  return 3 * one_minus_t * one_minus_t * (P1 - P0) +
         6 * one_minus_t * t * (P2 - P1) + 3 * t * t * (P3 - P2);
}

}  // namespace renderer
}  // namespace cobalt