1// Copyright 2014 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#include "ui/events/gestures/fling_curve.h"
6
7#include <algorithm>
8#include <cmath>
9
10#include "base/logging.h"
11
12namespace {
13
14const float kDefaultAlpha = -5.70762e+03f;
15const float kDefaultBeta = 1.72e+02f;
16const float kDefaultGamma = 3.7e+00f;
17
18inline double GetPositionAtTime(double t) {
19  return kDefaultAlpha * exp(-kDefaultGamma * t) - kDefaultBeta * t -
20         kDefaultAlpha;
21}
22
23inline double GetVelocityAtTime(double t) {
24  return -kDefaultAlpha * kDefaultGamma * exp(-kDefaultGamma * t) -
25         kDefaultBeta;
26}
27
28inline double GetTimeAtVelocity(double v) {
29  return -log((v + kDefaultBeta) / (-kDefaultAlpha * kDefaultGamma)) /
30         kDefaultGamma;
31}
32
33}  // namespace
34
35namespace ui {
36
37FlingCurve::FlingCurve(const gfx::Vector2dF& velocity,
38                       base::TimeTicks start_timestamp)
39    : curve_duration_(GetTimeAtVelocity(0)),
40      start_timestamp_(start_timestamp),
41      time_offset_(0),
42      position_offset_(0) {
43  float max_start_velocity = std::max(fabs(velocity.x()), fabs(velocity.y()));
44  if (max_start_velocity > GetVelocityAtTime(0))
45    max_start_velocity = GetVelocityAtTime(0);
46  CHECK_GT(max_start_velocity, 0);
47
48  displacement_ratio_ = gfx::Vector2dF(velocity.x() / max_start_velocity,
49                                       velocity.y() / max_start_velocity);
50  time_offset_ = GetTimeAtVelocity(max_start_velocity);
51  position_offset_ = GetPositionAtTime(time_offset_);
52  last_timestamp_ = start_timestamp_ + base::TimeDelta::FromSecondsD(
53                                           curve_duration_ - time_offset_);
54}
55
56FlingCurve::~FlingCurve() {
57}
58
59gfx::Vector2dF FlingCurve::GetScrollAmountAtTime(base::TimeTicks current) {
60  if (current < start_timestamp_)
61    return gfx::Vector2dF();
62
63  float displacement = 0;
64  if (current < last_timestamp_) {
65    float time = time_offset_ + (current - start_timestamp_).InSecondsF();
66    CHECK_LT(time, curve_duration_);
67    displacement = GetPositionAtTime(time) - position_offset_;
68  } else {
69    displacement = GetPositionAtTime(curve_duration_) - position_offset_;
70  }
71
72  gfx::Vector2dF scroll(displacement * displacement_ratio_.x(),
73                        displacement * displacement_ratio_.y());
74  gfx::Vector2dF scroll_increment(scroll.x() - cumulative_scroll_.x(),
75                                  scroll.y() - cumulative_scroll_.y());
76  cumulative_scroll_ = scroll;
77  return scroll_increment;
78}
79
80}  // namespace ui
81