athena: Add support for fling-scroll in overview mode.

ui/events/fling_curve.cc

Index: ui/events/fling_curve.cc
diff --git a/ui/events/fling_curve.cc b/ui/events/fling_curve.cc
new file mode 100644
index 0000000000000000000000000000000000000000..e433143f11dff6a8322d83b986277d0a185d88e3
--- /dev/null
+++ b/ui/events/fling_curve.cc
@@ -0,0 +1,80 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/events/fling_curve.h"
+
+#include <algorithm>
+#include <cmath>
+
+#include "base/logging.h"
+
+namespace {
+
+const float kDefaultAlpha = -5.70762e+03f;
+const float kDefaultBeta = 1.72e+02f;
+const float kDefaultGamma = 3.7e+00f;
+
+inline double GetPositionAtTime(double t) {
+  return kDefaultAlpha * exp(-kDefaultGamma * t) - kDefaultBeta * t -
+         kDefaultAlpha;
+}
+
+inline double GetVelocityAtTime(double t) {
+  return -kDefaultAlpha * kDefaultGamma * exp(-kDefaultGamma * t) -
+         kDefaultBeta;
+}
+
+inline double GetTimeAtVelocity(double v) {
+  return -log((v + kDefaultBeta) / (-kDefaultAlpha * kDefaultGamma)) /
+         kDefaultGamma;
+}
+
+}  // namespace
+
+namespace ui {
+
+FlingCurve::FlingCurve(const gfx::Vector2dF& velocity,
+                       base::TimeTicks start_timestamp)
+    : curve_duration_(GetTimeAtVelocity(0)),
+      start_timestamp_(start_timestamp),
+      time_offset_(0),
+      position_offset_(0) {
+  float max_start_velocity = std::max(fabs(velocity.x()), fabs(velocity.y()));
+  if (max_start_velocity > GetVelocityAtTime(0))
+    max_start_velocity = GetVelocityAtTime(0);
+  CHECK_GT(max_start_velocity, 0);
+
+  displacement_ratio_ = gfx::Vector2dF(velocity.x() / max_start_velocity,
+                                       velocity.y() / max_start_velocity);
+  time_offset_ = GetTimeAtVelocity(max_start_velocity);
+  position_offset_ = GetPositionAtTime(time_offset_);
+  last_timestamp_ = start_timestamp_ + base::TimeDelta::FromSecondsD(
+                                           curve_duration_ - time_offset_);
+}
+
+FlingCurve::~FlingCurve() {
+}
+
+gfx::Vector2dF FlingCurve::GetScrollAmountAtTime(base::TimeTicks current) {
+  if (current < start_timestamp_)
+    return gfx::Vector2dF();
+
+  float displacement = 0;
+  if (current < last_timestamp_) {
+    float time = time_offset_ + (current - start_timestamp_).InSecondsF();
+    CHECK_LT(time, curve_duration_);
+    displacement = GetPositionAtTime(time) - position_offset_;
+  } else {
+    displacement = GetPositionAtTime(curve_duration_) - position_offset_;
+  }
+
+  gfx::Vector2dF scroll(displacement * displacement_ratio_.x(),
+                        displacement * displacement_ratio_.y());
+  gfx::Vector2dF scroll_increment(scroll.x() - cumulative_scroll_.x(),
+                                  scroll.y() - cumulative_scroll_.y());
+  cumulative_scroll_ = scroll;
+  return scroll_increment;
+}
+
+}  // namespace ui