Port sky-scrollable to Dart

sky/framework/fling-curve.dart

+// Copyright 2015 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.
+
+import "dart:math" as math;
+
+const double _kDefaultAlpha = -5707.62;
+const double _kDefaultBeta = 172.0;
+const double _kDefaultGamma = 3.7;
+
+double _positionAtTime(double t) {
+  return kDefaultAlpha * math.exp(-kDefaultGamma * t) - kDefaultBeta * t - kDefaultAlpha;
+}
+
+double _velocityAtTime(double t) {
+  return -kDefaultAlpha * kDefaultGamma * math.exp(-kDefaultGamma * t) - kDefaultBeta;
+}
+
+double _timeAtVelocity(double v) {
+  return -math.log((v + kDefaultBeta) / (-kDefaultAlpha * kDefaultGamma)) / kDefaultGamma;
+}
+
+final double _kMaxVelocity = _velocityAtTime(0.0);
+final double _kCurveDuration = _timeAtVelocity(0.0);
+
+class FlingCurve {
+  double _timeOffset;
+  double _positionOffset;
+  double _startTime;
+  double _previousPosition;
+  double _direction;
+
+  FlingCurve(double velocity, double startTime) {
+    double startingVelocity = math.min(_kMaxVelocity, velocity.abs());
+    _timeOffset = _velocityAtTime(startingVelocity);
+    _positionOffset = _positionAtTime(_timeOffset);
+    _startTime = startTime / 1000.0;
+    _previousPosition = 0.0;
+    _direction = velocity.sign;
+  }
+
+  double update(double timeStamp) {
+    double t = timeStamp / 1000.0 - _startTime + _timeOffset;
+    if (t >= _kCurveDuration)
+      return 0.0;
+    double position = _positionAtTime(t) - _positionOffset;
+    double positionDelta = position - _previousPosition;
+    _previousPosition = position;
+    return _direction * math.max(0.0, positionDelta);
+  }
+}