Port of Android platform gesture detection code to C++

ui/events/gesture_detection/velocity_tracker.cc

+// 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/gesture_detection/velocity_tracker.h"
+
+#include <cmath>
+
+#include "base/logging.h"
+#include "ui/events/gesture_detection/motion_event.h"
+
+using base::TimeDelta;
+using base::TimeTicks;
+
+namespace ui {
+
+// Implements a particular velocity tracker algorithm.
+class VelocityTrackerStrategy {
+ public:
+  virtual ~VelocityTrackerStrategy() {}
+
+  virtual void Clear() = 0;
+  virtual void ClearPointers(BitSet32 id_bits) = 0;
+  virtual void AddMovement(const base::TimeTicks& event_time,
+                           BitSet32 id_bits,
+                           const Position* positions) = 0;
+  virtual bool GetEstimator(uint32_t id, Estimator* out_estimator) const = 0;
+
+ protected:
+  VelocityTrackerStrategy() {}
+};
+
+namespace {
+
+COMPILE_ASSERT(MotionEvent::MAX_POINTER_ID < 32, max_pointer_id_too_large);
+
+struct Position {
+  float x, y;
+};
+
+struct Estimator {
+  enum { MAX_DEGREE = 4 };
+
+  // Estimator time base.
+  TimeTicks time;
+
+  // Polynomial coefficients describing motion in X and Y.
+  float xcoeff[MAX_DEGREE + 1], ycoeff[MAX_DEGREE + 1];
+
+  // Polynomial degree (number of coefficients), or zero if no information is
+  // available.
+  uint32_t degree;
+
+  // Confidence (coefficient of determination), between 0 (no fit)
+  // and 1 (perfect fit).
+  float confidence;
+
+  inline void Clear() {
+    time = TimeTicks();
+    degree = 0;
+    confidence = 0;
+    for (size_t i = 0; i <= MAX_DEGREE; i++) {
+      xcoeff[i] = 0;
+      ycoeff[i] = 0;
+    }
+  }
+};
+
+// Threshold for determining that a pointer has stopped moving.
+// Some input devices do not send ACTION_MOVE events in the case where a pointer
+// hasstopped.  We need to detect this case so that we can accurately predict
+// the velocity after the pointer starts moving again.
+const TimeDelta ASSUME_POINTER_STOPPED_TIME = TimeDelta::FromMilliseconds(40);
+
+static float VectorDot(const float* a, const float* b, uint32_t m) {
+  float r = 0;
+  while (m--) {
+    r += *(a++) * *(b++);
+  }
+  return r;
+}
+
+static float VectorNorm(const float* a, uint32_t m) {
+  float r = 0;
+  while (m--) {
+    float t = *(a++);
+    r += t * t;
+  }
+  return sqrtf(r);
+}
+
+// Velocity tracker algorithm based on least-squares linear regression.
+class LeastSquaresVelocityTrackerStrategy : public VelocityTrackerStrategy {
+ public:
+  enum Weighting {
+    // No weights applied.  All data points are equally reliable.
+    WEIGHTING_NONE,
+
+    // Weight by time delta.  Data points clustered together are weighted less.
+    WEIGHTING_DELTA,
+
+    // Weight such that points within a certain horizon are weighed more than
+    // those outside of that horizon.
+    WEIGHTING_CENTRAL,
+
+    // Weight such that points older than a certain amount are weighed less.
+    WEIGHTING_RECENT,
+  };
+
+  // Number of samples to keep.
+  enum { HISTORY_SIZE = 20 };
+
+  // Degree must be no greater than Estimator::MAX_DEGREE.
+  LeastSquaresVelocityTrackerStrategy(uint32_t degree,
+                                      Weighting weighting = WEIGHTING_NONE);
+  virtual ~LeastSquaresVelocityTrackerStrategy();
+
+  virtual void Clear() OVERRIDE;
+  virtual void ClearPointers(BitSet32 id_bits) OVERRIDE;
+  virtual void AddMovement(const TimeTicks& event_time,
+                           BitSet32 id_bits,
+                           const Position* positions) OVERRIDE;
+  virtual bool GetEstimator(uint32_t id,
+                            Estimator* out_estimator) const OVERRIDE;
+
+ private:
+  // Sample horizon.
+  // We don't use too much history by default since we want to react to quick
+  // changes in direction.
+  static const TimeDelta HORIZON;
+
+  struct Movement {
+    TimeTicks event_time;
+    BitSet32 id_bits;
+    Position positions[VelocityTracker::MAX_POINTERS];
+
+    inline const Position& GetPosition(uint32_t id) const {
+      return positions[id_bits.get_index_of_bit(id)];
+    }
+  };
+
+  float ChooseWeight(uint32_t index) const;
+
+  const uint32_t degree_;
+  const Weighting weighting_;
+  uint32_t index_;
+  Movement movements_[HISTORY_SIZE];
+};
+
+// Velocity tracker algorithm that uses an IIR filter.
+class IntegratingVelocityTrackerStrategy : public VelocityTrackerStrategy {
+ public:
+  // Degree must be 1 or 2.
+  explicit IntegratingVelocityTrackerStrategy(uint32_t degree);
+  virtual ~IntegratingVelocityTrackerStrategy();
+
+  virtual void Clear() OVERRIDE;
+  virtual void ClearPointers(BitSet32 id_bits) OVERRIDE;
+  virtual void AddMovement(const TimeTicks& event_time,
+                           BitSet32 id_bits,
+                           const Position* positions) OVERRIDE;
+  virtual bool GetEstimator(uint32_t id,
+                            Estimator* out_estimator) const OVERRIDE;
+
+ private:
+  // Current state estimate for a particular pointer.
+  struct State {
+    TimeTicks update_time;
+    uint32_t degree;
+
+    float xpos, xvel, xaccel;
+    float ypos, yvel, yaccel;
+  };
+
+  const uint32_t degree_;
+  BitSet32 pointer_id_bits_;
+  State mPointerState[MotionEvent::MAX_POINTER_ID + 1];
+
+  void InitState(State& state,
+                 const TimeTicks& event_time,
+                 float xpos,
+                 float ypos) const;
+  void UpdateState(State& state,
+                   const TimeTicks& event_time,
+                   float xpos,
+                   float ypos) const;
+  void PopulateEstimator(const State& state, Estimator* out_estimator) const;
+};
+
+VelocityTrackerStrategy* CreateStrategy(VelocityTracker::Strategy strategy) {
+  switch (strategy) {
+    case VelocityTracker::LSQ1:
+      return new LeastSquaresVelocityTrackerStrategy(1);
+    case VelocityTracker::LSQ2:
+      return new LeastSquaresVelocityTrackerStrategy(2);
+    case VelocityTracker::LSQ3:
+      return new LeastSquaresVelocityTrackerStrategy(3);
+    case VelocityTracker::WLSQ2_DELTA:
+      return new LeastSquaresVelocityTrackerStrategy(
+          2, LeastSquaresVelocityTrackerStrategy::WEIGHTING_DELTA);
+    case VelocityTracker::WLSQ2_CENTRAL:
+      return new LeastSquaresVelocityTrackerStrategy(
+          2, LeastSquaresVelocityTrackerStrategy::WEIGHTING_CENTRAL);
+    case VelocityTracker::WLSQ2_RECENT:
+      return new LeastSquaresVelocityTrackerStrategy(
+          2, LeastSquaresVelocityTrackerStrategy::WEIGHTING_RECENT);
+    case VelocityTracker::INT1:
+      return new IntegratingVelocityTrackerStrategy(1);
+    case VelocityTracker::INT2:
+      return new IntegratingVelocityTrackerStrategy(2);
+  }
+  NOTREACHED() << "Unrecognized velocity tracker strategy: " << strategy;
+  return CreateStrategy(VelocityTracker::STRATEGY_DEFAULT);
+}
+
+}  // namespace
+
+// --- VelocityTracker ---
+
+VelocityTracker::VelocityTracker()
+    : current_pointer_id_bits_(0),
+      active_pointer_id_(-1),
+      strategy_(CreateStrategy(STRATEGY_DEFAULT)) {}
+
+VelocityTracker::VelocityTracker(Strategy strategy)
+    : current_pointer_id_bits_(0),
+      active_pointer_id_(-1),
+      strategy_(CreateStrategy(strategy)) {}
+
+VelocityTracker::~VelocityTracker() {}
+
+void VelocityTracker::Clear() {
+  current_pointer_id_bits_.clear();
+  active_pointer_id_ = -1;
+  strategy_->Clear();
+}
+
+void VelocityTracker::ClearPointers(BitSet32 id_bits) {
+  BitSet32 remaining_id_bits(current_pointer_id_bits_.value & ~id_bits.value);
+  current_pointer_id_bits_ = remaining_id_bits;
+
+  if (active_pointer_id_ >= 0 && id_bits.has_bit(active_pointer_id_)) {
+    active_pointer_id_ = !remaining_id_bits.is_empty()
+                             ? remaining_id_bits.first_marked_bit()
+                             : -1;
+  }
+
+  strategy_->ClearPointers(id_bits);
+}
+
+void VelocityTracker::AddMovement(const TimeTicks& event_time,
+                                  BitSet32 id_bits,
+                                  const Position* positions) {
+  while (id_bits.count() > MAX_POINTERS)
+    id_bits.clear_last_marked_bit();
+
+  if ((current_pointer_id_bits_.value & id_bits.value) &&
+      event_time >= (last_event_time_ + ASSUME_POINTER_STOPPED_TIME)) {
+    // We have not received any movements for too long.  Assume that all
+    // pointers
+    // have stopped.
+    strategy_->Clear();
+  }
+  last_event_time_ = event_time;
+
+  current_pointer_id_bits_ = id_bits;
+  if (active_pointer_id_ < 0 || !id_bits.has_bit(active_pointer_id_))
+    active_pointer_id_ = id_bits.is_empty() ? -1 : id_bits.first_marked_bit();
+
+  strategy_->AddMovement(event_time, id_bits, positions);
+}
+
+void VelocityTracker::AddMovement(const MotionEvent& event) {
+  int32_t actionMasked = event.GetAction();
+
+  switch (actionMasked) {
+    case MotionEvent::ACTION_DOWN:
+      // case MotionEvent::HOVER_ENTER:
+      // Clear all pointers on down before adding the new movement.
+      Clear();
+      break;
+    case MotionEvent::ACTION_POINTER_DOWN: {
+      // Start a new movement trace for a pointer that just went down.
+      // We do this on down instead of on up because the client may want to
+      // query the final velocity for a pointer that just went up.
+      BitSet32 downIdBits;
+      downIdBits.mark_bit(event.GetPointerId(event.GetActionIndex()));
+      ClearPointers(downIdBits);
+      break;
+    }
+    case MotionEvent::ACTION_MOVE:
+      // case MotionEvent::ACTION_HOVER_MOVE:
+      break;
+    default:
+      // Ignore all other actions because they do not convey any new information
+      // about pointer movement.  We also want to preserve the last known
+      // velocity of the pointers.
+      // Note that ACTION_UP and ACTION_POINTER_UP always report the last known
+      // position of the pointers that went up.  ACTION_POINTER_UP does include
+      // the new position of pointers that remained down but we will also
+      // receive an ACTION_MOVE with this information if any of them actually
+      // moved.  Since we don't know how many pointers will be going up at once
+      // it makes sense to just wait for the following ACTION_MOVE before adding
+      // the movement.
+      return;
+  }
+
+  size_t pointer_count = event.GetPointerCount();
+  if (pointer_count > MAX_POINTERS) {
+    pointer_count = MAX_POINTERS;
+  }
+
+  BitSet32 id_bits;
+  for (size_t i = 0; i < pointer_count; i++) {
+    id_bits.mark_bit(event.GetPointerId(i));
+  }
+
+  uint32_t pointer_index[MAX_POINTERS];
+  for (size_t i = 0; i < pointer_count; i++) {
+    pointer_index[i] = id_bits.get_index_of_bit(event.GetPointerId(i));
+  }
+
+  Position positions[MAX_POINTERS];
+  size_t historySize = event.GetHistorySize();
+  for (size_t h = 0; h < historySize; h++) {
+    for (size_t i = 0; i < pointer_count; i++) {
+      uint32_t index = pointer_index[i];
+      positions[index].x = event.GetHistoricalX(i, h);
+      positions[index].y = event.GetHistoricalY(i, h);
+    }
+    AddMovement(event.GetHistoricalEventTime(h), id_bits, positions);
+  }
+
+  for (size_t i = 0; i < pointer_count; i++) {
+    uint32_t index = pointer_index[i];
+    positions[index].x = event.GetX(i);
+    positions[index].y = event.GetY(i);
+  }
+  AddMovement(event.GetEventTime(), id_bits, positions);
+}
+
+bool VelocityTracker::GetVelocity(uint32_t id,
+                                  float* out_vx,
+                                  float* out_vy) const {
+  Estimator estimator;
+  if (GetEstimator(id, &estimator) && estimator.degree >= 1) {
+    *out_vx = estimator.xcoeff[1];
+    *out_vy = estimator.ycoeff[1];
+    return true;
+  }
+  *out_vx = 0;
+  *out_vy = 0;
+  return false;
+}
+
+void LeastSquaresVelocityTrackerStrategy::AddMovement(
+    const TimeTicks& event_time,
+    BitSet32 id_bits,
+    const Position* positions) {
+  if (++index_ == HISTORY_SIZE) {
+    index_ = 0;
+  }
+
+  Movement& movement = movements_[index_];
+  movement.event_time = event_time;
+  movement.id_bits = id_bits;
+  uint32_t count = id_bits.count();
+  for (uint32_t i = 0; i < count; i++) {
+    movement.positions[i] = positions[i];
+  }
+}
+
+bool VelocityTracker::GetEstimator(uint32_t id,
+                                   Estimator* out_estimator) const {
+  return strategy_->GetEstimator(id, out_estimator);
+}
+
+// --- LeastSquaresVelocityTrackerStrategy ---
+
+const TimeDelta LeastSquaresVelocityTrackerStrategy::HORIZON =
+    TimeDelta::FromMilliseconds(100);
+
+LeastSquaresVelocityTrackerStrategy::LeastSquaresVelocityTrackerStrategy(
+    uint32_t degree,
+    Weighting weighting)
+    : degree_(degree), weighting_(weighting) {
+  DCHECK_LT(degree_, static_cast<uint32_t>(Estimator::MAX_DEGREE));
+  Clear();
+}
+
+LeastSquaresVelocityTrackerStrategy::~LeastSquaresVelocityTrackerStrategy() {}
+
+void LeastSquaresVelocityTrackerStrategy::Clear() {
+  index_ = 0;
+  movements_[0].id_bits.clear();
+}
+
+/**
+ * Solves a linear least squares problem to obtain a N degree polynomial that
+ * fits the specified input data as nearly as possible.
+ *
+ * Returns true if a solution is found, false otherwise.
+ *
+ * The input consists of two vectors of data points X and Y with indices 0..m-1
+ * along with a weight vector W of the same size.
+ *
+ * The output is a vector B with indices 0..n that describes a polynomial
+ * that fits the data, such the sum of W[i] * W[i] * abs(Y[i] - (B[0] + B[1]
+ * X[i] * + B[2] X[i]^2 ... B[n] X[i]^n)) for all i between 0 and m-1 is
+ * minimized.
+ *
+ * Accordingly, the weight vector W should be initialized by the caller with the
+ * reciprocal square root of the variance of the error in each input data point.
+ * In other words, an ideal choice for W would be W[i] = 1 / var(Y[i]) = 1 /
+ * stddev(Y[i]).
+ * The weights express the relative importance of each data point.  If the
+ * weights are* all 1, then the data points are considered to be of equal
+ * importance when fitting the polynomial.  It is a good idea to choose weights
+ * that diminish the importance of data points that may have higher than usual
+ * error margins.
+ *
+ * Errors among data points are assumed to be independent.  W is represented
+ * here as a vector although in the literature it is typically taken to be a
+ * diagonal matrix.
+ *
+ * That is to say, the function that generated the input data can be
+ * approximated by y(x) ~= B[0] + B[1] x + B[2] x^2 + ... + B[n] x^n.
+ *
+ * The coefficient of determination (R^2) is also returned to describe the
+ * goodness of fit of the model for the given data.  It is a value between 0
+ * and 1, where 1 indicates perfect correspondence.
+ *
+ * This function first expands the X vector to a m by n matrix A such that
+ * A[i][0] = 1, A[i][1] = X[i], A[i][2] = X[i]^2, ..., A[i][n] = X[i]^n, then
+ * multiplies it by w[i]./
+ *
+ * Then it calculates the QR decomposition of A yielding an m by m orthonormal
+ * matrix Q and an m by n upper triangular matrix R.  Because R is upper
+ * triangular (lower part is all zeroes), we can simplify the decomposition into
+ * an m by n matrix Q1 and a n by n matrix R1 such that A = Q1 R1.
+ *
+ * Finally we solve the system of linear equations given by
+ * R1 B = (Qtranspose W Y) to find B.
+ *
+ * For efficiency, we lay out A and Q column-wise in memory because we
+ * frequently operate on the column vectors.  Conversely, we lay out R row-wise.
+ *
+ * http://en.wikipedia.org/wiki/Numerical_methods_for_linear_least_squares
+ * http://en.wikipedia.org/wiki/Gram-Schmidt
+ */
+static bool SolveLeastSquares(const float* x,
+                              const float* y,
+                              const float* w,
+                              uint32_t m,
+                              uint32_t n,
+                              float* out_b,
+                              float* out_det) {
+  // MSVC does not support variable-length arrays (used by the original Android
+  // implementation of this function).
+#if defined(COMPILER_MSVC)
+  enum {
+    M_ARRAY_LENGTH = LeastSquaresVelocityTrackerStrategy::HISTORY_SIZE,
+    N_ARRAY_LENGTH = Estimator::MAX_DEGREE
+  };
+  DCHECK_LE(m, static_cast<uint32_t>(M_ARRAY_LENGTH));
+  DCHECK_LE(n, static_cast<uint32_t>(N_ARRAY_LENGTH));
+#else
+  const uint32_t M_ARRAY_LENGTH = m;
+  const uint32_t N_ARRAY_LENGTH = n;
+#endif
+
+  // Expand the X vector to a matrix A, pre-multiplied by the weights.
+  float a[N_ARRAY_LENGTH][M_ARRAY_LENGTH];  // column-major order
+  for (uint32_t h = 0; h < m; h++) {
+    a[0][h] = w[h];
+    for (uint32_t i = 1; i < n; i++) {
+      a[i][h] = a[i - 1][h] * x[h];
+    }
+  }
+
+  // Apply the Gram-Schmidt process to A to obtain its QR decomposition.
+
+  // Orthonormal basis, column-major order.
+  float q[N_ARRAY_LENGTH][M_ARRAY_LENGTH];
+  // Upper triangular matrix, row-major order.
+  float r[N_ARRAY_LENGTH][N_ARRAY_LENGTH];
+  for (uint32_t j = 0; j < n; j++) {
+    for (uint32_t h = 0; h < m; h++) {
+      q[j][h] = a[j][h];
+    }
+    for (uint32_t i = 0; i < j; i++) {
+      float dot = VectorDot(&q[j][0], &q[i][0], m);
+      for (uint32_t h = 0; h < m; h++) {
+        q[j][h] -= dot * q[i][h];
+      }
+    }
+
+    float norm = VectorNorm(&q[j][0], m);
+    if (norm < 0.000001f) {
+      // vectors are linearly dependent or zero so no solution
+      return false;
+    }
+
+    float invNorm = 1.0f / norm;
+    for (uint32_t h = 0; h < m; h++) {
+      q[j][h] *= invNorm;
+    }
+    for (uint32_t i = 0; i < n; i++) {
+      r[j][i] = i < j ? 0 : VectorDot(&q[j][0], &a[i][0], m);
+    }
+  }
+
+  // Solve R B = Qt W Y to find B.  This is easy because R is upper triangular.
+  // We just work from bottom-right to top-left calculating B's coefficients.
+  float wy[M_ARRAY_LENGTH];
+  for (uint32_t h = 0; h < m; h++) {
+    wy[h] = y[h] * w[h];
+  }
+  for (uint32_t i = n; i-- != 0;) {
+    out_b[i] = VectorDot(&q[i][0], wy, m);
+    for (uint32_t j = n - 1; j > i; j--) {
+      out_b[i] -= r[i][j] * out_b[j];
+    }
+    out_b[i] /= r[i][i];
+  }
+
+  // Calculate the coefficient of determination as 1 - (SSerr / SStot) where
+  // SSerr is the residual sum of squares (variance of the error),
+  // and SStot is the total sum of squares (variance of the data) where each
+  // has been weighted.
+  float ymean = 0;
+  for (uint32_t h = 0; h < m; h++) {
+    ymean += y[h];
+  }
+  ymean /= m;
+
+  float sserr = 0;
+  float sstot = 0;
+  for (uint32_t h = 0; h < m; h++) {
+    float err = y[h] - out_b[0];
+    float term = 1;
+    for (uint32_t i = 1; i < n; i++) {
+      term *= x[h];
+      err -= term * out_b[i];
+    }
+    sserr += w[h] * w[h] * err * err;
+    float var = y[h] - ymean;
+    sstot += w[h] * w[h] * var * var;
+  }
+  *out_det = sstot > 0.000001f ? 1.0f - (sserr / sstot) : 1;
+  return true;
+}
+
+void LeastSquaresVelocityTrackerStrategy::ClearPointers(BitSet32 id_bits) {
+  BitSet32 remaining_id_bits(movements_[index_].id_bits.value & ~id_bits.value);
+  movements_[index_].id_bits = remaining_id_bits;
+}
+
+bool LeastSquaresVelocityTrackerStrategy::GetEstimator(
+    uint32_t id,
+    Estimator* out_estimator) const {
+  out_estimator->Clear();
+
+  // Iterate over movement samples in reverse time order and collect samples.
+  float x[HISTORY_SIZE];
+  float y[HISTORY_SIZE];
+  float w[HISTORY_SIZE];
+  float time[HISTORY_SIZE];
+  uint32_t m = 0;
+  uint32_t index = index_;
+  const Movement& newest_movement = movements_[index_];
+  do {
+    const Movement& movement = movements_[index];
+    if (!movement.id_bits.has_bit(id))
+      break;
+
+    TimeDelta age = newest_movement.event_time - movement.event_time;
+    if (age > HORIZON)
+      break;
+
+    const Position& position = movement.GetPosition(id);
+    x[m] = position.x;
+    y[m] = position.y;
+    w[m] = ChooseWeight(index);
+    time[m] = -age.InSecondsF();
+    index = (index == 0 ? HISTORY_SIZE : index) - 1;
+  } while (++m < HISTORY_SIZE);
+
+  if (m == 0)
+    return false;  // no data
+
+  // Calculate a least squares polynomial fit.
+  uint32_t degree = degree_;
+  if (degree > m - 1)
+    degree = m - 1;
+
+  if (degree >= 1) {
+    float xdet, ydet;
+    uint32_t n = degree + 1;
+    if (SolveLeastSquares(time, x, w, m, n, out_estimator->xcoeff, &xdet) &&
+        SolveLeastSquares(time, y, w, m, n, out_estimator->ycoeff, &ydet)) {
+      out_estimator->time = newest_movement.event_time;
+      out_estimator->degree = degree;
+      out_estimator->confidence = xdet * ydet;
+      return true;
+    }
+  }
+
+  // No velocity data available for this pointer, but we do have its current
+  // position.
+  out_estimator->xcoeff[0] = x[0];
+  out_estimator->ycoeff[0] = y[0];
+  out_estimator->time = newest_movement.event_time;
+  out_estimator->degree = 0;
+  out_estimator->confidence = 1;
+  return true;
+}
+
+float LeastSquaresVelocityTrackerStrategy::ChooseWeight(uint32_t index) const {
+  switch (weighting_) {
+    case WEIGHTING_DELTA: {
+      // Weight points based on how much time elapsed between them and the next
+      // point so that points that "cover" a shorter time span are weighed less.
+      //   delta  0ms: 0.5
+      //   delta 10ms: 1.0
+      if (index == index_) {
+        return 1.0f;
+      }
+      uint32_t next_index = (index + 1) % HISTORY_SIZE;
+      float delta_millis =
+          static_cast<float>((movements_[next_index].event_time -
+                              movements_[index].event_time).InMillisecondsF());
+      if (delta_millis < 0)
+        return 0.5f;
+      if (delta_millis < 10)
+        return 0.5f + delta_millis * 0.05;
+
+      return 1.0f;
+    }
+
+    case WEIGHTING_CENTRAL: {
+      // Weight points based on their age, weighing very recent and very old
+      // points less.
+      //   age  0ms: 0.5
+      //   age 10ms: 1.0
+      //   age 50ms: 1.0
+      //   age 60ms: 0.5
+      float age_millis =
+          static_cast<float>((movements_[index_].event_time -
+                              movements_[index].event_time).InMillisecondsF());
+      if (age_millis < 0)
+        return 0.5f;
+      if (age_millis < 10)
+        return 0.5f + age_millis * 0.05;
+      if (age_millis < 50)
+        return 1.0f;
+      if (age_millis < 60)
+        return 0.5f + (60 - age_millis) * 0.05;
+
+      return 0.5f;
+    }
+
+    case WEIGHTING_RECENT: {
+      // Weight points based on their age, weighing older points less.
+      //   age   0ms: 1.0
+      //   age  50ms: 1.0
+      //   age 100ms: 0.5
+      float age_millis =
+          static_cast<float>((movements_[index_].event_time -
+                              movements_[index].event_time).InMillisecondsF());
+      if (age_millis < 50) {
+        return 1.0f;
+      }
+      if (age_millis < 100) {
+        return 0.5f + (100 - age_millis) * 0.01f;
+      }
+      return 0.5f;
+    }
+
+    case WEIGHTING_NONE:
+    default:
+      return 1.0f;
+  }
+}
+
+// --- IntegratingVelocityTrackerStrategy ---
+
+IntegratingVelocityTrackerStrategy::IntegratingVelocityTrackerStrategy(
+    uint32_t degree)
+    : degree_(degree) {
+  DCHECK_LT(degree_, static_cast<uint32_t>(Estimator::MAX_DEGREE));
+}
+
+IntegratingVelocityTrackerStrategy::~IntegratingVelocityTrackerStrategy() {}
+
+void IntegratingVelocityTrackerStrategy::Clear() { pointer_id_bits_.clear(); }
+
+void IntegratingVelocityTrackerStrategy::ClearPointers(BitSet32 id_bits) {
+  pointer_id_bits_.value &= ~id_bits.value;
+}
+
+void IntegratingVelocityTrackerStrategy::AddMovement(
+    const TimeTicks& event_time,
+    BitSet32 id_bits,
+    const Position* positions) {
+  uint32_t index = 0;
+  for (BitSet32 iter_id_bits(id_bits); !iter_id_bits.is_empty();) {
+    uint32_t id = iter_id_bits.clear_first_marked_bit();
+    State& state = mPointerState[id];
+    const Position& position = positions[index++];
+    if (pointer_id_bits_.has_bit(id))
+      UpdateState(state, event_time, position.x, position.y);
+    else
+      InitState(state, event_time, position.x, position.y);
+  }
+
+  pointer_id_bits_ = id_bits;
+}
+
+bool IntegratingVelocityTrackerStrategy::GetEstimator(
+    uint32_t id,
+    Estimator* out_estimator) const {
+  out_estimator->Clear();
+
+  if (pointer_id_bits_.has_bit(id)) {
+    const State& state = mPointerState[id];
+    PopulateEstimator(state, out_estimator);
+    return true;
+  }
+
+  return false;
+}
+
+void IntegratingVelocityTrackerStrategy::InitState(State& state,
+                                                   const TimeTicks& event_time,
+                                                   float xpos,
+                                                   float ypos) const {
+  state.update_time = event_time;
+  state.degree = 0;
+  state.xpos = xpos;
+  state.xvel = 0;
+  state.xaccel = 0;
+  state.ypos = ypos;
+  state.yvel = 0;
+  state.yaccel = 0;
+}
+
+void IntegratingVelocityTrackerStrategy::UpdateState(
+    State& state,
+    const TimeTicks& event_time,
+    float xpos,
+    float ypos) const {
+  const base::TimeDelta MIN_TIME_DELTA = TimeDelta::FromMicroseconds(2);
+  const float FILTER_TIME_CONSTANT = 0.010f;  // 10 milliseconds
+
+  if (event_time <= state.update_time + MIN_TIME_DELTA)
+    return;
+
+  float dt = static_cast<float>((event_time - state.update_time).InSecondsF());
+  state.update_time = event_time;
+
+  float xvel = (xpos - state.xpos) / dt;
+  float yvel = (ypos - state.ypos) / dt;
+  if (state.degree == 0) {
+    state.xvel = xvel;
+    state.yvel = yvel;
+    state.degree = 1;
+  } else {
+    float alpha = dt / (FILTER_TIME_CONSTANT + dt);
+    if (degree_ == 1) {
+      state.xvel += (xvel - state.xvel) * alpha;
+      state.yvel += (yvel - state.yvel) * alpha;
+    } else {
+      float xaccel = (xvel - state.xvel) / dt;
+      float yaccel = (yvel - state.yvel) / dt;
+      if (state.degree == 1) {
+        state.xaccel = xaccel;
+        state.yaccel = yaccel;
+        state.degree = 2;
+      } else {
+        state.xaccel += (xaccel - state.xaccel) * alpha;
+        state.yaccel += (yaccel - state.yaccel) * alpha;
+      }
+      state.xvel += (state.xaccel * dt) * alpha;
+      state.yvel += (state.yaccel * dt) * alpha;
+    }
+  }
+  state.xpos = xpos;
+  state.ypos = ypos;
+}
+
+void IntegratingVelocityTrackerStrategy::PopulateEstimator(
+    const State& state,
+    Estimator* out_estimator) const {
+  out_estimator->time = state.update_time;
+  out_estimator->confidence = 1.0f;
+  out_estimator->degree = state.degree;
+  out_estimator->xcoeff[0] = state.xpos;
+  out_estimator->xcoeff[1] = state.xvel;
+  out_estimator->xcoeff[2] = state.xaccel / 2;
+  out_estimator->ycoeff[0] = state.ypos;
+  out_estimator->ycoeff[1] = state.yvel;
+  out_estimator->ycoeff[2] = state.yaccel / 2;
+}
+
+}  // namespace ui