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diff --git a/cc/input/page_scale_animation.h b/cc/input/page_scale_animation.h
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+// Copyright 2011 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.
+
+#ifndef CC_INPUT_PAGE_SCALE_ANIMATION_H_
+#define CC_INPUT_PAGE_SCALE_ANIMATION_H_
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "ui/gfx/size.h"
+#include "ui/gfx/vector2d_f.h"
+
+namespace cc {
+
+// A small helper class that does the math for zoom animations, primarily for
+// double-tap zoom. Initialize it with starting and ending scroll/page scale
+// positions and an animation length time, then call ...AtTime() at every frame
+// to obtain the current interpolated position.
+//
+// All sizes and vectors in this class's public methods are in the root scroll
+// layer's coordinate space.
+class PageScaleAnimation {
+ public:
+  // Construct with the state at the beginning of the animation.
+  static scoped_ptr<PageScaleAnimation> Create(
+      gfx::Vector2dF start_scroll_offset,
+      float start_page_scale_factor,
+      gfx::SizeF viewport_size,
+      gfx::SizeF root_layer_size,
+      double start_time);
+
+  ~PageScaleAnimation();
+
+  // The following methods initialize the animation. Call one of them
+  // immediately after construction to set the final scroll and page scale.
+
+  // Zoom while explicitly specifying the top-left scroll position.
+  void ZoomTo(gfx::Vector2dF target_scroll_offset,
+              float target_page_scale_factor,
+              double duration);
+
+  // Zoom based on a specified anchor. The animator will attempt to keep it
+  // at the same position on the physical display throughout the animation,
+  // unless the edges of the root layer are hit. The anchor is specified
+  // as an offset from the content layer.
+  void ZoomWithAnchor(gfx::Vector2dF anchor,
+                      float target_page_scale_factor,
+                      double duration);
+
+  // Call these functions while the animation is in progress to output the
+  // current state.
+  gfx::Vector2dF ScrollOffsetAtTime(double time) const;
+  float PageScaleFactorAtTime(double time) const;
+  bool IsAnimationCompleteAtTime(double time) const;
+
+  // The following methods return state which is invariant throughout the
+  // course of the animation.
+  double start_time() const { return start_time_; }
+  double duration() const { return duration_; }
+  double end_time() const { return start_time_ + duration_; }
+  gfx::Vector2dF target_scroll_offset() const { return target_scroll_offset_; }
+  float target_page_scale_factor() const { return target_page_scale_factor_; }
+
+ protected:
+  PageScaleAnimation(gfx::Vector2dF start_scroll_offset,
+                     float start_page_scale_factor,
+                     gfx::SizeF viewport_size,
+                     gfx::SizeF root_layer_size,
+                     double start_time);
+
+ private:
+  void ClampTargetScrollOffset();
+  void InferTargetScrollOffsetFromStartAnchor();
+  void InferTargetAnchorFromScrollOffsets();
+
+  gfx::SizeF StartViewportSize() const;
+  gfx::SizeF TargetViewportSize() const;
+  float InterpAtTime(double time) const;
+  gfx::SizeF ViewportSizeAt(float interp) const;
+  gfx::Vector2dF ScrollOffsetAt(float interp) const;
+  gfx::Vector2dF AnchorAt(float interp) const;
+  gfx::Vector2dF ViewportRelativeAnchorAt(float interp) const;
+  float PageScaleFactorAt(float interp) const;
+
+  float start_page_scale_factor_;
+  float target_page_scale_factor_;
+  gfx::Vector2dF start_scroll_offset_;
+  gfx::Vector2dF target_scroll_offset_;
+
+  gfx::Vector2dF start_anchor_;
+  gfx::Vector2dF target_anchor_;
+
+  gfx::SizeF viewport_size_;
+  gfx::SizeF root_layer_size_;
+
+  double start_time_;
+  double duration_;
+
+  DISALLOW_COPY_AND_ASSIGN(PageScaleAnimation);
+};
+
+}  // namespace cc
+
+#endif  // CC_INPUT_PAGE_SCALE_ANIMATION_H_