Compute snap offsets according to CSS Scroll Snap Spec Level 1

third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h

Index: third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h b/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h
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+// Copyright 2016 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 SnapCoordinator_h
+#define SnapCoordinator_h
+
+#include "core/layout/LayoutBox.h"
+#include "platform/heap/Handle.h"
+#include "wtf/Vector.h"
+
+namespace blink {
+
+class ComputedStyle;
+class Element;
+struct LengthPoint;
+
+// Snap Coordinator keeps track of snap containers and all of their associated
+// snap areas. It also contains the logic to generate the list of valid snap
+// positions for a given snap container.
+//
+// Snap container:
+//   An scroll container that has 'scroll-snap-type' value other
+//   than 'none'.
+// Snap area:
+//   A snap container's descendant that contributes snap positions. An element
+//   only contributes snap positions to its nearest ancestor (on the element’s
+//   containing block chain) scroll container.
+//
+// For more information see spec: https://drafts.csswg.org/css-snappoints/
+class CORE_EXPORT SnapCoordinator final : public GarbageCollectedFinalized<SnapCoordinator> {
+    WTF_MAKE_NONCOPYABLE(SnapCoordinator);
+
+public:
+    static SnapCoordinator* create();
+    ~SnapCoordinator();
+    DEFINE_INLINE_TRACE() {}
+
+    void snapContainerDidChange(LayoutBox&, ScrollSnapType);
+    void snapAreaDidChange(LayoutBox&, const Vector<LengthPoint>& snapCoordinates);
+
+#ifndef NDEBUG
+    void showSnapAreaMap();
+    void showSnapAreasFor(const LayoutBox*);
+#endif
+
+private:
+    friend class SnapCoordinatorTest;
+    explicit SnapCoordinator();
+
+    Vector<double> snapOffsets(const ContainerNode&, ScrollbarOrientation);
+
+    HashSet<const LayoutBox*> m_snapContainers;
+};
+
+} // namespace blink
+
+#endif // SnapCoordinator_h