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diff --git a/src/third_party/s2/s2region.h b/src/third_party/s2/s2region.h
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+// Copyright 2005 Google Inc. All Rights Reserved.
+
+#ifndef UTIL_GEOMETRY_S2REGION_H_
+#define UTIL_GEOMETRY_S2REGION_H_
+
+#include "s2.h"
+
+class Decoder;
+class Encoder;
+
+class S2Cap;
+class S2Cell;
+class S2LatLngRect;
+
+// An S2Region represents a two-dimensional region over the unit sphere.
+// It is an abstract interface with various concrete subtypes.
+//
+// The main purpose of this interface is to allow complex regions to be
+// approximated as simpler regions.  So rather than having a wide variety
+// of virtual methods that are implemented by all subtypes, the interface
+// is restricted to methods that are useful for computing approximations.
+class S2Region {
+ public:
+  virtual ~S2Region();
+
+  // Return a deep copy of this region.  If you want to narrow the result to a
+  // specific known region type, use down_cast<T*> from basictypes.h.
+  // Subtypes return pointers to that subtype from their Clone() methods.
+  virtual S2Region* Clone() const = 0;
+
+  // Return a bounding spherical cap. This is not guaranteed to be exact.
+  virtual S2Cap GetCapBound() const = 0;
+
+  // Return a bounding latitude-longitude rectangle that contains the region.
+  // The bounds are not guaranteed to be tight.
+  virtual S2LatLngRect GetRectBound() const = 0;
+
+  // If this method returns true, the region completely contains the given
+  // cell.  Otherwise, either the region does not contain the cell or the
+  // containment relationship could not be determined.
+  virtual bool Contains(S2Cell const& cell) const = 0;
+
+  // If this method returns false, the region does not intersect the given
+  // cell.  Otherwise, either region intersects the cell, or the intersection
+  // relationship could not be determined.
+  virtual bool MayIntersect(S2Cell const& cell) const = 0;
+
+  // Return true if and only if the given point is contained by the region.
+  // The point 'p' is generally required to be unit length, although some
+  // subtypes may relax this restriction.
+  // NOTE: If you will be calling this function on one specific subtype only,
+  // or if performance is a consideration, please use the non-virtual
+  // method Contains(S2Point const& p) declared below!
+  virtual bool VirtualContainsPoint(S2Point const& p) const = 0;
+
+  // Use encoder to generate a serialized representation of this region.
+  // Assumes that encoder can be enlarged using calls to Ensure(int).
+  //
+  // The representation chosen is left up to the sub-classes but it should
+  // satisfy the following constraints:
+  // - It should encode a version number.
+  // - It should be deserializable using the corresponding Decode method.
+  // - Performance, not space, should be the chief consideration. Encode() and
+  //   Decode() should be implemented such that the combination is equivalent
+  //   to calling Clone().
+  virtual void Encode(Encoder* const encoder) const = 0;
+
+  // Reconstruct a region from the serialized representation generated by
+  // Encode(). Note that since this method is virtual, it requires that a
+  // Region object of the appropriate concrete type has already been
+  // constructed. It is not possible to decode regions of unknown type.
+  //
+  // Whenever the Decode method is changed to deal with new serialized
+  // representations, it should be done so in a manner that allows for
+  // older versions to be decoded i.e. the version number in the serialized
+  // representation should be used to decide how to decode the data.
+  //
+  // Returns true on success.
+  virtual bool Decode(Decoder* const decoder) = 0;
+
+  // Provide the same functionality as Decode, except that decoded regions are
+  // allowed to point directly into the Decoder's memory buffer rather than
+  // copying the data.  This method can be much faster for regions that have
+  // a lot of data (such as polygons), but the decoded region is only valid
+  // within the scope (lifetime) of the Decoder's memory buffer.
+  // Default implementation just calls Decode.
+  virtual bool DecodeWithinScope(Decoder* const decoder);
+
+  /////////////////////////////////////////////////////////////////////////
+  // The following are NON-VIRTUAL methods (for efficiency reasons) that
+  // happen to be implemented by all subclasses.  You cannot call these
+  // methods unless you have an object of a particular subtype.
+  //
+  // bool Contains(S2Point const& p) const;
+  //
+  // Return true if and only if the given point is contained by the region.
+  // The point 'p' is generally required to be unit length, although some
+  // subtypes may relax this restriction.
+};
+
+#endif  // UTIL_GEOMETRY_S2REGION_H_