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diff --git a/src/DHTBucketTree.h b/src/DHTBucketTree.h
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+/* <!-- copyright */
+/*
+ * aria2 - The high speed download utility
+ *
+ * Copyright (C) 2010 Tatsuhiro Tsujikawa
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * In addition, as a special exception, the copyright holders give
+ * permission to link the code of portions of this program with the
+ * OpenSSL library under certain conditions as described in each
+ * individual source file, and distribute linked combinations
+ * including the two.
+ * You must obey the GNU General Public License in all respects
+ * for all of the code used other than OpenSSL.  If you modify
+ * file(s) with this exception, you may extend this exception to your
+ * version of the file(s), but you are not obligated to do so.  If you
+ * do not wish to do so, delete this exception statement from your
+ * version.  If you delete this exception statement from all source
+ * files in the program, then also delete it here.
+ */
+/* copyright --> */
+#ifndef D_DHT_BUCKET_TREE_H
+#define D_DHT_BUCKET_TREE_H
+
+#include "common.h"
+
+#include <vector>
+
+#include "SharedHandle.h"
+#include "DHTConstants.h"
+
+namespace aria2 {
+
+class DHTBucket;
+class DHTNode;
+
+// This class represents Kademlia DHT routing tree.  The leaf nodes
+// have bucket which contains DHT node.  The tree is binary tree but
+// highly unbalanced.
+class DHTBucketTreeNode {
+public:
+  // Ctor for internal node
+  DHTBucketTreeNode(DHTBucketTreeNode* left, DHTBucketTreeNode* right);
+  // Ctor for leaf node
+  DHTBucketTreeNode(const SharedHandle<DHTBucket>& bucket);
+  ~DHTBucketTreeNode();
+  // Returns child node, left or right, which contains key.  If dig is
+  // called against leaf node, then returns 0.
+  DHTBucketTreeNode* dig(const unsigned char* key);
+  bool isInRange(const unsigned char* key) const;
+  // Returns true iff this is a leaf node.
+  bool leaf() const { return bucket_; }
+  const unsigned char* getMaxId() const { return maxId_; }
+  const unsigned char* getMinId() const { return minId_; }
+  DHTBucketTreeNode* getParent() const { return parent_; }
+  DHTBucketTreeNode* getLeft() const { return left_; }
+  DHTBucketTreeNode* getRight() const { return right_; }
+  const SharedHandle<DHTBucket>& getBucket() const { return bucket_; }
+  // Splits this object's bucket using DHTBucket::split() and create
+  // left and right child node to hold buckets. The bucket of current
+  // node is reseted so this node becomes internal node after this
+  // call.
+  void split();
+private:
+  // Reset relation of children and minId_ and maxId_.
+  void resetRelation();
+  void setParent(DHTBucketTreeNode* parent) { parent_ = parent; }
+  DHTBucketTreeNode* parent_;
+  DHTBucketTreeNode* left_;
+  DHTBucketTreeNode* right_;
+  SharedHandle<DHTBucket> bucket_;
+  unsigned char minId_[DHT_ID_LENGTH];
+  unsigned char maxId_[DHT_ID_LENGTH];
+};
+
+namespace dht {
+
+// Returns leaf node where key fits between node's min and max ID
+// range.
+DHTBucketTreeNode* findTreeNodeFor
+(DHTBucketTreeNode* root, const unsigned char* key);
+
+// Returns bucket where key fits between bucket's min and max ID
+// range. This function first use findTreeNodeFor and returns its
+// bucket_.
+SharedHandle<DHTBucket> findBucketFor
+(DHTBucketTreeNode* root, const unsigned char* key);
+
+// Stores most closest K nodes against key in nodes. K is
+// DHTBucket::K. This function may returns less than K nodes because
+// the routing tree contains less than K nodes. The order of nodes is
+// arbitrary.  Caller must pass empty nodes.
+void findClosestKNodes
+(std::vector<SharedHandle<DHTNode> >& nodes,
+ DHTBucketTreeNode* root,
+ const unsigned char* key);
+
+// Stores all buckets in buckets.
+void enumerateBucket
+(std::vector<SharedHandle<DHTBucket> >& buckets, DHTBucketTreeNode* root);
+
+} // namespace dht
+
+} // namespace aria2
+
+#endif // D_DHT_BUCKET_TREE_H