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+// Copyright 2008, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef BASE_ID_MAP_H__
+#define BASE_ID_MAP_H__
+
+// hash map is common (GCC4 and Dinkumware also support it, for example), but
+// is not strictly part of the C++ standard. MS puts it into a funny namespace,
+// although most other vendors seem to use std. This may have to change if
+// other platforms are supported.
+#include <hash_map>
+using stdext::hash_map;
+
+#include "base/basictypes.h"
+#include "base/logging.h"
+
+// This object maintains a list of IDs that can be quickly converted to
+// pointers to objects. It is implemented as a hash table, optimized for
+// relatively small data sets (in the common case, there will be exactly one
+// item in the list).
+//
+// Items can be inserted into the container with arbitrary ID, but the caller
+// must ensure they are unique. Inserting IDs and relying on automatically
+// generated ones is not allowed because they can collide.
+template<class T>
+class IDMap {
+ private:
+  typedef hash_map<int32, T*> HashTable;
+
+ public:
+  IDMap() : next_id_(1) {
+  }
+  IDMap(const IDMap& other) : next_id_(other.next_id_),
+                                        data_(other.data_) {
+  }
+
+  // support const iterators over the items
+  // Note, use iterator->first to get the ID, iterator->second to get the T*
+  typedef typename HashTable::const_iterator const_iterator;
+  const_iterator begin() const {
+    return data_.begin();
+  }
+  const_iterator end() const {
+    return data_.end();
+  }
+
+  // Adds a view with an automatically generated unique ID. See AddWithID.
+  int32 Add(T* data) {
+    int32 this_id = next_id_;
+    DCHECK(data_.find(this_id) == data_.end()) << "Inserting duplicate item";
+    data_[this_id] = data;
+    next_id_++;
+    return this_id;
+  }
+
+  // Adds a new data member with the specified ID. The ID must not be in
+  // the list. The caller either must generate all unique IDs itself and use
+  // this function, or allow this object to generate IDs and call Add. These
+  // two methods may not be mixed, or duplicate IDs may be generated
+  void AddWithID(T* data, int32 id) {
+    DCHECK(data_.find(id) == data_.end()) << "Inserting duplicate item";
+    data_[id] = data;
+  }
+
+  void Remove(int32 id) {
+    HashTable::iterator i = data_.find(id);
+    if (i == data_.end()) {
+      NOTREACHED() << "Attempting to remove an item not in the list";
+      return;
+    }
+    data_.erase(i);
+  }
+
+  bool IsEmpty() const {
+    return data_.empty();
+  }
+
+  T* Lookup(int32 id) const {
+    HashTable::const_iterator i = data_.find(id);
+    if (i == data_.end())
+      return NULL;
+    return i->second;
+  }
+
+  size_t size() const {
+    return data_.size();
+  }
+
+ protected:
+  // The next ID that we will return from Add()
+  int32 next_id_;
+
+  HashTable data_;
+};
+
+#endif  // BASE_ID_MAP_H__