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diff --git a/db/projection.h b/db/projection.h
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+// projection.h
+
+/*    Copyright 2009 10gen Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#pragma once
+
+#include "pch.h"
+#include "jsobj.h"
+
+namespace mongo {
+
+    /**
+     * given a document and a projection specification
+     * can transform the document
+     * currently supports specifying which fields and $slice
+     */
+    class Projection {
+    public:
+
+        class KeyOnly {
+        public:
+
+            KeyOnly() : _stringSize(0) {}
+
+            BSONObj hydrate( const BSONObj& key ) const;
+
+            void addNo() { _add( false , "" ); }
+            void addYes( const string& name ) { _add( true , name ); }
+
+        private:
+
+            void _add( bool b , const string& name ) {
+                _include.push_back( b );
+                _names.push_back( name );
+                _stringSize += name.size();
+            }
+
+            vector<bool> _include; // one entry per field in key.  true iff should be in output
+            vector<string> _names; // name of field since key doesn't have names
+
+            int _stringSize;
+        };
+
+        Projection() :
+            _include(true) ,
+            _special(false) ,
+            _includeID(true) ,
+            _skip(0) ,
+            _limit(-1) ,
+            _hasNonSimple(false) {
+        }
+
+        /**
+         * called once per lifetime
+         * e.g. { "x" : 1 , "a.y" : 1 }
+         */
+        void init( const BSONObj& spec );
+
+        /**
+         * @return the spec init was called with
+         */
+        BSONObj getSpec() const { return _source; }
+
+        /**
+         * transforms in according to spec
+         */
+        BSONObj transform( const BSONObj& in ) const;
+
+
+        /**
+         * transforms in according to spec
+         */
+        void transform( const BSONObj& in , BSONObjBuilder& b ) const;
+
+
+        /**
+         * @return if the keyPattern has all the information needed to return then
+         *         return a new KeyOnly otherwise null
+         *         NOTE: a key may have modified the actual data
+         *               which has to be handled above this (arrays, geo)
+         */
+        KeyOnly* checkKey( const BSONObj& keyPattern ) const;
+
+    private:
+
+        /**
+         * appends e to b if user wants it
+         * will descend into e if needed
+         */
+        void append( BSONObjBuilder& b , const BSONElement& e ) const;
+
+
+        void add( const string& field, bool include );
+        void add( const string& field, int skip, int limit );
+        void appendArray( BSONObjBuilder& b , const BSONObj& a , bool nested=false) const;
+
+        bool _include; // true if default at this level is to include
+        bool _special; // true if this level can't be skipped or included without recursing
+
+        //TODO: benchmark vector<pair> vs map
+        typedef map<string, boost::shared_ptr<Projection> > FieldMap;
+        FieldMap _fields;
+        BSONObj _source;
+        bool _includeID;
+
+        // used for $slice operator
+        int _skip;
+        int _limit;
+
+        bool _hasNonSimple;
+    };
+
+
+}