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+/**
+ *    Copyright (C) 2014 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    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 Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    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 program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects for
+ *    all of the code used other than as permitted herein. 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 in the license file.
+ */
+
+#pragma once
+
+#include <boost/scoped_ptr.hpp>
+
+#include "mongo/db/exec/plan_stage.h"
+#include "mongo/db/diskloc.h"
+#include "mongo/db/index/btree_index_cursor.h"
+#include "mongo/db/index/index_access_method.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/db/matcher/expression.h"
+#include "mongo/db/query/index_bounds.h"
+#include "mongo/platform/unordered_set.h"
+
+namespace mongo {
+
+    class IndexAccessMethod;
+    class IndexCursor;
+    class IndexDescriptor;
+    class WorkingSet;
+
+    struct DistinctParams {
+        DistinctParams() : descriptor(NULL),
+                           direction(1),
+                           fieldNo(0) { }
+
+        // What index are we traversing?
+        const IndexDescriptor* descriptor;
+
+        // And in what direction?
+        int direction;
+
+        // What are the bounds?
+        IndexBounds bounds;
+
+        // What field in the index's key pattern is the one we're distinct-ing over?
+        // For example:
+        // If we have an index {a:1, b:1} we could use it to distinct over either 'a' or 'b'.
+        // If we distinct over 'a' the position is 0.
+        // If we distinct over 'b' the position is 1.
+        int fieldNo;
+    };
+
+    /**
+     * Used by the distinct command.  Executes a mutated index scan over the provided bounds.
+     * However, rather than looking at every key in the bounds, it skips to the next value of the
+     * _params.fieldNo-th indexed field.  This is because distinct only cares about distinct values
+     * for that field, so there is no point in examining all keys with the same value for that
+     * field.
+     *
+     * Only created through the getDistinctRunner path.  See db/query/get_runner.cpp
+     */
+    class DistinctScan : public PlanStage {
+    public:
+        /**
+         * Keeps track of what this distinct scan is currently doing so that it
+         * can do the right thing on the next call to work().
+         */
+        enum ScanState {
+            // Need to initialize the underlying index traversal machinery.
+            INITIALIZING,
+
+            // Skipping keys in order to check whether we have reached the end.
+            CHECKING_END,
+
+            // Retrieving the next key, and applying the filter if necessary.
+            GETTING_NEXT,
+
+            // The index scan is finished.
+            HIT_END
+        };
+
+        DistinctScan(const DistinctParams& params, WorkingSet* workingSet);
+        virtual ~DistinctScan() { }
+
+        virtual StageState work(WorkingSetID* out);
+        virtual bool isEOF();
+        virtual void prepareToYield();
+        virtual void recoverFromYield();
+        virtual void invalidate(const DiskLoc& dl, InvalidationType type);
+
+        virtual PlanStageStats* getStats();
+
+    private:
+        /**
+         * Initialize the underlying IndexCursor
+         */
+        void initIndexCursor();
+
+        /** See if the cursor is pointing at or past _endKey, if _endKey is non-empty. */
+        void checkEnd();
+
+        // The WorkingSet we annotate with results.  Not owned by us.
+        WorkingSet* _workingSet;
+
+        // Index access.
+        const IndexDescriptor* _descriptor; // owned by Collection -> IndexCatalog
+        const IndexAccessMethod* _iam; // owned by Collection -> IndexCatalog
+
+        // The cursor we use to navigate the tree.
+        boost::scoped_ptr<BtreeIndexCursor> _btreeCursor;
+
+        // Keeps track of what work we need to do next.
+        ScanState _scanState;
+
+        // For yielding.
+        BSONObj _savedKey;
+        DiskLoc _savedLoc;
+
+        DistinctParams _params;
+
+        // _checker gives us our start key and ensures we stay in bounds.
+        boost::scoped_ptr<IndexBoundsChecker> _checker;
+        int _keyEltsToUse;
+        bool _movePastKeyElts;
+        vector<const BSONElement*> _keyElts;
+        vector<bool> _keyEltsInc;
+
+        // Stats
+        CommonStats _commonStats;
+        DistinctScanStats _specificStats;
+    };
+
+}  // namespace mongo