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/**
* Copyright (C) 2013 10gen 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 <vector>
#include "mongo/db/diskloc.h"
#include "mongo/db/jsobj.h"
namespace mongo {
struct CursorOptions;
/**
* An IndexCursor is the interface through which one traverses the entries of a given
* index. The internal structure of an index is kept isolated.
*
* The cursor must be initiailized by seek()ing to a given entry in the index. The index is
* traversed by calling next() or skip()-ping ahead.
*
* The set of predicates a given index can understand is known a priori. These predicates may
* be simple (a key location for a Btree index) or rich ($within for a geo index).
*
* Locking is the responsibility of the caller. The IndexCursor keeps state. If the caller
* wishes to yield or unlock, it must call savePosition() first. When it decides to unyield it
* must call restorePosition(). The cursor may be EOF after a restorePosition().
*/
class IndexCursor {
public:
virtual ~IndexCursor() { }
/**
* Set options on the cursor (direction). See CursorOptions below.
*/
virtual Status setOptions(const CursorOptions& options) = 0;
/**
* A cursor doesn't point anywhere by default. You must seek to the start position.
* The provided position must be a predicate that the index understands. The
* predicate must describe one value, though there may be several instances
*
* Possible return values:
* 1. Success: seeked to the position.
* 2. Success: seeked to 'closest' key oriented according to the cursor's direction.
* 3. Error: can't seek to the position.
*/
virtual Status seek(const BSONObj& position) = 0;
//
// Iteration support
//
// Are we out of documents?
virtual bool isEOF() const = 0;
// Move to the next key/value pair. Assumes !isEOF().
virtual void next() = 0;
//
// Accessors
//
// Current key we point at. Assumes !isEOF().
virtual BSONObj getKey() const = 0;
// Current value we point at. Assumes !isEOF().
virtual DiskLoc getValue() const = 0;
//
// Yielding support
//
/**
* Yielding semantics:
* If the entry that a cursor points at is not deleted during a yield, the cursor will
* point at that entry after a restore.
* An entry inserted during a yield may or may not be returned by an in-progress scan.
* An entry deleted during a yield may or may not be returned by an in-progress scan.
* An entry modified during a yield may or may not be returned by an in-progress scan.
* An entry that is not inserted or deleted during a yield will be returned, and only once.
* If the index returns entries in a given order (Btree), this order will be mantained even
* if the entry corresponding to a saved position is deleted during a yield.
*/
/**
* Save our current position in the index. Assumes that we are currently pointing to a
* valid position in the index.
* If not, we error. Otherwise, succeed.
*/
virtual Status savePosition() = 0;
/**
* Restore the saved position. Errors if there is no saved position.
* The cursor may be EOF after a restore.
*/
virtual Status restorePosition() = 0;
// Return a string describing the cursor.
virtual string toString() = 0;
/**
* Add debugging info to the provided builder.
* TODO(hk): We can do this better, perhaps with a more structured format.
*/
virtual void explainDetails(BSONObjBuilder* b) { }
};
// All the options we might want to set on a cursor.
struct CursorOptions {
// Set the direction of the scan. Ignored if the cursor doesn't have directions (geo).
enum Direction {
DECREASING = -1,
INCREASING = 1,
};
Direction direction;
// 2d indices need to know exactly how many results you want beforehand.
// Ignored by every other index.
int numWanted;
};
} // namespace mongo
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