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/* hasher.h
*
* Defines a simple hash function class
*/
/**
* Copyright (C) 2012 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 "mongo/base/disallow_copying.h"
#include "mongo/bson/bsonelement.h"
namespace mongo {
typedef int HashSeed;
class BSONElementHasher {
MONGO_DISALLOW_COPYING(BSONElementHasher);
public:
/* The hash function we use can be given a seed, to effectively randomize it
* by choosing from among a family of hash functions. When it is not specified,
* use this.
*
* WARNING: do not change the hash see value. Hash-based sharding clusters will
* expect that value to be zero.
*/
static const int DEFAULT_HASH_SEED = 0;
/* This computes a 64-bit hash of the value part of BSONElement "e",
* preceded by the seed "seed". Squashes element (and any sub-elements)
* of the same canonical type, so hash({a:{b:4}}) will be the same
* as hash({a:{b:4.1}}). In particular, this squashes doubles to 64-bit long
* ints via truncation, so floating point values round towards 0 to the
* nearest int representable as a 64-bit long.
*
* This function is used in the computation of hashed indexes
* and hashed shard keys, and thus should not be changed unless
* the associated "getKeys" and "makeSingleKey" method in the
* hashindex type is changed accordingly.
*/
static long long int hash64(const BSONElement& e, HashSeed seed);
private:
BSONElementHasher();
};
}
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