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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/>.
*/
#pragma once
#include "mongo/pch.h"
#include "mongo/bson/bsonelement.h"
#include "mongo/util/md5.hpp"
namespace mongo {
typedef int HashSeed;
typedef unsigned char HashDigest[16];
class Hasher : private boost::noncopyable {
public:
explicit Hasher( HashSeed seed );
~Hasher() { };
//pointer to next part of input key, length in bytes to read
void addData( const void * keyData , size_t numBytes );
//finish computing the hash, put the result in the digest
//only call this once per Hasher
void finish( HashDigest out );
private:
md5_state_t _md5State;
HashSeed _seed;
};
class HasherFactory : private boost::noncopyable {
public:
/* Eventually this may be a more sophisticated factory
* for creating other hashers, but for now use MD5.
*/
static Hasher* createHasher( HashSeed seed ) {
return new Hasher( seed );
}
private:
HasherFactory();
};
class BSONElementHasher : private boost::noncopyable {
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();
/* This incrementally computes the hash of BSONElement "e"
* using hash function "h". If "includeFieldName" is true,
* then the name of the field is hashed in between the type of
* the element and the element value. The hash function "h"
* is applied recursively to any sub-elements (arrays/sub-documents),
* squashing elements of the same canonical type.
* Used as a helper for hash64 above.
*/
static void recursiveHash( Hasher* h , const BSONElement& e , bool includeFieldName );
};
}
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