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I am using boost::hash to get hash value for a string.
But it is giving different hash values for same string on Windows 32-bit and Debian 64-bit systems.
So how can I get same hash value (32-bit or 64-bit) using boost::hash irrespective of platform?
387
The cryptographic hashing of a value cannot be inverted to find the original value. Given a value, it is infeasible to find another value with the same cryptographic hash.
The 32-bit long hash value is a hexadecimal number of 8 characters. MD4 is a Message Digest Algorithm developed by Ronald L. Rivest from RSA Data Security, Inc. Currently it's considered insecure, but it's very fast on 32-bit mashines and it's used for calculating EDonkey 2000 hashes in the EDonkey p2p network.
Hashing involves applying a hashing algorithm to a data item, known as the hashing key, to create a hash value. Hashing algorithms take a large range of values (such as all possible strings or all possible files) and map them onto a smaller set of values (such as a 128 bit number). Hashing has two main applications.
x); boost::hash_combine (seed, p. y); return seed; } ... }; Calls to hash_combine incrementally build the hash from the different members of point, it can be repeatedly called for any number of elements. It calls hash_value on the supplied element, and combines it with the seed.
What is the guarantee concerning boost::hash? I don't see any
guarantees that a generated hash code is usable outside of the
process which generates it. (This is frequently the case with
hash functions.) If you need a hash value for external data,
valid over different programs and different platforms (e.g. for
a hashed access to data on disk), then you'll have to write your
own. Something like:
uint32_t
hash( std::string const& key )
{
uint32_t results = 12345;
for ( auto current = key.begin(); current != key.end(); ++ current ) {
results = 127 * results + static_cast<unsigned char>( *current );
}
return results;
}
should do the trick, as long as you don't have to worry about
porting to some exotic mainframes (which might not support
uint32_t).
101
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