Unexpected collision with std::hash

Question

I know hashing infinite number of string into 32b int must generate collision, but I expect from hashing function some nice distribution.

Isn't it weird that these 2 strings have the same hash?

size_t hash0 = std::hash<std::string>()("generated_id_0");
size_t hash1 = std::hash<std::string>()("generated_id_1");
//hash0 == hash1

I know I can use boost::hash<std::string> or others, but I want to know what is wrong with std::hash. Am I using it wrong? Shouldn't I somehow "seed" it?

Answer 1

There's nothing wrong with your usage of std::hash. The problem is that the specialization std::hash<std::string> provided by the standard library implementation bundled with Visual Studio 2010 only takes a subset of the string's characters to determine the hash value (presumably for performance reasons). Coincidentally the last character of a string with 14 characters is not part of this set, which is why both strings yield the same hash value.

As far as I know this behaviour is in conformance with the standard, which demands only that multiple calls to the hash function with the same argument must always return the same value. However, the probability of a hash collision should be minimal. The VS2010 implementation fulfills the mandatory part, yet fails to account for the optional one.

For details, see the implementation in the header file xfunctional (starting at line 869 in my copy) and §17.6.3.4 of the C++ standard (latest public draft).

If you absolutely need a better hash function for strings, you should implement it yourself. It's actually not that hard.

Answer 2

The exact hash algorithm isn't specified by the standard, so the results will vary. The algorithm used by VC10 doesn't seem to take all of the characters into account if the string is longer than 10 characters; it advances with an increment of 1 + s.size() / 10. This is legal, albeit from a QoI point of view, rather disappointing; such hash codes are known to perform very poorly for some typical sets of data (like URLs). I'd strongly suggest you replace it with either a FNV hash or one based on a Mersenne prime:

FNV hash:

struct hash
{
    size_t operator()( std::string const& s ) const
    {
        size_t result = 2166136261U ;
        std::string::const_iterator end = s.end() ;
        for ( std::string::const_iterator iter = s.begin() ;
              iter != end ;
              ++ iter ) {
            result = (16777619 * result)
                    ^ static_cast< unsigned char >( *iter ) ;
        }
        return result ;
    }
};

Mersenne prime hash:

struct hash
{
    size_t operator()( std::string const& s ) const
    {
        size_t result = 2166136261U ;
        std::string::const_iterator end = s.end() ;
        for ( std::string::const_iterator iter = s.begin() ;
              iter != end ;
              ++ iter ) {
            result = 127 * result
                   + static_cast< unsigned char >( *iter ) ;
        }
        return result ;
    }
};

(The FNV hash is supposedly better, but the Mersenne prime hash will be faster on a lot of machines, because multiplying by 127 is often significantly faster than multiplying by 16777619.)