is there any function in C++ that calculates a fingerprint or hash of a string that's guaranteed to be at least 64 bits wide?

Question

is there any function in C++ that calculates a fingerprint or hash of a string that's guaranteed to be at least 64 bits wide?

I'd like to replace my unordered_map<string, int> with unordered_map<long long, int>.

Given the answers that I'm getting (thanks Stack Overflow community...) the technique that I'm describing is not well-known. The reason that I want an unordered map of fingerprints instead of strings is for space and speed. The second map does not have to store strings and when doing the lookup, it doesn't incur any extra cache misses to fetch those strings. The only downside is the slight chance of a collision. That's why the key has to be 64 bits: a probability of 2^(-64) is basically an impossibility. Of course, this is predicated on a good hash function, which is exactly what my question is seeking.

Thanks again, Stack Overflowers.

Answer 1

What exactly is it that you seek to achieve? Your map is not gonna work any better with a "bigger" hash function. Not notably, anyway.

Answer 2

unordered_map always hashes the key into a size_t variable. This is independent from the type of the key and depends solely on the architecture you are working with.

Answer 3

If you want to map any string to a unique integer:

typedef std::map<string,long long> Strings;
static Strings s_strings;
long long s_highWaterMark = 0;
long long my_function(const string& s)
{
  Strings::const_iterator it = s_strings.find(s);
  if (it != s_strings.end())
  {
    //we've previously returned a fingerprint for this string
    //now return the same fingerprint again
    return it->second;
  }
  //else new fingerprint
  long long rc = ++s_highWaterMark;
  //... remember it for next time
  s_strings.insert(Strings::value_type(s, rc));
  //... and return it this time
  return rc;
}

Answer 4

c++ has no native 128 Bit type, nor does it have native hashing support. Such extensions for hashing are supposed to be added in TR1, but as far as I am aware 128 bit ints aren't supported my many compilers. (Microsoft supports an __int128 type -- only on x64 platforms though)

I'd expect the functions included with unordered_map would be faster in any case.

If you really do want to do things that way, MD5 provides a good 128 bit hash.