Hash Function

[Elkvis]

I came up with a string hashing algorithm which I haven't seen before, and I was hoping to get some opinions on whether it has merit as a usable hashing function for things like cryptography.

unsigned long hash(const std::string& str)
{
  std::string::iterator it;
  unsigned long hashed = 0;
  for(it = str.begin(); it != str.end(); it++)
  {
    hashed = ((hashed - (*it)) << ((*it) &#37; 7)) + (*it);
  }
  return hashed;
}

[laserlight]

Algorithm aside, that certainly would not be good enough for use as a cryptographic hash since the output is limited to 64 bits, at most, on typical platforms. It would be far too vulnerable to a birthday attack.

The algorithm itself looks like one of those used for hash tables.

[master5001]

It looks solid for making a hash table. Though for that purpose it looks like it may have some collisions with higher valued characters.

[CornedBee]

Is the modulo operation truly necessary? Modulo is still among the slowest integer operations you can execute.

[matsp]

Is the modulo operation truly necessary? Modulo is still among the slowest integer operations you can execute.

You could get a similar result by & 7 - but it extends the range a little bit from 0..6 to 0..7 - I have no idea if that matters much here.

--
Mats