Note that this isn't just some user-supplied bad hashers. Hash for std::string in GNU library is not much harder to hack for collisions than your example. And a

Note that this isn't just some user-supplied bad hashers. Hash for std::string in GNU library is not much harder to hack for collisions than your example. And also it uses the whole string, so is slow on long strings. I didn't investigate if other implementations of STL are any better.