Question

I need to specialize the hash function for unordered_map so I can use int arrays as keys. The array values are usually 0 or 1, e.g. int array = {0, 1, 0, 1}, but technically not bounded.

Can someone recommend a good hash function in this case? Alternatively, I can always convert the int array into a string and avoid specialization. But I am concerned about performance since I may have several million of these arrays.

Answer 1

C++ TR1 contains a hash template function.

If you don’t have that yet, you can use Boost Hash.

Idea for a handy helper:

#include <boost/functional/hash.hpp>

template <typename T, int N>
    static std::size_t hasharray(const T (&arr)[N])
{
     return boost::hash_range(arr, arr+N);
}

This would be (roughly?) equivalent to

 size_t seed = 0;
 for (const T* it=arr; it!=(arr+N); ++it)
     boost::hash_combine(seed, *it);
 return seed;

Don’t forget to implement proper equality comparison operations if you’re using this hash for lookup