Question

My main data object is a array of doubles of a length that depends on a specific instantiation of my class. I would like to construct a very simple hash table to store/retrieve these objects, and we can assume that the numbers are generated in a way that is free of numerical error.

int main() {
  std::tr1::unordered_map<double*, double*> cache;

  double x1[] = { 1.0, 3.14 };
  double x2[] = { 1.0, 3.14 };

  cache[x1] = x1;

  std::cout << "x1: " << cache.count(x1) << std::endl;
  std::cout << "x2: " << cache.count(x2) << std::endl;

  return 0;
}

The above obviously only compares the pointers, giving the output:

> ./tmp
x1: 1
x2: 0

When I really want to see:

> ./tmp
x1: 1
x2: 1

It’s pretty clear how to create custom hashing and equality functions when the size of the arrays are fixed at compile time but I do not know how to make custom functions that depend on a specific instantiation… I created a class below, but I’m not sure if it’s useful, or how it could be used.

class Hash_double_vec {

public:
  int dim;
  Hash_double_vec(int d) { dim = d; }

  size_t operator()(const double *x) const{
    std::tr1::hash<double> hash_fn;
    size_t r = hash_fn(x[0]);
    for(int i=1;i<dim;i++) r ^= hash_fn(x[i]);
    return r;
  }

  bool operator()(const double *x, const double *y) const{
    for(int i=1;i<dim;i++) if (fabs(x[i]-y[i]) > 1e-10) return false;
    return true;
  }
};

Answer 1

One way would be to create struct to hold the pointer to the sequence of doubles:

struct DoubleRegion
{
    double* p;
    size_t size;
};

bool operator==(DoubleRegion a, DoubleRegion b)
{
    return a.size == b.size && memcmp(a.p, b.p, a.size) == 0;
}

size_t hash(DoubleRegion dr) 
{
    size_t h = 0;
    for (double* p = dr.p; p != dr.p + dr.size; ++p)
        h ^= hash(*p);
    return h;
}

And then use it:

unordered_map<DoubleRegion, DoubleRegion> cache;

Of course it is your problem to make sure the lifetime of the backing memory is a superset of the lifetime of the DoubleRegion.

Old Answer:

If you don’t know until runtime how big the key and value is going to be, use a std::vector:

unordered_map<vector<double>, vector<double>> cache;

If you know at compile-time how big you can use an std::array:

unordered_map<array<double, N>, array<double, N>> cache;

In both cases the default hashing function will work by value as you want, and you do not need to define a custom one.