the default hash function for floats/doubles is not very good.

the default hash for floats works by casting the value to size_t:

	template <> struct hash<float>
		{ size_t operator()(float val) const { return static_cast<size_t>(val); } };

this is quite bad because anything that is between two integers gets hashed to the same value. my hash table essentially turned into a linear array because most of the values happened to be in the [0; 1] range.

the same applies to hashes for other floating point types (double, long double etc.).

when fixing this, keep in mind that floating point numbers have two representations for 0 (0 and -0) - those two bit patterns should hash to the same value because they represent the same number.