I was trying to replace my use of std::nth_element with miniselect's implementation of Andrei Alexandrescu's adaptive quickselect, but I ran into an issue while doing so: it seems that one of the median_index overloads does not consistently use the given comparator to compare elements (lines marked with HERE):

template <class Iter, class Compare,
          class DiffType = typename std::iterator_traits<Iter>::difference_type>
inline DiffType median_index(const Iter r, DiffType a, DiffType b, DiffType c,
                             Compare&& comp) {
  if (r[a] > r[c]) std::swap(a, c); // HERE
  if (r[b] > r[c]) return c; // HERE
  if (comp(r[b], r[a])) return a;
  return b;
}

I stumbled upon this issue while trying to sort with a custom predicate a collection of a type that doesn't have comparison operators, so I fortunately got into a compile-time error, but I believe that it can result into more subtle runtime errors too when the types to sort do implement comparison operators.

Anyway, thanks again for this library, I'm glad to see selection algorithms getting some love 😄

Currently, it is size_t, works well for most cases but not standard compliant