is_permutation_hash_2.cpp

template <
  typename LhsForwardIterator,
  typename RhsForwardIterator,
  typename Equal = std::equal_to<typename LhsForwardIterator::value_type>,
  typename Hash = std::hash<typename LhsForwardIterator::value_type>
>
bool is_permutation_hash_2(
  LhsForwardIterator lhs_first, LhsForwardIterator lhs_last,
  RhsForwardIterator rhs_first, RhsForwardIterator rhs_last,
  Equal equal = Equal(),
  Hash hash = Hash())
{
  auto lhs_size = std::distance(lhs_first, lhs_last);
  if (lhs_size != std::distance(rhs_first, rhs_last))
    return false;

  using value_type = typename LhsForwardIterator::value_type;	
  std::unordered_map<value_type, int, Hash, Equal> frequencies;
  frequencies.reserve(lhs_size);
	
  for (; lhs_first != lhs_last; ++lhs_first) frequencies[*lhs_first] += 1;
  for (; rhs_first != rhs_last; ++rhs_first)
  {
    auto where = frequencies.find(*rhs_first);
    if (where == frequencies.end()) return false;
    --where->second;
  }
	
  return std::all_of(begin(frequencies), end(frequencies),
                     [](auto const& p) { return p.second == 0; });
}