is_permutation_hash_1.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(
  LhsForwardIterator lhs_first, LhsForwardIterator lhs_last,
  RhsForwardIterator rhs_first, RhsForwardIterator rhs_last,
  Equal equal = Equal(),
  Hash hash = Hash())
{
  using value_type = typename LhsForwardIterator::value_type;
  std::unordered_map<value_type, int, Hash, Equal> frequencies;
  
  for (; lhs_first != lhs_last; ++lhs_first) frequencies[*lhs_first] += 1;
  for (; rhs_first != rhs_last; ++rhs_first) frequencies[*rhs_first] -= 1;
  
  return std::all_of(begin(frequencies), end(frequencies),
                     [](auto const& p) { return p.second == 0; });
}