enumerate.h

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// https://www.reedbeta.com/blog/python-like-enumerate-in-cpp17/
// std::vector<Thing> things;
// ...
// for (auto [i, thing] : enumerate(things))
// {
//     .. `i` gets the index and `thing` gets the Thing in each iteration
// }
 
#include <tuple>
 
template <typename T, typename TIter = decltype(std::begin(std::declval<T>())),
          typename = decltype(std::end(std::declval<T>()))>
constexpr auto enumerate(T &&iterable)
{
  struct iterator
  {
    size_t i;
    TIter  iter;
    bool   operator!=(const iterator &other) const { return iter != other.iter; }
    void   operator++()
    {
      ++i;
      ++iter;
    }
    auto operator*() const { return std::tie(i, *iter); }
  };
  struct iterable_wrapper
  {
    T    iterable;
    auto begin() { return iterator{0, std::begin(iterable)}; }
    auto end() { return iterator{0, std::end(iterable)}; }
  };
  return iterable_wrapper{std::forward<T>(iterable)};
}