enumerate.hh

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#ifndef ENUMERATE_HH
#define ENUMERATE_HH
 
#include <cstddef>
#include <iterator>
#include <tuple>
#include <utility>
 
template<typename Iterable,
         typename Iter = decltype(std::begin(std::declval<Iterable>())),
         typename      = decltype(std::end  (std::declval<Iterable>()))>
constexpr auto enumerate(Iterable&& iterable)
{
        struct iterator {
                size_t i;
                Iter iter;
 
                constexpr bool operator==(const iterator& other) const {
                        return iter == other.iter;
                }
                constexpr bool operator!=(const iterator& other) const {
                        return iter != other.iter;
                }
 
                constexpr bool operator==(const Iter& other) const {
                        return iter == other;
                }
                constexpr bool operator!=(const Iter& other) const {
                        return iter != other;
                }
 
                constexpr void operator++() {
                        ++i;
                        ++iter;
                }
                constexpr auto operator*() const {
                        return std::tie(i, *iter);
                }
        };
 
        struct iterable_wrapper {
                Iterable iterable;
 
                constexpr auto begin() {
                        return iterator{0, std::begin(iterable)};
                }
                constexpr auto end() {
                        return std::end(iterable);
                }
        };
 
        return iterable_wrapper{std::forward<Iterable>(iterable)};
}
 
#endif