src: Implement the enumerate() range filter

src/ranges.hh

@@ -5,6 +5,7 @@
 #include <utility>
 #include <iterator>
 #include <numeric>
+#include <tuple>
 
 #include "constexpr_utils.hh"
 
@@ -155,6 +156,53 @@ inline auto filter(Filter f)
     });
 }
 
+template<typename Range>
+struct EnumerateView
+{
+    using RangeIt = IteratorOf<Range>;
+
+    struct Iterator : std::iterator<std::forward_iterator_tag,
+                                    typename std::iterator_traits<RangeIt>::value_type>
+    {
+        Iterator(size_t index, RangeIt it, RangeIt end)
+            : m_index{index}, m_it{std::move(it)}, m_end{std::move(end)} {}
+
+        decltype(auto) operator*() { return std::make_tuple(m_index, *m_it); }
+        Iterator& operator++() { ++m_index; ++m_it; return *this; }
+        Iterator operator++(int) { auto copy = *this; ++(*this); return copy; }
+
+        friend bool operator==(const Iterator& lhs, const Iterator& rhs)
+        {
+            return lhs.m_index == rhs.m_index and lhs.m_it == rhs.m_it;
+        }
+
+        friend bool operator!=(const Iterator& lhs, const Iterator& rhs)
+        {
+            return not (lhs == rhs);
+        }
+
+        const RangeIt& base() const { return m_it; }
+
+    private:
+        size_t m_index;
+        RangeIt m_it;
+        RangeIt m_end;
+    };
+
+    Iterator begin() const { return {0, std::begin(m_range), std::end(m_range)}; }
+    Iterator end()   const { return {std::size(m_range), std::end(m_range), std::end(m_range)}; }
+
+    Range m_range;
+};
+
+inline auto enumerate()
+{
+    return make_view_factory([](auto&& range) {
+        using Range = decltype(range);
+        return EnumerateView<decay_range<Range>>{std::forward<Range>(range)};
+    });
+}
+
 template<typename Range, typename Transform>
 struct TransformView
 {