use a counter

lib/include/aocpp/Counter.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <cstdint>
+#include <iterator>
+
+template <typename T>
+struct Counter
+{
+  using value_type        = T;
+  using difference_type   = std::ptrdiff_t;
+  using pointer           = T*;
+  using reference         = T&;
+  using iterator_category = std::output_iterator_tag;
+
+  struct EmptyRef {
+    auto operator=(T const&) const -> void { }
+  };
+
+  std::size_t n;
+  
+  auto operator++() -> Counter& { n++; return *this; }
+  auto operator++(int) -> Counter { Counter temp; n++; return temp; }
+  auto operator*() const -> EmptyRef { return EmptyRef{}; }
+  auto operator-(Counter const& rhs) const -> std::ptrdiff_t {
+    return std::ptrdiff_t{n} - std::ptrdiff_t{rhs.n};
+  }
+  operator std::size_t() const { return n; }
+};
+
+static_assert(std::output_iterator<Counter<int>, int>);