From d66b3d3561de127f6d995e4b0af0aaffac33d701 Mon Sep 17 00:00:00 2001
From: Eric Mertens <emertens@galois.com>
Date: Mon, 4 Dec 2023 15:45:17 -0800
Subject: [PATCH] use a counter

---
 2023/04.cpp                   |  6 +++---
 lib/include/aocpp/Counter.hpp | 30 ++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 3 deletions(-)
 create mode 100644 lib/include/aocpp/Counter.hpp

diff --git a/2023/04.cpp b/2023/04.cpp
index e62afc4..0855b5e 100644
--- a/2023/04.cpp
+++ b/2023/04.cpp
@@ -12,6 +12,7 @@
 
 #include <aocpp/Parsing.hpp>
 #include <aocpp/Startup.hpp>
+#include <aocpp/Counter.hpp>
 
 namespace {
 
@@ -47,11 +48,10 @@ auto CountWins(std::vector<Card>& cards) -> std::vector<std::size_t>
 
   for (auto& card : cards)
   {
-    std::vector<unsigned long long> xs;
     std::sort(card.mine.begin(), card.mine.end());
     std::sort(card.winners.begin(), card.winners.end());
-    std::set_intersection(card.mine.begin(), card.mine.end(), card.winners.begin(), card.winners.end(), std::back_inserter(xs));
-    result.push_back(xs.size());
+    auto const c = std::set_intersection(card.mine.begin(), card.mine.end(), card.winners.begin(), card.winners.end(), Counter<std::size_t>{});
+    result.push_back(c);
   }
 
   return result;
diff --git a/lib/include/aocpp/Counter.hpp b/lib/include/aocpp/Counter.hpp
new file mode 100644
index 0000000..e257db5
--- /dev/null
+++ b/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>);