feat(pipeline): add MetaStack

include/mosh-me/pipeline.hpp

@@ -10,6 +10,47 @@ enum class PipeStatus {
   PIPE_ERROR,
 };
 
+struct EmptyMeta {
+  template <typename GetID> auto get() {
+    static_assert(false, "ID not found in the MetaStack");
+  }
+};
+
+template <typename ParentMeta, typename ID, typename Data> class MetaStack {
+private:
+  Data data_;
+  ParentMeta parent_;
+
+public:
+  using ID_t = ID;
+  using Data_t = Data;
+
+  template <typename D, typename P>
+  MetaStack(D &&data, P &&parent)
+      : data_(std::forward<D>(data)), parent_(std::forward<P>(parent)) {}
+
+  template <typename GetID> auto get() {
+    if constexpr (std::is_same_v<ID_t, GetID>) {
+      return data_;
+    } else {
+      return parent_.template get<GetID>();
+    }
+  }
+};
+
+template <typename ID, typename Data>
+MetaStack<EmptyMeta, ID, std::decay_t<Data>> makeMeta(Data &&d) {
+  return MetaStack<EmptyMeta, ID, std::decay_t<Data>>(std::forward<Data>(d),
+                                                      EmptyMeta{});
+}
+
+template <typename ID, typename ParentMeta, typename Data>
+MetaStack<std::decay_t<ParentMeta>, ID, std::decay_t<Data>>
+makeMeta(ParentMeta &&p, Data &&d) {
+  return MetaStack<std::decay_t<ParentMeta>, ID, std::decay_t<Data>>(
+      std::forward<Data>(d), std::forward<ParentMeta>(p));
+}
+
 template <typename Data> using PipeData = std::expected<Data, PipeStatus>;
 
 template <typename Output, typename OMeta> class PipeOutput {

test/pipeline.cpp

@@ -69,3 +69,13 @@ TEST_CASE("Pipeline") {
 
   REQUIRE(count == 5);
 }
+
+TEST_CASE("MetaStack") {
+  struct IntId;
+  struct StringId;
+
+  auto s = makeMeta<IntId>(makeMeta<StringId>(std::string("test")), 10);
+
+  REQUIRE(s.get<StringId>() == "test");
+  REQUIRE(s.get<IntId>() == 10);
+}