Add FilterTypes to TypePack functionality

(Not currently used.)

src/pbrt/util/containers.h

@@ -88,6 +88,10 @@ template <typename T, typename... Ts>
 struct Prepend<T, TypePack<Ts...>> {
     using type = TypePack<T, Ts...>;
 };
+template <typename... Ts>
+struct Prepend<void, TypePack<Ts...>> {
+    using type = TypePack<Ts...>;
+};
 
 template <int index, typename T, typename... Ts>
 struct TakeFirstN;
@@ -113,6 +117,38 @@ struct MapType<M, TypePack<T, Ts...>> {
     using type = typename Prepend<M<T>, typename MapType<M, TypePack<Ts...>>::type>::type;
 };
 
+template <template <typename> class Pred, typename... Ts>
+struct FilterTypes;
+
+namespace internal {
+
+template <typename T, bool>
+struct FilterTypesHelper;
+
+template <typename T>
+struct FilterTypesHelper<T, true> {
+    using type = T;
+};
+template <typename T>
+struct FilterTypesHelper<T, false> {
+    using type = void;
+};
+
+};  // namespace internal
+
+template <template <typename> class Pred, typename T>
+struct FilterTypes<Pred, TypePack<T>> {
+    using type = typename TypePack<
+        typename internal::FilterTypesHelper<T, Pred<T>::value>::type>::type;
+};
+
+template <template <typename> class Pred, typename T, typename... Ts>
+struct FilterTypes<Pred, TypePack<T, Ts...>> {
+    using type =
+        typename Prepend<typename internal::FilterTypesHelper<T, Pred<T>::value>::type,
+                         TypePack<Ts...>>::type;
+};
+
 template <typename F, typename... Ts>
 void ForEachType(F func, TypePack<Ts...>);
 template <typename F, typename T, typename... Ts>

src/pbrt/util/containers_test.cpp

@@ -149,3 +149,12 @@ TEST(TypePack, Map) {
     static_assert(std::is_same_v<TypePack<Set<double>>,
                   typename TakeFirstN<1, typename RemoveFirstN<2, SetPack>::type>::type>);
 }
+
+TEST(TypePack, Filter) {
+    using Pack = TypePack<signed int, float, double>;
+    using FilteredPack = typename FilterTypes<std::is_floating_point, Pack>::type;
+
+    static_assert(std::is_same_v<TypePack<float>, typename TakeFirstN<1, FilteredPack>::type>);
+    static_assert(std::is_same_v<TypePack<double>,
+                  typename TakeFirstN<1, typename RemoveFirstN<1, FilteredPack>::type>::type>);
+}