✨ Add filtered_index

CMakeLists.txt

@@ -72,6 +72,7 @@ target_sources(
               include/stdx/intrusive_list.hpp
               include/stdx/iterator.hpp
               include/stdx/latched.hpp
+              include/stdx/meta.hpp
               include/stdx/numeric.hpp
               include/stdx/optional.hpp
               include/stdx/panic.hpp
@@ -89,6 +90,10 @@ target_sources(
               include/stdx/utility.hpp)
 
 if(PROJECT_IS_TOP_LEVEL)
+    set_target_properties(stdx PROPERTIES VERIFY_INTERFACE_HEADER_SETS true)
+    add_dependencies(${INFRA_TARGET_NAMESPACE}quality
+                     stdx_verify_interface_header_sets)
+
     include(CTest)
     add_docs(docs)
     add_subdirectory(test)

docs/design/meta.md

@@ -0,0 +1,41 @@
+# meta
+
+Source code: https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/meta.hpp  
+Documentation: https://intel.github.io/cpp-std-extensions/#_meta_hpp
+
+## Filtering out `void`
+
+`filtered_index` solves a subproblem of aggregating function results. You have a
+list of functions:
+
+```cpp
+// f0 :: () -> int
+// f1 :: () -> void
+// f2 :: () -> int
+
+using func_list_t = std::tuple<F0, F1, F2>;
+```
+
+Some of those functions return `void`. It is easy enough to make a `tuple` of
+results with the `void` elements filtered out:
+
+```cpp
+template <typename F>
+using returns_void = std::is_void<std::invoke_result_t<F>>;
+
+using non_void_func_list_t = boost::mp11::mp_remove_if<func_list_t, returns_void>;
+// -> std::tuple<F0, F2>
+
+using results_t = boost::mp11::mp_transform<std::invoke_result_t, non_void_func_list_t>;
+// -> std::tuple<int, int>
+```
+
+The problem is: after running say `f2` and getting the result, where in the
+results tuple should it be placed? This is what `filtered_index` answers: where
+does the original index corresponding to `f2` map to in the filtered list?
+
+If this problem seems solvable a simpler way, imagine that the functions are
+asynchronous, we need to provision the results tuple space up front, and handle
+asynchronous placement of the results as they come in. We need to deal with
+indices because any of the function types or result types might coincide, i.e.
+we can't find them reliably by type.

docs/header_graph.mmd

@@ -6,13 +6,15 @@ flowchart BT
   array(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/array.hpp">array.hpp</a>)
   atomic(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/atomic.hpp">atomic.hpp</a>)
   ct_conversions(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/ct_conversions.hpp">ct_conversions.hpp</a>)
-  priority(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/priority.hpp">priority.hpp</a>)
+  meta(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/meta.hpp">meta.hpp</a>)
   numeric(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/numeric.hpp">numeric.hpp</a>)
+  priority(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/priority.hpp">priority.hpp</a>)
   array ~~~ compiler
   atomic ~~~ compiler
   ct_conversions --> compiler
-  priority ~~~ compiler
+  meta ~~~ compiler
   numeric ~~~ compiler
+  priority ~~~ compiler
 
   %% level 2
   type_traits(<a href="https://github.com/intel/cpp-std-extensions/tree/main/include/stdx/type_traits.hpp">type_traits.hpp</a>)

docs/index.adoc

@@ -33,6 +33,7 @@ include::intrusive_forward_list.adoc[]
 include::intrusive_list.adoc[]
 include::iterator.adoc[]
 include::latched.adoc[]
+include::meta.adoc[]
 include::numeric.adoc[]
 include::optional.adoc[]
 include::panic.adoc[]

docs/intro.adoc

@@ -19,6 +19,7 @@ The following compilers are supported:
 * clang 18
 * clang 19
 * clang 20
+* clang 21
 * clang 22
 * gcc 12
 * gcc 13
@@ -88,6 +89,7 @@ The following headers are available:
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/intrusive_list.hpp[`intrusive_list.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/iterator.hpp[`iterator.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/latched.hpp[`latched.hpp`]
+* https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/meta.hpp[`meta.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/numeric.hpp[`numeric.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/optional.hpp[`optional.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/panic.hpp[`panic.hpp`]

docs/meta.adoc

@@ -0,0 +1,40 @@
+
+== `meta.hpp`
+
+https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/meta.hpp[`meta.hpp`]
+defines some metafunctions intended for compatibility with https://www.boost.org/libs/mp11[`boost::mp11`].
+
+The metafunctions are in the `stdx::mp` namespace.
+
+=== `filtered_index`
+
+`filtered_index` answers the question: given a list and a predicate, if the list
+were filtered (as if by `boost::mp11::mp_copy_if`) by that predicate, where
+would an index in the unfiltered list map to in the filtered list?
+
+`filtered_index_c` is like `filtered_index`, but instead of taking a
+`std::integral_constant`, it takes a plain integral type.
+
+[source,cpp]
+----
+// list: (boost::mp_int<> of) 0, 1, 2, 3, 4
+using list_t = boost::mp11::mp_iota_c<5>;
+
+// predicate
+template <typename T> using is_even = boost::mp11::mp_bool<T::value % 2 == 0>;
+
+// filtered list would be (boost::mp_int<> of) 0, 2, 4
+using index_of_0 = stdx::mp::filtered_index_c<list_t, is_even, 0>;
+using index_of_2 = stdx::mp::filtered_index_c<list_t, is_even, 2>;
+using index_of_4 = stdx::mp::filtered_index_c<list_t, is_even, 4>;
+static_assert(std::same_as<index_of_0, boost::mp11::mp_size_t<0>>);
+static_assert(std::same_as<index_of_2, boost::mp11::mp_size_t<1>>);
+static_assert(std::same_as<index_of_4, boost::mp11::mp_size_t<2>>);
+
+// filtered items don't exist in the list -- by boost convention, the index
+// returned is equal to the (filtered) list size
+using index_of_1 = stdx::mp::filtered_index_c<list_t, is_even, 1>;
+using index_of_3 = stdx::mp::filtered_index_c<list_t, is_even, 3>;
+static_assert(std::same_as<index_of_1, boost::mp11::mp_size_t<3>>);
+static_assert(std::same_as<index_of_3, boost::mp11::mp_size_t<3>>);
+----

include/stdx/meta.hpp

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/function.hpp>
+#include <boost/mp11/integral.hpp>
+
+#include <limits>
+
+namespace stdx {
+inline namespace v1 {
+namespace mp {
+namespace detail {
+template <template <typename...> typename P> struct mask_bit_q {
+    template <typename T>
+    using fn = boost::mp11::mp_size_t<P<T>::value ? 1 : 0>;
+};
+
+template <typename L, template <typename...> typename P>
+using list_mask_t = boost::mp11::mp_transform_q<mask_bit_q<P>, L>;
+
+template <typename L, template <typename...> typename P>
+using list_mask_scan_t = boost::mp11::mp_partial_sum<
+    list_mask_t<L, P>,
+    boost::mp11::mp_size_t<std::numeric_limits<std::size_t>::max()>,
+    boost::mp11::mp_plus>;
+} // namespace detail
+
+template <typename L, template <typename...> typename P, typename I>
+using filtered_index =
+    boost::mp11::mp_if<P<boost::mp11::mp_at<L, I>>,
+                       boost::mp11::mp_at<detail::list_mask_scan_t<L, P>, I>,
+                       boost::mp11::mp_size<boost::mp11::mp_copy_if<L, P>>>;
+
+template <typename L, template <typename...> typename P, std::size_t I>
+using filtered_index_c = filtered_index<L, P, boost::mp11::mp_size_t<I>>;
+} // namespace mp
+} // namespace v1
+} // namespace stdx

test/CMakeLists.txt

@@ -55,6 +55,7 @@ add_tests(
     intrusive_list_properties
     iterator
     latched
+    meta
     numeric
     optional
     overload

test/meta.cpp

@@ -0,0 +1,44 @@
+#include <stdx/meta.hpp>
+
+#include <boost/mp11/algorithm.hpp>
+#include <catch2/catch_test_macros.hpp>
+
+namespace {
+using list_t = boost::mp11::mp_iota_c<5>;
+template <typename T> using is_even = std::bool_constant<T::value % 2 == 0>;
+} // namespace
+
+TEST_CASE("filtered_index (items that pass the predicate)", "[meta]") {
+    STATIC_CHECK(
+        std::same_as<stdx::mp::filtered_index<list_t, is_even,
+                                              boost::mp11::mp_size_t<0>>,
+                     boost::mp11::mp_size_t<0>>);
+    STATIC_CHECK(
+        std::same_as<stdx::mp::filtered_index<list_t, is_even,
+                                              boost::mp11::mp_size_t<2>>,
+                     boost::mp11::mp_size_t<1>>);
+    STATIC_CHECK(
+        std::same_as<stdx::mp::filtered_index<list_t, is_even,
+                                              boost::mp11::mp_size_t<4>>,
+                     boost::mp11::mp_size_t<2>>);
+}
+
+TEST_CASE("filtered_index (items that fail the predicate)", "[meta]") {
+    STATIC_CHECK(
+        std::same_as<stdx::mp::filtered_index<list_t, is_even,
+                                              boost::mp11::mp_size_t<1>>,
+                     boost::mp11::mp_size_t<3>>);
+    STATIC_CHECK(
+        std::same_as<stdx::mp::filtered_index<list_t, is_even,
+                                              boost::mp11::mp_size_t<3>>,
+                     boost::mp11::mp_size_t<3>>);
+}
+
+TEST_CASE("filtered_index_c", "[meta]") {
+    STATIC_CHECK(std::same_as<stdx::mp::filtered_index_c<list_t, is_even, 0>,
+                              boost::mp11::mp_size_t<0>>);
+    STATIC_CHECK(std::same_as<stdx::mp::filtered_index_c<list_t, is_even, 2>,
+                              boost::mp11::mp_size_t<1>>);
+    STATIC_CHECK(std::same_as<stdx::mp::filtered_index_c<list_t, is_even, 4>,
+                              boost::mp11::mp_size_t<2>>);
+}