From eda7ea303a0957f901cb02006891cfc8c35ea281 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2@gmail.com>
Date: Mon, 26 May 2014 14:21:29 -0400
Subject: [PATCH] Add a more efficient at_index algorithm.

---
 include/boost/hana/detail/at_index/best.hpp   | 22 ++++++++++++
 .../detail/at_index/overload_resolution.hpp   | 36 +++++++++++++++++++
 include/boost/hana/list.hpp                   |  8 +++++
 test/CMakeLists.txt                           |  1 +
 test/detail/at_index.cpp                      | 33 +++++++++++++++++
 5 files changed, 100 insertions(+)
 create mode 100644 include/boost/hana/detail/at_index/best.hpp
 create mode 100644 include/boost/hana/detail/at_index/overload_resolution.hpp
 create mode 100644 test/detail/at_index.cpp

diff --git a/include/boost/hana/detail/at_index/best.hpp b/include/boost/hana/detail/at_index/best.hpp
new file mode 100644
index 000000000..97da4572c
--- /dev/null
+++ b/include/boost/hana/detail/at_index/best.hpp
@@ -0,0 +1,22 @@
+/*!
+ * @file
+ * Defines `boost::hana::detail::at_index::best`.
+ *
+ *
+ * @copyright Louis Dionne 2014
+ * Distributed under the Boost Software License, Version 1.0.
+ *         (See accompanying file LICENSE.md or copy at
+ *             http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_DETAIL_AT_INDEX_BEST_HPP
+#define BOOST_HANA_DETAIL_AT_INDEX_BEST_HPP
+
+#include <boost/hana/detail/at_index/overload_resolution.hpp>
+
+
+namespace boost { namespace hana { namespace detail { namespace at_index {
+    BOOST_HANA_CONSTEXPR_LAMBDA auto best = overload_resolution;
+}}}}
+
+#endif // !BOOST_HANA_DETAIL_AT_INDEX_BEST_HPP
diff --git a/include/boost/hana/detail/at_index/overload_resolution.hpp b/include/boost/hana/detail/at_index/overload_resolution.hpp
new file mode 100644
index 000000000..5ed06202b
--- /dev/null
+++ b/include/boost/hana/detail/at_index/overload_resolution.hpp
@@ -0,0 +1,36 @@
+/*!
+ * @file
+ * Defines `boost::hana::detail::at_index::overload_resolution`.
+ *
+ *
+ * @copyright Louis Dionne 2014
+ * Distributed under the Boost Software License, Version 1.0.
+ *         (See accompanying file LICENSE.md or copy at
+ *             http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_DETAIL_AT_INDEX_OVERLOAD_RESOLUTION_HPP
+#define BOOST_HANA_DETAIL_AT_INDEX_OVERLOAD_RESOLUTION_HPP
+
+#include <boost/hana/detail/constexpr.hpp>
+#include <boost/hana/detail/integer_sequence.hpp>
+
+#include <cstddef>
+
+
+namespace boost { namespace hana { namespace detail { namespace at_index {
+    struct eat_pod { eat_pod(...) { } };
+
+    template <std::size_t ...ignore>
+    constexpr auto overload_impl(index_sequence<ignore...>) {
+        return [](decltype(ignore, eat_pod{})..., auto nth, ...) {
+            return nth;
+        };
+    }
+
+    BOOST_HANA_CONSTEXPR_LAMBDA auto overload_resolution = [](auto n, auto ...xs) {
+        return overload_impl(make_index_sequence<n>{})(xs...);
+    };
+}}}} // end namespace boost::hana::detail::at_index
+
+#endif // !BOOST_HANA_DETAIL_AT_INDEX_OVERLOAD_RESOLUTION_HPP
diff --git a/include/boost/hana/list.hpp b/include/boost/hana/list.hpp
index fb6f86e81..e309963dc 100644
--- a/include/boost/hana/list.hpp
+++ b/include/boost/hana/list.hpp
@@ -13,6 +13,7 @@
 #define BOOST_HANA_LIST_HPP
 
 #include <boost/hana/comparable.hpp>
+#include <boost/hana/detail/at_index/best.hpp>
 #include <boost/hana/detail/comparable_from_iterable.hpp>
 #include <boost/hana/detail/constexpr.hpp>
 #include <boost/hana/detail/foldable_from_iterable.hpp>
@@ -68,6 +69,13 @@ namespace boost { namespace hana {
                 return size_t<sizeof...(xs)>;
             });
         }
+
+        template <typename Index>
+        static constexpr auto at_impl(Index n, List<Storage> xs) {
+            return xs.into([=](auto ...xs) {
+                return detail::at_index::best(n, xs...);
+            });
+        }
     };
 
     template <typename Storage>
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index dc6b1f1da..0202f9708 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -19,6 +19,7 @@ boost_hana_add_test(adapted.std_integer_sequence.iterable)
 boost_hana_add_test(adapted.std_tuple.functor)
 boost_hana_add_test(adapted.std_tuple.iterable)
 
+boost_hana_add_test(detail.at_index)
 boost_hana_add_test(detail.comparable_from_iterable)
 boost_hana_add_test(detail.foldable_from_iterable)
 boost_hana_add_test(detail.integer_sequence)
diff --git a/test/detail/at_index.cpp b/test/detail/at_index.cpp
new file mode 100644
index 000000000..ebf835eef
--- /dev/null
+++ b/test/detail/at_index.cpp
@@ -0,0 +1,33 @@
+/*
+ * Copyright Louis Dionne 2014
+ * Distributed under the Boost Software License, Version 1.0.
+ *         (See accompanying file LICENSE.md or copy at
+ *             http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include <boost/hana/detail/at_index/overload_resolution.hpp>
+
+#include <boost/hana/detail/static_assert.hpp>
+#include <boost/hana/integral.hpp>
+using namespace boost::hana;
+
+
+template <typename At>
+constexpr void test_at(At at) {
+    BOOST_HANA_STATIC_ASSERT(at(size_t<0>, int_<0>) == int_<0>);
+    BOOST_HANA_STATIC_ASSERT(at(size_t<0>, int_<0>, int_<1>) == int_<0>);
+    BOOST_HANA_STATIC_ASSERT(at(size_t<0>, int_<0>, int_<1>, int_<2>) == int_<0>);
+
+    BOOST_HANA_STATIC_ASSERT(at(size_t<1>, int_<0>, int_<1>) == int_<1>);
+    BOOST_HANA_STATIC_ASSERT(at(size_t<1>, int_<0>, int_<1>, int_<2>) == int_<1>);
+
+    BOOST_HANA_STATIC_ASSERT(at(size_t<2>, int_<0>, int_<1>, int_<2>) == int_<2>);
+
+    BOOST_HANA_STATIC_ASSERT(at(size_t<0>, 0) == 0);
+    BOOST_HANA_STATIC_ASSERT(at(size_t<1>, 0, '1') == '1');
+    BOOST_HANA_STATIC_ASSERT(at(size_t<2>, 0, '1', "2") == "2");
+}
+
+int main() {
+    test_at(detail::at_index::overload_resolution);
+}