Add views::successor

include/range/v3/view/successor.hpp

@@ -0,0 +1,116 @@
+/// \file
+// Range v3 library
+//
+//  Copyright Semir Vrana 2020-present
+//
+//  Use, modification and distribution is subject to the
+//  Boost Software License, Version 1.0. (See accompanying
+//  file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt)
+//
+// Project home: https://github.com/ericniebler/range-v3
+//
+
+#ifndef RANGES_V3_VIEW_SUCCESSOR_HPP
+#define RANGES_V3_VIEW_SUCCESSOR_HPP
+
+#include <type_traits>
+#include <utility>
+
+#include <meta/meta.hpp>
+
+#include <range/v3/range_fwd.hpp>
+
+#include <range/v3/functional/invoke.hpp>
+#include <range/v3/iterator/unreachable_sentinel.hpp>
+#include <range/v3/range/access.hpp>
+#include <range/v3/range/primitives.hpp>
+#include <range/v3/range/traits.hpp>
+#include <range/v3/utility/optional.hpp>
+#include <range/v3/utility/semiregular_box.hpp>
+#include <range/v3/utility/static_const.hpp>
+#include <range/v3/view/facade.hpp>
+
+#include <range/v3/detail/prologue.hpp>
+
+namespace ranges
+{
+    /// \addtogroup group-views
+    /// @{
+
+    /// Iteratively calls a function on an initial value
+    /// successor_view(f, x) -> [x, f(x), f(f(x)), ...]
+    template<typename G, typename T>
+    struct successor_view : view_facade<successor_view<G, T>, infinite>
+    {
+    private:
+        friend range_access;
+        using result_t = T;
+        semiregular_box_t<G> gen_;
+        semiregular_box_t<T> val_;
+        struct cursor
+        {
+        private:
+            successor_view * view_;
+
+        public:
+            cursor() = default;
+            explicit cursor(successor_view * view)
+              : view_(view)
+            {}
+            const result_t & read() const
+            {
+                return view_->val_;
+            }
+            void next()
+            {
+                view_->val_ = ranges::invoke(view_->gen_, std::move(view_->val_));
+            }
+        };
+        cursor begin_cursor()
+        {
+            return cursor{this};
+        }
+        unreachable_sentinel_t end_cursor() const
+        {
+            return {};
+        }
+
+    public:
+        successor_view() = default;
+        explicit successor_view(G g, T x)
+          : gen_(std::move(g))
+          , val_(std::move(x))
+        {}
+        const result_t & cached()
+        {
+            return *val_;
+        }
+    };
+
+    namespace views
+    {
+        struct successor_fn
+        {
+            template<typename G, typename T>
+            auto operator()(G g, T x) const -> CPP_ret(successor_view<G, T>)( //
+                requires invocable<G &, T &&> && copy_constructible<G> &&
+                    copy_constructible<T> &&
+                        assignable_from<T &, invoke_result_t<G &, T &&>>)
+            {
+                return successor_view<G, T>{std::move(g), std::move(x)};
+            }
+        };
+
+        /// \relates successor_fn
+        /// \ingroup group-views
+        RANGES_INLINE_VARIABLE(successor_fn, successor)
+    } // namespace views
+      /// \@}
+} // namespace ranges
+
+#include <range/v3/detail/epilogue.hpp>
+#include <range/v3/detail/satisfy_boost_range.hpp>
+RANGES_SATISFY_BOOST_RANGE(::ranges::successor_view)
+
+#endif

test/view/CMakeLists.txt

@@ -54,6 +54,7 @@ rv3_add_test(test.view.span view.span span.cpp)
 rv3_add_test(test.view.split view.split split.cpp)
 rv3_add_test(test.view.stride view.stride stride.cpp)
 rv3_add_test(test.view.subrange view.subrange subrange.cpp)
+rv3_add_test(test.view.successor view.successor successor.cpp)
 rv3_add_test(test.view.tail view.tail tail.cpp)
 rv3_add_test(test.view.take view.take take.cpp)
 rv3_add_test(test.view.take_exactly view.take_exactly take_exactly.cpp)

test/view/successor.cpp

@@ -0,0 +1,62 @@
+// Range v3 library
+//
+//  Copyright Semir Vrana 2020-present
+//
+//  Use, modification and distribution is subject to the
+//  Boost Software License, Version 1.0. (See accompanying
+//  file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt)
+//
+// Project home: https://github.com/ericniebler/range-v3
+
+#include <range/v3/core.hpp>
+#include <range/v3/view/drop_exactly.hpp>
+#include <range/v3/view/successor.hpp>
+#include <range/v3/view/take_exactly.hpp>
+
+#include "../simple_test.hpp"
+#include "../test_utils.hpp"
+
+namespace views = ranges::views;
+
+int main()
+{
+    {
+        auto powers_of_10 = views::successor([](int x) { return x * 10; }, 1);
+        CPP_assert(ranges::input_range<decltype(powers_of_10)> &&
+                   ranges::view_<decltype(powers_of_10)>);
+        check_equal(powers_of_10 | views::take_exactly(5), {1, 10, 100, 1000, 10000});
+    }
+
+    // Test that we only call the function once for each dereferenceable position
+    {
+        int i = 0;
+        auto rng = views::successor(
+            [&i](int n) {
+                ++i;
+                return n + 1;
+            },
+            0);
+        auto rng2 = std::move(rng);
+        auto it = rng2.begin();
+        CHECK(i == 0);
+        CHECK(*it == 0);
+        CHECK(i == 0);
+        ++it;
+        CHECK(i == 1);
+        CHECK(*it == 1);
+        CHECK(i == 1);
+    }
+
+    // Test that skipping past positions works correctly
+    {
+        auto times_10 = [](int x) { return x * 10; };
+        auto rng = ranges::views::successor(times_10, 1) //
+                   | ranges::views::drop_exactly(3)      //
+                   | ranges::views::take_exactly(2);
+
+        check_equal(rng, {1000, 10000});
+    }
+
+    return test_result();
+}