Implement most ranges partition algorithms (#976)

Includes `ranges::is_partitioned`, `ranges::partition`, `ranges::partition_copy`, and `ranges::partition_point`.

tests/std/include/range_algorithm_support.hpp

@@ -40,6 +40,9 @@ namespace detail {
 } // namespace detail
 constexpr bool is_permissive = detail::Derived<int>::test();
 
+template <class T>
+inline constexpr T* nullptr_to = nullptr;
+
 template <bool>
 struct borrowed { // borrowed<true> is a borrowed_range; borrowed<false> is not
     int* begin() const;

tests/std/test.lst

@@ -263,6 +263,9 @@ tests\P0896R4_ranges_alg_minmax
 tests\P0896R4_ranges_alg_mismatch
 tests\P0896R4_ranges_alg_move
 tests\P0896R4_ranges_alg_none_of
+tests\P0896R4_ranges_alg_partition
+tests\P0896R4_ranges_alg_partition_copy
+tests\P0896R4_ranges_alg_partition_point
 tests\P0896R4_ranges_alg_replace_if
 tests\P0896R4_ranges_alg_search
 tests\P0896R4_ranges_alg_search_n

tests/std/tests/P0896R4_ranges_alg_partition/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\concepts_matrix.lst

tests/std/tests/P0896R4_ranges_alg_partition/test.cpp

@@ -0,0 +1,115 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Covers ranges::is_partitioned and ranges::partition (and bits of ranges::partition_point)
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <numeric>
+#include <ranges>
+#include <utility>
+
+#include <range_algorithm_support.hpp>
+
+using namespace std;
+
+#define ASSERT(...) assert((__VA_ARGS__))
+
+using P = pair<int, int>;
+
+constexpr auto is_even = [](int i) { return i % 2 == 0; };
+
+// Validate dangling story
+STATIC_ASSERT(same_as<decltype(ranges::partition(borrowed<false>{}, is_even)), ranges::dangling>);
+STATIC_ASSERT(same_as<decltype(ranges::partition(borrowed<true>{}, is_even)), ranges::subrange<int*>>);
+
+struct empty_test {
+    template <ranges::input_range Range>
+    static constexpr void call() {
+        // Validate empty ranges
+        using ranges::is_partitioned, ranges::partition, ranges::partition_point;
+        {
+            Range range{};
+            ASSERT(is_partitioned(range, is_even, get_first));
+        }
+        {
+            Range range{};
+            ASSERT(is_partitioned(range.begin(), range.end(), is_even, get_first));
+        }
+
+        if constexpr (ranges::forward_range<Range>) {
+            const Range range{};
+            {
+                const auto result = partition(range, is_even, get_first);
+                ASSERT(result.begin() == range.end());
+                ASSERT(result.end() == range.end());
+            }
+            {
+                const auto result = partition(range.begin(), range.end(), is_even, get_first);
+                ASSERT(result.begin() == range.end());
+                ASSERT(result.end() == range.end());
+            }
+        }
+    }
+};
+
+struct partition_test {
+    static constexpr array elements = {P{0, 42}, P{1, 42}, P{2, 42}, P{3, 42}, P{4, 42}, P{5, 42}, P{6, 42}, P{7, 42}};
+
+    template <ranges::input_range Range>
+    static constexpr void call() {
+        using ranges::is_partitioned, ranges::partition, ranges::partition_point, ranges::is_permutation,
+            ranges::subrange;
+
+        auto pairs = elements;
+
+        {
+            Range range{pairs};
+            ASSERT(!is_partitioned(range, is_even, get_first));
+        }
+        {
+            Range range{pairs};
+            ASSERT(!is_partitioned(range.begin(), range.end(), is_even, get_first));
+        }
+
+#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, VSO-938163
+        if (!is_constant_evaluated())
+#endif // TRANSITION, VSO-938163
+        {
+            if constexpr (ranges::forward_range<Range>) {
+                const Range range{pairs};
+                const auto mid = ranges::next(range.begin(), 4);
+
+                {
+                    auto result = partition(range, is_even, get_first);
+                    ASSERT(result.begin() == mid);
+                    ASSERT(result.end() == range.end());
+                }
+                ASSERT(is_permutation(subrange{range.begin(), mid}, array{0, 2, 4, 6}, ranges::equal_to{}, get_first));
+                ASSERT(is_partitioned(range, is_even, get_first));
+                ASSERT(partition_point(range, is_even, get_first) == mid);
+
+                pairs = elements;
+
+                {
+                    auto result = partition(range.begin(), range.end(), is_even, get_first);
+                    ASSERT(result.begin() == mid);
+                    ASSERT(result.end() == range.end());
+                }
+                ASSERT(is_permutation(subrange{range.begin(), mid}, array{0, 2, 4, 6}, ranges::equal_to{}, get_first));
+                ASSERT(is_partitioned(range.begin(), range.end(), is_even, get_first));
+                ASSERT(partition_point(range.begin(), range.end(), is_even, get_first) == mid);
+            }
+        }
+    }
+};
+
+int main() {
+    STATIC_ASSERT((test_in<empty_test, P>(), true));
+    test_in<empty_test, P>();
+
+    STATIC_ASSERT((test_in<partition_test, P>(), true));
+    test_in<partition_test, P>();
+}

tests/std/tests/P0896R4_ranges_alg_partition_copy/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\concepts_matrix.lst

tests/std/tests/P0896R4_ranges_alg_partition_copy/test.cpp

@@ -0,0 +1,134 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <numeric>
+#include <ranges>
+#include <utility>
+
+#include <range_algorithm_support.hpp>
+
+using namespace std;
+
+#define ASSERT(...) assert((__VA_ARGS__))
+
+using P = pair<int, int>;
+
+constexpr auto is_even = [](int i) { return i % 2 == 0; };
+
+// Validate that partition_copy_result aliases in_out_out_result
+STATIC_ASSERT(
+    same_as<ranges::partition_copy_result<int, double, void*>, ranges::in_out_out_result<int, double, void*>>);
+
+// Validate dangling story
+STATIC_ASSERT(same_as<decltype(ranges::partition_copy(borrowed<false>{}, nullptr_to<int>, nullptr_to<long>, is_even)),
+    ranges::partition_copy_result<ranges::dangling, int*, long*>>);
+STATIC_ASSERT(same_as<decltype(ranges::partition_copy(borrowed<true>{}, nullptr_to<int>, nullptr_to<long>, is_even)),
+    ranges::partition_copy_result<int*, int*, long*>>);
+
+struct empty_test {
+    template <ranges::input_range Range, indirectly_writable<ranges::range_reference_t<Range>> Out1,
+        indirectly_writable<ranges::range_reference_t<Range>> Out2>
+    static constexpr void call() {
+        // Validate empty ranges
+        using ranges::partition_copy, ranges::partition_copy_result, ranges::iterator_t;
+
+        {
+            Range range{};
+            auto result = partition_copy(range, Out1{}, Out2{}, is_even, get_first);
+            STATIC_ASSERT(same_as<decltype(partition_copy(range, Out1{}, Out2{}, is_even, get_first)),
+                partition_copy_result<iterator_t<Range>, Out1, Out2>>);
+            ASSERT(result.in == range.end());
+            ASSERT(result.out1.peek() == nullptr);
+            ASSERT(result.out2.peek() == nullptr);
+        }
+        {
+            Range range{};
+            auto result = partition_copy(range.begin(), range.end(), Out1{}, Out2{}, is_even, get_first);
+            STATIC_ASSERT(
+                same_as<decltype(partition_copy(range.begin(), range.end(), Out1{}, Out2{}, is_even, get_first)),
+                    partition_copy_result<iterator_t<Range>, Out1, Out2>>);
+            ASSERT(result.in == range.end());
+            ASSERT(result.out1.peek() == nullptr);
+            ASSERT(result.out2.peek() == nullptr);
+        }
+    }
+};
+
+struct partition_copy_test {
+    static constexpr int N = 32;
+
+    template <ranges::input_range R, indirectly_writable<ranges::range_reference_t<R>> O1,
+        indirectly_writable<ranges::range_reference_t<R>> O2>
+    static constexpr void call() {
+        using ranges::partition_copy;
+
+        P source[N];
+        for (int i = 0; i < N; ++i) {
+            source[i] = {i, 42};
+        }
+
+        for (int i = 0; i < N; ++i) {
+            P dest[N];
+            ranges::fill(dest, P{-1, 13});
+
+            const R range{source};
+            auto result = partition_copy(
+                range, O1{dest}, O2{dest + i}, [i](int x) { return x < i; }, get_first);
+            assert(result.in == range.end());
+            assert(result.out1.peek() == dest + i);
+            assert(result.out2.peek() == dest + N);
+            assert(ranges::equal(source, dest));
+        }
+    }
+};
+
+template <class Instantiator, class Elem>
+constexpr void run_tests() {
+    // Call Instantiator::template call</*...stuff...*/>() with a range whose element type is Elem, and two iterators to
+    // which Elem is writable, whose properties are "interesting" for ranges::partition_copy. What combinations of
+    // properties are "interesting"?
+
+    // For the input range, the algorithm simply unwraps iterators and chugs through looking for the end. It doesn't
+    // * take advantage of any capabilities provided by stronger-than-input categories,
+    // * care if the sentinel and iterator have the same type,
+    // * care if it can difference iterators with sentinels or each other, or
+    // * care about the size of the input range at all. (It can't even use size info to check the outputs, because we
+    // don't how many of the input elements will be written through each output.)
+    // TLDR: One input range with a proxy reference type and no other notable properties (the so-called "weakest" input
+    // range) suffices.
+
+    // For the outputs, both of which are treated equivalently, the algorithm is similarly oblivious to properties other
+    // than reference type and the ability to unwrap/rewrap. These could simply be the "weakest" writable iterator type
+    // in with_writable_iterators.
+
+    // Out of simple paranoia, let's permute ProxyRef; seven extra pointless tests won't hurt.
+
+    using test::range, test::iterator, test::input, test::output, test::CanCompare, test::CanDifference, test::Common,
+        test::ProxyRef, test::Sized;
+
+    using proxy_range     = range<input, Elem, Sized::no, CanDifference::no, Common::no, CanCompare::no, ProxyRef::yes>;
+    using non_proxy_range = range<input, Elem, Sized::no, CanDifference::no, Common::no, CanCompare::no, ProxyRef::no>;
+    using proxy_iterator  = iterator<output, remove_const_t<Elem>, CanDifference::no, CanCompare::no, ProxyRef::yes>;
+    using non_proxy_iterator = iterator<output, remove_const_t<Elem>, CanDifference::no, CanCompare::no, ProxyRef::no>;
+
+    Instantiator::template call<proxy_range, proxy_iterator, proxy_iterator>();
+    Instantiator::template call<proxy_range, proxy_iterator, non_proxy_iterator>();
+    Instantiator::template call<proxy_range, non_proxy_iterator, proxy_iterator>();
+    Instantiator::template call<proxy_range, non_proxy_iterator, non_proxy_iterator>();
+    Instantiator::template call<non_proxy_range, proxy_iterator, proxy_iterator>();
+    Instantiator::template call<non_proxy_range, proxy_iterator, non_proxy_iterator>();
+    Instantiator::template call<non_proxy_range, non_proxy_iterator, proxy_iterator>();
+    Instantiator::template call<non_proxy_range, non_proxy_iterator, non_proxy_iterator>();
+}
+
+int main() {
+    STATIC_ASSERT((run_tests<empty_test, const P>(), true));
+    run_tests<empty_test, const P>();
+
+    STATIC_ASSERT((run_tests<partition_copy_test, const P>(), true));
+    run_tests<partition_copy_test, const P>();
+}

tests/std/tests/P0896R4_ranges_alg_partition_point/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\concepts_matrix.lst

tests/std/tests/P0896R4_ranges_alg_partition_point/test.cpp

@@ -0,0 +1,64 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <numeric>
+#include <ranges>
+#include <span>
+#include <utility>
+
+#include <range_algorithm_support.hpp>
+
+using namespace std;
+
+#define ASSERT(...) assert((__VA_ARGS__))
+
+using P = pair<int, int>;
+
+constexpr auto is_even = [](int i) { return i % 2 == 0; };
+
+// Validate dangling story
+STATIC_ASSERT(same_as<decltype(ranges::partition_point(borrowed<false>{}, is_even)), ranges::dangling>);
+STATIC_ASSERT(same_as<decltype(ranges::partition_point(borrowed<true>{}, is_even)), int*>);
+
+struct empty_test {
+    template <ranges::forward_range Range>
+    static constexpr void call() {
+        // Validate empty ranges
+        using ranges::partition_point;
+
+        const Range range{};
+        ASSERT(partition_point(range, is_even, get_first) == range.end());
+        ASSERT(partition_point(range.begin(), range.end(), is_even, get_first) == range.end());
+    }
+};
+
+struct partition_point_test {
+    template <ranges::forward_range R>
+    static constexpr void call() {
+        using ranges::partition_point;
+        int elements[200];
+        iota(ranges::begin(elements), ranges::end(elements), 0);
+
+        // to avoid constant expression step limits
+        const size_t bound = elements[0] + (is_constant_evaluated() ? 10 : ranges::size(elements));
+
+        for (size_t i = 0; i < bound; ++i) {
+            const R range{span{elements}.first(i)};
+            for (size_t j = 0; j < i; ++j) {
+                assert(partition_point(range, [j](int x) { return x < static_cast<int>(j); }).peek() == elements + j);
+            }
+        }
+    }
+};
+
+int main() {
+    STATIC_ASSERT((test_fwd<empty_test, P>(), true));
+    test_fwd<empty_test, P>();
+
+    STATIC_ASSERT((test_fwd<partition_point_test, const int>(), true));
+    test_fwd<partition_point_test, const int>();
+}