[libcxx-commits] [libcxx] [libc++] Partially implement P2846R6: `reserve_hint` (PR #206385)

[Hristo Hristov via libcxx-commits]

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+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// constexpr auto reserve_hint()
+//     requires approximately_sized_range<V>
+// constexpr auto reserve_hint() const
+//     requires approximately_sized_range<const V>
+
+#include <cassert>
+#include <ranges>
+
+#include "types.h"
+
+// There is no reserve_hint function on a stride view over a view that
+// is *not* an approximately sized range
+static_assert(!std::ranges::approximately_sized_range<BasicTestView<cpp17_input_iterator<int*>>>);
+static_assert(
+    !std::ranges::approximately_sized_range<std::ranges::stride_view<BasicTestView<cpp17_input_iterator<int*>>>>);
+
+// There is a reserve_hint function on a stride view over a view that
+// *is *an approximately sized range
+static_assert(std::ranges::approximately_sized_range<
+              BasicTestView<int*, sentinel_wrapper<int*>, /*IsSized=*/false, /*IsApproximatelySized=*/true>>);
+static_assert(std::ranges::approximately_sized_range<std::ranges::stride_view<
+                  BasicTestView<int*, sentinel_wrapper<int*>, /*IsSized=*/false, /*IsApproximatelySized=*/true>>>);
+
+using Iota = std::ranges::iota_view<int, int>;
+using ApproximatelySizedView =
+    BasicTestView<std::ranges::iterator_t<Iota>,
+                  std::ranges::sentinel_t<Iota>,
+                  /*IsSized=*/false,
+                  /*IsApproximatelySized=*/true>;
+
+constexpr ApproximatelySizedView make_approximately_sized(Iota&& iota) {
+  return ApproximatelySizedView(std::ranges::begin(iota), std::ranges::end(iota));
+}
+
+constexpr bool test() {
+  {
+    // Test with stride as exact multiple of number of elements in view strided over.
+    auto view    = make_approximately_sized(std::views::iota(0, 12));
+    auto strided = std::views::stride(view, 3);
+    static_assert(std::ranges::approximately_sized_range<decltype(strided)>);
+    assert(strided.reserve_hint() == 4);
+  }
+
+  {
+    // Test with stride as inexact multiple of number of elements in view strided over.
+    auto view = make_approximately_sized(std::views::iota(0, 22));
+    static_assert(std::ranges::approximately_sized_range<decltype(view)>);
+    auto strided = std::views::stride(view, 3);
+    static_assert(std::ranges::approximately_sized_range<decltype(strided)>);
+    assert(strided.size() == 8);
+  }
+
+  {
+    // Empty range.
+    auto view    = make_approximately_sized(std::views::iota(0, 0));
+    auto strided = view | std::views::stride(3);
+    assert(strided.reserve_hint() == 0);
+  }
+
+  {
+    // Stride larger than range size.
+    auto view    = make_approximately_sized(std::views::iota(0, 3));
+    auto strided = view | std::views::stride(10);
+    assert(strided.reserve_hint() == 1);
+  }
+
+  {
+    // Stride equal to range size.
+    auto view    = make_approximately_sized(std::views::iota(0, 3));
+    auto strided = view | std::views::stride(5);
----------------
Zingam wrote:

Is this true?

https://github.com/llvm/llvm-project/pull/206385