[libcxx-commits] [libcxx] [libcxx] adds ranges::fold_left_with_iter and ranges::fold_left (PR #75259)

[via libcxx-commits]

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@@ -0,0 +1,93 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// template<input_iterator I, sentinel_for<I> S, class T,
+//          indirectly-binary-left-foldable<T, I> F>
+//   constexpr see below ranges::fold_left(I first, S last, T init, F f);
+//
+// template<input_range R, class T, indirectly-binary-left-foldable<T, iterator_t<R>> F>
+//   constexpr see below ranges::fold_left(R&& r, T init, F f);
+
+#include <algorithm>
+#include <cassert>
+#include <vector>
+#include <functional>
+
+#include "test_range.h"
+#include "../gaussian_sum.h"
+
+constexpr bool test() {
+  {
+    auto data         = std::vector<int>{1, 2, 3, 4};
+    auto const result = std::ranges::fold_left(data.begin(), data.begin(), 0, std::plus());
+
+    assert(result == 0);
+
+    auto range = std::span(data.data(), 0);
+    assert(std::ranges::fold_left(range, 0, std::plus()) == result);
+  }
+  {
+    auto data           = std::vector<int>{1, 2, 3, 4};
+    constexpr auto init = 100.1;
+    auto const result   = std::ranges::fold_left(data.begin(), data.begin(), init, std::plus());
+
+    assert(result == init);
+
+    auto range = std::span(data.data(), 0);
+    assert(std::ranges::fold_left(range, init, std::plus()) == result);
+  }
+  {
+    auto data         = std::vector{1, 3, 5, 7, 9};
+    auto const result = std::ranges::fold_left(data.begin(), data.end(), 0, std::plus());
+
+    assert(result == gaussian_sum(data)); // sum of n ascending odd numbers = n^2
+    assert(std::ranges::fold_left(data, 0, std::plus()) == result);
+  }
+  {
+    auto data         = std::vector{2, 4, 6, 8, 10, 12};
+    auto const result = std::ranges::fold_left(data.begin(), data.end(), 0L, std::plus<long>());
+
+    assert(result == gaussian_sum(data));
+    assert(std::ranges::fold_left(data, 0, std::plus()) == result);
+  }
+  {
+    auto data         = std::vector{-1.1, -2.2, -3.3, -4.4, -5.5, -6.6};
+    auto plus         = [](int const x, double const y) { return x + y; };
+    auto const result = std::ranges::fold_left(data.begin(), data.end(), 0.0, plus);
+
+    assert(result == -21.6); // int(  0.0) + -1.1 =   0 + -1.1 =  -1.1
----------------
EricWF wrote:

I really like the comment here. Nice.

https://github.com/llvm/llvm-project/pull/75259