[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 +1,116 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_with_iter(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_with_iter(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 result = std::ranges::fold_left_with_iter(data.begin(), data.begin(), 0, std::plus());
+
+    assert(result.in == data.begin());
+    assert(result.result == 0);
+
+    auto range   = std::span(data.data(), 0);
+    auto rresult = std::ranges::fold_left_with_iter(range, 0, std::plus());
+
+    assert(rresult.in == result.in);
+    assert(rresult.result == result.result);
+  }
+  {
+    auto data           = std::vector<int>{1, 2, 3, 4};
+    constexpr auto init = 100.1;
+    auto result         = std::ranges::fold_left_with_iter(data.begin(), data.begin(), init, std::plus());
+
+    assert(result.in == data.begin());
+    assert(result.result == init);
+
+    auto range   = std::span(data.data(), 0);
+    auto rresult = std::ranges::fold_left_with_iter(range, init, std::plus());
+
+    assert(rresult.in == result.in);
+    assert(rresult.result == result.result);
+  }
+  {
+    auto data   = std::vector{1, 3, 5, 7, 9};
+    auto result = std::ranges::fold_left_with_iter(data.begin(), data.end(), 0, std::plus());
+
+    assert(result.in == data.end());
+    assert(result.result == gaussian_sum(data)); // sum of n ascending odd numbers = n^2
----------------
EricWF wrote:

The Gaussian sum tests are **brilliant!**
I'm so in love with them.
I had to do a ton of work to get there, bit I'm glad I did. There's an elegance and beauty usings maths to avoid the trap of computing the expected result the same way twice, both incorrectly.

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