[libcxx-commits] [libcxx] Fix ranges_rotate.pass.cpp complexity checks (PR #158144)

[Jonathan Wakely via libcxx-commits]

https://github.com/jwakely created https://github.com/llvm/llvm-project/pull/158144

The complexity is "at most N swaps" _for each invocation of `rotate`_, but the tests currently assert that the total number of swaps for N calls is at most N. The standard allows that to be N squared, so the test is either requiring more than the standard (and the comment in the test) promises, or somebody just forgot to reset the counter on each iteration.

If the intent really is to verify the libc++ guarantee, not the standard one, then shouldn't `expected` be set to N/2 i.e. 5, since that's what libc++ actually does?

Either way, to be portable to other implementations, swaps should be reset on each loop. If you want to guard that with `#ifndef _LIBCPP_VERSION` it would not change the test for libc++.

>From 950a034845088298d1ff0c7af9a094fd6d6dfd8f Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <github at kayari.org>
Date: Thu, 11 Sep 2025 21:06:04 +0100
Subject: [PATCH] Fix ranges_rotate.pass.cpp complexity checks

The complexity is "at most N swaps" _for each invocation of `rotate`_, but the tests currently assert that the total number of swaps for N calls is at most N. The standard allows that to be N squared, so the test is either requiring more than the standard (and the comment in the test) promises, or somebody just forgot to reset the counter on each iteration.

If the intent really is to verify the libc++ guarantee, not the standard one, then shouldn't `expected` be set to N/2 i.e. 5, since that's what libc++ actually does?

Either way, to be portable to other implementations, swaps should be reset on each loop. If you want to guard that with `#ifndef _LIBCPP_VERSION` it would not change the test for libc++.
---
 .../alg.modifying.operations/alg.rotate/ranges_rotate.pass.cpp  | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/ranges_rotate.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/ranges_rotate.pass.cpp
index 5f594400e8321..574e96dea46a0 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/ranges_rotate.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/ranges_rotate.pass.cpp
@@ -173,6 +173,7 @@ constexpr bool test() {
       auto end   = adl::Iterator::TrackSwaps(in.data() + in.size(), swaps);
 
       for (std::size_t mid = 0; mid != input.size(); ++mid) {
+        swaps = 0;
         std::ranges::rotate(begin, begin + mid, end);
         assert(swaps <= expected);
       }
@@ -186,6 +187,7 @@ constexpr bool test() {
       auto range = std::ranges::subrange(begin, end);
 
       for (std::size_t mid = 0; mid != input.size(); ++mid) {
+        swaps = 0;
         std::ranges::rotate(range, begin + mid);
         assert(swaps <= expected);
       }