[libcxx-commits] [libcxx] [libc++][ranges] Avoid using `std::distance` in `ranges::contains_subrange` (PR #87155)

[A. Jiang via libcxx-commits]

https://github.com/frederick-vs-ja updated https://github.com/llvm/llvm-project/pull/87155

>From 88ef5c870608c25e19a9bdc9a1c806f9dc3935f5 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34 at live.cn>
Date: Sat, 30 Mar 2024 22:09:01 +0800
Subject: [PATCH] [libc++][ranges] Avoid using `std::distance` in
 `ranges::contains_subrange`

Either `std::distance` or `ranges::distance` are inefficient for
non-sized ranges. Also, calculation the range in `int` type is seriously
problematic.

This patch avoids using `distance` and calculation of the length of
non-sized ranges.
---
 .../include/__algorithm/ranges_contains_subrange.h  | 13 +++++--------
 .../alg.contains/ranges.contains_subrange.pass.cpp  |  4 ++++
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/libcxx/include/__algorithm/ranges_contains_subrange.h b/libcxx/include/__algorithm/ranges_contains_subrange.h
index 4cd03cbb537060..d1167371c340c6 100644
--- a/libcxx/include/__algorithm/ranges_contains_subrange.h
+++ b/libcxx/include/__algorithm/ranges_contains_subrange.h
@@ -15,7 +15,6 @@
 #include <__functional/ranges_operations.h>
 #include <__functional/reference_wrapper.h>
 #include <__iterator/concepts.h>
-#include <__iterator/distance.h>
 #include <__iterator/indirectly_comparable.h>
 #include <__iterator/projected.h>
 #include <__ranges/access.h>
@@ -53,8 +52,7 @@ struct __fn {
       _Pred __pred   = {},
       _Proj1 __proj1 = {},
       _Proj2 __proj2 = {}) {
-    auto __n2 = ranges::distance(__first2, __last2);
-    if (__n2 == 0)
+    if (__first2 == __last2)
       return true;
 
     auto __ret = ranges::search(
@@ -70,14 +68,13 @@ struct __fn {
     requires indirectly_comparable<iterator_t<_Range1>, iterator_t<_Range2>, _Pred, _Proj1, _Proj2>
   _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool static
   operator()(_Range1&& __range1, _Range2&& __range2, _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) {
-    auto __n2 = 0;
     if constexpr (sized_range<_Range2>) {
-      __n2 = ranges::size(__range2);
+      if (ranges::size(__range2) == 0)
+        return true;
     } else {
-      __n2 = std::distance(cbegin(__range2), cend(__range2));
+      if (ranges::begin(__range2) == ranges::end(__range2))
+        return true;
     }
-    if (__n2 == 0)
-      return true;
 
     auto __ret = ranges::search(__range1, __range2, __pred, std::ref(__proj1), std::ref(__proj2));
     return __ret.empty() == false;
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp
index d48ee9e4e7e02e..761691c2afdcb9 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp
@@ -309,6 +309,10 @@ constexpr bool test() {
     });
   });
 
+  assert(std::ranges::contains_subrange(
+      std::views::iota(0, 5), std::views::iota(0, 5) | std::views::filter([](int) { return true; })));
+  assert(!std::ranges::contains_subrange(std::views::iota(0ULL, 42ULL), std::views::iota(0ULL, 1ULL << 32)));
+
   return true;
 }