libstdc++: Fix complexity of drop_view::begin() const [PR112641]

Views are required to have a amortized O(1) begin(), but our drop_view's
const begin overload is O(n) for non-common ranges with a non-sized
sentinel.  This patch reimplements it so that it's O(1) always.  See
also LWG 4009.

	PR libstdc++/112641

libstdc++-v3/ChangeLog:

	* include/std/ranges (drop_view::begin): Reimplement const
	overload so that it's O(1) always.
	* testsuite/std/ranges/adaptors/drop.cc (test10): New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index cebe10683f91fe45335a108efdc753e9fffd63e1..743429dbceae77b03fd4ecc0edf25f5776cd1032 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2664,8 +2664,8 @@ namespace views::__adaptor
       begin() const
        requires random_access_range<const _Vp> && sized_range<const _Vp>
       {
-       return ranges::next(ranges::begin(_M_base), _M_count,
-                           ranges::end(_M_base));
+       return ranges::begin(_M_base) + ranges::min(ranges::distance(_M_base),
+                                                   _M_count);
       }
 
       constexpr auto

diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/drop.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/drop.cc
index c9987c61e3c126c94551caa1c36a4de98bebe2fb..0bd5bebb785d41527d9d9d942368a7553673b1ca 100644 (file)
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/drop.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/drop.cc
@@ -274,6 +274,17 @@ test09()
   static_assert(!requires { views::all | drop; });
 }
 
+constexpr bool
+test10()
+{
+  // PR libstdc++/112641 - drop_view::begin const may have O(n) complexity
+  const auto s = ranges::subrange(views::iota(size_t(1)), size_t(-1));
+  const auto r = ranges::drop_view(s, s.size() - 1);
+  const auto b = r.begin(); // time out
+  VERIFY( *b == size_t(-1) );
+  return true;
+}
+
 int
 main()
 {
@@ -286,4 +297,5 @@ main()
   test07();
   test08();
   test09();
+  static_assert(test10());
 }