[gcc r11-5096] libstdc++: Fix ranges::search_n for random access iterators [PR97828]

[gcc r11-5096] libstdc++: Fix ranges::search_n for random access iterators [PR97828]

[Patrick Palka]

https://gcc.gnu.org/g:8661f4faa875f361cd22a197774c1fa04cd0580b

commit r11-5096-g8661f4faa875f361cd22a197774c1fa04cd0580b
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Nov 17 10:28:20 2020 -0500

    libstdc++: Fix ranges::search_n for random access iterators [PR97828]
    
    My ranges transcription of the std::search_n implementation for random
    access iterators missed a crucial part of the algorithm which the
    existing tests didn't exercise.  When __remainder is less than __count
    at the start of an iteration of the outer while loop, it means we're
    continuing a partial match of __count - __remainder elements from the
    previous iteration.  If at the end of the iteration we don't complete
    this partial match, we need to reset __remainder so that it's only
    offset by the size of the most recent partial match before starting the
    next iteration.
    
    This patch fixes this appropriately, mirroring how it's done in the
    corresponding std::search_n implementation.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/97828
            * include/bits/ranges_algo.h (__search_n_fn::operator()): Check
            random_access_iterator before using the backtracking
            implementation.  When the backwards scan fails prematurely,
            reset __remainder appropriately.
            * testsuite/25_algorithms/search_n/97828.cc: New test.

Diff:
---
 libstdc++-v3/include/bits/ranges_algo.h            |  4 +-
 .../testsuite/25_algorithms/search_n/97828.cc      | 58 ++++++++++++++++++++++
 2 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index f1a4cc24c0d..3905fe44fb2 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -579,7 +579,8 @@ namespace ranges
 	      }
 	  }
 
-	if constexpr (sized_sentinel_for<_Sent, _Iter>)
+	if constexpr (sized_sentinel_for<_Sent, _Iter>
+		      && random_access_iterator<_Iter>)
 	  {
 	    auto __tail_size = __last - __first;
 	    auto __remainder = __count;
@@ -594,6 +595,7 @@ namespace ranges
 		    if (--__remainder == 0)
 		      return {__first - __count, __first};
 		  }
+		__remainder = __count + 1 - (__first - __backtrack);
 	      }
 	    auto __i = __first + __tail_size;
 	    return {__i, __i};
diff --git a/libstdc++-v3/testsuite/25_algorithms/search_n/97828.cc b/libstdc++-v3/testsuite/25_algorithms/search_n/97828.cc
new file mode 100644
index 00000000000..f692369cd3d
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/search_n/97828.cc
@@ -0,0 +1,58 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+// PR libstdc++/97828
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+using __gnu_test::test_sized_range;
+using __gnu_test::forward_iterator_wrapper;
+using __gnu_test::random_access_iterator_wrapper;
+
+template<template<typename> typename Wrapper>
+void
+test01()
+{
+  int x[] = {0,42,42,0,42,42,42};
+  test_sized_range<int, Wrapper> rx(x);
+  auto res = std::ranges::search_n(rx, 3, 42);
+  VERIFY( res.begin().ptr == x+4 && res.end().ptr == x+7 );
+}
+
+template<template<typename> typename Wrapper>
+void
+test02()
+{
+  int x[] = {0,42,42,0,42};
+  test_sized_range<int, Wrapper> rx(x);
+  auto res = std::ranges::search_n(rx, 3, 42);
+  VERIFY( res.begin().ptr == x+5 && res.end().ptr == x+5 );
+}
+
+int
+main()
+{
+  test01<forward_iterator_wrapper>();
+  test01<random_access_iterator_wrapper>();
+  test02<forward_iterator_wrapper>();
+  test02<random_access_iterator_wrapper>();
+}