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.

1 files changed, 58 insertions, 0 deletions

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>();
+}