[gcc(refs/users/ppalka/heads/libstdcxx-constrained-algos)] Add ranges::search_n (WIP)

[Patrick Palka]

https://gcc.gnu.org/g:0d01e2c08307d4e640fb48b680f8320c2c8cc324

commit 0d01e2c08307d4e640fb48b680f8320c2c8cc324
Author: Patrick Palka <ppalka@gcc.gnu.org>
Date:   Fri Jan 17 17:39:13 2020 -0500

    Add ranges::search_n (WIP)

Diff:
---
 libstdc++-v3/include/bits/ranges_algo.h            | 60 +++++++++++++++++
 .../25_algorithms/search_n/constrained.cc          | 77 ++++++++++++++++++++++
 2 files changed, 137 insertions(+)

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index 1cfa399..73c7a7f 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -418,6 +418,66 @@ namespace ranges
 			    std::move(__proj1), std::move(__proj2));
     }
 
+  template<forward_iterator _Iter, sentinel_for<_Iter> _Sent, class _Tp,
+	   class _Pred = ranges::equal_to, class _Proj = identity>
+    requires indirectly_comparable<_Iter, const _Tp*, _Pred, _Proj>
+    constexpr subrange<_Iter>
+    search_n(_Iter __first, _Sent __last, iter_difference_t<_Iter> __count,
+	     const _Tp& __value, _Pred __pred = {}, _Proj __proj = {})
+    {
+      if (__count <= 0)
+	return {__first, __first};
+
+      auto __value_comp = [&] <typename _Rp> (_Rp&& __arg) {
+	  return std::__invoke(__pred, std::forward<_Rp>(__arg), __value);
+      };
+      if (__count == 1)
+	{
+	  __first = ranges::find_if(std::move(__first), __last,
+				    std::move(__value_comp), std::move(__proj));
+	  if (__first == __last)
+	    return {__last, __last};
+	  else
+	    {
+	      __last = __first;
+	      ++__last;
+	      return {__first, __last};
+	    }
+	}
+
+      __first = ranges::find_if(__first, __last, __value_comp, __proj);
+      while (__first != __last)
+	{
+	  auto __n = __count;
+	  auto __i = __first;
+	  ++__i;
+	  while (__i != __last && __n != 1
+		 && std::__invoke(__pred, __value, std::__invoke(__proj, *__i)))
+	    {
+	      ++__i;
+	      --__n;
+	    }
+	  if (__n == 1)
+	    return {__first, __i};
+	  if (__i == __last)
+	    return {__last, __last};
+	  __first = ranges::find_if(__first, __last, __value_comp, __proj);
+	}
+      return {__last, __last};
+    }
+
+  template<forward_range _Range, class _Tp,
+	   class _Pred = ranges::equal_to, class _Proj = identity>
+    requires indirectly_comparable<iterator_t<_Range>, const _Tp*, _Pred, _Proj>
+    constexpr safe_subrange_t<_Range>
+    search_n(_Range&& __r, range_difference_t<_Range> __count,
+	     const _Tp& __value, _Pred __pred = {}, _Proj __proj = {})
+    {
+      return ranges::search_n(ranges::begin(__r), ranges::end(__r),
+			      std::move(__count), __value,
+			      std::move(__pred), std::move(__proj));
+    }
+
   template<forward_iterator _Iter1, sentinel_for<_Iter1> _Sent1,
 	   forward_iterator _Iter2, sentinel_for<_Iter2> _Sent2,
 	   class _Pred = ranges::equal_to,
diff --git a/libstdc++-v3/testsuite/25_algorithms/search_n/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/search_n/constrained.cc
new file mode 100644
index 0000000..b0733a6
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/search_n/constrained.cc
@@ -0,0 +1,77 @@
+// 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 } }
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+using __gnu_test::test_container;
+using __gnu_test::test_range;
+using __gnu_test::input_iterator_wrapper;
+using __gnu_test::forward_iterator_wrapper;
+using __gnu_test::bidirectional_iterator_wrapper;
+
+namespace ranges = std::ranges;
+
+struct X { int i; };
+
+void
+test01()
+{
+  int x[] = { {2}, {2}, {6}, {8}, {10}, {11} };
+  auto res = ranges::search_n(x+0, x+6, 2, 2);
+  VERIFY( res.begin() == x+0 && res.end() == x+2 );
+
+  int z[] = { {1}, {2}, {2}, {4}, {5}, {6} };
+  res = ranges::search_n(z, 3, 3, std::greater<int>());
+  VERIFY( res.begin() == z+3 && res.end() == z+6 );
+
+  test_container<int, forward_iterator_wrapper> cx(x);
+  auto res2 = ranges::search_n(cx, 2, 2);
+  VERIFY( res2.begin() == cx.begin() && *res2.end() == 6 );
+
+  int y[] = { {2}, {2}, {8}, {2}, {2}, {2}, {5} };
+  test_range<int, forward_iterator_wrapper> ry(y);
+  auto res3 = ranges::search_n(ry, 2, 3);
+  VERIFY( *res3.begin() == 2 && *res3.end() == 5 );
+}
+
+void
+test02()
+{
+  static constexpr X x[] = { {2}, {2}, {6}, {8}, {10}, {2} };
+  static constexpr X y[] = { {2}, {6}, {8}, {8}, {8}, {2} };
+  static constexpr int z[] = { {2}, {6}, {8}, {10}, {2}, {2} };
+
+  static_assert(ranges::search_n(z, 0, 5).end() == z+0);
+  static_assert(ranges::search_n(z, 1, 5).begin() == z+6);
+  static_assert(ranges::search_n(x, 2, 3, {}, &X::i).begin() == x+6);
+  static_assert(ranges::search_n(x, 2, 2, {}, &X::i).end() == x+2);
+  static_assert(ranges::search_n(y, 3, 8, {}, &X::i).begin() == y+2);
+  static_assert(ranges::search_n(y, 3, 8, {}, &X::i).end() == y+5);
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}
+