Add non-parallel version of for_each_n (+tests) from the Parallelism TS

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@303833 91177308-0d34-0410-b5e6-96231b3b80d8

include/algorithm

@@ -35,6 +35,9 @@ template <class InputIterator, class Function>
     Function
     for_each(InputIterator first, InputIterator last, Function f);
 
+template<class InputIterator, class Size, class Function>
+    InputIterator for_each_n(InputIterator first, Size n, Function f); // C++17
+
 template <class InputIterator, class T>
     InputIterator
     find(InputIterator first, InputIterator last, const T& value);
@@ -961,6 +964,24 @@ for_each(_InputIterator __first, _InputIterator __last, _Function __f)
     return __f;
 }
 
+// for_each_n
+
+template <class _InputIterator, class _Size, class _Function>
+inline _LIBCPP_INLINE_VISIBILITY
+_InputIterator
+for_each_n(_InputIterator __first, _Size __orig_n, _Function __f)
+{
+    typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;
+    _IntegralSize __n = __orig_n;
+    while (__n > 0)
+    {
+         __f(*__first);
+         ++__first;
+         --__n;
+    }
+    return __first;
+}
+
 // find
 
 template <class _InputIterator, class _Tp>

test/std/algorithms/alg.nonmodifying/alg.foreach/for_each_n.pass.cpp

@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// template<class InputIterator, class Size, class Function>
+//    InputIterator for_each_n(InputIterator first, Size n, Function f);
+
+
+#include <algorithm>
+#include <cassert>
+
+#include "test_iterators.h"
+
+struct for_each_test
+{
+    for_each_test(int c) : count(c) {}
+    int count;
+    void operator()(int& i) {++i; ++count;}
+};
+
+int main()
+{
+    typedef input_iterator<int*> Iter;
+    int ia[] = {0, 1, 2, 3, 4, 5};
+    const unsigned s = sizeof(ia)/sizeof(ia[0]);
+
+    {
+    auto f = for_each_test(0);
+    Iter it = std::for_each_n(Iter(ia), 0, std::ref(f));
+    assert(it == Iter(ia));
+    assert(f.count == 0);    
+    }
+
+    {
+    auto f = for_each_test(0);
+    Iter it = std::for_each_n(Iter(ia), s, std::ref(f));
+    
+    assert(it == Iter(ia+s));
+    assert(f.count == s);    
+    for (unsigned i = 0; i < s; ++i)
+        assert(ia[i] == static_cast<int>(i+1));
+    }
+
+    {
+    auto f = for_each_test(0);
+    Iter it = std::for_each_n(Iter(ia), 1, std::ref(f));
+    
+    assert(it == Iter(ia+1));
+    assert(f.count == 1);    
+    for (unsigned i = 0; i < 1; ++i)
+        assert(ia[i] == static_cast<int>(i+2));
+    }
+}