Implement std::experimental::sample.

Following specification in "C++ Extensions for Library Fundamentals":
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html#alg.random.sample



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@237264 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/algorithm

@@ -0,0 +1,114 @@
+// -*- C++ -*-
+//===-------------------------- algorithm ---------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_EXPERIMENTAL_ALGORITHM
+#define _LIBCPP_EXPERIMENTAL_ALGORITHM
+
+/*
+   experimental/algorithm synopsis
+
+#include <algorithm>
+
+namespace std {
+namespace experimental {
+inline namespace fundamentals_v1 {
+
+template <class ForwardIterator, class Searcher>
+ForwardIterator search(ForwardIterator first, ForwardIterator last,
+                       const Searcher &searcher);
+template <class PopulationIterator, class SampleIterator, class Distance,
+          class UniformRandomNumberGenerator>
+SampleIterator sample(PopulationIterator first, PopulationIterator last,
+                      SampleIterator out, Distance n,
+                      UniformRandomNumberGenerator &&g);
+
+} // namespace fundamentals_v1
+} // namespace experimental
+} // namespace std
+
+*/
+
+#include <experimental/__config>
+#include <algorithm>
+#include <type_traits>
+
+#include <__undef_min_max>
+
+#include <__debug>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_LFTS
+
+
+template <class _PopulationIterator, class _SampleIterator, class _Distance,
+          class _UniformRandomNumberGenerator>
+_LIBCPP_INLINE_VISIBILITY
+_SampleIterator __sample(_PopulationIterator __first,
+                         _PopulationIterator __last, _SampleIterator __out,
+                         _Distance __n,
+                         _UniformRandomNumberGenerator &&__g,
+                         input_iterator_tag) {
+
+  _Distance __k = 0;
+  for (; __first != __last && __k < __n; ++__first, (void)++__k)
+    __out[__k] = *__first;
+  _Distance __sz = __k;
+  for (; __first != __last; ++__first, (void)++__k) {
+    _Distance __r = _VSTD::uniform_int_distribution<_Distance>(0, __k)(__g);
+    if (__r < __sz)
+      __out[__r] = *__first;
+  }
+  return __out + _VSTD::min(__n, __k);
+}
+
+template <class _PopulationIterator, class _SampleIterator, class _Distance,
+          class _UniformRandomNumberGenerator>
+_LIBCPP_INLINE_VISIBILITY
+_SampleIterator __sample(_PopulationIterator __first,
+                         _PopulationIterator __last, _SampleIterator __out,
+                         _Distance __n,
+                         _UniformRandomNumberGenerator &&__g,
+                         forward_iterator_tag) {
+  _Distance __unsampled_sz = _VSTD::distance(__first, __last);
+  for (__n = _VSTD::min(__n, __unsampled_sz); __n != 0; ++__first) {
+    _Distance __r =
+        _VSTD::uniform_int_distribution<_Distance>(0, --__unsampled_sz)(__g);
+    if (__r < __n) {
+      *__out++ = *__first;
+      --__n;
+    }
+  }
+  return __out;
+}
+
+template <class _PopulationIterator, class _SampleIterator, class _Distance,
+          class _UniformRandomNumberGenerator>
+_LIBCPP_INLINE_VISIBILITY
+_SampleIterator sample(_PopulationIterator __first,
+                         _PopulationIterator __last, _SampleIterator __out,
+                         _Distance __n, _UniformRandomNumberGenerator &&__g) {
+  typedef typename iterator_traits<_PopulationIterator>::iterator_category
+        _PopCategory;
+  typedef typename iterator_traits<_PopulationIterator>::difference_type
+        _Difference;
+  typedef typename common_type<_Distance, _Difference>::type _CommonType;
+  _LIBCPP_ASSERT(__n >= 0, "N must be a positive number.");
+  return _VSTD_LFTS::__sample(
+      __first, __last, __out, _CommonType(__n),
+      _VSTD::forward<_UniformRandomNumberGenerator>(__g),
+      _PopCategory());
+}
+
+_LIBCPP_END_NAMESPACE_LFTS
+
+#endif /* _LIBCPP_EXPERIMENTAL_ALGORITHM */