New algorithm 'partition_subrange'. Name not 100% final, bue we've got tests. No docs yet.

include/boost/algorithm/sort_subrange.hpp

@@ -67,6 +67,43 @@ namespace boost { namespace algorithm {
 
 /// range versions?
 
+
+/// \fn partition_subrange ( T const& val, 
+///               Iterator first,     Iterator last, 
+///               Iterator sub_first, Iterator sub_last, 
+///               Pred p )
+/// \brief Gather the elements of the subrange [sub_first, sub_last) that is 
+///     inside the range [first, last) as if you had sorted the entire range.
+/// 
+/// \param first       The start of the larger range
+/// \param last        The end of the larger range
+/// \param sub_first   The start of the sub range
+/// \param sub_last    The end of the sub range
+/// \param p           A predicate to use to compare the values.
+///                        p ( a, b ) returns a boolean.
+///
+  template<typename Iterator, typename Pred> 
+  void partition_subrange (
+  	Iterator first,     Iterator last, 
+  	Iterator sub_first, Iterator sub_last,
+  	Pred p)
+  {
+  	if (sub_first != first) {
+  		(void) std::nth_element(first, sub_first, last, p);
+  		++sub_first;
+  		}
+  	
+  	if (sub_last != last)
+  		(void) std::nth_element(sub_first, sub_last, last, p);
+  }
+
+  template<typename Iterator> 
+  void partition_subrange (Iterator first, Iterator last, Iterator sub_first, Iterator sub_last)
+  {
+  	typedef typename std::iterator_traits<Iterator>::value_type value_type;
+  	return partition_subrange(first, last, sub_first, sub_last, std::less<value_type>());
+  }
+
 }}
 
 #endif // BOOST_ALGORITHM_SORT_SUBRANGE_HPP