1 //---------------------------------------------------------------------------//
2 // Copyright (c) 2013 Kyle Lutz <kyle.r.lutz@gmail.com>
3 //
4 // Distributed under the Boost Software License, Version 1.0
5 // See accompanying file LICENSE_1_0.txt or copy at
6 // http://www.boost.org/LICENSE_1_0.txt
7 //
8 // See http://boostorg.github.com/compute for more information.
9 //---------------------------------------------------------------------------//
10
11 #ifndef BOOST_COMPUTE_ALGORITHM_NTH_ELEMENT_HPP
12 #define BOOST_COMPUTE_ALGORITHM_NTH_ELEMENT_HPP
13
14 #include <boost/static_assert.hpp>
15
16 #include <boost/compute/command_queue.hpp>
17 #include <boost/compute/algorithm/fill_n.hpp>
18 #include <boost/compute/algorithm/find.hpp>
19 #include <boost/compute/algorithm/partition.hpp>
20 #include <boost/compute/algorithm/sort.hpp>
21 #include <boost/compute/functional/bind.hpp>
22 #include <boost/compute/type_traits/is_device_iterator.hpp>
23
24 namespace boost {
25 namespace compute {
26
27 /// Rearranges the elements in the range [\p first, \p last) such that
28 /// the \p nth element would be in that position in a sorted sequence.
29 ///
30 /// Space complexity: \Omega(3n)
31 template<class Iterator, class Compare>
nth_element(Iterator first,Iterator nth,Iterator last,Compare compare,command_queue & queue=system::default_queue ())32 inline void nth_element(Iterator first,
33 Iterator nth,
34 Iterator last,
35 Compare compare,
36 command_queue &queue = system::default_queue())
37 {
38 BOOST_STATIC_ASSERT(is_device_iterator<Iterator>::value);
39 if(nth == last) return;
40
41 typedef typename std::iterator_traits<Iterator>::value_type value_type;
42
43 while(1)
44 {
45 value_type value = nth.read(queue);
46
47 using boost::compute::placeholders::_1;
48 Iterator new_nth = partition(
49 first, last, ::boost::compute::bind(compare, _1, value), queue
50 );
51
52 Iterator old_nth = find(new_nth, last, value, queue);
53
54 value_type new_value = new_nth.read(queue);
55
56 fill_n(new_nth, 1, value, queue);
57 fill_n(old_nth, 1, new_value, queue);
58
59 new_value = nth.read(queue);
60
61 if(value == new_value) break;
62
63 if(std::distance(first, nth) < std::distance(first, new_nth))
64 {
65 last = new_nth;
66 }
67 else
68 {
69 first = new_nth;
70 }
71 }
72 }
73
74 /// \overload
75 template<class Iterator>
nth_element(Iterator first,Iterator nth,Iterator last,command_queue & queue=system::default_queue ())76 inline void nth_element(Iterator first,
77 Iterator nth,
78 Iterator last,
79 command_queue &queue = system::default_queue())
80 {
81 BOOST_STATIC_ASSERT(is_device_iterator<Iterator>::value);
82 if(nth == last) return;
83
84 typedef typename std::iterator_traits<Iterator>::value_type value_type;
85
86 less<value_type> less_than;
87
88 return nth_element(first, nth, last, less_than, queue);
89 }
90
91 } // end compute namespace
92 } // end boost namespace
93
94 #endif // BOOST_COMPUTE_ALGORITHM_NTH_ELEMENT_HPP
95