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spreadsort_common.hpp
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1 // Contains get_min_count, the core optimization of the spreadsort algorithm.
2 // Also has other helper functions commonly useful across variants.
3
4 // Copyright Steven J. Ross 2001 - 2014.
5 // Distributed under the Boost Software License, Version 1.0.
6 // (See accompanying file LICENSE_1_0.txt or copy at
7 // http://www.boost.org/LICENSE_1_0.txt)
8
9 // See http://www.boost.org/libs/sort for library home page.
10
11 /*
12 Some improvements suggested by:
13 Phil Endecott and Frank Gennari
14 */
15
16 #ifndef BOOST_SORT_SPREADSORT_DETAIL_SPREAD_SORT_COMMON_HPP
17 #define BOOST_SORT_SPREADSORT_DETAIL_SPREAD_SORT_COMMON_HPP
18 #include <algorithm>
19 #include <vector>
20 #include <cstring>
21 #include <limits>
22 #include <functional>
23 #include <boost/static_assert.hpp>
24 #include <boost/serialization/static_warning.hpp>
25 #include <boost/sort/spreadsort/detail/constants.hpp>
26 #include <boost/cstdint.hpp>
27
28 namespace boost {
29 namespace sort {
30 namespace detail {
31 //This only works on unsigned data types
32 template <typename T>
33 inline unsigned
34 rough_log_2_size(const T& input)
35 {
36 unsigned result = 0;
37 //The && is necessary on some compilers to avoid infinite loops
38 //it doesn't significantly impair performance
39 while ((input >> result) && (result < (8*sizeof(T)))) ++result;
40 return result;
41 }
42
43 //Gets the minimum size to call spreadsort on to control worst-case runtime.
44 //This is called for a set of bins, instead of bin-by-bin, to minimize
45 //runtime overhead.
46 //This could be replaced by a lookup table of sizeof(Div_type)*8 but this
47 //function is more general.
48 template<unsigned log_mean_bin_size,
49 unsigned log_min_split_count, unsigned log_finishing_count>
50 inline size_t
51 get_min_count(unsigned log_range)
52 {
53 const size_t typed_one = 1;
54 const unsigned min_size = log_mean_bin_size + log_min_split_count;
55 //Assuring that constants have valid settings
56 BOOST_STATIC_ASSERT(log_min_split_count <= max_splits &&
57 log_min_split_count > 0);
58 BOOST_STATIC_ASSERT(max_splits > 1 &&
59 max_splits < (8 * sizeof(unsigned)));
60 BOOST_STATIC_ASSERT(max_finishing_splits >= max_splits &&
61 max_finishing_splits < (8 * sizeof(unsigned)));
62 BOOST_STATIC_ASSERT(log_mean_bin_size >= 0);
63 BOOST_STATIC_ASSERT(log_finishing_count >= 0);
64 //if we can complete in one iteration, do so
65 //This first check allows the compiler to optimize never-executed code out
66 if (log_finishing_count < min_size) {
67 if (log_range <= min_size && log_range <= max_splits) {
68 //Return no smaller than a certain minimum limit
69 if (log_range <= log_finishing_count)
70 return typed_one << log_finishing_count;
71 return typed_one << log_range;
72 }
73 }
74 const unsigned base_iterations = max_splits - log_min_split_count;
75 //sum of n to n + x = ((x + 1) * (n + (n + x)))/2 + log_mean_bin_size
76 const unsigned base_range =
77 ((base_iterations + 1) * (max_splits + log_min_split_count))/2
78 + log_mean_bin_size;
79 //Calculating the required number of iterations, and returning
80 //1 << (iteration_count + min_size)
81 if (log_range < base_range) {
82 unsigned result = log_min_split_count;
83 for (unsigned offset = min_size; offset < log_range;
84 offset += ++result);
85 //Preventing overflow; this situation shouldn't occur
86 if ((result + log_mean_bin_size) >= (8 * sizeof(size_t)))
87 return typed_one << ((8 * sizeof(size_t)) - 1);
88 return typed_one << (result + log_mean_bin_size);
89 }
90 //A quick division can calculate the worst-case runtime for larger ranges
91 unsigned remainder = log_range - base_range;
92 //the max_splits - 1 is used to calculate the ceiling of the division
93 unsigned bit_length = ((((max_splits - 1) + remainder)/max_splits)
94 + base_iterations + min_size);
95 //Preventing overflow; this situation shouldn't occur
96 if (bit_length >= (8 * sizeof(size_t)))
97 return typed_one << ((8 * sizeof(size_t)) - 1);
98 //n(log_range)/max_splits + C, optimizing worst-case performance
99 return typed_one << bit_length;
100 }
101
102 // Resizes the bin cache and bin sizes, and initializes each bin size to 0.
103 // This generates the memory overhead to use in radix sorting.
104 template <class RandomAccessIter>
105 inline RandomAccessIter *
106 size_bins(size_t *bin_sizes, std::vector<RandomAccessIter>
107 &bin_cache, unsigned cache_offset, unsigned &cache_end, unsigned bin_count)
108 {
109 // Clear the bin sizes
110 for (size_t u = 0; u < bin_count; u++)
111 bin_sizes[u] = 0;
112 //Make sure there is space for the bins
113 cache_end = cache_offset + bin_count;
114 if (cache_end > bin_cache.size())
115 bin_cache.resize(cache_end);
116 return &(bin_cache[cache_offset]);
117 }
118 }
119 }
120 }
121
122 #endif
boost
Definition: constants.hpp:11
boost::sort::detail::max_finishing_splits
Definition: constants.hpp:22
boost::sort::detail::get_min_count
size_t get_min_count(unsigned log_range)
Definition: spreadsort_common.hpp:51
boost::sort::detail::max_splits
Definition: constants.hpp:19
boost::sort::detail::rough_log_2_size
unsigned rough_log_2_size(const T &input)
Definition: spreadsort_common.hpp:34
constants.hpp
boost::sort::detail::size_bins
RandomAccessIter * size_bins(size_t *bin_sizes, std::vector< RandomAccessIter > &bin_cache, unsigned cache_offset, unsigned &cache_end, unsigned bin_count)
Definition: spreadsort_common.hpp:106