// Contains get_min_count, the core optimization of the spreadsort algorithm. | |
// Also has other helper functions commonly useful across variants. | |
// Copyright Steven J. Ross 2001 - 2014. | |
// Distributed under the Boost Software License, Version 1.0. | |
// (See accompanying file LICENSE_1_0.txt or copy at | |
// http://www.boost.org/LICENSE_1_0.txt) | |
// See http://www.boost.org/libs/sort for library home page. | |
/* | |
Some improvements suggested by: | |
Phil Endecott and Frank Gennari | |
*/ | |
#ifndef BOOST_SORT_SPREADSORT_DETAIL_SPREAD_SORT_COMMON_HPP | |
#define BOOST_SORT_SPREADSORT_DETAIL_SPREAD_SORT_COMMON_HPP | |
#include <algorithm> | |
#include <vector> | |
#include <cstring> | |
#include <limits> | |
#include <functional> | |
#include <boost/static_assert.hpp> | |
#include <boost/serialization/static_warning.hpp> | |
#include <boost/sort/spreadsort/detail/constants.hpp> | |
#include <boost/cstdint.hpp> | |
namespace boost { | |
namespace sort { | |
namespace spreadsort { | |
namespace detail { | |
//This only works on unsigned data types | |
template <typename T> | |
inline unsigned | |
rough_log_2_size(const T& input) | |
{ | |
unsigned result = 0; | |
//The && is necessary on some compilers to avoid infinite loops | |
//it doesn't significantly impair performance | |
while ((input >> result) && (result < (8*sizeof(T)))) ++result; | |
return result; | |
} | |
//Gets the minimum size to call spreadsort on to control worst-case runtime. | |
//This is called for a set of bins, instead of bin-by-bin, to minimize | |
//runtime overhead. | |
//This could be replaced by a lookup table of sizeof(Div_type)*8 but this | |
//function is more general. | |
template<unsigned log_mean_bin_size, | |
unsigned log_min_split_count, unsigned log_finishing_count> | |
inline size_t | |
get_min_count(unsigned log_range) | |
{ | |
const size_t typed_one = 1; | |
const unsigned min_size = log_mean_bin_size + log_min_split_count; | |
//Assuring that constants have valid settings | |
BOOST_STATIC_ASSERT(log_min_split_count <= max_splits && | |
log_min_split_count > 0); | |
BOOST_STATIC_ASSERT(max_splits > 1 && | |
max_splits < (8 * sizeof(unsigned))); | |
BOOST_STATIC_ASSERT(max_finishing_splits >= max_splits && | |
max_finishing_splits < (8 * sizeof(unsigned))); | |
BOOST_STATIC_ASSERT(log_mean_bin_size >= 0); | |
BOOST_STATIC_ASSERT(log_finishing_count >= 0); | |
//if we can complete in one iteration, do so | |
//This first check allows the compiler to optimize never-executed code out | |
if (log_finishing_count < min_size) { | |
if (log_range <= min_size && log_range <= max_splits) { | |
//Return no smaller than a certain minimum limit | |
if (log_range <= log_finishing_count) | |
return typed_one << log_finishing_count; | |
return typed_one << log_range; | |
} | |
} | |
const unsigned base_iterations = max_splits - log_min_split_count; | |
//sum of n to n + x = ((x + 1) * (n + (n + x)))/2 + log_mean_bin_size | |
const unsigned base_range = | |
((base_iterations + 1) * (max_splits + log_min_split_count))/2 | |
+ log_mean_bin_size; | |
//Calculating the required number of iterations, and returning | |
//1 << (iteration_count + min_size) | |
if (log_range < base_range) { | |
unsigned result = log_min_split_count; | |
for (unsigned offset = min_size; offset < log_range; | |
offset += ++result); | |
//Preventing overflow; this situation shouldn't occur | |
if ((result + log_mean_bin_size) >= (8 * sizeof(size_t))) | |
return typed_one << ((8 * sizeof(size_t)) - 1); | |
return typed_one << (result + log_mean_bin_size); | |
} | |
//A quick division can calculate the worst-case runtime for larger ranges | |
unsigned remainder = log_range - base_range; | |
//the max_splits - 1 is used to calculate the ceiling of the division | |
unsigned bit_length = ((((max_splits - 1) + remainder)/max_splits) | |
+ base_iterations + min_size); | |
//Preventing overflow; this situation shouldn't occur | |
if (bit_length >= (8 * sizeof(size_t))) | |
return typed_one << ((8 * sizeof(size_t)) - 1); | |
//n(log_range)/max_splits + C, optimizing worst-case performance | |
return typed_one << bit_length; | |
} | |
// Resizes the bin cache and bin sizes, and initializes each bin size to 0. | |
// This generates the memory overhead to use in radix sorting. | |
template <class RandomAccessIter> | |
inline RandomAccessIter * | |
size_bins(size_t *bin_sizes, std::vector<RandomAccessIter> | |
&bin_cache, unsigned cache_offset, unsigned &cache_end, unsigned bin_count) | |
{ | |
// Clear the bin sizes | |
for (size_t u = 0; u < bin_count; u++) | |
bin_sizes[u] = 0; | |
//Make sure there is space for the bins | |
cache_end = cache_offset + bin_count; | |
if (cache_end > bin_cache.size()) | |
bin_cache.resize(cache_end); | |
return &(bin_cache[cache_offset]); | |
} | |
} | |
} | |
} | |
} | |
#endif |