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