8114104a43
Rename module partition to partitioner. This cultivates naming used in existing modules like extractor, customizer, etc. - noun vs verb (word partition is both though).
56 lines
1.8 KiB
C++
56 lines
1.8 KiB
C++
#ifndef OSRM_PARTITIONER_REORDER_FIRST_LAST_HPP
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#define OSRM_PARTITIONER_REORDER_FIRST_LAST_HPP
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#include <boost/assert.hpp>
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#include <algorithm>
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#include <cstddef>
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#include <iterator>
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namespace osrm
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{
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namespace partitioner
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{
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// Reorders the first n elements in the range to satisfy the comparator,
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// and the last n elements to satisfy the comparator with arguments flipped.
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// Note: no guarantees to the element's ordering inside the reordered ranges.
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template <typename RandomIt, typename Comparator>
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void reorderFirstLast(RandomIt first, RandomIt last, std::size_t n, Comparator comp)
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{
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BOOST_ASSERT_MSG(n <= (last - first) / std::size_t{2}, "overlapping subranges not allowed");
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if (n == 0 || (last - first < 2))
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return;
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// Reorder first n: guarantees that the predicate holds for the first elements.
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std::nth_element(first, first + (n - 1), last, comp);
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// Reorder last n: guarantees that the flipped predicate holds for the last k elements.
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// We reorder from the end backwards up to the end of the already reordered range.
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// We can not use std::not2, since then e.g. std::less<> would lose its irreflexive
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// requirements.
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std::reverse_iterator<RandomIt> rfirst{last}, rlast{first + n};
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const auto flipped = [](auto fn) {
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return [fn](auto &&lhs, auto &&rhs) {
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return fn(std::forward<decltype(lhs)>(rhs), std::forward<decltype(rhs)>(lhs));
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};
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};
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std::nth_element(rfirst, rfirst + (n - 1), rlast, flipped(comp));
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}
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template <typename RandomAccessRange, typename Compare>
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void reorderFirstLast(RandomAccessRange &rng, std::size_t n, Compare comp)
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{
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using std::begin;
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using std::end;
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return reorderFirstLast(begin(rng), end(rng), n, comp);
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}
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} // ns partition
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} // ns osrm
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#endif
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