cpp-sort

Additional sorting algorithms for C++14

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It would be nice if only one or two of the sorting methods would dominate all of the others, regardless of application or the computer being used. But in fact, each method has its own peculiar virtues. […] Thus we find that nearly all of the algorithms deserve to be remembered, since there are some applications in which they turn out to be best. — Donald Knuth, The Art Of Computer Programming, Volume 3

cpp-sort is a generic C++14 header-only sorting library. It revolves around one main generic sorting interface and provides several small tools to pick and/or design sorting algorithms. Using its basic sorting features should be trivial enough:

#include <array>
#include <iostream>
#include <cpp-sort/sort.h>

int main()
{
    std::array<int, 5u> arr = { 5, 8, 3, 2, 9 };
    cppsort::sort(arr);
    
    // prints 2 3 5 8 9
    for (int val: arr)
    {
        std::cout << val << ' ';
    }
}

cpp-sort actually provides a full set of sorting-related features. The most important one is probably the concept of sorters and sorter adapters. Sorters are function objects implementing a sorting algorithm and sorter adapters are special class templates designed to adapt sorters and alter their behaviour in some specific manner. The library provides sorters implementing common and not-so-common sorting algorithms as well as some specific adapters. It also provides fixed-size sorters and tools such as sorter facade or sorter traits, designed to craft your own sorters. Here is a more complete example of what the library can do:

#include <algorithm>
#include <cassert>
#include <forward_list>
#include <functional>
#include <iterator>
#include <vector>
#include <cpp-sort/adapters.h>
#include <cpp-sort/sort.h>
#include <cpp-sort/sorters.h>

int main()
{
    struct wrapper { int value; }

    std::forward_list<wrapper> li = { {5}, {8}, {3}, {2}, {9} };
    std::vector<wrapper> vec = { {5}, {8}, {3}, {2}, {9} };
    
    // When used, this sorter will use a pattern-defeating quicksort
    // to sort random-access collections, and a mergesort otherwise
    using sorter = cppsort::hybrid_adapter<
        cppsort::pdq_sorter,
        cppsort::merge_sorter
    >;
    
    // Sort li and vec in reverse order using their value member
    cppsort::sort(sorter{}, li, std::greater<>{}, &wrapper::value);
    cppsort::sort(sorter{}, vec, std::greater<>{}, &wrapper::value);

    assert(std::equal(
        std::begin(li), std::end(li),
        std::begin(vec), std::end(vec)
    ));
}

You can read more about all the available tools and find some tutorials about using and extending cpp-sort in the wiki.

Benchmarks

The following graph has been generated with a script found in the benchmarks directory. It shows the time needed for one sorting algorithm to sort one million shuffled std::array<int, N> of sizes 0 to 15. It compares the sorters generally used to sort small arrays:

shuffled int arrays

These results were generated with MinGW g++ 5.2 with the compiler options -std=c++14 -O3 -march=native. This benchmark is just an example to make the introduction look good. You can find more commented benchmarks in the dedicated wiki page.

Compiler support

cpp-sort currently works with g++5 and clang++3.8. It uses some of the most recent (and not widely supported) C++14 features and will probably use the C++17 features once they are available. The overall goal is to make sure that the library works with the latest g++ and clang++ versions, without going out of its way to support older releases.

In the future, the branches will follow the following pattern: the master branch will remain C++14 and there will be a C++17 branch. There will be other branches forking the C++17 one for some of the published Technical Specifications (for example, there will likely be a branch for the concepts TS); these branches will eventually be merged in the C++17 one when the corresponding technical specifications are merged into the current C++ working draft (or in a C++20 branch if the specifications do not make it in time for the C++17 release). Of course the creation of such branches will depend on compiler support: if a feature isn’t supported by either the latest g++ or clang++, I won’t use it before the following release.

Thanks

I got a new car. I just need to put it together. They’re easier to steal piece by piece. — Jarod Kintz, $3.33

Even though some parts of the library are original research and some others correspond to custom and rather naive implementations of standard sorting algorithms, cpp-sort also reuses a great deal of code from open-source projects, often slightly altered to integrate seamlessly into the library. Here is a list of the external resources used to create this library. I hope that the many different licenses are compatible. If it is not the case, please contact me (or submit an issue) and we will see what can be done about it:

  • Some of the algorithms used by insertion_sorter and pdq_sorter come from Orson Peters’ pattern-defeating quicksort. Some parts of the benchmarks come from there as well.

  • The algorithm used by tim_sorter comes from Goro Fuji’s (gfx) implementation of a Timsort.

  • The three algorithms used by spread_sorter come from Steven Ross Boost.Sort module with some modifications so that they do not depend on Boost anymore.

  • utility::as_function, utility::static_const, and several projection-enhanced helper algorithms come from Eric Niebler’s Range v3 library. Several ideas such as proxy iterators, customization points and projections, as well as a few other utility functions also come from that library or from the related articles and standard C++ proposals.

  • Many enhanced standard algorithms are directly adapted from their counterparts in libc++, enhanced to handle both projections and proxy iterators.

  • The algorithm used by utility::inplace_merge is an implementation of a merge algorithm proposed by Dudziński and Dydek, and implemented by Alexander Stepanov and Paul McJones in their book Elements of Programming.

  • The implementation of Dijkstra’s smoothsort used by smooth_sorter has been directly adapted from Keith Schwarz’s implementation of the algorithm.

  • The algorithm used by block_sorter has been adapted from BonzaiThePenguin’s WikiSort.

  • The algorithm used by grail_sorter has been adapted from Mrrl’s GrailSort, hence the name.

  • The algorithms 17 to 22 used by sorting_network_sorter correspond to the ones found by Symmetry and Evolution based Network Sort Optimization (SENSO) published in Using Symmetry and Evolutionary Search to Minimize Sorting Networks by Valsalam and Miikkulainen.

  • The algorithms 0 to 16 used by sorting_network_sorter have been generated with Perl’s Algorithm::Networksort module.

  • Some of the optimizations used by sorting_network_sorter come from this discussion on StackOverflow and are backed by the article Applying Sorting Networks to Synthesize Optimized Sorting Libraries.

  • The LaTeX scripts used to draw the sorting networks are modified versions of kaayy’s sortingnetwork.tex, slightly adapted to be 0-based and draw the network from top to bottom.

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