dlib C++ Library - disjoint_subsets.h

    // Copyright (C) 2011  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_DISJOINT_SUBsETS_Hh_
    #define DLIB_DISJOINT_SUBsETS_Hh_

    #include "disjoint_subsets_abstract.h"
    #include <vector>
    #include "../algs.h"

    namespace dlib
    {

    // ----------------------------------------------------------------------------------------

        class disjoint_subsets
        {
        public:

            void clear (
            ) noexcept
            {
                items.clear();
            }

            void set_size (
                unsigned long new_size
            )
            {
                items.resize(new_size);
                for (unsigned long i = 0; i < items.size(); ++i)
                {
                    items[i].parent = i;
                    items[i].rank = 0;
                }
            }

            size_t size (
            ) const noexcept
            {
                return items.size();
            }

            unsigned long find_set (
                unsigned long item
            ) const
            {
                // make sure requires clause is not broken
                DLIB_ASSERT(item < size(),
                    "\t unsigned long disjoint_subsets::find_set()"
                    << "\n\t item must be less than size()"
                    << "\n\t item: " << item
                    << "\n\t size(): " << size()
                    << "\n\t this: " << this
                    );

                if (items[item].parent == item)
                {
                    return item;
                }
                else
                {
                    // find root of item
                    unsigned long x = item;
                    do
                    {
                        x = items[x].parent;
                    } while (items[x].parent != x);

                    // do path compression
                    const unsigned long root = x;
                    x = item;
                    while (items[x].parent != x)
                    {
                        const unsigned long prev = x;
                        x = items[x].parent;
                        items[prev].parent = root;
                    }

                    return root;
                }
            }

            unsigned long merge_sets (
                unsigned long a,
                unsigned long b
            )
            {
                // make sure requires clause is not broken
                DLIB_ASSERT(a != b &&
                            a < size() &&
                            b < size() &&
                            find_set(a) == a &&
                            find_set(b) == b,
                    "\t unsigned long disjoint_subsets::merge_sets(a,b)"
                    << "\n\t invalid arguments were given to this function"
                    << "\n\t a: " << a
                    << "\n\t b: " << b
                    << "\n\t size(): " << size()
                    << "\n\t find_set(a): " << find_set(a)
                    << "\n\t find_set(b): " << find_set(b)
                    << "\n\t this: " << this
                    );

                if (items[a].rank > items[b].rank)
                {
                    items[b].parent = a;
                    return a;
                }
                else
                {
                    items[a].parent = b;
                    if (items[a].rank == items[b].rank)
                    {
                        items[b].rank = items[b].rank + 1;
                    }
                    return b;
                }
            }

        private:

            /*
                See the book Introduction to Algorithms by Cormen, Leiserson, Rivest and Stein
                for a discussion of how this algorithm works.
            */

            struct data
            {
                unsigned long rank;
                unsigned long parent;
            };

            mutable std::vector<data> items;

        };

    // ----------------------------------------------------------------------------------------

    }

    #endif // DLIB_DISJOINT_SUBsETS_Hh_