Boost C++ Libraries

boost/heap/skew_heap.hpp

// boost heap: skew heap
//
// Copyright (C) 2010 Tim Blechmann
//
// 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)

#ifndef BOOST_HEAP_SKEW_HEAP_HPP
#define BOOST_HEAP_SKEW_HEAP_HPP

#include <algorithm>
#include <array>
#include <type_traits>
#include <utility>

#include <boost/assert.hpp>

#include <boost/heap/detail/heap_comparison.hpp>
#include <boost/heap/detail/heap_node.hpp>
#include <boost/heap/detail/stable_heap.hpp>
#include <boost/heap/detail/tree_iterator.hpp>

#ifdef BOOST_HAS_PRAGMA_ONCE
#    pragma once
#endif

#ifndef BOOST_DOXYGEN_INVOKED
#    ifdef BOOST_HEAP_SANITYCHECKS
#        define BOOST_HEAP_ASSERT BOOST_ASSERT
#    else
#        define BOOST_HEAP_ASSERT( expression ) static_assert( true, "force semicolon" )
#    endif
#endif

namespace boost { namespace heap {
namespace detail {

template < typename node_pointer, bool store_parent_pointer >
struct parent_holder
{
    parent_holder( void ) :
        parent_( nullptr )
    {}

    void set_parent( node_pointer parent )
    {
        BOOST_HEAP_ASSERT( static_cast< node_pointer >( this ) != parent );
        parent_ = parent;
    }

    node_pointer get_parent( void ) const
    {
        return parent_;
    }

    node_pointer parent_;
};

template < typename node_pointer >
struct parent_holder< node_pointer, false >
{
    void set_parent( node_pointer /*parent*/ )
    {}

    node_pointer get_parent( void ) const
    {
        return nullptr;
    }
};


template < typename value_type, bool store_parent_pointer >
struct skew_heap_node : parent_holder< skew_heap_node< value_type, store_parent_pointer >*, store_parent_pointer >
{
    typedef parent_holder< skew_heap_node< value_type, store_parent_pointer >*, store_parent_pointer > super_t;

    typedef std::array< skew_heap_node*, 2 >         child_list_type;
    typedef typename child_list_type::iterator       child_iterator;
    typedef typename child_list_type::const_iterator const_child_iterator;

    skew_heap_node( value_type const& v ) :
        value( v )
    {
        children.fill( 0 );
    }

    skew_heap_node( value_type&& v ) :
        value( v )
    {
        children.fill( 0 );
    }

    template < typename Alloc >
    skew_heap_node( skew_heap_node const& rhs, Alloc& allocator, skew_heap_node* parent ) :
        value( rhs.value )
    {
        super_t::set_parent( parent );
        node_cloner< skew_heap_node, skew_heap_node, Alloc > cloner( allocator );
        clone_child( 0, rhs, cloner );
        clone_child( 1, rhs, cloner );
    }

    template < typename Cloner >
    void clone_child( int index, skew_heap_node const& rhs, Cloner& cloner )
    {
        if ( rhs.children[ index ] )
            children[ index ] = cloner( *rhs.children[ index ], this );
        else
            children[ index ] = nullptr;
    }

    template < typename Alloc >
    void clear_subtree( Alloc& alloc )
    {
        node_disposer< skew_heap_node, skew_heap_node, Alloc > disposer( alloc );
        dispose_child( children[ 0 ], disposer );
        dispose_child( children[ 1 ], disposer );
    }

    template < typename Disposer >
    void dispose_child( skew_heap_node* node, Disposer& disposer )
    {
        if ( node )
            disposer( node );
    }

    std::size_t count_children( void ) const
    {
        size_t ret = 1;
        if ( children[ 0 ] )
            ret += children[ 0 ]->count_children();
        if ( children[ 1 ] )
            ret += children[ 1 ]->count_children();

        return ret;
    }

    template < typename HeapBase >
    bool is_heap( typename HeapBase::value_compare const& cmp ) const
    {
        for ( const_child_iterator it = children.begin(); it != children.end(); ++it ) {
            const skew_heap_node* child = *it;

            if ( child == nullptr )
                continue;

            if ( store_parent_pointer )
                BOOST_HEAP_ASSERT( child->get_parent() == this );

            if ( cmp( HeapBase::get_value( value ), HeapBase::get_value( child->value ) )
                 || !child->is_heap< HeapBase >( cmp ) )
                return false;
        }
        return true;
    }

    value_type                       value;
    std::array< skew_heap_node*, 2 > children;
};


typedef parameter::parameters< boost::parameter::optional< tag::allocator >,
                               boost::parameter::optional< tag::compare >,
                               boost::parameter::optional< tag::stable >,
                               boost::parameter::optional< tag::store_parent_pointer >,
                               boost::parameter::optional< tag::stability_counter_type >,
                               boost::parameter::optional< tag::constant_time_size >,
                               boost::parameter::optional< tag::mutable_ > >
    skew_heap_signature;

template < typename T, typename BoundArgs >
struct make_skew_heap_base
{
    static const bool constant_time_size
        = parameter::binding< BoundArgs, tag::constant_time_size, std::true_type >::type::value;

    typedef typename make_heap_base< T, BoundArgs, constant_time_size >::type               base_type;
    typedef typename make_heap_base< T, BoundArgs, constant_time_size >::allocator_argument allocator_argument;
    typedef typename make_heap_base< T, BoundArgs, constant_time_size >::compare_argument   compare_argument;

    static const bool is_mutable = extract_mutable< BoundArgs >::value;
    static const bool store_parent_pointer
        = parameter::binding< BoundArgs, tag::store_parent_pointer, boost::false_type >::type::value || is_mutable;

    typedef skew_heap_node< typename base_type::internal_type, store_parent_pointer > node_type;

    typedef typename boost::allocator_rebind< allocator_argument, node_type >::type allocator_type;

    struct type : base_type, allocator_type
    {
        type( compare_argument const& arg ) :
            base_type( arg )
        {}

        type( allocator_type const& arg ) :
            allocator_type( arg )
        {}

        type( type&& rhs ) :
            base_type( std::move( static_cast< base_type& >( rhs ) ) ),
            allocator_type( std::move( static_cast< allocator_type& >( rhs ) ) )
        {}

        type( type const& rhs ) :
            base_type( rhs ),
            allocator_type( rhs )
        {}

        type& operator=( type&& rhs )
        {
            base_type::operator=( std::move( static_cast< base_type& >( rhs ) ) );
            allocator_type::operator=( std::move( static_cast< allocator_type& >( rhs ) ) );
            return *this;
        }

        type& operator=( type const& rhs )
        {
            base_type::operator=( static_cast< base_type const& >( rhs ) );
            allocator_type::operator=( static_cast< allocator_type const& >( rhs ) );
            return *this;
        }
    };
};

} /* namespace detail */

/**
 * \class skew_heap
 * \brief skew heap
 *
 *
 * The template parameter T is the type to be managed by the container.
 * The user can specify additional options and if no options are provided default options are used.
 *
 * The container supports the following options:
 * - \c boost::heap::compare<>, defaults to \c compare<std::less<T> >
 * - \c boost::heap::stable<>, defaults to \c stable<false>
 * - \c boost::heap::stability_counter_type<>, defaults to \c stability_counter_type<boost::uintmax_t>
 * - \c boost::heap::allocator<>, defaults to \c allocator<std::allocator<T> >
 * - \c boost::heap::constant_time_size<>, defaults to \c constant_time_size<true>
 * - \c boost::heap::store_parent_pointer<>, defaults to \c store_parent_pointer<true>. Maintaining a parent pointer
 * adds some maintenance and size overhead, but iterating a heap is more efficient.
 * - \c boost::heap::mutable<>, defaults to \c mutable<false>.
 *
 */
#ifdef BOOST_DOXYGEN_INVOKED
template < class T, class... Options >
#else
template < typename T,
           class A0 = boost::parameter::void_,
           class A1 = boost::parameter::void_,
           class A2 = boost::parameter::void_,
           class A3 = boost::parameter::void_,
           class A4 = boost::parameter::void_,
           class A5 = boost::parameter::void_,
           class A6 = boost::parameter::void_ >
#endif
class skew_heap :
    private detail::make_skew_heap_base< T, typename detail::skew_heap_signature::bind< A0, A1, A2, A3, A4, A5, A6 >::type >::type
{
    typedef typename detail::skew_heap_signature::bind< A0, A1, A2, A3, A4, A5, A6 >::type bound_args;
    typedef detail::make_skew_heap_base< T, bound_args >                                   base_maker;
    typedef typename base_maker::type                                                      super_t;

    typedef typename super_t::internal_type         internal_type;
    typedef typename super_t::size_holder_type      size_holder;
    typedef typename base_maker::allocator_argument allocator_argument;

    static const bool store_parent_pointer = base_maker::store_parent_pointer;
    template < typename Heap1, typename Heap2 >
    friend struct heap_merge_emulate;

    struct implementation_defined : detail::extract_allocator_types< typename base_maker::allocator_argument >
    {
        typedef T value_type;

        typedef typename base_maker::compare_argument value_compare;
        typedef typename base_maker::allocator_type   allocator_type;

        typedef typename base_maker::node_type                                  node;
        typedef typename boost::allocator_pointer< allocator_type >::type       node_pointer;
        typedef typename boost::allocator_const_pointer< allocator_type >::type const_node_pointer;

        typedef detail::value_extractor< value_type, internal_type, super_t > value_extractor;

        typedef std::array< node_pointer, 2 >      child_list_type;
        typedef typename child_list_type::iterator child_list_iterator;

        typedef typename std::conditional<
            false,
            detail::recursive_tree_iterator< node,
                                             child_list_iterator,
                                             const value_type,
                                             value_extractor,
                                             detail::list_iterator_converter< node, child_list_type > >,
            detail::tree_iterator< node,
                                   const value_type,
                                   allocator_type,
                                   value_extractor,
                                   detail::dereferencer< node >,
                                   true,
                                   false,
                                   value_compare > >::type iterator;

        typedef iterator const_iterator;

        typedef detail::
            tree_iterator< node, const value_type, allocator_type, value_extractor, detail::dereferencer< node >, true, true, value_compare >
                ordered_iterator;

        typedef typename detail::extract_allocator_types< typename base_maker::allocator_argument >::reference reference;
        typedef detail::node_handle< node_pointer, super_t, reference > handle_type;
    };

    typedef typename implementation_defined::value_extractor value_extractor;
    typedef typename implementation_defined::node            node;
    typedef typename implementation_defined::node_pointer    node_pointer;

public:
    typedef T value_type;

    typedef typename implementation_defined::size_type       size_type;
    typedef typename implementation_defined::difference_type difference_type;
    typedef typename implementation_defined::value_compare   value_compare;
    typedef typename implementation_defined::allocator_type  allocator_type;
    typedef typename implementation_defined::reference       reference;
    typedef typename implementation_defined::const_reference const_reference;
    typedef typename implementation_defined::pointer         pointer;
    typedef typename implementation_defined::const_pointer   const_pointer;

    /// \copydoc boost::heap::priority_queue::iterator
    typedef typename implementation_defined::iterator         iterator;
    typedef typename implementation_defined::const_iterator   const_iterator;
    typedef typename implementation_defined::ordered_iterator ordered_iterator;

    static const bool constant_time_size    = super_t::constant_time_size;
    static const bool has_ordered_iterators = true;
    static const bool is_mergable           = true;
    static const bool is_stable             = detail::extract_stable< bound_args >::value;
    static const bool has_reserve           = false;
    static const bool is_mutable            = detail::extract_mutable< bound_args >::value;

    typedef
        typename std::conditional< is_mutable, typename implementation_defined::handle_type, void* >::type handle_type;

    /// \copydoc boost::heap::priority_queue::priority_queue(value_compare const &)
    explicit skew_heap( value_compare const& cmp = value_compare() ) :
        super_t( cmp ),
        root( nullptr )
    {}

    /// \copydoc boost::heap::priority_queue::priority_queue(allocator_type const &)
    explicit skew_heap( allocator_type const& alloc ) :
        super_t( alloc ),
        root( 0 )
    {}

    /// \copydoc boost::heap::priority_queue::priority_queue(priority_queue const &)
    skew_heap( skew_heap const& rhs ) :
        super_t( rhs ),
        root( 0 )
    {
        if ( rhs.empty() )
            return;

        clone_tree( rhs );
        size_holder::set_size( rhs.get_size() );
    }

    /// \copydoc boost::heap::priority_queue::operator=(priority_queue const & rhs)
    skew_heap& operator=( skew_heap const& rhs )
    {
        clear();
        size_holder::set_size( rhs.get_size() );
        static_cast< super_t& >( *this ) = rhs;

        clone_tree( rhs );
        return *this;
    }

    /// \copydoc boost::heap::priority_queue::priority_queue(priority_queue &&)
    skew_heap( skew_heap&& rhs ) :
        super_t( std::move( rhs ) ),
        root( rhs.root )
    {
        rhs.root = nullptr;
    }

    /// \copydoc boost::heap::priority_queue::operator=(priority_queue &&)
    skew_heap& operator=( skew_heap&& rhs )
    {
        super_t::operator=( std::move( rhs ) );
        root     = rhs.root;
        rhs.root = nullptr;
        return *this;
    }

    ~skew_heap( void )
    {
        clear();
    }

    /**
     * \b Effects: Adds a new element to the priority queue.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * */
    typename std::conditional< is_mutable, handle_type, void >::type push( value_type const& v )
    {
        typedef typename std::conditional< is_mutable, push_handle, push_void >::type push_helper;
        return push_helper::push( this, v );
    }

    /**
     * \b Effects: Adds a new element to the priority queue. The element is directly constructed in-place.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * */
    template < typename... Args >
    typename std::conditional< is_mutable, handle_type, void >::type emplace( Args&&... args )
    {
        typedef typename std::conditional< is_mutable, push_handle, push_void >::type push_helper;
        return push_helper::emplace( this, std::forward< Args >( args )... );
    }

    /// \copydoc boost::heap::priority_queue::empty
    bool empty( void ) const
    {
        return root == nullptr;
    }

    /// \copydoc boost::heap::binomial_heap::size
    size_type size( void ) const
    {
        if ( constant_time_size )
            return size_holder::get_size();

        if ( root == nullptr )
            return 0;
        else
            return root->count_children();
    }

    /// \copydoc boost::heap::priority_queue::max_size
    size_type max_size( void ) const
    {
        const allocator_type& alloc = *this;
        return boost::allocator_max_size( alloc );
    }

    /// \copydoc boost::heap::priority_queue::clear
    void clear( void )
    {
        if ( empty() )
            return;

        root->template clear_subtree< allocator_type >( *this );
        root->~node();
        allocator_type& alloc = *this;
        alloc.deallocate( root, 1 );
        root = nullptr;
        size_holder::set_size( 0 );
    }

    /// \copydoc boost::heap::priority_queue::get_allocator
    allocator_type get_allocator( void ) const
    {
        return *this;
    }

    /// \copydoc boost::heap::priority_queue::swap
    void swap( skew_heap& rhs )
    {
        super_t::swap( rhs );
        std::swap( root, rhs.root );
    }

    /// \copydoc boost::heap::priority_queue::top
    const_reference top( void ) const
    {
        BOOST_ASSERT( !empty() );

        return super_t::get_value( root->value );
    }

    /**
     * \b Effects: Removes the top element from the priority queue.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * */
    void pop( void )
    {
        BOOST_ASSERT( !empty() );

        node_pointer top = root;

        root = merge_children( root );
        size_holder::decrement();

        if ( root )
            BOOST_HEAP_ASSERT( root->get_parent() == nullptr );
        else
            BOOST_HEAP_ASSERT( size_holder::get_size() == 0 );

        top->~node();
        allocator_type& alloc = *this;
        alloc.deallocate( top, 1 );
        sanity_check();
    }

    /// \copydoc boost::heap::priority_queue::begin
    iterator begin( void ) const
    {
        return iterator( root, super_t::value_comp() );
    }

    /// \copydoc boost::heap::priority_queue::end
    iterator end( void ) const
    {
        return iterator();
    }

    /// \copydoc boost::heap::fibonacci_heap::ordered_begin
    ordered_iterator ordered_begin( void ) const
    {
        return ordered_iterator( root, super_t::value_comp() );
    }

    /// \copydoc boost::heap::fibonacci_heap::ordered_begin
    ordered_iterator ordered_end( void ) const
    {
        return ordered_iterator( 0, super_t::value_comp() );
    }

    /**
     * \b Effects: Merge all elements from rhs into this
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * */
    void merge( skew_heap& rhs )
    {
        if ( rhs.empty() )
            return;

        merge_node( rhs.root );

        size_holder::add( rhs.get_size() );
        rhs.set_size( 0 );
        rhs.root = nullptr;
        sanity_check();

        super_t::set_stability_count( ( std::max )( super_t::get_stability_count(), rhs.get_stability_count() ) );
        rhs.set_stability_count( 0 );
    }

    /// \copydoc boost::heap::priority_queue::value_comp
    value_compare const& value_comp( void ) const
    {
        return super_t::value_comp();
    }

    /// \copydoc boost::heap::priority_queue::operator<(HeapType const & rhs) const
    template < typename HeapType >
    bool operator<( HeapType const& rhs ) const
    {
        return detail::heap_compare( *this, rhs );
    }

    /// \copydoc boost::heap::priority_queue::operator>(HeapType const & rhs) const
    template < typename HeapType >
    bool operator>( HeapType const& rhs ) const
    {
        return detail::heap_compare( rhs, *this );
    }

    /// \copydoc boost::heap::priority_queue::operator>=(HeapType const & rhs) const
    template < typename HeapType >
    bool operator>=( HeapType const& rhs ) const
    {
        return !operator<( rhs );
    }

    /// \copydoc boost::heap::priority_queue::operator<=(HeapType const & rhs) const
    template < typename HeapType >
    bool operator<=( HeapType const& rhs ) const
    {
        return !operator>( rhs );
    }

    /// \copydoc boost::heap::priority_queue::operator==(HeapType const & rhs) const
    template < typename HeapType >
    bool operator==( HeapType const& rhs ) const
    {
        return detail::heap_equality( *this, rhs );
    }

    /// \copydoc boost::heap::priority_queue::operator!=(HeapType const & rhs) const
    template < typename HeapType >
    bool operator!=( HeapType const& rhs ) const
    {
        return !( *this == rhs );
    }


    /// \copydoc boost::heap::d_ary_heap::s_handle_from_iterator
    static handle_type s_handle_from_iterator( iterator const& it )
    {
        node* ptr = const_cast< node* >( it.get_node() );
        return handle_type( ptr );
    }

    /**
     * \b Effects: Removes the element handled by \c handle from the priority_queue.
     *
     * \b Complexity: Logarithmic (amortized).
     * */
    void erase( handle_type object )
    {
        BOOST_STATIC_ASSERT( is_mutable );
        node_pointer this_node = object.node_;

        unlink_node( this_node );
        size_holder::decrement();

        sanity_check();
        this_node->~node();
        allocator_type& alloc = *this;
        alloc.deallocate( this_node, 1 );
    }

    /**
     * \b Effects: Assigns \c v to the element handled by \c handle & updates the priority queue.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * */
    void update( handle_type handle, const_reference v )
    {
        BOOST_STATIC_ASSERT( is_mutable );
        if ( super_t::operator()( super_t::get_value( handle.node_->value ), v ) )
            increase( handle, v );
        else
            decrease( handle, v );
    }

    /**
     * \b Effects: Updates the heap after the element handled by \c handle has been changed.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * \b Note: If this is not called, after a handle has been updated, the behavior of the data structure is undefined!
     * */
    void update( handle_type handle )
    {
        BOOST_STATIC_ASSERT( is_mutable );
        node_pointer this_node = handle.node_;

        if ( this_node->get_parent() ) {
            if ( super_t::operator()( super_t::get_value( this_node->get_parent()->value ),
                                      super_t::get_value( this_node->value ) ) )
                increase( handle );
            else
                decrease( handle );
        } else
            decrease( handle );
    }

    /**
     * \b Effects: Assigns \c v to the element handled by \c handle & updates the priority queue.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * \b Note: The new value is expected to be greater than the current one
     * */
    void increase( handle_type handle, const_reference v )
    {
        BOOST_STATIC_ASSERT( is_mutable );
        handle.node_->value = super_t::make_node( v );
        increase( handle );
    }

    /**
     * \b Effects: Updates the heap after the element handled by \c handle has been changed.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * \b Note: If this is not called, after a handle has been updated, the behavior of the data structure is undefined!
     * */
    void increase( handle_type handle )
    {
        BOOST_STATIC_ASSERT( is_mutable );
        node_pointer this_node = handle.node_;

        if ( this_node == root )
            return;

        node_pointer parent = this_node->get_parent();

        if ( this_node == parent->children[ 0 ] )
            parent->children[ 0 ] = nullptr;
        else
            parent->children[ 1 ] = nullptr;

        this_node->set_parent( nullptr );
        merge_node( this_node );
    }

    /**
     * \b Effects: Assigns \c v to the element handled by \c handle & updates the priority queue.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * \b Note: The new value is expected to be less than the current one
     * */
    void decrease( handle_type handle, const_reference v )
    {
        BOOST_STATIC_ASSERT( is_mutable );
        handle.node_->value = super_t::make_node( v );
        decrease( handle );
    }

    /**
     * \b Effects: Updates the heap after the element handled by \c handle has been changed.
     *
     * \b Complexity: Logarithmic (amortized).
     *
     * \b Note: The new value is expected to be less than the current one. If this is not called, after a handle has
     * been updated, the behavior of the data structure is undefined!
     * */
    void decrease( handle_type handle )
    {
        BOOST_STATIC_ASSERT( is_mutable );
        node_pointer this_node = handle.node_;

        unlink_node( this_node );
        this_node->children.fill( 0 );
        this_node->set_parent( nullptr );
        merge_node( this_node );
    }

private:
#if !defined( BOOST_DOXYGEN_INVOKED )
    struct push_void
    {
        static void push( skew_heap* self, const_reference v )
        {
            self->push_internal( v );
        }

        template < class... Args >
        static void emplace( skew_heap* self, Args&&... args )
        {
            self->emplace_internal( std::forward< Args >( args )... );
        }
    };

    struct push_handle
    {
        static handle_type push( skew_heap* self, const_reference v )
        {
            return handle_type( self->push_internal( v ) );
        }

        template < class... Args >
        static handle_type emplace( skew_heap* self, Args&&... args )
        {
            return handle_type( self->emplace_internal( std::forward< Args >( args )... ) );
        }
    };

    node_pointer push_internal( const_reference v )
    {
        size_holder::increment();

        allocator_type& alloc = *this;
        node_pointer    n     = alloc.allocate( 1 );
        new ( n ) node( super_t::make_node( v ) );
        merge_node( n );
        return n;
    }

    template < class... Args >
    node_pointer emplace_internal( Args&&... args )
    {
        size_holder::increment();

        allocator_type& alloc = *this;
        node_pointer    n     = alloc.allocate( 1 );
        new ( n ) node( super_t::make_node( std::forward< Args >( args )... ) );
        merge_node( n );
        return n;
    }

    void unlink_node( node_pointer node )
    {
        node_pointer parent          = node->get_parent();
        node_pointer merged_children = merge_children( node );

        if ( parent ) {
            if ( node == parent->children[ 0 ] )
                parent->children[ 0 ] = merged_children;
            else
                parent->children[ 1 ] = merged_children;
        } else
            root = merged_children;
    }

    void clone_tree( skew_heap const& rhs )
    {
        BOOST_HEAP_ASSERT( root == nullptr );
        if ( rhs.empty() )
            return;

        allocator_type& alloc = *this;
        root                  = alloc.allocate( 1 );
        new ( root ) node( *rhs.root, alloc, nullptr );
    }

    void merge_node( node_pointer other )
    {
        BOOST_HEAP_ASSERT( other );
        if ( root != nullptr )
            root = merge_nodes( root, other, nullptr );
        else
            root = other;
    }

    node_pointer merge_nodes( node_pointer node1, node_pointer node2, node_pointer new_parent )
    {
        if ( node1 == nullptr ) {
            if ( node2 )
                node2->set_parent( new_parent );
            return node2;
        }
        if ( node2 == nullptr ) {
            node1->set_parent( new_parent );
            return node1;
        }

        node_pointer merged = merge_nodes_recursive( node1, node2, new_parent );
        return merged;
    }

    node_pointer merge_children( node_pointer node )
    {
        node_pointer parent          = node->get_parent();
        node_pointer merged_children = merge_nodes( node->children[ 0 ], node->children[ 1 ], parent );

        return merged_children;
    }

    node_pointer merge_nodes_recursive( node_pointer node1, node_pointer node2, node_pointer new_parent )
    {
        if ( super_t::operator()( node1->value, node2->value ) )
            std::swap( node1, node2 );

        node* parent = node1;
        node* child  = node2;

        if ( parent->children[ 1 ] ) {
            node* merged          = merge_nodes( parent->children[ 1 ], child, parent );
            parent->children[ 1 ] = merged;
            merged->set_parent( parent );
        } else {
            parent->children[ 1 ] = child;
            child->set_parent( parent );
        }


        std::swap( parent->children[ 0 ], parent->children[ 1 ] );
        parent->set_parent( new_parent );
        return parent;
    }

    void sanity_check( void )
    {
#    ifdef BOOST_HEAP_SANITYCHECKS
        if ( root )
            BOOST_HEAP_ASSERT( root->template is_heap< super_t >( super_t::value_comp() ) );

        if ( constant_time_size ) {
            size_type stored_size = size_holder::get_size();

            size_type counted_size;
            if ( root == nullptr )
                counted_size = 0;
            else
                counted_size = root->count_children();

            BOOST_HEAP_ASSERT( counted_size == stored_size );
        }
#    endif
    }

    node_pointer root;
#endif
};

}} // namespace boost::heap

#undef BOOST_HEAP_ASSERT
#endif /* BOOST_HEAP_SKEW_HEAP_HPP */