// Copyright (c) 2016, the SDSL Project Authors.  All rights reserved.
// Please see the AUTHORS file for details.  Use of this source code is governed
// by a BSD license that can be found in the LICENSE file.
/*!\file wm_int.hpp
 * \brief wm_int.hpp contains a specialized class for a wavelet tree for
 * sequences over large alphabets.
 * \author Simon Gog
 */
#ifndef INCLUDED_SDSL_WM_INT
#define INCLUDED_SDSL_WM_INT

#include <algorithm> // for std::swap
#include <map>       // for mapping a symbol to its lexicographical index
#include <queue>
#include <set> // for calculating the alphabet size
#include <stdexcept>
#include <utility>
#include <vector>

#include <sdsl/int_vector.hpp>
#include <sdsl/rank_support_v.hpp>
#include <sdsl/sdsl_concepts.hpp>
#include <sdsl/select_support_mcl.hpp>
#include <sdsl/util.hpp>
#include <sdsl/wt_helper.hpp>

//! Namespace for the succinct data structure library.
namespace sdsl
{

//! A wavelet tree class for integer sequences.
/*!
 * \tparam t_bitvector   Type of the bitvector used for representing the wavelet tree.
 * \tparam t_rank        Type of the support structure for rank on pattern `1`.
 * \tparam t_select      Type of the support structure for select on pattern `1`.
 * \tparam t_select_zero Type of the support structure for select on pattern `0`.
 *
 * This wavelet tree variant does not store the two children of a node v aligned
 * with v; it is also known as wavelet matrix.
 *
 * \par References
 *      [1] F. Claude, G. Navarro: ,,The Wavelet Matrix'', Proceedings of
 *          SPIRE 2012.
 *
 *   @ingroup wt
 */
template <class t_bitvector = bit_vector,
          class t_rank = typename t_bitvector::rank_1_type,
          class t_select = typename t_bitvector::select_1_type,
          class t_select_zero = typename t_bitvector::select_0_type>
class wm_int
{
  public:
    typedef int_vector<>::size_type size_type;
    typedef int_vector<>::value_type value_type;
    typedef typename t_bitvector::difference_type difference_type;
    typedef random_access_const_iterator<wm_int> const_iterator;
    typedef const_iterator iterator;
    typedef t_bitvector bit_vector_type;
    typedef t_rank rank_1_type;
    typedef t_select select_1_type;
    typedef t_select_zero select_0_type;
    typedef wt_tag index_category;
    typedef int_alphabet_tag alphabet_category;
    enum
    {
        lex_ordered = 0
    };

    typedef std::pair<value_type, size_type> point_type;
    typedef std::vector<point_type> point_vec_type;
    typedef std::pair<size_type, point_vec_type> r2d_res_type;

    struct node_type;

  protected:
    size_type m_size = 0;
    size_type m_sigma = 0;        //<- \f$ |\Sigma| \f$
    bit_vector_type m_tree;       // bit vector to store the wavelet tree
    rank_1_type m_tree_rank;      // rank support for the wavelet tree bit vector
    select_1_type m_tree_select1; // select support for the wavelet tree bit vector
    select_0_type m_tree_select0;
    uint32_t m_max_level = 0;
    int_vector<64> m_zero_cnt;   // m_zero_cnt[i] contains the number of zeros in level i
    int_vector<64> m_rank_level; // m_rank_level[i] contains m_tree_rank(i*size())

  public:
    const size_type & sigma = m_sigma;        //!< Effective alphabet size of the wavelet tree.
    const bit_vector_type & tree = m_tree;    //!< A concatenation of all bit vectors of the wavelet tree.
    const uint32_t & max_level = m_max_level; //!< Maximal level of the wavelet tree.

    //! Default constructor
    wm_int() = default;

    //! Construct the wavelet tree from a sequence defined by two interators
    /*!
     * \param begin   Iterator to the start of the input.
     * \param end     Iterator one past the end of the input.
     * \param tmp_dir Temporary directory for intermediate results.
     *    \par Time complexity
     *        \f$ \Order{n\log|\Sigma|}\f$, where \f$n=size\f$
     *        I.e. we need \Order{n\log n} if rac is a permutation of 0..n-1.
     */
    template <typename t_it>
    wm_int(t_it begin, t_it end, std::string tmp_dir = ram_file_name(""))
      : m_size(std::distance(begin, end))
    {
        if (0 == m_size) return;
        m_sigma = 0;

        value_type max_elem = 1; // variable for the biggest value in rac
        for (auto it = begin; it != end; ++it)
        {
            value_type value = *it;
            if (value > max_elem) max_elem = value;
        }
        m_max_level = bits::hi(max_elem) + 1;
        std::string tree_out_buf_file_name = tmp_file(tmp_dir, "_m_tree");
        {
            int_vector<> rac(m_size, 0, m_max_level);
            std::copy(begin, end, rac.begin());

            // buffer for elements in the right node
            std::string zero_buf_file_name = tmp_file(tmp_dir, "_zero_buf");
            osfstream tree_out_buf(tree_out_buf_file_name,
                                   std::ios::binary | std::ios::trunc | std::ios::out); // open buffer for tree
            size_type bit_size = m_size * m_max_level;
            int_vector<1>::write_header(bit_size, 1, tree_out_buf); // write bv header

            size_type tree_pos = 0;
            uint64_t tree_word = 0;

            m_zero_cnt = int_vector<64>(m_max_level, 0); // zeros at level i

            for (uint32_t k = 0; k < m_max_level; ++k)
            {
                uint8_t width = m_max_level - k - 1;
                const uint64_t mask = 1ULL << width;
                uint64_t x = 0;
                size_type zeros = 0;
                int_vector_buffer<> zero_buf(zero_buf_file_name, std::ios::out, 1024 * 1024, m_max_level);
                for (size_t i = 0; i < m_size; ++i)
                {
                    x = rac[i];
                    if (x & mask)
                    {
                        tree_word |= (1ULL << (tree_pos & 0x3FULL));
                        zero_buf.push_back(x);
                    }
                    else
                    {
                        rac[zeros++] = x;
                    }
                    ++tree_pos;
                    if ((tree_pos & 0x3FULL) == 0)
                    { // if tree_pos % 64 == 0 write old word
                        tree_out_buf.write((char *)&tree_word, sizeof(tree_word));
                        tree_word = 0;
                    }
                }
                m_zero_cnt[k] = zeros;
                for (size_t i = zeros; i < m_size; ++i) { rac[i] = zero_buf[i - zeros]; }
            }
            if ((tree_pos & 0x3FULL) != 0)
            { // if tree_pos % 64 > 0 => there are remaining entries we have to write
                tree_out_buf.write((char *)&tree_word, sizeof(tree_word));
            }
            sdsl::remove(zero_buf_file_name);
            tree_out_buf.close();
            m_sigma = std::unique(rac.begin(), rac.end()) - rac.begin();
        }
        bit_vector tree;
        load_from_file(tree, tree_out_buf_file_name);
        sdsl::remove(tree_out_buf_file_name);
        m_tree = bit_vector_type(std::move(tree));
        util::init_support(m_tree_rank, &m_tree);
        util::init_support(m_tree_select0, &m_tree);
        util::init_support(m_tree_select1, &m_tree);
        m_rank_level = int_vector<64>(m_max_level, 0);
        for (uint32_t k = 0; k < m_rank_level.size(); ++k) { m_rank_level[k] = m_tree_rank(k * m_size); }
    }

    //! Copy constructor
    wm_int(const wm_int & wt)
      : m_size(wt.m_size)
      , m_sigma(wt.m_sigma)
      , m_tree(wt.m_tree)
      , m_tree_rank(wt.m_tree_rank)
      , m_tree_select1(wt.m_tree_select1)
      , m_tree_select0(wt.m_tree_select0)
      , m_max_level(wt.m_max_level)
      , m_zero_cnt(wt.m_zero_cnt)
      , m_rank_level(wt.m_rank_level)
    {
        m_tree_rank.set_vector(&m_tree);
        m_tree_select1.set_vector(&m_tree);
        m_tree_select0.set_vector(&m_tree);
    }

    //! Move copy constructor
    wm_int(wm_int && wt)
      : m_size(wt.m_size)
      , m_sigma(wt.m_sigma)
      , m_tree(std::move(wt.m_tree))
      , m_tree_rank(std::move(wt.m_tree_rank))
      , m_tree_select1(std::move(wt.m_tree_select1))
      , m_tree_select0(std::move(wt.m_tree_select0))
      , m_max_level(wt.m_max_level)
      , m_zero_cnt(wt.m_zero_cnt)
      , m_rank_level(wt.m_rank_level)
    {
        m_tree_rank.set_vector(&m_tree);
        m_tree_select1.set_vector(&m_tree);
        m_tree_select0.set_vector(&m_tree);
    }

    //! Assignment operator
    wm_int & operator=(const wm_int & wt)
    {
        if (this != &wt)
        {
            m_size = wt.m_size;
            m_sigma = wt.m_sigma;
            m_tree = wt.m_tree;
            m_tree_rank = wt.m_tree_rank;
            m_tree_rank.set_vector(&m_tree);
            m_tree_select1 = wt.m_tree_select1;
            m_tree_select1.set_vector(&m_tree);
            m_tree_select0 = wt.m_tree_select0;
            m_tree_select0.set_vector(&m_tree);
            m_max_level = wt.m_max_level;
            m_zero_cnt = wt.m_zero_cnt;
            m_rank_level = wt.m_rank_level;
        }
        return *this;
    }

    //! Assignment move operator
    wm_int & operator=(wm_int && wt)
    {
        if (this != &wt)
        {
            m_size = wt.m_size;
            m_sigma = wt.m_sigma;
            m_tree = std::move(wt.m_tree);
            m_tree_rank = std::move(wt.m_tree_rank);
            m_tree_rank.set_vector(&m_tree);
            m_tree_select1 = std::move(wt.m_tree_select1);
            m_tree_select1.set_vector(&m_tree);
            m_tree_select0 = std::move(wt.m_tree_select0);
            m_tree_select0.set_vector(&m_tree);
            m_max_level = std::move(wt.m_max_level);
            m_zero_cnt = std::move(wt.m_zero_cnt);
            m_rank_level = std::move(wt.m_rank_level);
        }
        return *this;
    }

    //! Returns the size of the original vector.
    size_type size() const { return m_size; }

    //! Returns whether the wavelet tree contains no data.
    bool empty() const { return m_size == 0; }

    //! Recovers the i-th symbol of the original vector.
    /*!\param i The index of the symbol in the original vector.
     *  \returns The i-th symbol of the original vector.
     *  \par Precondition
     *       \f$ i < size() \f$
     */
    value_type operator[](size_type i) const
    {
        assert(i < size());
        value_type res = 0;
        for (uint32_t k = 0; k < m_max_level; ++k)
        {
            res <<= 1;
            size_type rank_ones = m_tree_rank(i) - m_rank_level[k];
            if (m_tree[i])
            { // one at position i => follow right child
                i = (k + 1) * m_size + m_zero_cnt[k] + rank_ones;
                res |= 1;
            }
            else
            { // zero at position i => follow left child
                auto rank_zeros = (i - k * m_size) - rank_ones;
                i = (k + 1) * m_size + rank_zeros;
            }
        }
        return res;
    };

    //! Calculates how many symbols c are in the prefix [0..i-1] of the supported vector.
    /*!
     *  \param i The exclusive index of the prefix range [0..i-1], so \f$i\in[0..size()]\f$.
     *  \param c The symbol to count the occurrences in the prefix.
     *    \returns The number of occurrences of symbol c in the prefix [0..i-1] of the supported vector.
     *  \par Time complexity
     *       \f$ \Order{\log |\Sigma|} \f$
     *  \par Precondition
     *       \f$ i \leq size() \f$
     */
    size_type rank(size_type i, value_type c) const
    {
        assert(i <= size());
        if (((1ULL) << (m_max_level)) <= c)
        { // c is greater than any symbol in wt
            return 0;
        }
        size_type b = 0; // start position of the interval
        uint64_t mask = (1ULL) << (m_max_level - 1);
        for (uint32_t k = 0; k < m_max_level and i; ++k)
        {
            size_type rank_b = m_tree_rank(b);
            size_type ones = m_tree_rank(b + i) - rank_b; // ones in [b..i)
            size_type ones_p = rank_b - m_rank_level[k];  // ones in [level_b..b)
            if (c & mask)
            { // search for a one at this level
                i = ones;
                b = (k + 1) * m_size + m_zero_cnt[k] + ones_p;
            }
            else
            { // search for a zero at this level
                i = i - ones;
                b = (k + 1) * m_size + (b - k * m_size - ones_p);
            }
            mask >>= 1;
        }
        return i;
    };

    //! Calculates how many occurrences of symbol wt[i] are in the prefix [0..i-1] of the original sequence.
    /*!
     *  \param i The index of the symbol.
     *  \return  Pair (rank(wt[i],i),wt[i])
     *  \par Precondition
     *       \f$ i < size() \f$
     */
    std::pair<size_type, value_type> inverse_select(size_type i) const
    {
        assert(i < size());
        value_type c = 0;
        size_type b = 0; // start position of the interval
        for (uint32_t k = 0; k < m_max_level; ++k)
        {
            size_type rank_b = m_tree_rank(b);
            size_type ones = m_tree_rank(b + i) - rank_b; // ones in [b..i)
            size_type ones_p = rank_b - m_rank_level[k];  // ones in [level_b..b)
            c <<= 1;
            if (m_tree[b + i])
            { // go to the right child
                i = ones;
                b = (k + 1) * m_size + m_zero_cnt[k] + ones_p;
                c |= 1;
            }
            else
            { // go to the left child
                i = i - ones;
                b = (k + 1) * m_size + (b - k * m_size - ones_p);
            }
        }
        return std::make_pair(i, c);
    }

    //! Calculates the i-th occurrence of the symbol c in the supported vector.
    /*!
     *  \param i The i-th occurrence.
     *  \param c The symbol c.
     *  \par Time complexity
     *       \f$ \Order{\log |\Sigma|} \f$
     *  \par Precondition
     *       \f$ 1 \leq i \leq rank(size(), c) \f$
     */
    size_type select(size_type i, value_type c) const
    {
        assert(1 <= i and i <= rank(size(), c));
        uint64_t mask = 1ULL << (m_max_level - 1);
        int_vector<64> m_path_off(max_level + 1);
        int_vector<64> m_path_rank_off(max_level + 1);
        m_path_off[0] = m_path_rank_off[0] = 0;
        size_type b = 0; // start position of the interval
        size_type r = i;
        for (uint32_t k = 0; k < m_max_level and i; ++k)
        {
            size_type rank_b = m_tree_rank(b);
            size_type ones = m_tree_rank(b + r) - rank_b; // ones in [b..i)
            size_type ones_p = rank_b - m_rank_level[k];  // ones in [0..b)
            if (c & mask)
            { // search for a one at this level
                r = ones;
                b = (k + 1) * m_size + m_zero_cnt[k] + ones_p;
            }
            else
            { // search for a zero at this level
                r = r - ones;
                b = (k + 1) * m_size + (b - k * m_size - ones_p);
            }
            mask >>= 1;
            m_path_off[k + 1] = b;
            m_path_rank_off[k] = rank_b;
        }
        mask = 1ULL;
        for (uint32_t k = m_max_level; k > 0; --k)
        {
            b = m_path_off[k - 1];
            size_type rank_b = m_path_rank_off[k - 1];
            if (c & mask)
            { // right child => search i'th one
                i = m_tree_select1(rank_b + i) - b + 1;
            }
            else
            { // left child => search i'th zero
                i = m_tree_select0(b - rank_b + i) - b + 1;
            }
            mask <<= 1;
        }
        return i - 1;
    };

    //! range_search_2d searches points in the index interval [lb..rb] and value interval [vlb..vrb].
    /*!\param lb     Left bound of index interval (inclusive)
     *  \param rb     Right bound of index interval (inclusive)
     *  \param vlb    Left bound of value interval (inclusive)
     *  \param vrb    Right bound of value interval (inclusive)
     *  \param report Should the matching points be returned?
     *  \return Pair (#of found points, vector of points), the vector is empty when
     *          report = false.
     */
    std::pair<size_type, std::vector<std::pair<value_type, size_type>>> range_search_2d(size_type lb,
                                                                                        size_type rb,
                                                                                        value_type vlb,
                                                                                        value_type vrb,
                                                                                        bool report = true) const
    {

        if (vrb > (1ULL << m_max_level)) vrb = (1ULL << m_max_level);
        if (vlb > vrb) return make_pair(0, point_vec_type());
        size_type cnt_answers = 0;
        point_vec_type point_vec;
        if (lb <= rb)
        {
            std::vector<size_type> is(m_max_level + 1);
            std::vector<size_type> rank_off(m_max_level + 1);
            _range_search_2d(root(), { { lb, rb } }, vlb, vrb, 0, is, rank_off, point_vec, report, cnt_answers);
        }
        return make_pair(cnt_answers, point_vec);
    }

    void _range_search_2d(node_type v,
                          range_type r,
                          value_type vlb,
                          value_type vrb,
                          size_type ilb,
                          std::vector<size_type> & is,
                          std::vector<size_type> & rank_off,
                          point_vec_type & point_vec,
                          bool report,
                          size_type & cnt_answers) const
    {
        using std::get;
        if (get<0>(r) > get<1>(r)) return;
        is[v.level] = v.offset + get<0>(r);

        if (v.level == m_max_level)
        {
            for (size_type j = 1; j <= sdsl::size(r) and report; ++j)
            {
                size_type i = j;
                size_type c = v.sym;
                for (uint32_t k = m_max_level; k > 0; --k)
                {
                    size_type offset = is[k - 1];
                    size_type rank_offset = rank_off[k - 1];
                    if (c & 1) { i = m_tree_select1(rank_offset + i) - offset + 1; }
                    else
                    {
                        i = m_tree_select0(offset - rank_offset + i) - offset + 1;
                    }
                    c >>= 1;
                }
                point_vec.emplace_back(is[0] + i - 1, v.sym);
            }
            cnt_answers += sdsl::size(r);
            return;
        }
        else
        {
            rank_off[v.level] = m_tree_rank(is[v.level]);
        }
        size_type irb = ilb + (1ULL << (m_max_level - v.level));
        size_type mid = (irb + ilb) >> 1;

        auto c_v = expand(v);
        auto c_r = expand(v, r);

        if (!sdsl::empty(get<0>(c_r)) and vlb < mid and mid)
        {
            _range_search_2d(get<0>(c_v),
                             get<0>(c_r),
                             vlb,
                             std::min(vrb, mid - 1),
                             ilb,
                             is,
                             rank_off,
                             point_vec,
                             report,
                             cnt_answers);
        }
        if (!sdsl::empty(get<1>(c_r)) and vrb >= mid)
        {
            _range_search_2d(get<1>(c_v),
                             get<1>(c_r),
                             std::max(mid, vlb),
                             vrb,
                             mid,
                             is,
                             rank_off,
                             point_vec,
                             report,
                             cnt_answers);
        }
    }

    //! Returns a const_iterator to the first element.
    const_iterator begin() const { return const_iterator(this, 0); }

    //! Returns a const_iterator to the element after the last element.
    const_iterator end() const { return const_iterator(this, size()); }

    //! Serializes the data structure into the given ostream
    size_type serialize(std::ostream & out, structure_tree_node * v = nullptr, std::string name = "") const
    {
        structure_tree_node * child = structure_tree::add_child(v, name, util::class_name(*this));
        size_type written_bytes = 0;
        written_bytes += write_member(m_size, out, child, "size");
        written_bytes += write_member(m_sigma, out, child, "sigma");
        written_bytes += m_tree.serialize(out, child, "tree");
        written_bytes += m_tree_rank.serialize(out, child, "tree_rank");
        written_bytes += m_tree_select1.serialize(out, child, "tree_select_1");
        written_bytes += m_tree_select0.serialize(out, child, "tree_select_0");
        written_bytes += write_member(m_max_level, out, child, "max_level");
        written_bytes += m_zero_cnt.serialize(out, child, "zero_cnt");
        written_bytes += m_rank_level.serialize(out, child, "rank_level");
        structure_tree::add_size(child, written_bytes);
        return written_bytes;
    }

    //! Loads the data structure from the given istream.
    void load(std::istream & in)
    {
        read_member(m_size, in);
        read_member(m_sigma, in);
        m_tree.load(in);
        m_tree_rank.load(in, &m_tree);
        m_tree_select1.load(in, &m_tree);
        m_tree_select0.load(in, &m_tree);
        read_member(m_max_level, in);
        m_zero_cnt.load(in);
        m_rank_level.load(in);
    }

    //!\brief Serialise (save) via cereal
    template <typename archive_t>
    void CEREAL_SAVE_FUNCTION_NAME(archive_t & ar) const
    {
        ar(CEREAL_NVP(m_size));
        ar(CEREAL_NVP(m_sigma));
        ar(CEREAL_NVP(m_max_level));
        ar(CEREAL_NVP(m_tree));
        ar(CEREAL_NVP(m_tree_rank));
        ar(CEREAL_NVP(m_tree_select1));
        ar(CEREAL_NVP(m_tree_select0));
        ar(CEREAL_NVP(m_zero_cnt));
        ar(CEREAL_NVP(m_rank_level));
    }

    //!\brief Load via cereal
    template <typename archive_t>
    void CEREAL_LOAD_FUNCTION_NAME(archive_t & ar)
    {
        ar(CEREAL_NVP(m_size));
        ar(CEREAL_NVP(m_sigma));
        ar(CEREAL_NVP(m_max_level));
        ar(CEREAL_NVP(m_tree));
        ar(CEREAL_NVP(m_tree_rank));
        m_tree_rank.set_vector(&m_tree);
        ar(CEREAL_NVP(m_tree_select1));
        m_tree_select1.set_vector(&m_tree);
        ar(CEREAL_NVP(m_tree_select0));
        m_tree_select0.set_vector(&m_tree);
        ar(CEREAL_NVP(m_zero_cnt));
        ar(CEREAL_NVP(m_rank_level));
    }

    //! Equality operator.
    bool operator==(wm_int const & other) const noexcept
    {
        return (m_size == other.m_size) && (m_sigma == other.m_sigma) && (m_max_level == other.m_max_level) &&
               (m_tree == other.m_tree) && (m_tree_rank == other.m_tree_rank) &&
               (m_tree_select1 == other.m_tree_select1) && (m_tree_select0 == other.m_tree_select0) &&
               (m_zero_cnt == other.m_zero_cnt) && (m_rank_level == other.m_rank_level);
    }

    //! Inequality operator.
    bool operator!=(wm_int const & other) const noexcept { return !(*this == other); }

    //! Represents a node in the wavelet tree
    struct node_type
    {
        size_type offset = 0;
        size_type size = 0;
        size_type level = 0;
        value_type sym = 0;

        // Default constructor
        node_type(size_type o = 0, size_type sz = 0, size_type l = 0, value_type sy = 0)
          : offset(o)
          , size(sz)
          , level(l)
          , sym(sy)
        {}

        // Copy constructor
        node_type(const node_type &) = default;

        // Move copy constructor
        node_type(node_type &&) = default;

        // Assignment operator
        node_type & operator=(const node_type &) = default;

        // Move assignment operator
        node_type & operator=(node_type &&) = default;

        // Comparator operator
        bool operator==(const node_type & v) const { return offset == v.offset; }

        // Smaller operator
        bool operator<(const node_type & v) const { return offset < v.offset; }

        // Greater operator
        bool operator>(const node_type & v) const { return offset > v.offset; }
    };

    //! Checks if the node is a leaf node
    bool is_leaf(const node_type & v) const { return v.level == m_max_level; }

    //! Symbol of leaf node v
    value_type sym(const node_type & v) const { return v.sym; }

    //! Random access container to bitvector of node v
    auto bit_vec(const node_type & v) const -> node_bv_container<t_bitvector>
    {
        return node_bv_container<t_bitvector>(begin(v), end(v));
    }

    //! Random access container to sequence of node v
    auto seq(const node_type & v) const -> random_access_container<std::function<value_type(size_type)>>
    {
        return random_access_container<std::function<value_type(size_type)>>(
                                                          [&v, this](size_type i) {
                                                              node_type vv = v;
                                                              while (!is_leaf(vv))
                                                              {
                                                                  auto vs = expand(vv);
                                                                  auto rs = expand(vv, { 0, i });
                                                                  bool bit = *(begin(vv) + i);
                                                                  i = std::get<1>(rs[bit]);
                                                                  vv = vs[bit];
                                                              }
                                                              return sym(vv);
                                                          },
                                                          size(v));
    }

    //! Indicates if node v is empty
    bool empty(const node_type & v) const { return v.size == (size_type)0; }

    //! Return the size of node v
    auto size(const node_type & v) const -> decltype(v.size) { return v.size; }

    //! Return the root node
    node_type root() const { return node_type(0, m_size, 0, 0); }

    //! Returns the two child nodes of an inner node
    /*!\param v An inner node of a wavelet tree.
     *  \return Return a pair of nodes (left child, right child).
     *  \pre !is_leaf(v)
     */
    std::array<node_type, 2> expand(const node_type & v) const
    {
        node_type v_right = v;
        return expand(std::move(v_right));
    }

    //! Returns the two child nodes of an inner node
    /*!\param v An inner node of a wavelet tree.
     *  \return Return a pair of nodes (left child, right child).
     *  \pre !is_leaf(v)
     */
    std::array<node_type, 2> expand(node_type && v) const
    {
        node_type v_left;
        size_type rank_b = m_tree_rank(v.offset);
        size_type ones = m_tree_rank(v.offset + v.size) - rank_b; // ones in [b..size)
        size_type ones_p = rank_b - m_rank_level[v.level];        // ones in [level_b..b)

        v_left.offset = (v.level + 1) * m_size + (v.offset - v.level * m_size) - ones_p;
        v_left.size = v.size - ones;
        v_left.level = v.level + 1;
        v_left.sym = v.sym << 1;

        v.offset = (v.level + 1) * m_size + m_zero_cnt[v.level] + ones_p;
        v.size = ones;
        v.level = v.level + 1;
        v.sym = (v.sym << 1) | 1;

        return { { std::move(v_left), v } };
    }

    //! Returns for each range its left and right child ranges
    /*!\param v      An inner node of an wavelet tree.
     *  \param ranges A vector of ranges. Each range [s,e]
     *                has to be contained in v=[v_s,v_e].
     *  \return A vector a range pairs. The first element of each
     *          range pair correspond to the original range
     *          mapped to the left child of v; the second element to the
     *          range mapped to the right child of v.
     *  \pre !is_leaf(v) and s>=v_s and e<=v_e
     */
    std::array<range_vec_type, 2> expand(const node_type & v, const range_vec_type & ranges) const
    {
        auto ranges_copy = ranges;
        return expand(v, std::move(ranges_copy));
    }

    //! Returns for each range its left and right child ranges
    /*!\param v      An inner node of an wavelet tree.
     *  \param ranges A vector of ranges. Each range [s,e]
     *                has to be contained in v=[v_s,v_e].
     *  \return A vector a range pairs. The first element of each
     *          range pair correspond to the original range
     *          mapped to the left child of v; the second element to the
     *          range mapped to the right child of v.
     *  \pre !is_leaf(v) and s>=v_s and e<=v_e
     */
    std::array<range_vec_type, 2> expand(const node_type & v, range_vec_type && ranges) const
    {
        auto v_sp_rank = m_tree_rank(v.offset); // this is already calculated in expand(v)
        range_vec_type res(ranges.size());
        size_t i = 0;
        for (auto & r : ranges)
        {
            auto sp_rank = m_tree_rank(v.offset + r[0]);
            auto right_size = m_tree_rank(v.offset + r[1] + 1) - sp_rank;
            auto left_size = (r[1] - r[0] + 1) - right_size;

            auto right_sp = sp_rank - v_sp_rank;
            auto left_sp = r[0] - right_sp;

            r = { { left_sp, left_sp + left_size - 1 } };
            res[i++] = { { right_sp, right_sp + right_size - 1 } };
        }
        return { { ranges, std::move(res) } };
    }

    //! Returns for a range its left and right child ranges
    /*!\param v An inner node of an wavelet tree.
     *  \param r A ranges [s,e], such that [s,e] is
     *           contained in v=[v_s,v_e].
     *  \return A range pair. The first element of the
     *          range pair correspond to the original range
     *          mapped to the left child of v; the second element to the
     *          range mapped to the right child of v.
     *  \pre !is_leaf(v) and s>=v_s and e<=v_e
     */
    std::array<range_type, 2> expand(const node_type & v, const range_type & r) const
    {
        auto v_sp_rank = m_tree_rank(v.offset); // this is already calculated in expand(v)
        auto sp_rank = m_tree_rank(v.offset + r[0]);
        auto right_size = m_tree_rank(v.offset + r[1] + 1) - sp_rank;
        auto left_size = (r[1] - r[0] + 1) - right_size;

        auto right_sp = sp_rank - v_sp_rank;
        auto left_sp = r[0] - right_sp;

        return { { { { left_sp, left_sp + left_size - 1 } }, { { right_sp, right_sp + right_size - 1 } } } };
    }

    //! return the path to the leaf for a given symbol
    std::pair<uint64_t, uint64_t> path(value_type c) const { return { m_max_level, c }; }

  private:
    //! Iterator to the begin of the bitvector of inner node v
    auto begin(const node_type & v) const -> decltype(m_tree.begin() + v.offset) { return m_tree.begin() + v.offset; }

    //! Iterator to the begin of the bitvector of inner node v
    auto end(const node_type & v) const -> decltype(m_tree.begin() + v.offset + v.size)
    {
        return m_tree.begin() + v.offset + v.size;
    }
};

} // end namespace sdsl
#endif
