ts_hash_map.h

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/* +------------------------------------------------------------------------+
   |                     Mobile Robot Programming Toolkit (MRPT)            |
   |                          https://www.mrpt.org/                         |
   |                                                                        |
   | Copyright (c) 2005-2019, Individual contributors, see AUTHORS file     |
   | See: https://www.mrpt.org/Authors - All rights reserved.               |
   | Released under BSD License. See: https://www.mrpt.org/License          |
   +------------------------------------------------------------------------+ */
#pragma once

#include <mrpt/core/common.h>  // remove MSVC warnings
#include <mrpt/core/integer_select.h>
#include <array>
#include <stdexcept>
#include <string_view>

namespace mrpt::containers
{
template <typename KEY, typename VALUE>
struct ts_map_entry
{
    bool used{false};
    KEY first;
    VALUE second;
    ts_map_entry() = default;
};

/** hash function used by ts_hash_map. Uses dbj2 method */
void reduced_hash(const std::string_view& value, uint8_t& hash);
void reduced_hash(const std::string_view& value, uint16_t& hash);
void reduced_hash(const std::string_view& value, uint32_t& hash);
void reduced_hash(const std::string_view& value, uint64_t& hash);

/** A thread-safe (ts) container which minimally emulates a std::map<>'s [] and
 * find() methods but which is implemented as a linear vector indexed by a hash
 * of KEY.
 * Any custom hash function can be implemented, we don't rely by default on
 * C++11 std::hash<> due to its limitation in some implementations.
 *
 * This implementation is much more efficient than std::map<> when the most
 * common operation is accesing elements
 *  by KEY with find() or [], and is also thread-safe if different threads
 * create entries with different hash values.
 *
 * The default underlying non-associative container is a "memory-aligned
 * std::vector<>", but it can be changed to a
 *  standard vector<> or to a deque<> (to avoid memory reallocations) by
 * changing the template parameter \a VECTOR_T.
 *
 * \note Defined in #include <mrpt/containers/ts_hash_map.h>
 * \ingroup mrpt_containers_grp
 */
template <
    typename KEY, typename VALUE, unsigned int NUM_BYTES_HASH_TABLE = 1,
    unsigned int NUM_HAS_TABLE_COLLISIONS_ALLOWED = 5,
    typename VECTOR_T = std::array<
        std::array<ts_map_entry<KEY, VALUE>, NUM_HAS_TABLE_COLLISIONS_ALLOWED>,
        1u << (8 * NUM_BYTES_HASH_TABLE)>>
class ts_hash_map
{
   public:
    /** @name Types
        @{ */
    using self_t = ts_hash_map<
        KEY, VALUE, NUM_BYTES_HASH_TABLE, NUM_HAS_TABLE_COLLISIONS_ALLOWED,
        VECTOR_T>;
    using key_type = KEY;
    using value_type = ts_map_entry<KEY, VALUE>;
    using vec_t = VECTOR_T;

    struct iterator;
    struct const_iterator
    {
       public:
        const_iterator() : m_vec(nullptr), m_parent(nullptr) {}
        const_iterator(
            const VECTOR_T& vec, const self_t& parent, int idx_outer,
            int idx_inner)
            : m_vec(const_cast<VECTOR_T*>(&vec)),
              m_parent(const_cast<self_t*>(&parent)),
              m_idx_outer(idx_outer),
              m_idx_inner(idx_inner)
        {
        }
        const_iterator& operator=(const const_iterator& o)
        {
            m_vec = o.m_vec;
            m_idx_outer = o.m_idx_outer;
            m_idx_inner = o.m_idx_inner;
            return *this;
        }
        bool operator==(const const_iterator& o) const
        {
            return m_vec == o.m_vec && m_idx_outer == o.m_idx_outer &&
                   m_idx_inner == o.m_idx_inner;
        }
        bool operator!=(const const_iterator& o) const { return !(*this == o); }
        const value_type& operator*() const
        {
            return (*m_vec)[m_idx_outer][m_idx_inner];
        }
        const value_type* operator->() const
        {
            return &(*m_vec)[m_idx_outer][m_idx_inner];
        }
        inline const_iterator operator++(int)
        { /* Post: it++ */
            const_iterator aux = *this;
            ++(*this);
            return aux;
        }
        inline const_iterator& operator++()
        { /* pre: ++it */
            incr();
            return *this;
        }

       protected:
        VECTOR_T* m_vec;
        self_t* m_parent;
        int m_idx_outer{0}, m_idx_inner{0};
        void incr()
        {
            // This loop ends with the first used entry in the nested arrays, or
            // an iterator pointing to "end()".
            do
            {
                if (++m_idx_inner >=
                    static_cast<int>(NUM_HAS_TABLE_COLLISIONS_ALLOWED))
                {
                    m_idx_inner = 0;
                    m_idx_outer++;
                }
            } while (m_idx_outer < static_cast<int>(m_parent->m_vec.size()) &&
                     !(*m_vec)[m_idx_outer][m_idx_inner].used);
        }
    };

    struct iterator : public const_iterator
    {
       public:
        iterator() : const_iterator() {}
        iterator(VECTOR_T& vec, self_t& parent, int idx_outer, int idx_inner)
            : const_iterator(vec, parent, idx_outer, idx_inner)
        {
        }
        value_type& operator*()
        {
            return (*const_iterator::m_vec)[const_iterator::m_idx_outer]
                                           [const_iterator::m_idx_inner];
        }
        value_type* operator->()
        {
            return &(*const_iterator::m_vec)[const_iterator::m_idx_outer]
                                            [const_iterator::m_idx_inner];
        }
        inline iterator operator++(int)
        { /* Post: it++ */
            iterator aux = *this;
            ++(*this);
            return aux;
        }
        inline iterator& operator++()
        { /* pre: ++it */
            const_iterator::incr();
            return *this;
        }
    };
    /** @} */
   private:
    /** The actual container */
    vec_t m_vec;
    /** Number of elements accessed with write access so far */
    size_t m_size{0};

   public:
    /** @name Constructors, read/write access and other operations
        @{ */
    //!< Default constructor */
    ts_hash_map() = default;
    /** Clear the contents of this container */
    void clear()
    {
        m_size = 0;
        for (size_t oi = 0; oi < m_vec.size(); oi++)
            for (size_t ii = 0; ii < NUM_HAS_TABLE_COLLISIONS_ALLOWED; ii++)
                m_vec[oi][ii] = value_type();
    }

    bool empty() const { return m_size == 0; }
    /** Write/read via [i] operator, that creates an element if it didn't exist
     * already. */
    VALUE& operator[](const KEY& key)
    {
        typename mrpt::uint_select_by_bytecount<NUM_BYTES_HASH_TABLE>::type
            hash;
        reduced_hash(key, hash);
        std::array<ts_map_entry<KEY, VALUE>, NUM_HAS_TABLE_COLLISIONS_ALLOWED>&
            match_arr = m_vec[hash];
        for (unsigned int i = 0; i < NUM_HAS_TABLE_COLLISIONS_ALLOWED; i++)
        {
            if (!match_arr[i].used)
            {
                m_size++;
                match_arr[i].used = true;
                match_arr[i].first = key;
                return match_arr[i].second;
            }
            if (match_arr[i].first == key) return match_arr[i].second;
        }
        throw std::runtime_error("ts_hash_map: too many hash collisions!");
    }
    const_iterator find(const KEY& key) const
    {
        typename mrpt::uint_select_by_bytecount<NUM_BYTES_HASH_TABLE>::type
            hash;
        reduced_hash(key, hash);
        const std::array<
            ts_map_entry<KEY, VALUE>, NUM_HAS_TABLE_COLLISIONS_ALLOWED>&
            match_arr = m_vec[hash];
        for (unsigned int i = 0; i < NUM_HAS_TABLE_COLLISIONS_ALLOWED; i++)
        {
            if (match_arr[i].used && match_arr[i].first == key)
                return const_iterator(m_vec, *this, hash, i);
        }
        return this->end();
    }

    const_iterator begin() const
    {
        const_iterator it(m_vec, *this, 0, -1);
        ++it;
        return it;
    }
    const_iterator end() const
    {
        return const_iterator(m_vec, *this, m_vec.size(), 0);
    }
    iterator begin()
    {
        iterator it(m_vec, *this, 0, -1);
        ++it;
        return it;
    }
    iterator end() { return iterator(m_vec, *this, m_vec.size(), 0); }
    /** @} */

};  // end class ts_hash_map

}  // namespace mrpt::containers