monero/src/lmdb/key_stream.h

// Copyright (c) 2018, The Monero Project
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#pragma once

#include <boost/range/iterator_range.hpp>
#include <cstdint>
#include <cstring>
#include <iterator>
#include <lmdb.h>
#include <utility>

#include "lmdb/value_stream.h"
#include "span.h"

namespace lmdb
{

    /*!
        An InputIterator for a fixed-sized LMDB key and value. `operator++`
        iterates over keys.

        \tparam K Key type in database records.
        \tparam V Value type in database records.

        \note This meets requirements for an InputIterator only. The iterator
            can only be incremented and dereferenced. All copies of an iterator
            share the same LMDB cursor, and therefore incrementing any copy will
            change the cursor state for all (incrementing an iterator will
            invalidate all prior copies of the iterator). Usage is identical
            to `std::istream_iterator`.
    */
    template<typename K, typename V>
    class key_iterator
    {
        MDB_cursor* cur;
        epee::span<const std::uint8_t> key;
        
        void increment()
        {
            // MDB_NEXT_MULTIPLE doesn't work if only one value is stored :/
            if (cur)
                key = lmdb::stream::get(*cur, MDB_NEXT_NODUP, sizeof(K), sizeof(V)).first;
        }

    public:
        using value_type = std::pair<K, boost::iterator_range<value_iterator<V>>>;
        using reference = value_type;
        using pointer = void;
        using difference_type = std::size_t;
        using iterator_category = std::input_iterator_tag;

        //! Construct an "end" iterator.
        key_iterator() noexcept
          : cur(nullptr), key()
        {}

        /*!
            \param cur Iterate over keys starting at this cursor position.
            \throw std::system_error if unexpected LMDB error. This can happen
                if `cur` is invalid.
        */
        key_iterator(MDB_cursor* cur)
          : cur(cur), key()
        {
            if (cur)
                key = lmdb::stream::get(*cur, MDB_GET_CURRENT, sizeof(K), sizeof(V)).first;
        }

        //! \return True if `this` is one-past the last key.
        bool is_end() const noexcept { return key.empty(); }

        //! \return True iff `rhs` is referencing `this` key.
        bool equal(key_iterator const& rhs) const noexcept
        {
            return
                (key.empty() && rhs.key.empty()) ||
                key.data() == rhs.key.data();
        }

        /*!
            Moves iterator to next key or end. Invalidates all prior copies of
            the iterator.
        */
        key_iterator& operator++()
        {
            increment();
            return *this;
        }

        /*!
            Moves iterator to next key or end.

            \return A copy that is already invalidated, ignore
        */
        key_iterator operator++(int)
        {
            key_iterator out{*this};
            increment();
            return out;
        }

        //! \pre `!is_end()` \return {current key, current value range}
        value_type operator*() const
        {
            return {get_key(), make_value_range()};
        }

        //! \pre `!is_end()` \return Current key
        K get_key() const noexcept
        {
            assert(!is_end());
            K out;
            std::memcpy(std::addressof(out), key.data(), sizeof(out));
            return out;
        }

        /*!
            Return a C++ iterator over database values from current cursor
            position that will reach `.is_end()` after the last duplicate key
            record. Calling `make_iterator()` will return an iterator whose
            `operator*` will return an entire value (`V`).
            `make_iterator<MONERO_FIELD(account, id)>()` will return an
            iterator whose `operator*` will return a `decltype(account.id)`
            object - the other fields in the struct `account` are never copied
            from the database.

            \throw std::system_error if LMDB has unexpected errors.
            \return C++ iterator starting at current cursor position.
        */
        template<typename T = V, typename F = T, std::size_t offset = 0>
        value_iterator<T, F, offset> make_value_iterator() const
        {
            static_assert(std::is_same<T, V>(), "bad MONERO_FIELD usage?");
            return {cur};
        }

        /*!
            Return a range from current cursor position until last duplicate
            key record. Useful in for-each range loops or in templated code
            expecting a range of elements. Calling `make_range()` will return
            a range of `T` objects. `make_range<MONERO_FIELD(account, id)>()`
            will return a range of `decltype(account.id)` objects - the other
            fields in the struct `account` are never copied from the database.

            \throw std::system_error if LMDB has unexpected errors.
            \return An InputIterator range over values at cursor position.
        */
        template<typename T = V, typename F = T, std::size_t offset = 0>
        boost::iterator_range<value_iterator<T, F, offset>> make_value_range() const
        {
            return {make_value_iterator<T, F, offset>(), value_iterator<T, F, offset>{}};
        }
    };

    /*!
        C++ wrapper for a LMDB read-only cursor on a fixed-sized key `K` and
        value `V`.

        \tparam K key type being stored by each record.
        \tparam V value type being stored by each record.
        \tparam D cleanup functor for the cursor; usually unique per db/table.
    */
    template<typename K, typename V, typename D>
    class key_stream
    {
        std::unique_ptr<MDB_cursor, D> cur;
    public:

        //! Take ownership of `cur` without changing position. `nullptr` valid.
        explicit key_stream(std::unique_ptr<MDB_cursor, D> cur)
          : cur(std::move(cur))
        {}

        key_stream(key_stream&&) = default;
        key_stream(key_stream const&) = delete;
        ~key_stream() = default;
        key_stream& operator=(key_stream&&) = default;
        key_stream& operator=(key_stream const&) = delete;

        /*!
            Give up ownership of the cursor. `make_iterator()` and
            `make_range()` can still be invoked, but return the empty set.

            \return Currently owned LMDB cursor.
        */
        std::unique_ptr<MDB_cursor, D> give_cursor() noexcept
        {
            return {std::move(cur)};
        }

        /*!
            Place the stream back at the first key/value. Newly created
            iterators will start at the first value again.

            \note Invalidates all current iterators, including those created
                with `make_iterator` or `make_range`. Also invalidates all
                `value_iterator`s created with `key_iterator`.
        */
        void reset()
        {
            if (cur)
                lmdb::stream::get(*cur, MDB_FIRST, 0, 0);
        }

        /*!
            \throw std::system_error if LMDB has unexpected errors.
            \return C++ iterator over database keys from current cursor
                position that will reach `.is_end()` after the last key.
        */
        key_iterator<K, V> make_iterator() const
        {
            return {cur.get()};
        }

        /*!
            \throw std::system_error if LMDB has unexpected errors.
            \return Range from current cursor position until last key record.
                Useful in for-each range loops or in templated code
        */
        boost::iterator_range<key_iterator<K, V>> make_range() const
        {
            return {make_iterator(), key_iterator<K, V>{}};
        }
    };

    template<typename K, typename V>
    inline
    bool operator==(key_iterator<K, V> const& lhs, key_iterator<K, V> const& rhs) noexcept
    {
        return lhs.equal(rhs);
    }

    template<typename K, typename V>
    inline
    bool operator!=(key_iterator<K, V> const& lhs, key_iterator<K, V> const& rhs) noexcept
    {
        return !lhs.equal(rhs);
    }
} // lmdb