ClickHouse/dbms/src/Common/HashTable/SmallTable.h

#pragma once

#include <Common/HashTable/HashMap.h>


namespace DB
{
    namespace ErrorCodes
    {
        extern const int INCORRECT_DATA;
    }
}


/** Replacement of the hash table for a small number (<10) of keys.
  * Implemented as an array with linear search.
  * The array is located inside the object.
  * The interface is a subset of the HashTable interface.
  *
  * Insert is possible only if the `full` method returns false.
  * With an unknown number of different keys,
  *  you should check if the table is not full,
  *  and do a `fallback` in this case (for example, use a real hash table).
  */
template
<
    typename Key,
    typename Cell,
    size_t capacity
>
class SmallTable :
    private boost::noncopyable,
    protected Cell::State
{
protected:
    friend class const_iterator;
    friend class iterator;
    friend class Reader;

    using Self = SmallTable;
    using cell_type = Cell;

    size_t m_size = 0;        /// Amount of elements.
    Cell buf[capacity];       /// A piece of memory for all elements.


    /// Find a cell with the same key or an empty cell, starting from the specified position and then by the collision resolution chain.
    const Cell * ALWAYS_INLINE findCell(const Key & x) const
    {
        const Cell * it = buf;
        while (it < buf + m_size)
        {
            if (it->keyEquals(x))
                break;
            ++it;
        }
        return it;
    }

    Cell * ALWAYS_INLINE findCell(const Key & x)
    {
        Cell * it = buf;
        while (it < buf + m_size)
        {
            if (it->keyEquals(x))
                break;
            ++it;
        }
        return it;
    }


public:
    using key_type = Key;
    using value_type = typename Cell::value_type;


    class Reader final : private Cell::State
    {
    public:
        Reader(DB::ReadBuffer & in_)
        : in(in_)
        {
        }

        Reader(const Reader &) = delete;
        Reader & operator=(const Reader &) = delete;

        bool next()
        {
            if (!is_initialized)
            {
                Cell::State::read(in);
                DB::readVarUInt(size, in);

                if (size > capacity)
                    throw DB::Exception("Illegal size", DB::ErrorCodes::INCORRECT_DATA);

                is_initialized = true;
            }

            if (read_count == size)
            {
                is_eof = true;
                return false;
            }

            cell.read(in);
            ++read_count;

            return true;
        }

        inline const value_type & get() const
        {
            if (!is_initialized || is_eof)
                throw DB::Exception("No available data", DB::ErrorCodes::NO_AVAILABLE_DATA);

            return cell.getValue();
        }

    private:
        DB::ReadBuffer & in;
        Cell cell;
        size_t read_count = 0;
        size_t size;
        bool is_eof = false;
        bool is_initialized = false;
    };

    class iterator
    {
        Self * container;
        Cell * ptr;

        friend class SmallTable;

    public:
        iterator() {}
        iterator(Self * container_, Cell * ptr_) : container(container_), ptr(ptr_) {}

        bool operator== (const iterator & rhs) const { return ptr == rhs.ptr; }
        bool operator!= (const iterator & rhs) const { return ptr != rhs.ptr; }

        iterator & operator++()
        {
            ++ptr;
            return *this;
        }

        value_type & operator* () const { return ptr->getValue(); }
        value_type * operator->() const { return &ptr->getValue(); }

        Cell * getPtr() const { return ptr; }
    };


    class const_iterator
    {
        const Self * container;
        const Cell * ptr;

        friend class SmallTable;

    public:
        const_iterator() {}
        const_iterator(const Self * container_, const Cell * ptr_) : container(container_), ptr(ptr_) {}
        const_iterator(const iterator & rhs) : container(rhs.container), ptr(rhs.ptr) {}

        bool operator== (const const_iterator & rhs) const { return ptr == rhs.ptr; }
        bool operator!= (const const_iterator & rhs) const { return ptr != rhs.ptr; }

        const_iterator & operator++()
        {
            ++ptr;
            return *this;
        }

        const value_type & operator* () const { return ptr->getValue(); }
        const value_type * operator->() const { return &ptr->getValue(); }

        const Cell * getPtr() const { return ptr; }
    };


    const_iterator begin() const     { return iteratorTo(buf); }
    iterator begin()                 { return iteratorTo(buf); }

    const_iterator end() const         { return iteratorTo(buf + m_size); }
    iterator end()                     { return iteratorTo(buf + m_size); }


protected:
    const_iterator iteratorTo(const Cell * ptr) const     { return const_iterator(this, ptr); }
    iterator iteratorTo(Cell * ptr)                     { return iterator(this, ptr); }


public:
    /** The table is full.
      * You can not insert anything into the full table.
      */
    bool full()
    {
        return m_size == capacity;
    }


    /// Insert the value. In the case of any more complex values, it is better to use the `emplace` function.
    std::pair<iterator, bool> ALWAYS_INLINE insert(const value_type & x)
    {
        std::pair<iterator, bool> res;

        emplace(Cell::getKey(x), res.first, res.second);

        if (res.second)
            res.first.ptr->setMapped(x);

        return res;
    }


    /** Insert the key,
      * return an iterator to a position that can be used for `placement new` of value,
      * as well as the flag - whether a new key was inserted.
      *
      * You have to make `placement new` of value if you inserted a new key,
      * since when destroying a hash table, a destructor will be called for it!
      *
      * Example usage:
      *
      * Map::iterator it;
      * bool inserted;
      * map.emplace(key, it, inserted);
      * if (inserted)
      *     new(&it->second) Mapped(value);
      */
    void ALWAYS_INLINE emplace(Key x, iterator & it, bool & inserted)
    {
        Cell * res = findCell(x);
        it = iteratorTo(res);
        inserted = res == buf + m_size;
        if (inserted)
        {
            new(res) Cell(x, *this);
            ++m_size;
        }
    }


    /// Same, but return false if it's full.
    bool ALWAYS_INLINE tryEmplace(Key x, iterator & it, bool & inserted)
    {
        Cell * res = findCell(x);
        it = iteratorTo(res);
        inserted = res == buf + m_size;
        if (inserted)
        {
            if (res == buf + capacity)
                return false;

            new(res) Cell(x, *this);
            ++m_size;
        }
        return true;
    }


    /// Copy the cell from another hash table. It is assumed that there was no such key in the table yet.
    void ALWAYS_INLINE insertUnique(const Cell * cell)
    {
        memcpy(&buf[m_size], cell, sizeof(*cell));
        ++m_size;
    }

    void ALWAYS_INLINE insertUnique(Key x)
    {
        new(&buf[m_size]) Cell(x, *this);
        ++m_size;
    }


    iterator ALWAYS_INLINE find(Key x)                 { return iteratorTo(findCell(x)); }
    const_iterator ALWAYS_INLINE find(Key x) const     { return iteratorTo(findCell(x)); }


    void write(DB::WriteBuffer & wb) const
    {
        Cell::State::write(wb);
        DB::writeVarUInt(m_size, wb);

        for (size_t i = 0; i < m_size; ++i)
            buf[i].write(wb);
    }

    void writeText(DB::WriteBuffer & wb) const
    {
        Cell::State::writeText(wb);
        DB::writeText(m_size, wb);

        for (size_t i = 0; i < m_size; ++i)
        {
            DB::writeChar(',', wb);
            buf[i].writeText(wb);
        }
    }

    void read(DB::ReadBuffer & rb)
    {
        Cell::State::read(rb);

        m_size = 0;

        size_t new_size = 0;
        DB::readVarUInt(new_size, rb);

        if (new_size > capacity)
            throw DB::Exception("Illegal size", DB::ErrorCodes::INCORRECT_DATA);

        for (size_t i = 0; i < new_size; ++i)
            buf[i].read(rb);

        m_size = new_size;
    }

    void readText(DB::ReadBuffer & rb)
    {
        Cell::State::readText(rb);

        m_size = 0;

        size_t new_size = 0;
        DB::readText(new_size, rb);

        if (new_size > capacity)
            throw DB::Exception("Illegal size", DB::ErrorCodes::INCORRECT_DATA);

        for (size_t i = 0; i < new_size; ++i)
        {
            DB::assertChar(',', rb);
            buf[i].readText(rb);
        }

        m_size = new_size;
    }


    size_t size() const
    {
        return m_size;
    }

    bool empty() const
    {
        return 0 == m_size;
    }

    void clear()
    {
        if (!std::is_trivially_destructible_v<Cell>)
            for (iterator it = begin(); it != end(); ++it)
                it.ptr->~Cell();

        m_size = 0;
    }

    size_t getBufferSizeInBytes() const
    {
        return sizeof(buf);
    }
};


struct HashUnused {};


template
<
    typename Key,
    size_t capacity
>
using SmallSet = SmallTable<Key, HashTableCell<Key, HashUnused>, capacity>;


template
<
    typename Key,
    typename Cell,
    size_t capacity
>
class SmallMapTable : public SmallTable<Key, Cell, capacity>
{
public:
    using key_type = Key;
    using mapped_type = typename Cell::Mapped;
    using value_type = typename Cell::value_type;

    mapped_type & ALWAYS_INLINE operator[](Key x)
    {
        typename SmallMapTable::iterator it;
        bool inserted;
        this->emplace(x, it, inserted);
        new(&it->second) mapped_type();
        return it->second;
    }
};


template
<
    typename Key,
    typename Mapped,
    size_t capacity
>
using SmallMap = SmallMapTable<Key, HashMapCell<Key, Mapped, HashUnused>, capacity>;