ClickHouse/base/ext/map.h

#pragma once

#include <type_traits>
#include <boost/iterator/transform_iterator.hpp>


namespace ext
{
    /// \brief Strip type off top level reference and cv-qualifiers thus allowing storage in containers
    template <typename T>
    using unqualified_t = std::remove_cv_t<std::remove_reference_t<T>>;

    /** \brief Returns collection of the same container-type as the input collection,
      *    with each element transformed by the application of `mapper`.
      */
    template <template <typename...> class Collection, typename... Params, typename Mapper>
    auto map(const Collection<Params...> & collection, const Mapper mapper)
    {
        using value_type = unqualified_t<decltype(mapper(*std::begin(collection)))>;

        return Collection<value_type>(
            boost::make_transform_iterator(std::begin(collection), mapper),
            boost::make_transform_iterator(std::end(collection), mapper));
    }

    /** \brief Returns collection of specified container-type,
      *    with each element transformed by the application of `mapper`.
      *    Allows conversion between different container-types, e.g. std::vector to std::list
      */
    template <template <typename...> class ResultCollection, typename Collection, typename Mapper>
    auto map(const Collection & collection, const Mapper mapper)
    {
        using value_type = unqualified_t<decltype(mapper(*std::begin(collection)))>;

        return ResultCollection<value_type>(
            boost::make_transform_iterator(std::begin(collection), mapper),
            boost::make_transform_iterator(std::end(collection), mapper));
    }

    /** \brief Returns collection of specified type,
      *    with each element transformed by the application of `mapper`.
      *    Allows leveraging implicit conversion between the result of applying `mapper` and R::value_type.
      */
    template <typename ResultCollection, typename Collection, typename Mapper>
    auto map(const Collection & collection, const Mapper mapper)
    {
        return ResultCollection(
            boost::make_transform_iterator(std::begin(collection), mapper),
            boost::make_transform_iterator(std::end(collection), mapper));
    }
}