ClickHouse/base/common/map.h

#pragma once

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

namespace collections
{

/// \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, Mapper && mapper)
{
    using value_type = unqualified_t<decltype(mapper(*std::begin(collection)))>;

    return Collection<value_type>(
        boost::make_transform_iterator(std::begin(collection), std::forward<Mapper>(mapper)),
        boost::make_transform_iterator(std::end(collection), std::forward<Mapper>(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, Mapper && mapper)
{
    using value_type = unqualified_t<decltype(mapper(*std::begin(collection)))>;

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

}