view.hh

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#ifndef VIEW_HH
#define VIEW_HH

#include "semiregular.hh"
#include <algorithm>
#include <iterator>
#include <tuple>
#include <type_traits>

namespace view {
namespace detail {

template<typename Iterator>
[[nodiscard]] Iterator safe_next(Iterator first, Iterator last, size_t n, std::input_iterator_tag)
{
        while (n-- && (first != last)) ++first;
        return first;
}

template<typename Iterator>
[[nodiscard]] Iterator safe_next(Iterator first, Iterator last, size_t n, std::random_access_iterator_tag)
{
        return first + std::min<size_t>(n, last - first);
}

template<typename Iterator>
[[nodiscard]] Iterator safe_prev(Iterator first, Iterator last, size_t n, std::bidirectional_iterator_tag)
{
        while (n-- && (first != last)) --last;
        return last;
}

template<typename Iterator>
[[nodiscard]] Iterator safe_prev(Iterator first, Iterator last, size_t n, std::random_access_iterator_tag)
{
        return last - std::min<size_t>(n, last - first);
}


template<typename Range>
class Drop
{
public:
        Drop(Range&& range_, size_t n_)
                : range(std::forward<Range>(range_))
                , n(n_)
        {
        }

        [[nodiscard]] auto begin() const {
                using Iterator = decltype(std::begin(range));
                return safe_next(std::begin(range), std::end(range), n,
                                 typename std::iterator_traits<Iterator>::iterator_category());
        }

        [[nodiscard]] auto end() const {
                return std::end(range);
        }

private:
        Range range;
        size_t n;
};


template<typename Range>
class DropBack
{
public:
        DropBack(Range&& range_, size_t n_)
                : range(std::forward<Range>(range_))
                , n(n_)
        {
        }

        [[nodiscard]] auto begin() const {
                return std::begin(range);
        }

        [[nodiscard]] auto end() const {
                using Iterator = decltype(std::begin(range));
                return safe_prev(std::begin(range), std::end(range), n,
                                 typename std::iterator_traits<Iterator>::iterator_category());
        }

private:
        Range range;
        size_t n;
};


template<typename Range>
class Reverse
{
public:
        explicit Reverse(Range&& range_)
                : range(std::forward<Range>(range_))
        {
        }

        [[nodiscard]] auto begin()  const { return range.rbegin(); }
        [[nodiscard]] auto begin()        { return range.rbegin(); }
        [[nodiscard]] auto end()    const { return range.rend(); }
        [[nodiscard]] auto end()          { return range.rend(); }
        [[nodiscard]] auto rbegin() const { return range.begin(); }
        [[nodiscard]] auto rbegin()       { return range.begin(); }
        [[nodiscard]] auto rend()   const { return range.end(); }
        [[nodiscard]] auto rend()         { return range.end(); }

private:
        Range range;
};


template<typename Iterator, typename UnaryOp> class TransformIterator
{
public:
        using return_type       = decltype(std::declval<UnaryOp>()(*std::declval<Iterator>()));
        using value_type        = std::remove_reference_t<return_type>;
        using reference         = value_type;
        using pointer           = value_type*;
        using difference_type   = typename std::iterator_traits<Iterator>::difference_type;
        using iterator_category = typename std::iterator_traits<Iterator>::iterator_category;

public:
        TransformIterator() = default;

        TransformIterator(Iterator it_, UnaryOp op_)
                : storage(it_, op_)
        {
        }

        // InputIterator, ForwardIterator

        [[nodiscard]] return_type operator*() const { return op()(*it()); }

        // pointer operator->() const   not defined

        TransformIterator& operator++()
        {
                ++it();
                return *this;
        }

        TransformIterator operator++(int)
        {
                auto copy = *this;
                ++it();
                return copy;
        }

        [[nodiscard]] friend bool operator==(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it() == y.it();
        }

        [[nodiscard]] friend bool operator!=(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it() != y.it();
        }

        // BidirectionalIterator

        TransformIterator& operator--()
        {
                --it();
                return *this;
        }

        TransformIterator operator--(int)
        {
                auto copy = *this;
                --it();
                return copy;
        }

        // RandomAccessIterator

        TransformIterator& operator+=(difference_type n)
        {
                it() += n;
                return *this;
        }

        TransformIterator& operator-=(difference_type n)
        {
                it() -= n;
                return *this;
        }

        [[nodiscard]] friend TransformIterator operator+(TransformIterator x, difference_type n)
        {
                x += n;
                return x;
        }
        [[nodiscard]] friend TransformIterator operator+(difference_type n, TransformIterator x)
        {
                x += n;
                return x;
        }

        [[nodiscard]] friend TransformIterator operator-(TransformIterator x, difference_type n)
        {
                x -= n;
                return x;
        }

        [[nodiscard]] friend difference_type operator-(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it() - y.it();
        }

        [[nodiscard]] reference operator[](difference_type n)
        {
                return *(*this + n);
        }

        [[nodiscard]] friend bool operator<(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it() < y.it();
        }
        [[nodiscard]] friend bool operator<=(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it() <= y.it();
        }
        [[nodiscard]] friend bool operator>(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it() > y.it();
        }
        [[nodiscard]] friend bool operator>=(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it() >= y.it();
        }

private:
        std::tuple<Iterator, semiregular_t<UnaryOp>> storage;

        [[nodiscard]]       Iterator& it()       { return std::get<0>(storage); }
        [[nodiscard]] const Iterator& it() const { return std::get<0>(storage); }
        [[nodiscard]]       UnaryOp&  op()       { return std::get<1>(storage); }
        [[nodiscard]] const UnaryOp&  op() const { return std::get<1>(storage); }
};

template<typename Range, typename UnaryOp> class Transform
{
public:
        Transform(Range&& range_, UnaryOp op_)
                : storage(std::forward<Range>(range_), op_)
        {
        }

        [[nodiscard]] auto begin() const
        {
                return TransformIterator(std::begin(range()), op());
        }
        [[nodiscard]] auto end() const
        {
                return TransformIterator(std::end(range()), op());
        }
        [[nodiscard]] auto rbegin() const
        {
                return TransformIterator(std::rbegin(range()), op());
        }
        [[nodiscard]] auto rend() const
        {
                return TransformIterator(std::rend(range()), op());
        }

        [[nodiscard]] auto size()  const { return range().size(); }
        [[nodiscard]] auto empty() const { return range().empty(); }

        [[nodiscard]] auto front() const { return op()(range().front()); }
        [[nodiscard]] auto back()  const { return op()(range().back()); }

        [[nodiscard]] auto operator[](size_t idx) const { return op()(range()[idx]); }

private:
        std::tuple<Range, UnaryOp> storage;

        [[nodiscard]] const Range& range() const { return std::get<0>(storage); }
        [[nodiscard]] const UnaryOp& op()  const { return std::get<1>(storage); }
};


} // namespace detail

template<typename Range>
[[nodiscard]] auto drop(Range&& range, size_t n)
{
        return detail::Drop<Range>(std::forward<Range>(range), n);
}

template<typename Range>
[[nodiscard]] auto drop_back(Range&& range, size_t n)
{
        return detail::DropBack<Range>(std::forward<Range>(range), n);
}

template<typename Range>
[[nodiscard]] auto reverse(Range&& range)
{
        return detail::Reverse<Range>(std::forward<Range>(range));
}

template<typename Range, typename UnaryOp>
[[nodiscard]] auto transform(Range&& range, UnaryOp op)
{
        return detail::Transform<Range, UnaryOp>(std::forward<Range>(range), op);
}

template<typename Map> [[nodiscard]] auto keys(Map&& map)
{
        return transform(std::forward<Map>(map),
                         [](const auto& t) -> auto& { return std::get<0>(t); });
}

template<typename Map> [[nodiscard]] auto values(Map&& map)
{
        return transform(std::forward<Map>(map),
                         [](const auto& t) -> auto& { return std::get<1>(t); });
}

} // namespace view

#endif