view.hh

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#ifndef VIEW_HH
#define VIEW_HH
 
#ifndef _MSC_VER
#include "semiregular.hh"
#endif
#include <algorithm>
#include <functional>
#include <iterator>
#include <tuple>
#include <type_traits>
 
namespace view {
namespace detail {
 
template<typename Iterator>
[[nodiscard]] constexpr 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]] constexpr 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]] constexpr 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]] constexpr 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:
        constexpr Drop(Range&& range_, size_t n_)
                : range(std::forward<Range>(range_))
                , n(n_)
        {
        }
 
        [[nodiscard]] constexpr 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]] constexpr auto end() const {
                return std::end(range);
        }
 
private:
        Range range;
        size_t n;
};
 
 
template<typename Range>
class DropBack
{
public:
        constexpr DropBack(Range&& range_, size_t n_)
                : range(std::forward<Range>(range_))
                , n(n_)
        {
        }
 
        [[nodiscard]] constexpr auto begin() const {
                return std::begin(range);
        }
 
        [[nodiscard]] constexpr 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:
        constexpr explicit Reverse(Range&& range_)
                : range(std::forward<Range>(range_))
        {
        }
 
        [[nodiscard]] constexpr auto begin()  const { return range.rbegin(); }
        [[nodiscard]] constexpr auto begin()        { return range.rbegin(); }
        [[nodiscard]] constexpr auto end()    const { return range.rend(); }
        [[nodiscard]] constexpr auto end()          { return range.rend(); }
        [[nodiscard]] constexpr auto rbegin() const { return range.begin(); }
        [[nodiscard]] constexpr auto rbegin()       { return range.begin(); }
        [[nodiscard]] constexpr auto rend()   const { return range.end(); }
        [[nodiscard]] constexpr auto rend()         { return range.end(); }
 
private:
        Range range;
};
 
 
template<typename Iterator, typename UnaryOp> class TransformIterator
{
public:
        using return_type       = std::invoke_result_t<UnaryOp, decltype(*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:
        constexpr TransformIterator() = default;
 
        constexpr TransformIterator(Iterator it_, UnaryOp op_)
                : it(it_), op(op_)
        {
        }
 
        // InputIterator, ForwardIterator
 
        [[nodiscard]] constexpr return_type operator*() const { return std::invoke(op, *it); }
 
        // pointer operator->() const   not defined
 
        constexpr TransformIterator& operator++()
        {
                ++it;
                return *this;
        }
 
        constexpr TransformIterator operator++(int)
        {
                auto copy = *this;
                ++it;
                return copy;
        }
 
        [[nodiscard]] constexpr friend bool operator==(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it == y.it;
        }
 
        [[nodiscard]] constexpr friend bool operator!=(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it != y.it;
        }
 
        // BidirectionalIterator
 
        constexpr TransformIterator& operator--()
        {
                --it;
                return *this;
        }
 
        constexpr TransformIterator operator--(int)
        {
                auto copy = *this;
                --it;
                return copy;
        }
 
        // RandomAccessIterator
 
        constexpr TransformIterator& operator+=(difference_type n)
        {
                it += n;
                return *this;
        }
 
        constexpr TransformIterator& operator-=(difference_type n)
        {
                it -= n;
                return *this;
        }
 
        [[nodiscard]] constexpr friend TransformIterator operator+(TransformIterator x, difference_type n)
        {
                x += n;
                return x;
        }
        [[nodiscard]] constexpr friend TransformIterator operator+(difference_type n, TransformIterator x)
        {
                x += n;
                return x;
        }
 
        [[nodiscard]] constexpr friend TransformIterator operator-(TransformIterator x, difference_type n)
        {
                x -= n;
                return x;
        }
 
        [[nodiscard]] constexpr friend difference_type operator-(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it - y.it;
        }
 
        [[nodiscard]] constexpr reference operator[](difference_type n)
        {
                return *(*this + n);
        }
 
        [[nodiscard]] constexpr friend bool operator<(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it < y.it;
        }
        [[nodiscard]] constexpr friend bool operator<=(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it <= y.it;
        }
        [[nodiscard]] constexpr friend bool operator>(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it > y.it;
        }
        [[nodiscard]] constexpr friend bool operator>=(const TransformIterator& x, const TransformIterator& y)
        {
                return x.it >= y.it;
        }
 
private:
        Iterator it;
 
#ifndef _MSC_VER
        // Wrapping in 'semiregular_t' is needed on libc++,
        // see commit e81b34b5ed9f for more details.
        using UO = semiregular_t<UnaryOp>;
#else
        // But it triggers compile errors on visual studio, though only in c++20
        // mode. Hopefully we can remove this workaround in the future when we
        // use std::views::transform().
        using UO = UnaryOp;
#endif
        [[no_unique_address]] UO op;
};
 
template<typename Range, typename UnaryOp> class Transform
{
public:
        constexpr Transform(Range&& range_, UnaryOp op_)
                : range(std::forward<Range>(range_)), op(op_)
        {
        }
 
        [[nodiscard]] constexpr auto begin() const
        {
                return TransformIterator(std::begin(range), op);
        }
        [[nodiscard]] constexpr auto end() const
        {
                return TransformIterator(std::end(range), op);
        }
        [[nodiscard]] constexpr auto rbegin() const
        {
                return TransformIterator(std::rbegin(range), op);
        }
        [[nodiscard]] constexpr auto rend() const
        {
                return TransformIterator(std::rend(range), op);
        }
 
        [[nodiscard]] constexpr auto size()  const { return range.size(); }
        [[nodiscard]] constexpr auto empty() const { return range.empty(); }
 
        [[nodiscard]] constexpr auto front() const { return op(range.front()); }
        [[nodiscard]] constexpr auto back()  const { return op(range.back()); }
 
        [[nodiscard]] constexpr auto operator[](size_t idx) const {
                return std::invoke(op, range[idx]);
        }
 
private:
        Range range;
        [[no_unique_address]] UnaryOp op;
};
 
 
template<typename Iterator, typename Sentinel, typename Predicate>
class FilteredIterator
{
public:
        using value_type = typename std::iterator_traits<Iterator>::value_type;
        using reference = typename std::iterator_traits<Iterator>::reference;
        using pointer = typename std::iterator_traits<Iterator>::pointer;
        using difference_type = typename std::iterator_traits<Iterator>::difference_type;
        using iterator_category = std::forward_iterator_tag;
 
        constexpr FilteredIterator(Iterator it_, Sentinel last_, Predicate pred_)
                : it(it_), last(last_), pred(pred_)
        {
                while (isFiltered()) ++it;
        }
 
        [[nodiscard]] constexpr friend bool operator==(const FilteredIterator& x, const FilteredIterator& y)
        {
                return x.it == y.it;
        }
        [[nodiscard]] constexpr friend bool operator!=(const FilteredIterator& x, const FilteredIterator& y)
        {
                return !(x == y);
        }
 
        [[nodiscard]] constexpr reference operator*() const { return *it; }
        [[nodiscard]] constexpr pointer operator->() const { return &*it; }
 
        constexpr FilteredIterator& operator++()
        {
                do {
                        ++it;
                } while (isFiltered());
                return *this;
        }
        constexpr FilteredIterator operator++(int)
        {
                FilteredIterator result = *this;
                ++(*this);
                return result;
        }
 
private:
        [[nodiscard]] constexpr bool isFiltered()
        {
                return (it != last) && !std::invoke(pred, *it);
        }
 
private:
        Iterator it;
        Sentinel last;
        [[no_unique_address]] Predicate pred;
};
 
template<typename Range, typename Predicate>
class Filter
{
public:
        using Iterator = decltype(std::begin(std::declval<Range>()));
        using Sentinel = decltype(std::end  (std::declval<Range>()));
        using F_Iterator = FilteredIterator<Iterator, Sentinel, Predicate>;
 
        constexpr Filter(Range&& range_, Predicate pred_)
                : range(std::forward<Range>(range_)), pred(pred_)
        {
        }
 
        [[nodiscard]] constexpr F_Iterator begin() const
        {
                return {std::begin(range), std::end(range), pred};
        }
        [[nodiscard]] constexpr F_Iterator end() const
        {
                // TODO should be a 'FilteredSentinel', but that only works well
                // in c++20 (and c++20 already has 'std::views::filter').
                return {std::end(range), std::end(range), pred};
        }
 
private:
        Range range;
        [[no_unique_address]] Predicate pred;
};
 
} // namespace detail
 
template<typename Range>
[[nodiscard]] constexpr auto drop(Range&& range, size_t n)
{
        return detail::Drop<Range>(std::forward<Range>(range), n);
}
 
template<typename Range>
[[nodiscard]] constexpr auto drop_back(Range&& range, size_t n)
{
        return detail::DropBack<Range>(std::forward<Range>(range), n);
}
 
template<typename Range>
[[nodiscard]] constexpr auto reverse(Range&& range)
{
        return detail::Reverse<Range>(std::forward<Range>(range));
}
 
template<typename Range, typename UnaryOp>
[[nodiscard]] constexpr auto transform(Range&& range, UnaryOp op)
{
        return detail::Transform<Range, UnaryOp>(std::forward<Range>(range), op);
}
 
template<typename Map> [[nodiscard]] constexpr auto keys(Map&& map)
{
        return transform(std::forward<Map>(map),
                         [](const auto& t) -> auto& { return std::get<0>(t); });
}
 
template<typename Map> [[nodiscard]] constexpr auto values(Map&& map)
{
        return transform(std::forward<Map>(map),
                         [](const auto& t) -> auto& { return std::get<1>(t); });
}
 
template<typename ForwardRange, typename Predicate>
[[nodiscard]] auto filter(ForwardRange&& range, Predicate pred)
{
    return detail::Filter<ForwardRange, Predicate>{std::forward<ForwardRange>(range), pred};
}
 
} // namespace view
 
#endif