distance.h

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#pragma once
 
#include "di/container/concepts/container.h"
#include "di/container/concepts/iterator.h"
#include "di/container/concepts/sentinel_for.h"
#include "di/container/concepts/sized_container.h"
#include "di/container/concepts/sized_sentinel_for.h"
#include "di/container/interface/begin.h"
#include "di/container/interface/end.h"
#include "di/container/interface/size.h"
#include "di/container/interface/ssize.h"
#include "di/container/meta/container_ssize_type.h"
#include "di/container/meta/iterator_ssize_type.h"
 
namespace di::container {
namespace detail {
    struct DistanceFunction {
        template<concepts::Iterator Iter, concepts::SentinelFor<Iter> Sent>
        requires(!concepts::SizedSentinelFor<Sent, Iter>)
        constexpr auto operator()(Iter iter, Sent sent) const -> meta::IteratorSSizeType<Iter> {
            auto distance = meta::IteratorSSizeType<Iter> { 0 };
            for (; iter != sent; ++iter) {
                ++distance;
            }
            return distance;
        }
 
        template<concepts::Iterator Iter, concepts::SizedSentinelFor<Iter> Sent>
        constexpr auto operator()(Iter const& iter, Sent const& sent) const {
            return sent - iter;
        }
 
        template<concepts::Container Con>
        constexpr auto operator()(Con&& container) const -> meta::ContainerSSizeType<Con> {
            if constexpr (concepts::SizedContainer<Con>) {
                return container::ssize(container);
            } else {
                return (*this)(begin(container), end(container));
            }
        }
    };
}
 
constexpr inline auto distance = detail::DistanceFunction {};
}
 
namespace di {
using container::distance;
}