Filter.h

#pragma once
 
#include "../Common.h"
#include "../util/TraitImpl.h"
 
namespace CXXIter {
 
    // ################################################################################################
    // FILTER
    // ################################################################################################
    namespace op {
        template<typename TChainInput, typename TFilterFn>
        class [[nodiscard(CXXITER_CHAINER_NODISCARD_WARNING)]] Filter : public IterApi<Filter<TChainInput, TFilterFn>> {
            friend struct trait::Iterator<Filter<TChainInput, TFilterFn>>;
            friend struct trait::DoubleEndedIterator<Filter<TChainInput, TFilterFn>>;
        private:
            using InputItem = typename TChainInput::Item;
 
            TChainInput input;
            TFilterFn filterFn;
        public:
            constexpr Filter(TChainInput&& input, TFilterFn filterFn) : input(std::move(input)), filterFn(filterFn) {}
        };
    }
    // ------------------------------------------------------------------------------------------------
    template<typename TChainInput, typename TFilterFn>
    struct trait::Iterator<op::Filter<TChainInput, TFilterFn>> {
        using ChainInputIterator = trait::Iterator<TChainInput>;
        // CXXIter Interface
        using Self = op::Filter<TChainInput, TFilterFn>;
        using Item = typename ChainInputIterator::Item;
 
        static constexpr inline IterValue<Item> next(Self& self) {
            while(true) {
                auto item = ChainInputIterator::next(self.input);
                if(!item.has_value()) [[unlikely]] { return {}; }
                if(self.filterFn(item.value())) { return item; }
            }
        }
        static constexpr inline SizeHint sizeHint(const Self& self) {
            SizeHint input = ChainInputIterator::sizeHint(self.input);
            return SizeHint(0, input.upperBound);
        }
        static constexpr inline size_t advanceBy(Self& self, size_t n) { return util::advanceByPull(self, n); }
    };
    template<CXXIterDoubleEndedIterator TChainInput, typename TFilterFn>
    struct trait::DoubleEndedIterator<op::Filter<TChainInput, TFilterFn>> {
        using ChainInputIterator = trait::DoubleEndedIterator<TChainInput>;
        // CXXIter Interface
        using Self = op::Filter<TChainInput, TFilterFn>;
        using Item = typename ChainInputIterator::Item;
 
        static constexpr inline IterValue<Item> nextBack(Self& self) {
            while(true) {
                auto item = ChainInputIterator::nextBack(self.input);
                if(!item.has_value()) [[unlikely]] { return {}; }
                if(self.filterFn(item.value())) { return item; }
            }
        }
    };
 
}