Unique.h

#pragma once
 
#include <unordered_set>
 
#include "../Common.h"
#include "../sources/ContainerSources.h"
#include "../util/TraitImpl.h"
 
namespace CXXIter {
 
    // ################################################################################################
    // UNIQUE
    // ################################################################################################
    namespace op {
        template<typename TChainInput, typename TMapFn>
        class [[nodiscard(CXXITER_CHAINER_NODISCARD_WARNING)]] Unique : public IterApi<Unique<TChainInput, TMapFn>> {
            friend struct trait::Iterator<Unique<TChainInput, TMapFn>>;
        private:
            using OwnedInputItem = typename TChainInput::ItemOwned;
 
            TChainInput input;
            TMapFn mapFn;
            std::unordered_set<OwnedInputItem> uniqueCache;
        public:
            constexpr Unique(TChainInput&& input, TMapFn mapFn) : input(std::move(input)), mapFn(mapFn) {
                uniqueCache.reserve(this->input.sizeHint().expectedResultSize());
            }
        };
    }
    // ------------------------------------------------------------------------------------------------
    template<typename TChainInput, typename TMapFn>
    struct trait::Iterator<op::Unique<TChainInput, TMapFn>> {
        using ChainInputIterator = trait::Iterator<TChainInput>;
        using InputItem = typename TChainInput::Item;
        using OwnedInputItem = typename TChainInput::ItemOwned;
        // CXXIter Interface
        using Self = op::Unique<TChainInput, TMapFn>;
        using Item = InputItem;
 
        static constexpr inline IterValue<Item> next(Self& self) {
            while(true) {
                auto item = ChainInputIterator::next(self.input);
                if(!item.has_value()) [[unlikely]] { return {}; } // reached end of input
 
                auto itemUniqueValue = self.mapFn(item.value());
                if(self.uniqueCache.contains(itemUniqueValue)) { continue; } // uniqueness-property violated, ignore item
                self.uniqueCache.insert(std::move(itemUniqueValue));
                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); }
    };
 
}