payload.h

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#pragma once
#include <scl/stream/details/deprecation.h>
#include <scl/stream/details/invalid_value.h>
#include <scl/utils/Either.h>
#include <scl/utils/Optional.h>
#include <functional>

namespace scl{
    namespace stream{
        namespace details{
            template <class T>
            class StreamIteratorPayload{
                protected:
                    void ensureGenerated() const{
                        if(!generated){
                            this->alt = gen();
                            generated = true;
                        }
                    }

                public:
                    using value_type = T;

                    using alternative = scl::utils::Optional<value_type>;

                    using producer = std::function<alternative(void)>;

                    explicit StreamIteratorPayload(producer prod) : gen{prod}, alt{}{
                    }

                    const alternative& value() const{
                        ensureGenerated();
                        return alt;
                    }

                    bool isInvalid() const{
                        ensureGenerated();
                        return !alt.hasValue();
                    }

                    bool isValid() const{
                        return !this->isInvalid();
                    }

                    constexpr static StreamIteratorPayload withValue(const T& value){
                        return StreamIteratorPayload{[=]{
                            return alternative::ref(value);
                        }};
                    }

                    constexpr static StreamIteratorPayload withoutValue(){
                        return StreamIteratorPayload{[]{
                            return scl::utils::none;
                        }};
                    }

                protected:
                    producer gen;

                    mutable alternative alt;

                    mutable bool generated = false;
            };
        }
    }
}