split_tuple_iterator.hpp
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split_tuple_iterator.hpp
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	//===-- rosa/support/iterator/split_tuple_iterator.hpp ----------- C++ --===/
	//
	// The RoSA Framework
	//
	//===----------------------------------------------------------------------===/
	///
	/// \file rosa/support/iterator/split_tuple_iterator.hpp
	///
	/// \authors David Juhasz (david.juhasz@tuwien.ac.at)
	///
	/// \date 2019
	///
	/// \brief Facilities to split iterators of \c std::tuple into iterators of
	/// their elements.
	///
	//===----------------------------------------------------------------------===/

	#ifndef ROSA_SUPPORT_ITERATOR_SPLIT_TUPLE_ITERATOR_HPP
	#define ROSA_SUPPORT_ITERATOR_SPLIT_TUPLE_ITERATOR_HPP

	#include "rosa/support/debug.hpp"
	#include "rosa/support/sequence.hpp"

	#include <memory>
	#include <tuple>
	#include <type_traits>
	#include <queue>

	namespace rosa {
	namespace iterator {

	/// Anonymous namespace providing implementation for splitting tuple iterators;
	/// consider it private.
	namespace {

	/// Iterator of values provided by a buffer based on an index
	///
	/// \tparam TB buffer providing values to iterate
	/// \tparam I index of the values to iterator from \p TB
	///
	/// \note \p TB is expected to implemnet an interface matching that of \c
	/// TupleIteratorBuffer.
	template <typename TB, size_t I>
	class SplittedElementIterator {
	public:
	/// Type alias for the values the iterator iterates.
	using T = typename TB::template element_type<I>;

	/// \defgroup SplittedElementIteratorTypedefs Typedefs of
	/// \c SplittedElementIterator
	///
	/// Standard `typedef`s for iterators.
	///
	///@{
	typedef std::input_iterator_tag
	iterator_category; ///< Category of the iterator.
	typedef T value_type; ///< Type of values iterated over.
	typedef std::size_t difference_type; ///< Type to identify distance.
	typedef T *pointer; ///< Pointer to the type iterated over.
	typedef T &reference; ///< Reference to the type iterated ver.
	///@}

	/// Creates a new instance.
	///
	/// \param Buffer buffer providing values for \p this object
	///
	/// \note \p Buffer is captured as a \c std::shared_ptr because it is shared
	/// among iterators for its element positions.
	SplittedElementIterator(std::shared_ptr<TB> Buffer)
	: Buffer(Buffer), Value() {
	// \c Value is initialized empty so the first incrementation here will read
	// the first value.
	++(*this);
	}

	/// Creates an empty new instance.
	SplittedElementIterator(void) noexcept : Buffer(), Value() {}

	/// Pre-increment operator.
	///
	/// The implementation reads the next value from \c Buffer. If no more values
	/// can be provided by \c Buffer, \p this object becomes empty and the
	/// operator has no further effect.
	///
	/// \return \p this object after incrementing it.
	SplittedElementIterator &operator++() {
	if (Buffer->template hasMore<I>()) {
	Value = Buffer->template next<I>();
	} else {
	// Reached end of range, remove reference of \c Buffer.
	Buffer.reset();
	}
	return *this;
	}

	/// Post-increment operator.
	///
	/// The implementation uses the pre-increment operator and returns a copy of
	/// the original state of \p this object.
	///
	/// \return \p this object before incrementing it.
	SplittedElementIterator operator++(int) {
	SplittedElementIterator<TB, I> Tmp(*this);
	++(*this);
	return Tmp;
	}

	/// Returns a constant reference to the current value.
	///
	/// \note Should not dereference the iterator when it is empty.
	///
	/// \return constant reference to the current value.
	const T &operator*(void)const noexcept { return Value; }

	/// Returns a constant pointer to the current value.
	///
	/// \note Should not dereference the iterator when it is empty.
	///
	/// \return constant pointer to the current value.
	const T *operator->(void)const noexcept { return &Value; }

	/// Tells if \p this object is equal to another one.
	///
	/// Two \c SplittedElementIterator instances are equal if and only if they are
	/// the same or both are empty.
	///
	/// \param RHS other object to compare to
	///
	/// \return whether \p this object is equal with \p RHS
	bool operator==(const SplittedElementIterator &RHS) const noexcept {
	return ((this == &RHS) \|\| (!Buffer && !RHS.Buffer));
	}

	/// Tells if \p this object is not equal to another one.
	///
	/// \see SplittedElementIterator::operator==
	///
	/// \param RHS other object to compare to
	///
	/// \return whether \p this object is not equal with \p RHS.
	bool operator!=(const SplittedElementIterator &RHS) const noexcept {
	return !((*this) == RHS);
	}

	private:
	std::shared_ptr<TB> Buffer; ///< Buffer providing values for \p this object
	T Value; ///< The current value of the iterator
	};

	///\defgroup TupleBufferContainer Type converter turning a \c std::tuple of
	///types into a \c std::tuple of container of those types.
	///
	/// The new type is used for buffering elements of tuples separately.
	///
	///@{

	/// Template declaration.
	///
	/// \tparam T type to convert
	///
	/// \note The template is defined only when \p T is a \c std::tuple.
	///
	/// Usage for a type \c Tuple as:\code
	/// typename TupleBufferContainer<Tuple>::Type
	/// \endcode
	template <typename T> struct TupleBufferContainer;

	/// Template definition for \c std::tuple.
	template <typename... Ts> struct TupleBufferContainer<std::tuple<Ts...>> {
	/// The converted type.
	using Type = std::tuple<std::queue<Ts>...>;
	};

	///@}

	/// Convenience template type alias for easy use of \c TupleBufferContainer.
	///
	/// Converts a \c std::tuple of types into a \c std::tuple of container of those
	/// types.
	///
	/// \tparam Tuple type to convert
	template <typename Tuple>
	using tuple_buffer_container_t = typename TupleBufferContainer<Tuple>::Type;

	/// Buffer for element values for iterator of \c std::tuple.
	///
	/// The class can be instantiated with a range of iterator of \c std::tuple and
	/// provides an interface for accessing elements of the iterated tuples one by
	/// one.
	///
	/// \note The class is utilized by \c SplittedElementIterator.
	///
	/// \tparam TupleIterator the iterator type that provides values of \c
	/// std::tuple
	///
	/// \todo Consider thread safety of the class.
	template <typename TupleIterator>
	class TupleIteratorBuffer {
	public:
	/// Type alias for the value type of \p TupleIterator.
	/// \note The type is expected to be \c std::tuple.
	using iterator_value_type = typename TupleIterator::value_type;

	/// The number of elements of \c iterator_value_type.
	static constexpr size_t iterator_value_size =
	std::tuple_size_v<iterator_value_type>;

	/// Template type alias to get element types of \c iterator_value_type by
	/// index.
	/// \tparam I the index of the element
	template <size_t I>
	using element_type =
	typename std::tuple_element<I, iterator_value_type>::type;

	/// Type alias for index sequence for accessing elements of \c
	/// iterator_value_type.
	using element_idx_seq_t = seq_t<iterator_value_size>;

	/// Creates a new instance.
	///
	/// \param Begin the beginning of the iterator range to buffer
	/// \param End the end of the iterator range to buffer
	TupleIteratorBuffer(TupleIterator &&Begin, const TupleIterator &End) noexcept
	: Iterator(Begin), End(End), Buffer() {}

	/// Tells whether there is any more value in the handled range for index \p I.
	///
	/// \tparam I the index of the element to check
	///
	/// \return whether there is more value for index \p I
	template <size_t I> bool hasMore(void) const noexcept {
	const auto &ElementBuffer = std::get<I>(Buffer);
	// There is more value if some has already been buffered or the end of the
	// handled range is not reached yet.
	return !ElementBuffer.empty() \|\| Iterator != End;
	}

	/// Gives the next value from the handled range for index \p I.
	///
	/// \note The function should not be called if there is no more value for
	/// index \p I (i.e., \c hasMore<I>() returns \c false). If it is called in
	/// that situation, it returns the default value of \c element_type<I>.
	///
	/// \tparam I the index of the element to fetch next value for
	///
	/// \return the next value for index \p I
	template <size_t I> element_type<I> next(void) noexcept {
	auto &ElementBuffer = std::get<I>(Buffer);
	if (ElementBuffer.empty()) {
	// This position needs a new value from Iterator if there is more.
	if (Iterator != End) {
	// Push next value for all elements.
	push(*Iterator++, element_idx_seq_t());
	} else {
	// No more values; called when hasMore<I>() is false.
	// Return default value.
	return element_type<I>();
	}
	}
	// Buffer is not empty here, return the next value.
	ASSERT(!ElementBuffer.empty() && "Unexpected condition");
	// Get the next value and also pop it from the buffer.
	const element_type<I> Elem = ElementBuffer.front();
	ElementBuffer.pop();
	return Elem;
	}

	private:
	/// Buffers next tuple value elementwise into element containers.
	///
	/// \tparam S0 indices for accessing elements of \c std::tuple
	///
	/// \param Tuple the values to buffer
	///
	/// \note The second parameter provides indices as template parameter \p S0
	/// and its actual value is ignored.
	template <size_t... S0>
	void push(const iterator_value_type &Tuple, Seq<S0...>) noexcept {
	(std::get<S0>(Buffer).push(std::get<S0>(Tuple)), ...);
	}

	TupleIterator Iterator; ///< Current input iterator
	const TupleIterator End; ///< End of iterator range to handle
	tuple_buffer_container_t<iterator_value_type>
	Buffer; ///< Container for elementwise buffering
	};

	/// Template type alias for iterator of an element based on buffered iterator of
	/// \c std::tuple.
	///
	/// The alias utilizes \c SplittedElementIterator for iterating over the values
	/// provided by \p TB.
	///
	/// \tparam TB buffer providing values to iterate
	/// \tparam I index of the values to iterator from \p TB
	template <typename TB, size_t I>
	using element_iterator_t = SplittedElementIterator<TB, I>;

	/// Template type alias for iterator-range of an element based on buffered
	/// iterator of \c std::tuple.
	///
	/// \tparam TB buffer providing values to iterate
	/// \tparam I index of the values to iterator from \p TB
	template <typename TB, size_t I>
	using element_iterator_range_t =
	std::pair<element_iterator_t<TB, I>, element_iterator_t<TB, I>>;

	///\defgroup ElementIteratorRanges Type converter turning a buffer of \c
	/// std::tuple into a \c std::tuple of corresponding \c
	/// element_iterator_range_t.
	///
	///@{

	/// Template declaration.
	///
	/// \tparam TB buffer providing values
	/// \tparam S type providing indices for accessing elements of \c std::tuple
	///
	/// \note \p TB is expected to implement an interface matching that of \c
	/// TupleIteratorBuffer.
	///
	/// \note The template is defined only when \p S is a \c rosa::Seq.
	///
	/// Usage for a proper buffer type \c TB:\code
	/// typename ElementIteratorRanges<TB, typename TB::element_idx_seq_t>::Type
	/// \endcode
	template <typename TB, typename S> struct ElementIteratorRanges;

	/// Template definition.
	template <typename TB, size_t... S0>
	struct ElementIteratorRanges<TB, Seq<S0...>> {
	/// The converted type.
	using Type = std::tuple<element_iterator_range_t<TB, S0>...>;
	};

	///@}

	/// Convenience template type alias for easy use of \c ElementIteratorRanges.
	///
	/// Converts a buffer of \c std::tuple into a \c std::tuple of corresponding \c
	/// element_iterator_range_t.
	///
	/// \tparam TB buffer providing values
	///
	/// \note \p TB is expected to implement an interface matching that of \c
	/// TupleIteratorBuffer.
	template <typename TB>
	using element_iterator_ranges_t =
	typename ElementIteratorRanges<TB, typename TB::element_idx_seq_t>::Type;

	/// Template type alias for turning an iterator of \c std::tuple into
	/// corresponding \c element_iterator_ranges_t.
	///
	/// The alias utilizes \c TupleIteratorBuffer for buffering the values provided
	/// by \p TupleIterator. The elementwise iterators are \c
	/// SplittedElementIterator.
	///
	/// \tparam TupleIterator iterator of \c std::tuple
	template <typename TupleIterator>
	using splitted_tuple_iterator_ranges_t =
	element_iterator_ranges_t<TupleIteratorBuffer<TupleIterator>>;

	/// Creates elementwise iterator ranges for an iterator range of \c std::tuple.
	///
	/// \note Implementation for \c rosa::iterator::splitTupleIterator.
	///
	/// \tparam TupleIterator iterator of \c std::tuple
	/// \tparam S0 indices for accessing elements of \c std::tuple
	///
	/// \param Begin the beginning of the iterator range to handle
	/// \param End the end of the iterator range to handle
	///
	/// \note The last parameter provides indices as template parameter \p S0 and
	/// its actual value is ignored.
	///
	/// \return \c std::tuple of elementwise iterator ranges corresponding to \p
	/// Begin and \p End
	template <typename TupleIterator, size_t... S0>
	splitted_tuple_iterator_ranges_t<TupleIterator>
	splitTupleIteratorImpl(TupleIterator &&Begin, const TupleIterator &End,
	Seq<S0...>) noexcept {
	using TB = TupleIteratorBuffer<TupleIterator>;
	// Create a buffer in shared pointer. The buffer will be destructed once
	// all corresponding element iterators (created below) have reached the end of
	// the handled range or have been destructed.
	auto Buffer = std::make_shared<TB>(std::move(Begin), End);
	return {std::make_pair(element_iterator_t<TB, S0>(Buffer),
	element_iterator_t<TB, S0>())...};
	}

	} // End namespace

	/// Gives the beginning of a range created by \c splitTupleIterator().
	///
	/// \see rosa::iterator::splitTupleIterator()
	///
	/// \note The signature of the template function uses implementation details.
	///
	/// \tparam TB buffer providing values to iterate
	/// \tparam I index of the values to iterator from \p TB
	///
	/// \param Range range to get the beginning of
	///
	/// \return beginning of \p Range
	template <typename TB, size_t I>
	element_iterator_t<TB, I> &begin(element_iterator_range_t<TB, I> &Range) {
	return Range.first;
	}

	/// Gives the end of a range created by \c splitTupleIterator().
	///
	/// \see rosa::iterator::splitTupleIterator()
	///
	/// \note The signature of the template function uses implementation details.
	///
	/// \tparam TB buffer providing values to iterate
	/// \tparam I index of the values to iterator from \p TB
	///
	/// \param Range range to get the end of
	///
	/// \return end of \p Range
	template <typename TB, size_t I>
	element_iterator_t<TB, I> &end(element_iterator_range_t<TB, I> &Range) {
	return Range.second;
	}

	/// Creates elementwise iterator ranges for an iterator range of \c std::tuple.
	///
	/// \note The implementation utilizes \c splitTupleIteratorImpl.
	///
	/// Obtain element iterator ranges for an iterator range \c Begin and \c End of
	/// iterator type \c TupleIterator of \c std::tuple<T1, T2, T3> as \code
	/// auto [T1Range, T2Range, T3Range] = splitTupleIterator(std::move(Begin), End);
	/// auto [T1Begin, T1End] = T1Range;
	/// auto [T2Begin, T2End] = T2Range;
	/// auto [T3Begin, T3End] = T3Range;
	/// \endcode
	/// One range can also be dissected by dedicated functions like \code
	/// auto T1Begin = begin(T1Range);
	/// auto T1End = end(T1Range);
	/// \endcode
	///
	/// The function returns a tuple of ranges (i.e., \c std::pair of iterators),
	/// which can be assigned to variables using structured binding. The ranges can
	/// be dissected into begin and end iterators by separate structured bindings.
	///
	/// The function moves its first argument (i.e., \p Begin cannot be used
	/// directly after splitting it). If the first argument is a variable, it needs
	/// to be moved explicitly by \c std::move() as in the example.
	///
	/// \tparam TupleIterator iterator of \c std::tuple
	///
	/// \param Begin the beginning of the iterator range to handle
	/// \param End the end of the iterator range to handle
	///
	/// \return \c std::tuple of elementwise iterator ranges corresponding to \p
	/// Begin and \p End
	template <typename TupleIterator>
	splitted_tuple_iterator_ranges_t<TupleIterator>
	splitTupleIterator(TupleIterator &&Begin, const TupleIterator &End) noexcept {
	return splitTupleIteratorImpl(
	std::move(Begin), End,
	seq_t<std::tuple_size_v<typename TupleIterator::value_type>>());
	}

	} // End namespace iterator
	} // End namespace rosa

	#endif // ROSA_SUPPORT_ITERATOR_SPLIT_TUPLE_ITERATOR_HPP

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