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	//===-- rosa/agent/History.hpp ----------------------------------- C++ --===//
	//
	// The RoSA Framework
	//
	//===----------------------------------------------------------------------===//
	///
	/// \file rosa/agent/History.hpp
	///
	/// \author David Juhasz (david.juhasz@tuwien.ac.at)
	///
	/// \date 2017
	///
	/// \brief Definition of history functionality.
	///
	//===----------------------------------------------------------------------===//

	#ifndef ROSA_AGENT_HISTORY_HPP
	#define ROSA_AGENT_HISTORY_HPP

	#include "rosa/agent/Module.h"

	#include "rosa/config/config.h"

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

	#include <array>

	namespace rosa {
	namespace agent {

	/// Retention policies defining what a \c rosa::agent::History instance should
	/// do when the number of recorded entries reached its capacity.
	enum class HistoryPolicy {
	SRWF, ///< Stop Recording When Full -- no new entry is recorded when full
	FIFO ///< First In First Out -- overwrite the earliest entry with a new one
	};

	/// Implements history by recording and storing values.
	///
	/// \note Not thread-safe implementation, which should not be a problem as any
	/// instance of \c rosa::agent::Module is an internal component of a
	/// \c rosa::Agent, which is the basic unit of concurrency.
	///
	/// \tparam T type of values to store
	/// \tparam N number of values to store at most
	/// \tparam P retention policy to follow when capacity is reached
	///
	/// \invariant The size of the underlying \c std::array is `N + 1`:\code
	/// max_size() == N + 1 && N == max_size() - 1
	/// \endcode
	template <typename T, size_t N, HistoryPolicy P>
	class History : public Module, private std::array<T, N + 1> {

	// Bring into scope inherited functions that are used.
	using std::array<T, N + 1>::max_size;
	using std::array<T, N + 1>::operator[];

	/// The index of the first data element in the circular buffer.
	size_t Data;

	/// The index of the first empty slot in the circular buffer.
	size_t Space;

	public:
	/// Creates an instances by initializing the indices for the circular buffer.
	History(void) noexcept : Data(0), Space(0) {}

	/// Destroys \p this object.
	~History(void) = default;

	/// Tells the retention policy applied to \p this object.
	///
	/// \return \c rosa::agent::History::P
	static constexpr HistoryPolicy policyOfHistory(void) noexcept { return P; }

	/// Tells how many entries may be recorded by \c this object.
	///
	/// \note The number of entries that are actually recorded may be smaller.
	///
	/// \return \c rosa::agent::History::N
	static constexpr size_t lengthOfHistory(void) noexcept { return N; }

	/// Tells how many entries are currently recorded by \p this object.
	///
	/// \return number of entries currently recorded by \p this object.
	///
	/// \post The returned value cannot be larger than the capacity of \p this
	/// object:\code
	/// 0 <= numberOfEntries() && numberOfEntries <= lengthOfHistory()
	/// \endcode
	inline size_t numberOfEntries(void) const noexcept {
	return Data <= Space ? Space - Data : max_size() - Data + Space;
	}

	/// Tells if \p this object has not recorded anything yet.
	///
	/// \return if \p this object has no entries recorded
	inline bool empty(void) const noexcept { return numberOfEntries() == 0; }

	/// Gives a constant lvalue reference to an entry stored in \p this object.
	///
	/// \note The recorded entries are indexed starting from the latest one.
	///
	/// \param I the index at which the stored entry to take from
	///
	/// \pre \p I is a valid index:\code
	/// 0 <= I && I <= numberOfEntries()
	/// \endcode
	const T &entry(const size_t I = 0) const noexcept {
	ASSERT(0 <= I && I < numberOfEntries()); // Boundary check.
	// Position counted back from the last recorded entry.
	typename std::make_signed<const size_t>::type Pos = Space - (1 + I);
	// Actual index wrapped around to the end of the buffer if negative.
	return (*this)[Pos >= 0 ? Pos : max_size() + Pos];
	}

	private:
	/// Tells if the circular buffer is full.
	///
	/// \return if the circular buffer is full.
	inline bool full(void) const noexcept { return numberOfEntries() == N; }

	/// Pushes a new entry into the circular buffer.
	///
	/// \note The earliest entry gets overwritten if the buffer is full.
	///
	/// \param V value to push into the buffer
	void pushBack(const T &V) noexcept {
	// Store value to the first empty slot and step Space index.
	(*this)[Space] = V;
	Space = (Space + 1) % max_size();
	if (Data == Space) {
	// Buffer was full, step Data index.
	Data = (Data + 1) % max_size();
	}
	}

	public:
	/// Adds a new entry to \p this object and tells if the operation was
	/// successful.
	///
	/// \note Success of the operation depends on the actual policy.
	///
	/// \param V value to store
	///
	/// \return if \p V was successfully stored
	bool addEntry(const T &V) noexcept {
	switch (P) {
	default:
	ROSA_CRITICAL("unkown HistoryPolicy");
	case HistoryPolicy::SRWF:
	if (full()) {
	return false;
	}
	// \note Fall through to FIFO which unconditionally pushes the new entry.
	case HistoryPolicy::FIFO:
	// FIFO and SRWF not full.
	pushBack(V);
	return true;
	}
	}

	/// Tells the trend set by the entries recorded by \p this object.
	///
	/// The number of steps to go back when calculating the trend is defined as
	/// argument to the function.
	///
	/// \note The number of steps that can be made is limited by the number of
	/// entries recorded by \p this object.
	///
	/// \note The function is made a template only to be able to use
	/// \c std::enable_if.
	///
	/// \tparam X always use the default!
	///
	/// \param D number of steps to go back in history
	///
	/// \return trend set by analyzed entries
	///
	/// \pre Statically, \p this object stores signed arithmetic values:\code
	/// std::is_arithmetic<T>::value && std::is_signed<T>::value
	/// \endcode Dynamically, \p D is a valid number of steps to take:\code
	/// 0 <= D && D < N
	/// \endcode
	template <typename X = T>
	typename std::enable_if<
	std::is_arithmetic<X>::value && std::is_signed<X>::value, X>::type
	trend(const size_t D = N - 1) const noexcept {
	STATIC_ASSERT((std::is_same<X, T>::value), "not default template arg");
	ASSERT(0 <= D && D < N); // Boundary check.
	if (numberOfEntries() < 2 \|\| D < 1) {
	// No entries for computing trend.
	return {}; // Zero element of \p T
	} else {
	// Here at least two entries.
	// \c S is the number of steps that can be done.
	const size_t S = std::min(numberOfEntries() - 1, D);
	size_t I = S;

	// Compute trend with linear regression.
	size_t SumIndices = 0;
	T SumEntries = {};
	T SumSquareEntries = {};
	T SumProduct = {};
	while (I > 0) {
	// \note Indexing for the regression starts in the past.
	const size_t Index = S - I;
	const T Entry = entry(--I);
	SumIndices += Index;
	SumEntries += Entry;
	SumSquareEntries += Entry * Entry;
	SumProduct += Entry * Index;
	}

	return (SumProduct * S - SumEntries * SumIndices) /
	(SumSquareEntries * S - SumEntries * SumEntries);
	}
	}

	/// Tells the average absolute difference between consequtive entries recorded
	/// by \p this object
	/// The number of steps to go back when calculating the average is defined as
	/// argument to the function.
	///
	/// \note The number of steps that can be made is limited by the number of
	/// entries recorded by \p this object.
	///
	/// \note The function is made a template only to be able to use
	/// \c std::enable_if.
	///
	/// \tparam X always use the default!
	///
	/// \param D number of steps to go back in history
	///
	/// \pre Statically, \p this object stores arithmetic values:\code
	/// std::is_arithmetic<T>::value
	/// \endcode Dynamically, \p D is a valid number of steps to take:\code
	/// 0 <= D && D < N
	/// \endcode
	template <typename X = T>
	typename std::enable_if<std::is_arithmetic<X>::value, unsigned_t<X>>::type
	averageAbsDiff(const size_t D = N - 1) const noexcept {
	STATIC_ASSERT((std::is_same<X, T>::value), "not default template arg");
	ASSERT(0 <= D && D < N); // Boundary check.
	if (numberOfEntries() < 2 \|\| D < 1) {
	// No difference to average.
	return {}; // Zero element of \p T
	} else {
	// Here at least two entries.
	// \c S is the number of steps that can be done.
	const size_t S = std::min(numberOfEntries() - 1, D);

	// Sum up differences as non-negative values only, hence using an
	// unsigned variable for that.
	unsigned_t<T> Diffs = {}; // Init to zero.
	// Count down entry indices and sum up all the absolute differences.
	size_t I = S;
	T Last = entry(I);
	while (I > 0) {
	T Next = entry(--I);
	Diffs += Last < Next ? Next - Last : Last - Next;
	Last = Next;
	}
	// Return the average of the summed differences.
	return Diffs / S;
	}
	}
	};

	/// Adds a new entry to a \c rosa::agent::History instance.
	///
	/// \note The result of \c rosa::agent::History::addEntry is ignored.
	///
	/// \tparam T type of values stored in \p H
	/// \tparam N number of values \p H is able to store
	/// \tparam P retention policy followed by \p H when capacity is reached
	///
	/// \param H to add a new entry to
	/// \param V value to add to \p H
	///
	/// \return \p H after adding \p V to it
	template <typename T, size_t N, HistoryPolicy P>
	History<T, N, P> &operator<<(History<T, N, P> &H, const T &V) noexcept {
	H.addEntry(V);
	return H;
	}

	} // End namespace agent
	} // End namespace rosa

	#endif // ROSA_AGENT_HISTORY_HPP

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