/***************************************************************************//**
 *
 * \file rosa/agent/History.hpp
 *
 * \author David Juhasz (david.juhasz@tuwien.ac.at)
 *
 * \date 2017
 *
 * \brief Definition of the *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 `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 `rosa::agent::Module` is an internal component of a
/// `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 `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 `this` object.
  ~History(void) = default;

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

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

  /// Tells how many entries are currently recorded by `this` object.
  ///
  /// \return number of entries currently recorded by `this` object.
  ///
  /// \post The returned value cannot be larger than the capacity of `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 `this` object has not recorded anything yet.
  ///
  /// \return if `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 `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 `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 `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 `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 `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 `this` object.
  ///
  /// \note The function is made a template only to be able to use
  /// `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, `this` object stores signed arithmetic values:\code
  /// std::is_arithmetic<T>::value && std::is_signed<T>::value
  /// \endcode Dynamically, `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 T
    } else {
      // Here at least two entries.
      // `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 `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 `this` object.
  ///
  /// \note The function is made a template only to be able to use
  /// `std::enable_if`.
  ///
  /// \tparam X always use the default!
  ///
  /// \param D number of steps to go back in *history*
  ///
  /// \pre Statically, `this` object stores arithmetic values:\code
  /// std::is_arithmetic<T>::value
  /// \endcode Dynamically, `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 T
    } else {
      // Here at least two entries.
      // `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 `rosa::agent::History` instance.
///
/// \note The result of `rosa::agent::History::addEntry` is ignored.
///
/// \tparam T type of values stored in `H`
/// \tparam N number of values `H` is able to store
/// \tparam P retention policy followed by `H` when capacity is reached
///
/// \param H to add a new entry to
/// \param V value to add to `H`
///
/// \return `H` after adding `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