/*
 * Copyright (c) 2019 NITK Surathkal
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 * Cobalt, the CoDel - BLUE - Alternate Queueing discipline
 * Based on linux code.
 *
 * Ported to ns-3 by: Vignesh Kannan <vignesh2496@gmail.com>
 *                    Harsh Lara <harshapplefan@gmail.com>
 *                    Jendaipou Palmei <jendaipoupalmei@gmail.com>
 *                    Shefali Gupta <shefaligups11@gmail.com>
 *                    Mohit P. Tahiliani <tahiliani@nitk.edu.in>
 *
 */

#ifndef COBALT_H
#define COBALT_H

#include "queue-disc.h"

#include "ns3/boolean.h"
#include "ns3/data-rate.h"
#include "ns3/nstime.h"
#include "ns3/random-variable-stream.h"
#include "ns3/simulator.h"
#include "ns3/string.h"
#include "ns3/trace-source-accessor.h"
#include "ns3/traced-value.h"

namespace ns3
{

#define REC_INV_SQRT_CACHE (16)
#define DEFAULT_COBALT_LIMIT 1000

class TraceContainer;

/**
 * @ingroup traffic-control
 *
 * @brief Cobalt packet queue disc
 *
 * Cobalt uses CoDel and BLUE algorithms in parallel, in order
 * to obtain the best features of each. CoDel is excellent on flows
 * which respond to congestion signals in a TCP-like way. BLUE is far
 * more effective on unresponsive flows.
 */
class CobaltQueueDisc : public QueueDisc
{
  public:
    /**
     * @brief Get the type ID.
     * @return the object TypeId
     */
    static TypeId GetTypeId();

    /**
     * @brief CobaltQueueDisc Constructor
     *
     * Create a Cobalt queue disc
     */
    CobaltQueueDisc();

    /**
     * @brief Destructor
     *
     * Destructor
     */
    ~CobaltQueueDisc() override;

    /**
     * @brief Get the target queue delay
     *
     * @returns The target queue delay
     */
    Time GetTarget() const;

    /**
     * @brief Get the interval
     *
     * @returns The interval
     */
    Time GetInterval() const;

    /**
     * @brief Get the time for next packet drop while in the dropping state
     *
     * @returns The time (in microseconds) for next packet drop
     */
    int64_t GetDropNext() const;

    static constexpr const char* TARGET_EXCEEDED_DROP =
        "Target exceeded drop";                                     //!< Sojourn time above target
    static constexpr const char* OVERLIMIT_DROP = "Overlimit drop"; //!< Overlimit dropped packet
    static constexpr const char* FORCED_MARK =
        "forcedMark"; //!< forced marks by Codel on ECN-enabled
    static constexpr const char* CE_THRESHOLD_EXCEEDED_MARK =
        "CE threshold exceeded mark"; //!< Sojourn time above CE threshold

    /**
     * @brief Get the drop probability of Blue
     *
     * @returns The current value of Blue's drop probability
     */
    double GetPdrop() const;

    /**
     * Assign a fixed random variable stream number to the random variables
     * used by this model.  Return the number of streams (possibly zero) that
     * have been assigned.
     *
     * @param stream first stream index to use
     * @return the number of stream indices assigned by this model
     */
    int64_t AssignStreams(int64_t stream);

    /**
     * Return the unsigned 32-bit integer representation of the input Time
     * object. Units are microseconds
     * @param t the input Time Object
     * @return the unsigned 32-bit integer representation
     */
    int64_t Time2CoDel(Time t) const;

  protected:
    /**
     * @brief Dispose of the object
     */
    void DoDispose() override;

  private:
    bool DoEnqueue(Ptr<QueueDiscItem> item) override;
    Ptr<QueueDiscItem> DoDequeue() override;
    Ptr<const QueueDiscItem> DoPeek() override;
    bool CheckConfig() override;

    /**
     * @brief Initialize the queue parameters.
     */
    void InitializeParams() override;

    /**
     * @brief Calculate the reciprocal square root of m_count by using Newton's method
     *  http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
     * m_recInvSqrt (new) = (m_recInvSqrt (old) / 2) * (3 - m_count * m_recInvSqrt^2)
     */
    void NewtonStep();

    /**
     * @brief Determine the time for next drop
     * CoDel control law is t + m_interval/sqrt(m_count).
     * Here, we use m_recInvSqrt calculated by Newton's method in NewtonStep() to avoid
     * both sqrt() and divide operations
     *
     * @param t Current next drop time
     * @returns The new next drop time:
     */
    int64_t ControlLaw(int64_t t);

    /**
     * @brief Updates the inverse square root
     */
    void InvSqrt();

    /**
     * There is a big difference in timing between the accurate values placed in
     * the cache and the approximations given by a single Newton step for small
     * count values, particularly when stepping from count 1 to 2 or vice versa.
     * Above 16, a single Newton step gives sufficient accuracy in either
     * direction, given the precision stored.
     *
     * The magnitude of the error when stepping up to count 2 is such as to give
     * the value that *should* have been produced at count 4.
     */
    void CacheInit();

    /**
     * Check if CoDel time a is successive to b
     * @param a left operand
     * @param b right operand
     * @return true if a is greater than b
     */

    bool CoDelTimeAfter(int64_t a, int64_t b);

    /**
     * Check if CoDel time a is successive or equal to b
     * @param a left operand
     * @param b right operand
     * @return true if a is greater than or equal to b
     */
    bool CoDelTimeAfterEq(int64_t a, int64_t b);

    /**
     * Called when the queue becomes full to alter the drop probabilities of Blue
     * @param now time in CoDel time units (microseconds)
     */
    void CobaltQueueFull(int64_t now);

    /**
     * Called when the queue becomes empty to alter the drop probabilities of Blue
     * @param now time in CoDel time units (microseconds)
     */
    void CobaltQueueEmpty(int64_t now);

    /**
     * Called to decide whether the current packet should be dropped based on decisions taken by
     * Blue and Codel working parallelly
     *
     * @return true if the packet should be dropped, false otherwise
     * @param item current packet
     * @param now time in CoDel time units (microseconds)
     */
    bool CobaltShouldDrop(Ptr<QueueDiscItem> item, int64_t now);

    // Common to CoDel and Blue
    // Maintained by Cobalt
    Stats m_stats; //!< Cobalt statistics

    // Codel parameters
    // Maintained by Cobalt
    TracedValue<uint32_t> m_count;   //!< Number of packets dropped since entering drop state
    TracedValue<int64_t> m_dropNext; //!< Time to drop next packet
    TracedValue<bool> m_dropping;    //!< True if in dropping state
    uint32_t m_recInvSqrt;           //!< Reciprocal inverse square root
    uint32_t m_recInvSqrtCache[REC_INV_SQRT_CACHE] = {
        0}; //!< Cache to maintain some initial values of InvSqrt

    // Supplied by user
    Time m_interval;      //!< sliding minimum time window width
    Time m_target;        //!< target queue delay
    bool m_useEcn;        //!< True if ECN is used (packets are marked instead of being dropped)
    Time m_ceThreshold;   //!< Threshold above which to CE mark
    bool m_useL4s;        //!< True if L4S is used (ECT1 packets are marked at CE threshold)
    Time m_blueThreshold; //!< Threshold to enable blue enhancement

    // Blue parameters
    // Maintained by Cobalt
    Ptr<UniformRandomVariable> m_uv; //!< Rng stream
    uint32_t m_lastUpdateTimeBlue;   //!< Blue's last update time for drop probability

    // Supplied by user
    double m_increment; //!< increment value for marking probability
    double m_decrement; //!< decrement value for marking probability
    double m_pDrop;     //!< Drop Probability
};

} // namespace ns3

#endif /* COBALT_H */
