stk_strategy_rrobin.h

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/*
 * SuperTinyKernel™ (STK): Lightweight High-Performance Deterministic C++ RTOS for Embedded Systems.
 *
 * Source: https://github.com/SuperTinyKernel-RTOS
 *
 * Copyright (c) 2022-2026 Neutron Code Limited <stk@neutroncode.com>. All Rights Reserved.
 * License: MIT License, see LICENSE for a full text.
 */
 
#ifndef STK_STRATEGY_RROBIN_H_
#define STK_STRATEGY_RROBIN_H_

 
#include "stk_common.h"
 
namespace stk {

class SwitchStrategyRoundRobin : public ITaskSwitchStrategy
{
public:
    enum EConfig
    {
        WEIGHT_API          = 0, 
        SLEEP_EVENT_API     = 1, 
        DEADLINE_MISSED_API = 0  
    };

    SwitchStrategyRoundRobin() : m_tasks(), m_sleep(), m_prev(nullptr)
    {}

    ~SwitchStrategyRoundRobin()
    {}

    void AddTask(IKernelTask *task)
    {
        STK_ASSERT(task != nullptr);
        STK_ASSERT(task->GetHead() == nullptr);
 
        bool tail = (m_prev == m_tasks.GetLast());
 
        m_tasks.LinkBack(task);
 
        // if pointer was pointing to the tail, become a tail
        if (tail)
            m_prev = task;
    }

    void RemoveTask(IKernelTask *task)
    {
        STK_ASSERT(task != nullptr);
        STK_ASSERT(GetSize() != 0);
        STK_ASSERT((task->GetHead() == &m_tasks) || (task->GetHead() == &m_sleep));
 
        if (task->GetHead() == &m_tasks)
            RemoveActive(task);
        else
            m_sleep.Unlink(task);
    }

    IKernelTask *GetNext()
    {
        IKernelTask *ret = m_prev;
 
        if (ret != nullptr)
        {
            ret    = (*ret->GetNext());
            m_prev = ret;
        }
 
        return ret;
    }

    IKernelTask *GetFirst() const
    {
        STK_ASSERT(GetSize() != 0U);
 
        if (!m_tasks.IsEmpty())
            return (*m_tasks.GetFirst());
        else
            return (*m_sleep.GetFirst());
    }

    size_t GetSize() const
    {
        return m_tasks.GetSize() + m_sleep.GetSize();
    }

    void OnTaskSleep(IKernelTask *task)
    {
        STK_ASSERT(task != nullptr);
        STK_ASSERT(task->IsSleeping());
        STK_ASSERT(task->GetHead() == &m_tasks);
 
        RemoveActive(task);
        m_sleep.LinkBack(task);
    }

    void OnTaskWake(IKernelTask *task)
    {
        STK_ASSERT(task != nullptr);
        STK_ASSERT(!task->IsSleeping());
        STK_ASSERT(task->GetHead() == &m_sleep);
 
        m_sleep.Unlink(task);
        AddActive(task);
    }

    bool OnTaskDeadlineMissed(IKernelTask */*task*/)
    {
        // Budget Overrun API unsupported
        STK_ASSERT(false);
        return false;
    }
 
protected:
    STK_NONCOPYABLE_CLASS(SwitchStrategyRoundRobin);

    void AddActive(IKernelTask *task)
    {
        m_tasks.LinkBack(task);
 
        // update pointer: if all tasks were sleeping, this task will change state
        // of the kernel to active
        if (m_prev == nullptr)
            m_prev = task;
    }

    void RemoveActive(IKernelTask *task)
    {
        IKernelTask *next = (*task->GetNext());
 
        m_tasks.Unlink(task);
 
        // update pointer: set to previous task so that GetNext() could return next,
        // if there are no tasks left GetNext() will return nullptr causing a sleep
        // state for the kernel
        if (next != task)
            m_prev = (*next->GetPrev());
        else
            m_prev = nullptr;
    }
 
    IKernelTask::ListHeadType m_tasks; 
    IKernelTask::ListHeadType m_sleep; 
    IKernelTask              *m_prev;  
};

typedef SwitchStrategyRoundRobin SwitchStrategyRR;
 
} // namespace stk
 
#endif /* STK_STRATEGY_RROBIN_H_ */