pop_from_queue.hpp

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/*  Copyright (C) 2022 Davide Faconti -  All Rights Reserved
*
*   Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"),
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*   and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
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*
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*/

#pragma once

#include <list>
#include <mutex>
#include "behaviortree_cpp_v3/action_node.h"
#include "behaviortree_cpp_v3/decorator_node.h"


namespace BT
{

template <typename T>
struct ProtectedQueue
{
    std::list<T> items;
    std::mutex mtx;
};

/*
 * Few words about why we represent the queue as std::shared_ptr<ProtectedQueue>:
 *
 * Since we will pop from the queue, the fact that the blackboard uses
 * a value semantic is not very convenient, since it would oblige us to
 * copy the entire std::list from the BB and than copy again a new one with one less element.
 *
 * We avoid this using reference semantic (wrapping the object in a shared_ptr).
 * Unfortunately, remember that this makes our access to the list not thread-safe!
 * This is the reason why we add a mutex to be used when modyfying the ProtectedQueue::items
 *
 * */


template <typename T>
class PopFromQueue : public SyncActionNode
{
  public:
    PopFromQueue(const std::string& name, const NodeConfiguration& config)
      : SyncActionNode(name, config)
    {
    }

    NodeStatus tick() override
    {
        std::shared_ptr<ProtectedQueue<T>> queue;
        if( getInput("queue", queue) && queue )
        {
            std::unique_lock<std::mutex> lk(queue->mtx);
            auto& items = queue->items;

            if( items.empty() )
            {
                return NodeStatus::FAILURE;
            }
            else{
                T val = items.front();
                items.pop_front();
                setOutput("popped_item", val);
                return NodeStatus::SUCCESS;
            }
        }
        else{
            return NodeStatus::FAILURE;
        }
    }

    static PortsList providedPorts()
    {
        return { InputPort<std::shared_ptr<ProtectedQueue<T>>>("queue"),
                 OutputPort<T>("popped_item")};
    }
};

template <typename T>
class QueueSize : public SyncActionNode
{
  public:
    QueueSize(const std::string& name, const NodeConfiguration& config)
      : SyncActionNode(name, config)
    {
    }

    NodeStatus tick() override
    {
        std::shared_ptr<ProtectedQueue<T>> queue;
        if( getInput("queue", queue) && queue )
        {
            std::unique_lock<std::mutex> lk(queue->mtx);
            auto& items = queue->items;

            if( items.empty() )
            {
                return NodeStatus::FAILURE;
            }
            else{
                setOutput("size", int(items.size()) );
                return NodeStatus::SUCCESS;
            }
        }
        return NodeStatus::FAILURE;
    }

    static PortsList providedPorts()
    {
        return { InputPort<std::shared_ptr<ProtectedQueue<T>>>("queue"),
                 OutputPort<int>("size")};
    }
};


}