/**
This example listens to TF data, computes velocity commands, and publishes them to turtle2.
turtle2->turtle1 = world->turtle*world->turtle2
*/

#include <ros/ros.h>
#include <tf/transform_listener.h>
#include <geometry_msgs/Twist.h>
#include <turtlesim/Spawn.h>

int main(int argc, char** argv)
{

    ros::init(argc, argv, "turtle1_turtle2_listener");// Initialize the ROS node.


    ros::NodeHandle node; // Create a node handle.

    // Call the service to spawn turtle2.
    ros::service::waitForService("/spawn");
    ros::ServiceClient add_turtle = node.serviceClient<turtlesim::Spawn>("/spawn");
    turtlesim::Spawn srv;
    add_turtle.call(srv);

    // Create a publisher for turtle2 velocity commands.
    ros::Publisher vel = node.advertise<geometry_msgs::Twist>("/turtle2/cmd_vel", 10);

    tf::TransformListener listener;// Create a TF listener.

    ros::Rate rate(10.0);

    while (node.ok())
    {
        // Get TF data between turtle1 and turtle2.
        tf::StampedTransform transform;
        try
        {
            listener.waitForTransform("/turtle2", "/turtle1", ros::Time(0), ros::Duration(3.0));
            listener.lookupTransform("/turtle2", "/turtle1", ros::Time(0), transform);
        }
        catch (tf::TransformException &ex)
        {
            ROS_ERROR("%s",ex.what());
            ros::Duration(1.0).sleep();
            continue;
        }

        // Compute angular and linear velocity from the relative pose between turtle1 and turtle2, then publish commands for turtle2.
        geometry_msgs::Twist turtle2_vel_msg;

        turtle2_vel_msg.angular.z = 6.0 * atan2(transform.getOrigin().y(),
                                        transform.getOrigin().x());
        turtle2_vel_msg.linear.x = 0.8 * sqrt(pow(transform.getOrigin().x(), 2) +
                                      pow(transform.getOrigin().y(), 2));
        vel.publish(turtle2_vel_msg);

        rate.sleep();
    }
    return 0;
};