5. Dynamic weights

This 5th step illustrates how to obtain the shortest path from a point to another one and to update the weights of an existing graph.

Formulation

• At initialization, the value of the destruction_coeff of the road agents will be equal to 1.
• Add a new parameter: the destroy parameter that represents the value of destruction when a people agent takes a road. By default, it is equal to 0.02.
• When a people agent arrive at its destination (home or work), it updates the destruction_coeff of the road agents it took to reach its destination: "destruction_coeff = destruction_coeff - destroy". Then, the graph is updated.

Model Definition

global section

We add the destroy parameter.

In the global section, we define the destroy variable:

float destroy <- 0.02;

In the experiment section, we add a parameter:

parameter "Value of destruction when a people agent takes a road" var: destroy category: "Road" ;

We define a new reflex that updates the graph at each simulation step. For that, we use the with_weights operator. This operator allows to update the weights of an existing graph.

global {
    ...
    reflex update_graph{
        map<road,float> weights_map <- road as_map (each:: (each.destruction_coeff * each.shape.perimeter));
        the_graph <- the_graph with_weights weights_map;
     }
}

people agents

At each time-step, after a people agent has moved over one or multiple road segments, it updates the value of the destruction coefficient of road agents crossed (i.e. roads belonging to the path followed). We have for that to set the argument return_path to true in the goto action to obtain the path followed, then to compute the list of agents concerned by this path with the operator agent_from_geometry.

species people skills: [moving]{
    ...
    reflex move when: the_target != nil {
	path path_followed <- goto(target: the_target, on:the_graph, return_path: true);
	list<geometry> segments <- path_followed.segments;
	loop line over: segments {
	    float dist <- line.perimeter;
	    ask road(path_followed agent_from_geometry line) { 
		destruction_coeff <- destruction_coeff + (destroy * dist / shape.perimeter);
	    }
	}
	if the_target = location {
	    the_target <- nil ;
	}
    }
    ...
}	

Complete Model

https://github.com/gama-platform/gama.old/blob/GAMA_1.9.3/msi.gama.models/models/Tutorials/Road%20Traffic/models/Model%2005.gaml