====== Homework 02 - Counting Agents ======


The implementation of ''agent_count'' is split into multiple parts via dispatch.
First, we can implement the methods for ''Plant'' and ''Animal'' as described in the homework text.

Second, the agent count of a vector is another method which can be conveniently implemented with
''sum(f::Function,v::Vector)''. This method applies ''f'' to each element in ''v'' and sums the result.
Using ''agent_count'' as the function that is passed to ''sum'' will result in the application of the methods
for ''Plant'' and ''Animal'' which we implemented already. 

Finally, we need a method for ''World''. We have seen many different ways of doing this in your homeworks.
Some of them hardcoded the types of ''Agent''s that are counted.
Below we demonstrate how to implement ''agent_count'' for an arbitrary number of ''Agent''s.
Note that the solution uses ''foldl'' but you can just as well do it with a for loop. 

<code julia>
agent_count(p::Plant) = size(p)/max_size(p)
agent_count(::Animal) = 1
agent_count(as::Vector{<:Agent}) = sum(agent_count,as)

function agent_count(w::World)
    function op(d::Dict,a::A) where A<:Agent
        n = nameof(A)
        if n in keys(d)
            d[n] += agent_count(a)
        else
            d[n] = agent_count(a)
        end
        return d
    end
    foldl(op, w.agents |> values |> collect, init=Dict{Symbol,Real}())
end
</code>