fibonacci_recursive_in_out_demo.puml

@startuml

!procedure $create_state($name, $instance_name, $value)
    /' just to demonstrate customization based on value, nothing special about this check. '/
    !if $name >= 4
        !$color = "#LightSalmon"
    !else
        !$color = ""
    !endif

    state "$name" as $instance_name $color
    $instance_name : $value
!endprocedure

!procedure $connect_states($instance1_name, $instance2_name)
    $instance1_name -down-> $instance2_name
!endprocedure

!function $fibonacci($n, $index = 0)
    !if $n == 0
        !$fibonacci_n = 0
    !elseif $n == 1
        !$fibonacci_n = 1
    !else
        /' preprocessor seems unable to maintain two unnamed temporary variables,
        hence create local variables to hold return values of the recursive calls. '/
        !$fibonacci_n_minus_1 = $fibonacci($n - 1, $index * 2 + 1)
        !$fibonacci_n_minus_2 = $fibonacci($n - 2, $index * 2 + 2)
        !$fibonacci_n = $fibonacci_n_minus_1 + $fibonacci_n_minus_2
    !endif

    $create_state($n, $index, $fibonacci_n)

    !if $n >= 2
        $connect_states($index, $index * 2 + 1)
        $connect_states($index, $index * 2 + 2)
    !endif

    !return $fibonacci_n
!endfunction

!$ignore = $fibonacci(10)

@enduml