The class Light

The class Light will represent a traffic light.

A traffic light is characterized by the following properties:

Its period, i.e. the number of time steps between two transitions from green to red.
Green period, i.e. the number of time steps that the light is green.

The sequence for a signal of period 7 and green period 3 can be illustrated as:

Time step:	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Internal clock:	0	1	2	3	4	5	6	0	1	2	3	4	5	6	0	1
Color:	G	G	G	R	R	R	R	G	G	G	R	R	R	R	G	G

(For simplicity, the lights should always start with the green period.)

Note that the internal clock is not stored as an instance variable since it can easily be computed with the help of the global time step clock wallclock.get_time(). By doing this we avoid writing a method that steps the traffic light.

We need to be able to ask the light if it is green or not, but this property does not need to be stored as an instance variable either, and can easily be computed every time the question is asked.

We thus arrive at the following class diagram:

Note: If you one wants to test the class separately, the wallclock time must be steped with the method wallclock.tick().

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