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Exercise 3.32

Using a queue (first in, first out) instead of a list (first in, last out) structure to store our events ensure that the correct state is propagated.

For example suppose we have an and-gate with input \(A=0\) and \(B=1\) and at a certain time both inputs \(A\) and \(B\) change value. Suppose our simulation change \(A\) to \(1\) first then \(B\) to \(0\) (during the same simulation time segment). Both these changes of state will add a procedure to the agenda. When \(A\) is set to \(1\) a procedure telling to change the output to \(1\) is added to the agenda (because both \(A\) and \(B\) equal \(1\) during evaluation). Then \(B\) change to \(1\) a procedure telling to change the output to \(0\) is added to the agenda. As you can see the last procedure added to the agenda gives us the correct value the output should change to. Hence we need to use a queue (first in, first out) to get the correct state at the end of each time segment.

Such pitfalls arise when mutating objects at the same simulation time.