2 Why We Need Model-Based Testing

124 Structuring Model Programs with Features and Composition
0
B() C()
1
Figure 7.7. Model program M2.
0
A()
1
B(2)
2
C()
3
Figure 7.8. Model program M1 × M2, the product of M1 and M2.
Let us explain exactly how the FSM of the product M1 × M2 in Figure 7.8 was
obtained. There is a systematic method for generating the product of two model
programs. First, identify the action vocabulary for each program, and the unshared
actions from the other program. For M1 the action vocabulary is A, B and there is
one unshared action, C;forM2 the action vocabulary is B, C and the unshared action
is A. Then, form the loop extension of each program: at each state, add a self-loop
transition for each of the unshared actions. The two loop extensions now have the
same action vocabulary. Moreover, in the loop extensions, actions with the same
action symbol are made to have the same arity (number of parameters) by extending
them with placeholder parameters indicated by (underscore). Here the action B()
from M2 becomes B( ) in the loop extension of M2, so it has the same arity as B(2) in
M1. Figures 7.9 and 7.10 show the loop extensions of M1 and M2, respectively. The
product is generated from these loop extensions.
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