finite_state_machine
->0
start
State 0 (Enter digits only)
0
State 1 (Enter digits, operators, period or equal)
1
State 2 (Enter digits only)
2
State 3 (Enter digits, operators or equal only)
3
State 4 (Enter operators only)
4
0->1
D
1->0
O
1->1
D
1->2
P
1->4
E
2->3
D
3->0
O
3->3
D
3->4
E
4->0
O
Clear
/
7
8
9
×
4
5
6
−
1
2
3
+
0
.
=
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