By SUTD Google Developers Student Club (Digital, Design and Development Club)

1. DDW Clinic Session 3
State Machine

2. Outline
1. What is a state machine?
2. How to construct a state diagram
3. Abstract state machine class
4. BFS

3. 1. What is a state machine?
● A computation machine where the output is a function of the current input
AND all the previous inputs
●

4. Pure function State machine

5. 2. How to construct a state transition diagram
Template Sample diagram
● Contains: State, Input, Output
○ Output may or may not be the next state

6. 3. Abstract state machine class
● Points to note:
○ Inherits from abc.ABC (python module for abstract base class)
○ 2 attributes:
■ self.start_state
■ self.state
○ 4 methods:
■ self.start()
■ self.get_next_values(state, inp) → next_state, output
■ self.step(inp)
■ self.tranduce() → output[]

7. Code.
Part 1.

8. Code.
Part 2.

9. Code.
Part 3.

10. 4. BFS
● Explores all
neighboring
vertices before
going in depth
(1) (2)
Source: DDW notes

11. 4. BFS
● Problem: Find shortest path from S to H
● Algorithm:
1. Dequeue SearchNode X from queue
2. For every nth neighbor of X:
a. Create SearchNode N
i. state=neighbor, action=n, parent=X
b. If N is goal → return path
c. Else if N is already explored → pass
d. Else → queue N
● Implementation using SM: StateSpaceSearch
○ State: Node
○ Input: Neighbor #
○ Output: Next node
Source: DDW notes

12. 4. BFS
● StateSpaceSearch
○ State: Node
○ Input: Neighbor #
○ Output: Next node

13. 4. BFS
● Implementation details:
○ Class Queue
○ Abstract class StateSpaceSearch inherits from StateMachine
○ Class MapSM inherits from StateSpaceSearch:
■ 2 attributes: state, start_state
■ 4 methods: start(), get_next_values(), step(), tranduce()
■ 2 properties: statemap, legal_inputs
○ Class Step:
■ 2 attributes: state, action
○ Class SearchNode:
■ 3 attributes: state, action, parent
■ 2 methods: path(), in_path()
○ Function sm_search()

14. Q&A
Go to slido.com
#ddwclinic3