Elevator Door Controller Simulation

Simulation

Simulate an elevator door controller using a finite state machine. The elevator door can be in one of four states: 'CLOSED', 'OPENING', 'OPEN', or 'CLOSING'.

The system processes a sequence of events. Each event is one of three strings: 'BUTTON', 'TIMER', or 'OBSTACLE'.

The state transition rules are defined as follows:

From 'CLOSED':
- 'BUTTON' transitions to 'OPENING'.
- 'TIMER' or 'OBSTACLE' keeps the state in 'CLOSED'.
From 'OPENING':
- 'TIMER' transitions to 'OPEN'.
- 'BUTTON' or 'OBSTACLE' keeps the state in 'OPENING'.
From 'OPEN':
- 'TIMER' transitions to 'CLOSING'.
- 'BUTTON' or 'OBSTACLE' keeps the state in 'OPEN'.
From 'CLOSING':
- 'BUTTON' or 'OBSTACLE' transitions to 'OPENING'.
- 'TIMER' transitions to 'CLOSED'.

Given the initial state initialState and a array of events events, return the final state of the elevator door after processing all events in order.

Example 1

Input

[
  "CLOSED",
  "[\"BUTTON\", \"TIMER\", \"TIMER\"]"
]

Output

CLOSING

Explanation. CLOSED + BUTTON -> OPENING; OPENING + TIMER -> OPEN; OPEN + TIMER -> CLOSING.

Example 2

Input

[
  "CLOSING",
  "[\"OBSTACLE\", \"TIMER\"]"
]

Output

OPEN

Explanation. CLOSING + OBSTACLE -> OPENING; OPENING + TIMER -> OPEN.

Example 3

Input

[
  "OPEN",
  "[\"BUTTON\", \"OBSTACLE\"]"
]

Output

OPEN

Explanation. From OPEN, BUTTON and OBSTACLE leave the door state in OPEN.

Example 4

Input

[
  "CLOSED",
  "[]"
]

Output

CLOSED

Explanation. With no events, the state remains unchanged.

Example 5

Input

[
  "CLOSING",
  "[\"TIMER\", \"BUTTON\", \"TIMER\", \"TIMER\"]"
]

Output

CLOSING

Explanation. CLOSING + TIMER -> CLOSED; CLOSED + BUTTON -> OPENING; OPENING + TIMER -> OPEN; OPEN + TIMER -> CLOSING.

Example 6

Input

[
  "OPENING",
  "[\"TIMER\", \"TIMER\", \"TIMER\"]"
]

Output

CLOSED

Explanation. OPENING + TIMER -> OPEN; OPEN + TIMER -> CLOSING; CLOSING + TIMER -> CLOSED.

Follow-up: How would you extend the state machine if each transition also logged timestamps and emitted state duration metrics?

Constraints:

`initialState` is one of 'CLOSED', 'OPENING', 'OPEN', or 'CLOSING'. `events` is an array of strings containing between 0 and 1000 elements. Each string in `events` is one of 'BUTTON', 'TIMER', or 'OBSTACLE'.

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