Turnstile tally

Easy

Online Assessment

Tally what a station turnstile did during a shift, given the log of events it saw.

The turnstile is always in one of three states and starts locked. Each character of events is one event:

C (a coin is inserted): if the turnstile is locked, it keeps the coin and becomes open. In any other state the coin is returned to the customer.
P (someone pushes): if the turnstile is open, one person passes through and it becomes locked again. In any other state nothing happens.
M (staff start maintenance): from any state the turnstile goes into maintenance. A coin already kept for a passage is not refunded.
R (staff resume): only a turnstile in maintenance reacts, and it returns to locked. In any other state nothing happens.

Input: events, a string of C, P, M and R characters (possibly empty).

Output: a list of three integers [passed, kept, returned]: how many people passed, how many coins were kept, and how many coins were returned.

Example 1

Input:: ["CPCCP"]
Output:: [2,2,1]
Explanation: C kept 1 coin (open), P passes one person (locked), C kept a second coin (open), the next C arrives while open and is returned, and the final P passes a second person: [2, 2, 1].

Input:: ["CMCRP"]
Output:: [0,1,1]
Explanation: C keeps a coin (open). M puts the turnstile in maintenance, and the kept coin is not refunded. The next C is returned, R locks it again, and the final P finds it locked: [0, 1, 1].

Input:: ["CRPPC"]
Output:: [1,2,0]
Explanation: C keeps a coin (open). The R does nothing because the turnstile is not in maintenance, so the first P still passes one person. The second P finds it locked. The last C is kept: [1, 2, 0].