Subscriptions on an Injected Clock

Subscription rules depend on today's date - and code that reads the wall clock directly can't be tested: you'd have to wait 30 days to see a subscription expire. The fix is Dependency Inversion: the service depends on a Clock abstraction, and whoever creates it decides which clock to pass - SystemClock in production, a ManualClock in tests.

The starter's constructor receives a Clock but ignores it and reads the real time instead. Store the injected clock and implement the rules using only clock.today():

• subscribe(user, days) - if the user is currently active, the new days are added to their existing expiry day; otherwise the subscription runs from today. Returns the new expiry day.
• A user is active while today < expiryDay.
• status(user) - never subscribed, active, <n> day(s) left, expired today (when today == expiryDay) or expired <n> day(s) ago.
• activeUsers() - the names of active users in alphabetical order.

In Python: the methods are subscribe, status and active_users; Clock is an abstract base class (any object with a today() method would do), and the real clock reads time.time().

Commands: SUBSCRIBE <user> <days>, ADVANCE <days> (moves the test clock forward; it starts at day 0), STATUS <user>, ACTIVE.

Example

Input:
6
SUBSCRIBE asha 30
ADVANCE 10
STATUS asha
ADVANCE 20
STATUS asha
STATUS ravi

Output:
asha active until day 30
day 10
asha active, 20 day(s) left
day 30
asha expired today
ravi never subscribed

• At most 1000 commands; 1 <= days <= 10^6.