Amazon Locker

medium · allocation (best fit), pickup codes, expiry

Brief-reported interview attribution (unverified): Amazon

Design an Amazon Locker bank. The bank has lockers in three sizes — small, medium, large. Packages also have sizes; a package fits a locker of its own size or larger. When a courier deposits a package, the system assigns the smallest locker that fits (best fit, so large lockers stay free for large packages) and returns a 6-digit pickup code. The customer enters the code to collect the package, which frees the locker. A package not collected within pickup_days (default 3) expires; an expired package is collected by a daily expiry job (expire(day)), which frees its locker and returns the list of expired package ids. Days are plain integers passed by the caller.

class AmazonLocker:
    def __init__(self, lockers, pickup_days=3):
        # lockers: {"small": n, "medium": n, "large": n}
    def deposit(self, package_id, package_size, day):
        # -> (locker_id, code) or None (no fitting locker free / duplicate package / bad size)
        # locker ids are "L1", "L2", ... in size order (small, then medium, then large);
        # codes are 6-digit strings starting at "100000", one per successful deposit
    def pickup(self, code, day):
        # -> package_id if the code is valid and not expired, else None
        # (a successful pickup frees the locker)
    def expire(self, day):
        # -> list of package ids that expired as of this day (in locker order); frees their lockers
    def available_lockers(self, package_size=None):
        # -> count of free lockers that fit the size (all free lockers if size is None; 0 for a bad size)
    def status(self, locker_id):
        # -> "available" | "occupied" | None for an unknown locker

Rules. Best fit: among free lockers that fit, pick the smallest size, then the lowest locker id. A package expires when day >= deposit_day + pickup_days. pickup on an expired code returns None and does not free the locker — only expire(day) frees expired packages (it is the daily job). A code is single-use. Depositing a package_id that is already stored returns None. Codes are assigned only on successful deposits.

Tests use explicit inputs and injected clocks/callbacks. No network, real threads or application-server execution is involved.

Public demo

# Public demo: run after implementing the class API above.
l = AmazonLocker({'small': 1, 'medium': 1, 'large': 1}, pickup_days=3)
print(l.deposit('P1', 'small', 0))

Starter code

class AmazonLocker:

    def __init__(self, lockers, pickup_days=3):
        """lockers: {"small": n, "medium": n, "large": n}"""
        pass

    def deposit(self, package_id, package_size, day):
        """-> (locker_id, code) or None (no fitting locker free / duplicate package / bad size) locker ids are "L1", "L2", ... in size order (small, then medium, then large); codes are 6-digit strings starting at "100000", one per successful deposit"""
        pass

    def pickup(self, code, day):
        """-> package_id if the code is valid and not expired, else None (a successful pickup frees the locker)"""
        pass

    def expire(self, day):
        """-> list of package ids that expired as of this day (in locker order); frees their lockers"""
        pass

    def available_lockers(self, package_size=None):
        """-> count of free lockers that fit the size (all free lockers if size is None; 0 for a bad size)"""
        pass

    def status(self, locker_id):
        """-> "available" | "occupied" | None for an unknown locker"""
        pass

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