Wash & Fold: Pricing, Pickup Lockers and Subscriptions

Wash & Fold: Pricing, Pickup Lockers and Subscriptions

You've joined the small team behind the counter software for a wash-and-fold laundry chain. Customers drop off a bag, it gets weighed in grams, and later the finished order is put in a pickup locker. Two things are happening this sprint: customers are complaining that prices at the counter don't match the price list (Part 1), and the new pickup lockers need software behind them (Part 2). After that, a subscription plan lands and changes some of the rules you just built (Part 3). All money is integer cents.

Codebase

How your code is run

run(operations: list[list]) -> list executes the operations in order against a fresh store and returns one result per operation. Every operation is [name, ts, ...args]. Timestamps are strings shaped exactly "YYYY-MM-DD HH:MM" on one local clock (no time zones, no daylight saving) and never decrease from one operation to the next. A malformed or impossible timestamp (including in ready_by) returns "error: invalid arguments". Errors are returned as "error: <code>" strings, never raised.

A price object is {"pounds", "covered", "base", "rush", "total"}; total = base + rush. drop_off returns the same object with an "order" key (the order id) added in front.

• An unknown operation name returns "error: unknown operation"; a wrong number of arguments returns "error: invalid arguments", as does a non-string or empty order_id/customer, and a grams value that is not an integer (JSON booleans are not integers).
• A valid weight is an integer from 1 to 50,000 grams. ready_by must be strictly later than ts.
• status returns state (dropped, in_locker, at_desk or collected), locker (a label such as "L03" while the order is in a locker, otherwise null) and due (cents still owed on the order: its total plus any re-delivery fee).

Part 1: Make counter prices match the price list

Finance has pinned down the price list below. Make quote and drop_off follow it exactly. The quote operation is the sticker price a customer is shown; drop_off prices and records the order.

1. Billable pounds are the weight in grams rounded up to the next whole pound, where 1 lb = 453.59237 g exactly. In integers: pounds = ceil(grams * 100000 / 45359237). (4535 g is 10 lb; 4536 g is 11 lb.)
2. Laundry charge: the first 10 lb of an order cost 199 cents per lb; every pound beyond 10 costs 149 cents per lb.
3. Minimum charge: the laundry charge is never below 1500 cents. The result of this step is base.
4. Rush: an order is a rush order when ready_by is at most 24 hours after ts (exactly 24:00 counts as rush; 24:01 does not). This is a real elapsed-time comparison, so it must hold across day, month and year boundaries.
5. Rush fee: for a rush order, rush is 25% of base (the charge *after* the minimum is applied), rounded half up to a whole cent. Otherwise rush is 0.
6. covered is always 0 for now (Part 3 uses it).

Worked example

[["quote", "2025-03-03 09:00", 5000, "2025-03-05 09:00"],
 ["quote", "2025-03-03 09:00", 1000, "2025-03-04 09:00"],
 ["drop_off", "2025-03-03 09:05", "o-1", "ana", 4536, "2025-03-04 09:06"],
 ["status", "2025-03-03 09:10", "o-1"]]

returns

[{"pounds": 12, "covered": 0, "base": 2288, "rush": 0, "total": 2288},
 {"pounds": 3, "covered": 0, "base": 1500, "rush": 375, "total": 1875},
 {"order": "o-1", "pounds": 11, "covered": 0, "base": 2139, "rush": 0, "total": 2139},
 {"state": "dropped", "locker": null, "due": 2139}]

5000 g is 12 lb (10 x 199 + 2 x 149). 1000 g is 3 lb = 597 cents, lifted to the 1500 minimum, and the order is wanted in exactly 24 hours, so it is rush: 25% of 1500 = 375. The third order is 11 lb and its 09:06 next-day deadline is 24h01 away, so no rush.

Part 2: Pickup lockers

Finished orders go into pickup lockers. Build the staff operations below. Orders are collected from a locker with a 6-digit code; codes expire, and an expired order is moved to the front desk for a re-delivery fee.

1. Placing an order. mark_ready puts a dropped order into the lowest-numbered free locker (8 lockers, shown as L01...L08) and sets its state to in_locker. Only a dropped order can be placed, otherwise invalid state. If every locker is taken the order stays dropped and the error is no free locker.
2. Codes. The code is make_pickup_code(order_id, locker_number, ts) from wash/lockers.py (already written; do not change it). It is returned as a string and printed on the receipt.
3. Expiry. A code is valid for 72 hours: it stops working at exactly placed + 72:00. expires_at is that moment as a timestamp.
4. Moving to the front desk. Expiry must be noticed by every operation, whatever it is: before any operation is handled, every order whose expiry time is at or before that operation's ts moves to the desk: its state becomes at_desk, its locker is freed (and may be handed out by that same operation), and it owes a re-delivery fee of 300 cents.
5. Collecting. pickup with the right code from a locker returns the locker label and paid (the order total); the order becomes collected and its locker is free again. A wrong code (any value that is not the order's code, including a non-string) is wrong code. An order that is at_desk answers code expired whatever code is given; an order that is dropped or already collected answers invalid state.
6. Front desk. desk_pickup works only on an at_desk order (anything else is invalid state), returns paid = total + re-delivery fee, and marks the order collected. status reports due as total + fee for an at_desk order.

Worked example

[["drop_off", "2025-03-03 09:00", "o-1", "ana", 5000, "2025-03-04 09:00"],
 ["drop_off", "2025-03-03 09:10", "o-2", "bo", 4536, "2025-03-05 09:00"],
 ["mark_ready", "2025-03-03 18:00", "o-1"],
 ["mark_ready", "2025-03-03 18:30", "o-2"],
 ["pickup", "2025-03-04 10:00", "o-1", "111111"],
 ["pickup", "2025-03-04 10:01", "o-1", "599854"],
 ["status", "2025-03-06 18:29", "o-2"],
 ["status", "2025-03-06 18:30", "o-2"],
 ["pickup", "2025-03-06 18:31", "o-2", "728340"],
 ["desk_pickup", "2025-03-07 08:00", "o-2"]]

returns (after the two drop_off results, which are the usual price objects)

[{"locker": "L01", "code": "599854", "expires_at": "2025-03-06 18:00"},
 {"locker": "L02", "code": "728340", "expires_at": "2025-03-06 18:30"},
 "error: wrong code",
 {"order": "o-1", "locker": "L01", "paid": 2860},
 {"state": "in_locker", "locker": "L02", "due": 2139},
 {"state": "at_desk", "locker": null, "due": 2439},
 "error: code expired",
 {"order": "o-2", "paid": 2439}]

o-2 expires at 18:30 on 03-06: one minute earlier it is still in its locker, and at 18:30 the status call itself finds it already at the desk with the 300-cent fee added.

Part 3: Subscriptions (requirements change)

Marketing is launching a subscription plan. A subscriber has a monthly allowance of pounds that pays for laundry before any money is charged. This replaces two earlier rules, so revisit Parts 1 and 2 as well as adding the new operations.

monthly_pounds must be an integer from 1 to 200 (a non-integer or boolean is invalid arguments; any integer outside the range is invalid allowance).

1. Months. Allowances follow calendar months (2025-03-31 23:59 and 2025-04-01 00:00 are different months). subscribe sets the customer's available pounds to monthly_pounds for the month of its ts; no earlier allowance exists to roll over.
2. Renewal and rollover. At each month boundary the available pounds become monthly_pounds + min(10, leftover): at most 10 unused pounds roll over. This applies once for every month that passes, including months with no activity (leftover 4 with a 20 lb plan gives 24 after one month, then 30 after a second). Renewal is evaluated lazily, at the month of the ts of the operation that looks at the allowance (drop_off or allowance).
3. Using the allowance. At drop_off, the customer's available pounds (after renewal) are applied to the order: covered = min(pounds, available). Covered pounds are removed from the allowance. The allowance pays for pounds at the cheaper rate first: covered pounds are taken from the tier-2 pounds (the pounds beyond the first 10), and only when those are all covered does it cover tier-1 pounds. The laundry charge is then the per-pound rates applied to the uncovered pounds only. quote ignores subscriptions entirely (it is the sticker price).
4. Minimum charge (changes Part 1). The 1500-cent minimum now applies only when at least one pound of the order is uncovered. A fully covered order has base 0. Everything else in Part 1 is unchanged: the rush fee is still 25% of base (so a fully covered rush order has no rush fee).
5. Re-delivery (changes Part 2). The 300-cent re-delivery fee is waived (0) for a subscriber. "Subscriber" is decided at the moment the order moves to the desk: that happens at the start of the first operation whose ts is at or after expiry, *before* that operation runs (so a subscribe at exactly the expiry minute is too late for that order).
6. A rejected drop_off (any error) must not consume any allowance.

Worked example

[["subscribe", "2025-03-01 08:00", "ana", 20],
 ["drop_off", "2025-03-03 09:00", "o-1", "ana", 7000, "2025-03-05 09:00"],
 ["allowance", "2025-03-03 09:01", "ana"],
 ["drop_off", "2025-03-04 09:00", "o-2", "ana", 5000, "2025-03-06 09:00"],
 ["allowance", "2025-03-04 09:01", "ana"],
 ["allowance", "2025-04-02 12:00", "ana"],
 ["quote", "2025-04-02 12:00", 7000, "2025-04-04 12:00"]]

returns

[true,
 {"order": "o-1", "pounds": 16, "covered": 16, "base": 0, "rush": 0, "total": 0},
 4,
 {"order": "o-2", "pounds": 12, "covered": 4, "base": 1592, "rush": 0, "total": 1592},
 0,
 20,
 {"pounds": 16, "covered": 0, "base": 2884, "rush": 0, "total": 2884}]

o-1 (16 lb) is fully covered by the 20 lb allowance, leaving 4. o-2 is 12 lb: 2 tier-2 pounds and 10 tier-1 pounds. The 4 remaining allowance pounds cover both tier-2 pounds first, then 2 tier-1 pounds, leaving 8 tier-1 pounds = 1592 (above the minimum). The allowance is then 0, and renews to 20 + min(10, 0) in April.