Parking Lot Fee Calculator
Problem
Design the pricing part of a parking lot as a small class. It is built with a rate table, mapping each vehicle kind to a price for the first hour and a price for every further started hour, and a daily cap per kind. fee(kind, minutes) returns what a stay costs: stays of 15 minutes or less are free, every full 24 hours costs exactly the daily cap, and the remaining part of a day is priced by the hourly rates but never above the cap. An unknown vehicle kind must raise a ValueError rather than be priced as something else.
Examples
Input: rates = car (4, 2), motorbike (2, 1); caps = car 20, motorbike 8
fee("car", 150)
Output: 8
Why: first hour 4, then 90 more minutes start 2 more hours at 2 eachInput: same lot, fee("car", 26 * 60)
Output: 26
Why: one full day at the cap of 20, then 2 hours for 4 + 2Input: same lot, fee("motorbike", 10)
Output: 0
Why: edge case, the stay is inside the 15-minute grace periodHints
0 / 3
Keep the numbers that change from one lot to another in the object, and keep the rules in methods. A new vehicle kind should mean a new table row, not a new if-statement.
Split a stay into whole days and the remainder with divmod. Whole days are simple; only the remainder needs the hourly rules and the cap.
For the remainder: free if it is zero, otherwise the first-hour price plus the extra-hour price times the number of started hours after the first, capped at the daily cap. Check the grace period against the whole stay, and reject unknown kinds up front.
Solution
The class holds data, the rate table and the caps, and one method applies the rules, so adding a van is a change to the table rather than to the code. A stay splits into whole days, each worth exactly the cap, and a remainder that goes through the hourly rule: the first hour, then every started hour after it, rounded up with negated floor division and capped for the day. The grace period is checked against the whole stay, so the 10 minutes after a full day are billed as a started hour instead of slipping through as grace. Raising on an unknown kind keeps a typo from being billed at some default rate. Each call is O(1).
class FeeSchedule:
GRACE, HOUR, DAY = 15, 60, 24 * 60
def __init__(self, rates, daily_cap):
self.rates = rates # kind -> (first hour, each further hour)
self.daily_cap = daily_cap
def fee(self, kind, minutes):
if kind not in self.rates:
raise ValueError("unknown vehicle kind: " + kind)
if minutes <= self.GRACE:
return 0
days, rest = divmod(minutes, self.DAY)
first, extra = self.rates[kind]
started = -(-max(0, rest - self.HOUR) // self.HOUR) # started hours after the first
part = min(self.daily_cap[kind], first + extra * started) if rest else 0
return days * self.daily_cap[kind] + part
lot = FeeSchedule({"car": (4, 2), "motorbike": (2, 1)}, {"car": 20, "motorbike": 8})
print(lot.fee("car", 150)) # -> 8
print(lot.fee("car", 26 * 60)) # -> 26
print(lot.fee("motorbike", 10)) # -> 0
print(lot.fee("car", 24 * 60 + 10)) # -> 24Stuck on the idea rather than the code? Designing a Parking Lot covers it.