Vending Machine

medium · state machine, exact change from a limited coin pool

Brief-reported interview attribution (unverified): Amazon, Microsoft

Design a vending machine. Prices and money are integer cents. Coins are 1, 5, 10, 25, 100. The machine holds a coin inventory and product quantities. A purchase takes the inserted coins into the inventory, dispenses the product, and returns change made from the coin pool. If exact change cannot be made, the purchase fails and the inserted money stays in the machine (the user cancels to get it back).

class VendingMachine:
    def __init__(self, coin_inventory=None):
        # coin_inventory: {denomination: count}
    def add_product(self, code, name, price_cents, quantity):
    def restock(self, code, quantity):
    def insert_coin(self, denomination):
        # -> True if the coin is accepted, False for an invalid denomination
    def inserted_total(self):
        # -> total cents currently inserted
    def cancel(self):
        # -> refunded amount (cents); resets the inserted coins to zero
    def select(self, code):
        # -> ("ok", {"product": name, "change": {denom: count}})
        #    or ("error", reason): "invalid selection", "out of stock",
        #    "insufficient funds", "cannot make change"

Rules. select checks, in order: the product exists, it is in stock, enough money is inserted, and change can be made from the inventory plus the inserted coins. Only then does it commit (coins in, change out, quantity down). Change must be *exact* change: a greedy pass is not enough, because with limited coins the greedy choice can fail while a solution exists (e.g. 30¢ from three dimes when a quarter is also present) — search the denomination counts. cancel returns the inserted total and resets it. An invalid coin is rejected without side effects.

Money units: all prices, coins and change are integer cents.

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.
v = VendingMachine({25: 10, 10: 10, 5: 10, 1: 10})
v.add_product('A1', 'Chips', 75, 2)
v.add_product('B2', 'Soda', 100, 0)
v.add_product('C3', 'Candy', 50, 5)
print(v.select('ZZ'))

Starter code

class VendingMachine:

    def __init__(self, coin_inventory=None):
        """coin_inventory: {denomination: count}"""
        pass

    def add_product(self, code, name, price_cents, quantity):
        """Implement the return value and rules in the API contract."""
        pass

    def restock(self, code, quantity):
        """Implement the return value and rules in the API contract."""
        pass

    def insert_coin(self, denomination):
        """-> True if the coin is accepted, False for an invalid denomination"""
        pass

    def inserted_total(self):
        """-> total cents currently inserted"""
        pass

    def cancel(self):
        """-> refunded amount (cents); resets the inserted coins to zero"""
        pass

    def select(self, code):
        '''-> ("ok", {"product": name, "change": {denom: count}}) or ("error", reason): "invalid selection", "out of stock", "insufficient funds", "cannot make change"'''
        pass

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