Class Definition

VendingMachine(List<String> products, List<Integer> acceptedDenominations)

• Each product record has the format "productCode,name,price,quantity,capacity".
• acceptedDenominations contains the cash denominations accepted by the machine.
• Prices and denominations are represented in the smallest currency unit, such as paise or cents.

Method Signatures

View Products

List<String> getProducts()

• Returns every product in the format "productCode,name,price,quantity".
• Products are returned in ascending lexicographical order of product code.
• Out-of-stock products are included with quantity 0.

Select a Product

String selectProduct(String productCode)

• Returns "SELECTED,productCode,price" when an available product is selected.
• Returns "INVALID_PRODUCT" when the product code does not exist.
• Returns "OUT_OF_STOCK" when the product has no available units.
• Returns "TRANSACTION_IN_PROGRESS" when another product is already selected.

Insert Cash

String insertCash(int denomination)

• Adds one cash unit of the specified denomination to the current transaction.
• Returns "ACCEPTED,totalInserted" when the cash is accepted.
• Returns "NO_PRODUCT_SELECTED" when no product is selected.
• Returns "UNSUPPORTED_DENOMINATION" when the denomination is not accepted by the machine.
• Returns "EXCEEDS_PRICE" when accepting the denomination would make the inserted total greater than the product price.
• A rejected denomination is not added to the transaction.

Complete a Purchase

String completePurchase()

• Dispenses the selected product only when the inserted total exactly equals its price.
• Returns "DISPENSED,productCode" when the purchase succeeds.
• Returns "INSUFFICIENT_PAYMENT,remainingAmount" when more cash is required. The transaction remains active.
• Returns "NO_ACTIVE_TRANSACTION" when no product is selected.

Cancel a Transaction

List<Integer> cancelTransaction()

• Cancels the current transaction before the product is dispensed.
• Returns the exact denominations inserted by the customer in insertion order.
• Returns an empty list when no cash was inserted or no transaction exists.
• The selected product and inserted total are cleared after cancellation.

Restock a Product

boolean restockProduct(String productCode, int quantity, int newPrice)

• Adds quantity units to an existing product slot and changes its price to newPrice.
• Returns true when both the quantity and price are updated.
• Returns false if the product does not exist, a parameter is invalid, the capacity would be exceeded or a customer transaction is active.
• The operation is atomic. If it fails, neither the quantity nor the price is changed.

Transaction Rules

• The machine handles one customer transaction at a time.
• Each successful transaction dispenses exactly one product.
• The customer must insert the exact product price.
• The machine never returns change.
• An insertion that would exceed the product price is immediately rejected.
• An insufficient-payment result does not clear the selected product or previously inserted cash.
• Cancelling returns the exact cash units inserted during the transaction.
• After a successful purchase or cancellation, the machine becomes ready for another transaction.
• Restocking is not allowed while a customer transaction is active.

Constraints

• 1 ≤ products.size() ≤ 1,000
• 1 ≤ acceptedDenominations.size() ≤ 100
• Every product record contains exactly five comma-separated fields.
• Product codes are unique, non-empty and contain at most 20 letters or digits.
• Product names contain at most 100 characters and do not contain commas.
• 1 ≤ price, newPrice ≤ 1,000,000,000
• 0 ≤ product quantity ≤ capacity ≤ 1,000,000
• Accepted denominations are unique positive integers.
• 1 ≤ denomination ≤ 1,000,000,000
• Every product price can be formed using one or more accepted denominations.
• For restockProduct, 1 ≤ quantity ≤ 1,000,000.
• Parameter values are never null.

Examples

Example 1: Exact Payment

VendingMachine(products = ["A1,Orange Juice,65,2,5", "B4,Granola Bar,40,1,4"], acceptedDenominations = [5, 10, 20, 50])
selectProduct(productCode = "A1")
Output: "SELECTED,A1,65"
insertCash(denomination = 50)
Output: "ACCEPTED,50"
insertCash(denomination = 10)
Output: "ACCEPTED,60"
insertCash(denomination = 5)
Output: "ACCEPTED,65"
completePurchase()
Output: "DISPENSED,A1"

Example 2: Insufficient Payment

VendingMachine(products = ["D8,Trail Mix,75,2,6"], acceptedDenominations = [5, 10, 20, 50])
selectProduct(productCode = "D8")
Output: "SELECTED,D8,75"
insertCash(denomination = 50)
Output: "ACCEPTED,50"
completePurchase()
Output: "INSUFFICIENT_PAYMENT,25"
insertCash(denomination = 20)
Output: "ACCEPTED,70"
insertCash(denomination = 5)
Output: "ACCEPTED,75"
completePurchase()
Output: "DISPENSED,D8"

Example 3: Insertion Exceeds the Price

VendingMachine(products = ["F2,Lemon Drink,60,3,5"], acceptedDenominations = [10, 20, 50])
selectProduct(productCode = "F2")
Output: "SELECTED,F2,60"
insertCash(denomination = 50)
Output: "ACCEPTED,50"
insertCash(denomination = 20)
Output: "EXCEEDS_PRICE"
insertCash(denomination = 10)
Output: "ACCEPTED,60"
completePurchase()
Output: "DISPENSED,F2"

Example 4: Cancel a Transaction

VendingMachine(products = ["M7,Baked Chips,90,3,5"], acceptedDenominations = [10, 20, 50, 100])
selectProduct(productCode = "M7")
Output: "SELECTED,M7,90"
insertCash(denomination = 20)
Output: "ACCEPTED,20"
insertCash(denomination = 50)
Output: "ACCEPTED,70"
cancelTransaction()
Output: [20, 50]

Example 5: Restock and Change the Price

VendingMachine(products = ["P5,Sparkling Water,45,1,3"], acceptedDenominations = [5, 10, 20, 50])
restockProduct(productCode = "P5", quantity = 2, newPrice = 50)
Output: true
getProducts()
Output: ["P5,Sparkling Water,50,3"]
restockProduct(productCode = "P5", quantity = 1, newPrice = 55)
Output: false
getProducts()
Output: ["P5,Sparkling Water,50,3"]