Core Requirements

• Bill: Create a bill for a customer using a list of cart items, compute a subtotal, and track bill state (open/paid).
• Discount: Apply one or more discount codes to an open bill and compute the payable amount deterministically.
• Point/Level calculation system: When a bill is paid successfully, award loyalty points and update the customer’s level.
• In-memory: No database; store customers, bills, applied discounts, and points in memory.
• Deterministic IDs: Bill IDs must be generated sequentially as B1, B2, B3, ... in creation order.

Data Format

Cart Item Encoding

• itemName is a non-empty string without the | character.
• unitPrice is an integer representing price per unit (in dollars).
• quantity is an integer.
• Subtotal contribution = unitPrice * quantity.

Supported Discount Codes

• P10: 10% off subtotal.
• P20: 20% off subtotal.
• FLAT100: Flat 100 off, applicable only if subtotal >= 500.
• REDEEM: Redeem customer points for additional discount (see rules below).

Discount Rules

• Percentage discount: At most one percentage code can be effective. If both P10 and P20 are applied, only the highest percentage is used.
• Flat discount: FLAT100 can be applied at most once and only if subtotal >= 500.
• Redemption discount: REDEEM can be applied at most once.
• Computation order:
  • Start with subtotal.
  • Apply the effective percentage discount (if any).
  • Apply FLAT100 (if applicable).
  • Apply REDEEM (if applied).
• Rounding: All calculations must use integer math. Percentage discount uses floor: percentDiscount = (subtotal * percent) / 100.
• Non-negative payable: Payable amount must not go below 0.
• Idempotency: Applying the same discount code multiple times must not stack. It should have no additional effect after the first time.

Point/Level Calculation System

Point Earning

• On successful payment, points earned = floor(payableAmount / 100).
• Points are awarded only if the bill transitions to paid.

Point Redemption

• When REDEEM is applied, the bill can use customer’s currently available points for discount at the rate 1 point = 1 dollar.
• Redemption amount is capped to 20% of the current payable amount (after percentage and flat discounts):
  redeemCap = floor(currentPayable * 20 / 100)
• Redemption amount = min(customerPoints, redeemCap).
• Points are deducted only if payment succeeds. If payment fails then no points are deducted.

Customer Levels

• BRONZE: 0 - 99 points
• SILVER: 100 - 499 points
• GOLD: 500 - 1999 points
• PLATINUM: 2000+ points
• Level is derived from current total points after any redemption deduction and newly earned points.

Error Handling

• If an operation cannot be completed due to invalid input or invalid state, return an error output as defined by the method contract.
• A bill can be paid at most once. Discounts can be applied only while the bill is open.

Method Signatures

1) Create bill

• customerId is a non-empty string.
• cartItems contains 1 or more strings, each in format "itemName|unitPrice|quantity".
• unitPrice >= 0 and quantity > 0 for every item.
• Returns the new bill ID ("B1", "B2", ...).
• If input is invalid, return "ERROR".

2) Apply discount

• billId must refer to an existing, open bill.
• discountCode must be one of: P10, P20, FLAT100, REDEEM.
• Discount application is idempotent per code.
• Returns the current payable amount after applying the code (and re-evaluating all applied codes).
• If billId is invalid or bill is already paid, return -1.
• If discountCode is unknown, ignore it and return the current payable amount (no change).

3) Pay bill (also updates points/level)

• billId must refer to an existing, open bill.
• amountPaid must be exactly equal to the current payable amount.
• On success, the bill becomes paid, points are redeemed (if REDEEM was applied), then new points are earned, and level is updated.
• Return receipt string in format:
  "PAID|final=<finalAmount>|pointsEarned=<x>|totalPoints=<y>|level=<LEVEL>"
• If billId is invalid, bill is already paid, or amountPaid mismatches, return "ERROR".

Constraints

• 1 <= cartItems.size() <= 10^5
• 0 <= unitPrice <= 10^9
• 1 <= quantity <= 10^6
• Total subtotal fits in 64-bit signed integer (long).
• At most 4 distinct discount codes can be applied to a bill (from the supported set).

Examples

Example 1: Basic bill + percentage discount + payment earns points

• createBill(customerId = "C1", cartItems = List.of("book|200|1", "pen|10|5")) returns "B1"
  Explanation: Subtotal = 200*1 + 10*5 = 250; bill ID starts from B1.
• applyDiscount(billId = "B1", discountCode = "P10") returns 225
  Explanation: 10% of 250 is 25; payable becomes 250 - 25 = 225.
• applyDiscount(billId = "B1", discountCode = "FLAT100") returns 225
  Explanation: FLAT100 requires subtotal >= 500; subtotal is 250 so it is not applicable.
• payBill(billId = "B1", amountPaid = 225) returns "PAID|final=225|pointsEarned=2|totalPoints=2|level=BRONZE"
  Explanation: Points earned = floor(225/100)=2; total points become 2; level stays BRONZE.

Example 2: Multiple discounts including REDEEM (points used + points earned)

• createBill(customerId = "C1", cartItems = List.of("shoes|600|1", "tshirt|200|2")) returns "B2"
  Explanation: Subtotal = 600 + 200*2 = 1000.
• applyDiscount(billId = "B2", discountCode = "P20") returns 800
  Explanation: 20% of 1000 is 200; payable becomes 1000 - 200 = 800.
• applyDiscount(billId = "B2", discountCode = "FLAT100") returns 700
  Explanation: Subtotal >= 500, so flat 100 applies; payable becomes 800 - 100 = 700.
• applyDiscount(billId = "B2", discountCode = "REDEEM") returns 698
  Explanation: Customer C1 currently has 2 points from Example 1. Redeem cap = floor(700*20/100)=140. Redeem amount = min(2,140)=2. Payable becomes 700 - 2 = 698.
• payBill(billId = "B2", amountPaid = 698) returns "PAID|final=698|pointsEarned=6|totalPoints=6|level=BRONZE"
  Explanation: First deduct redeemed 2 points (2 - 2 = 0), then earn floor(698/100)=6. Total points become 6; level remains BRONZE.

Example 3: Invalid payment amount does not change state or points

• createBill(customerId = "C2", cartItems = List.of("mouse|499|1")) returns "B3"
  Explanation: Subtotal = 499.
• applyDiscount(billId = "B3", discountCode = "P10") returns 450
  Explanation: floor(499*10/100)=49; payable becomes 499 - 49 = 450.
• payBill(billId = "B3", amountPaid = 449) returns "ERROR"
  Explanation: Amount paid must match payable exactly; bill remains open and no points/level changes occur.