Practice/Amazon/Design an LLD of Splitwise

Design an LLD of Splitwise

Object Oriented DesignMust

Problem

Design a low-level system for tracking shared expenses among multiple users, similar to expense-sharing applications. The system should allow users to add expenses, split costs in different ways (equally, by exact amounts, or by percentages), track who owes whom, and settle debts between users.

Your implementation should handle the core functionality of managing users, recording expenses with various split strategies, calculating balances, and processing settlements. The system needs to maintain accurate records of all financial transactions and compute net balances efficiently.

Requirements

1. User Management: Support adding and managing users in the system

2. Expense Recording: Allow users to record expenses with a payer and total amount

3. Split Strategies: Implement three types of expense splitting:

   • Equal Split: Divide the expense equally among all participants
   • Exact Split: Specify exact amounts for each participant
   • Percentage Split: Divide based on percentage shares for each participant

4. Balance Tracking: Calculate and maintain balances showing who owes money to whom

5. Debt Settlement: Allow users to settle debts by recording payments between users

6. Balance Simplification: Compute net balances to minimize the number of transactions needed

Constraints

• User IDs and names are unique strings
• Expense amounts are positive floating-point numbers (up to 2 decimal places)
• For equal splits, handle division that doesn't result in whole numbers
• For percentage splits, percentages must sum to 100
• For exact splits, amounts must sum to the total expense amount
• The system should handle up to 1000 users and 10000 expenses efficiently
• Balance calculations should be accurate to 2 decimal places

Examples

Example 1: Equal Split

` Users: Alice, Bob, Charlie Expense: Alice pays $90 for dinner, split equally among all three

Calculation:

• Each person's share: $90 / 3 = $30
• Alice paid $90 but owes $30, so she's owed $60
• Bob owes $30 (his share)
• Charlie owes $30 (his share)

Balances:

• Alice: +$60
• Bob: -$30
• Charlie: -$30 `

Example 2: Exact Split

` Users: Alice, Bob, Charlie Expense: Bob pays $100, split as Alice=$20, Bob=$50, Charlie=$30

Calculation:

• Bob paid $100 but owes $50, so he's owed $50
• Alice owes $20
• Charlie owes $30

Balances:

• Alice: -$20
• Bob: +$50
• Charlie: -$30 `

Example 3: Percentage Split

` Users: Alice, Bob, Charlie Expense: Charlie pays $150, split as Alice=50%, Bob=30%, Charlie=20%

Calculation:

• Alice's share: $150 × 0.50 = $75
• Bob's share: $150 × 0.30 = $45
• Charlie's share: $150 × 0.20 = $30
• Charlie paid $150 but owes $30, so he's owed $120

Balances:

• Alice: -$75
• Bob: -$45
• Charlie: +$120 `

Example 4: Multiple Expenses

` Users: Alice, Bob Transaction 1: Alice pays $60, split equally

• Alice: +$30, Bob: -$30

Transaction 2: Bob pays $40, split equally

• Alice: -$20, Bob: +$20

Net Balances:

• Alice: +$10 (owed by Bob)
• Bob: -$10 (owes Alice) `