Basic Swap Function

Basic Swap Function

What You Are Building

The swap function is the core of any DEX. A user deposits one token and receives the other. The contract calculates the output amount using the constant product formula, applies fees, and transfers tokens.

In this section, you will implement:

• getAmountOut(): calculates output tokens for a given input (with 0.3% fee)
• swap(): executes the trade with slippage protection
• Invariant verification (k should only increase due to fees)
• Reserve updates after each swap

The Swap Flow

1. User calls swap(tokenIn, amountIn, minAmountOut)
2. Contract determines direction (token0 → token1 or vice versa)
3. Contract transfers input tokens from user to pool
4. Contract calculates output using getAmountOut()
5. Contract verifies amountOut >= minAmountOut (slippage protection)
6. Contract transfers output tokens to user
7. Contract updates reserves
8. Contract verifies k has not decreased

Security: Checks-Effects-Interactions

The order of operations matters. Update state (reserves) before making external calls (token transfers). This prevents reentrancy attacks where a malicious token's transfer callback could re-enter the swap function with stale reserves.

Your Task

Complete the swap implementation in the editor. The TODO comments show each step.