SimpleDEX.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IERC20 {
    function transfer(address to, uint256 amount) external returns (bool);
    function transferFrom(address from, address to, uint256 amount) external returns (bool);
}

contract SimpleDEX {
    address public tokenA;
    address public tokenB;
    uint256 public exchangeRate; // tokenA -> tokenB

    constructor(address _tokenA, address _tokenB, uint256 _exchangeRate) {
        tokenA = _tokenA;
        tokenB = _tokenB;
        exchangeRate = _exchangeRate;
    }

    function swapAtoB(uint256 amountA) public {
        uint256 amountB = amountA * exchangeRate;
        require(IERC20(tokenA).transferFrom(msg.sender, address(this), amountA), "Token A transfer failed");
        require(IERC20(tokenB).transfer(msg.sender, amountB), "Token B transfer failed");
    }

    function swapBtoA(uint256 amountB) public {
        uint256 amountA = amountB / exchangeRate;
        require(IERC20(tokenB).transferFrom(msg.sender, address(this), amountB), "Token B transfer failed");
        require(IERC20(tokenA).transfer(msg.sender, amountA), "Token A transfer failed");
    }
}