// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

interface IERC20 {
    function transfer(address, uint256) external returns (bool);
    function balanceOf(address) external view returns (uint256);
}

contract SimpleAMM {
    IERC20 public tokenA;
    IERC20 public tokenB;
    uint256 public reserveA;
    uint256 public reserveB;

    constructor(address _a, address _b) {
        tokenA = IERC20(_a);
        tokenB = IERC20(_b);
    }

    function swapAforB(uint256 amountIn) external {
        require(amountIn > 0, "Zero input");
        tokenA.transferFrom(msg.sender, address(this), amountIn);

        // Constant product: x * y = k
        // (reserveA + amountIn) * (reserveB - amountOut) = reserveA * reserveB
        uint256 amountOut = (reserveB * amountIn) /
            (reserveA + amountIn);

        require(amountOut < reserveB, "Insufficient liquidity");
        reserveA += amountIn;
        reserveB -= amountOut;

        tokenB.transfer(msg.sender, amountOut);
    }
}