Smart contract that implements swapping different tokens using order book.

orderSwap.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/IERC20.sol";

contract Swap {
    struct Order {
        address orderOwner;
        address fromToken;
        address toToken;
        uint256 amountIn;
        uint256 amountOut;
        bool orderOpened;
    }

    uint256 ID = 1;

    mapping(uint => Order) orders;

    function createOrder(address _fromToken, address _toToken, uint256 _amountIn, uint256 _amountOut) external {
        require(IERC20(_fromToken).balanceOf(msg.sender) >= _amountIn, "insufficient funds");
        require(IERC20(_fromToken).transferFrom(msg.sender, address(this), _amountIn), "transfer failed");
        Order storage _order = orders[ID];
        _order.orderOwner = msg.sender;
        _order.fromToken = _fromToken;
        _order.toToken = _toToken;
        _order.amountIn = _amountIn;
        _order.amountOut = _amountOut;
        _order.orderOpened = true;

        ID++;
    }

    function executeOrder(uint256 _ID) external {
        Order storage _order = orders[_ID];
        assert(_order.orderOpened);
        require(IERC20(_order.toToken).balanceOf(msg.sender) > _order.amountOut, "insufficient funds");
        require(IERC20(_order.toToken).transferFrom(msg.sender, address(this), _order.amountOut), "transfer failed");

        _order.orderOpened = false;

        IERC20(_order.toToken).transfer(_order.orderOwner, _order.amountOut);
        IERC20(_order.fromToken).transfer(msg.sender, _order.amountIn);
    }
}