Dollars-to-donuts exchange. Demo: Circle <> Tether. Smol Swap Lego.

CTX.sol

// SPDX-License-Identifier: AGPL-3.0-only
pragma solidity ^0.8.19;

/// @notice Dollars-to-donuts exchange.
/// Put in one thing for another thing.
contract CTX {
    IERC20 constant C = IERC20(0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48); // Circle dollars.
    IERC20 constant T = IERC20(0xdAC17F958D2ee523a2206206994597C13D831ec7); // Tether dollars.
    uint256 constant F = 1; // Fee is just a lil bit. Placeholder. Someone should do the math.

    /// @dev So, we just assume same price. One in. One out.
    function swap(bool cForT, uint256 amount) public payable {
        unchecked {
            // Pull in some tokens. We check whether it is C or T.
            if (cForT) {
                C.transferFrom(msg.sender, address(this), amount);
                T.transfer(msg.sender, amount - F);
            } else {
                T.transferFrom(msg.sender, address(this), amount);
                C.transfer(msg.sender, amount - F);
            }
        }
    }

    /// @dev Enter the pool with your amount, or exit fees.
    function lp(bool enter, uint256 amount) public payable {
        unchecked {
            if (enter) {
                C.transferFrom(msg.sender, address(this), amount);
                T.transferFrom(msg.sender, address(this), amount);
            } else {/*Do the withdrawal stuff.*/}
        }
    }
}

/// @dev Fungible token transfer interface.
interface IERC20 {
    function transfer(address, uint256) external returns (bool);
    function transferFrom(address, address, uint256) external returns (bool);
}