[ERC3156] Ethereumチェーン上でフラッシュローンを実装する仕組みを理解しよう！

pragma solidity ^0.7.0 || ^0.8.0;
import "./IERC3156FlashBorrower.sol";


interface IERC3156FlashLender {

    /**
     * @dev The amount of currency available to be lent.
     * @param token The loan currency.
     * @return The amount of `token` that can be borrowed.
     */
    function maxFlashLoan(
        address token
    ) external view returns (uint256);

    /**
     * @dev The fee to be charged for a given loan.
     * @param token The loan currency.
     * @param amount The amount of tokens lent.
     * @return The amount of `token` to be charged for the loan, on top of the returned principal.
     */
    function flashFee(
        address token,
        uint256 amount
    ) external view returns (uint256);

    /**
     * @dev Initiate a flash loan.
     * @param receiver The receiver of the tokens in the loan, and the receiver of the callback.
     * @param token The loan currency.
     * @param amount The amount of tokens lent.
     * @param data Arbitrary data structure, intended to contain user-defined parameters.
     */
    function flashLoan(
        IERC3156FlashBorrower receiver,
        address token,
        uint256 amount,
        bytes calldata data
    ) external returns (bool);
}

function flashLoan(
    IERC3156FlashBorrower receiver,
    address token,
    uint256 amount,
    bytes calldata data
) external returns (bool) {
  ...
  require(
      receiver.onFlashLoan(msg.sender, token, amount, fee, data) == keccak256("ERC3156FlashBorrower.onFlashLoan"),
      "IERC3156: Callback failed"
  );
  ...
}

pragma solidity ^0.7.0 || ^0.8.0;


interface IERC3156FlashBorrower {

    /**
     * @dev Receive a flash loan.
     * @param initiator The initiator of the loan.
     * @param token The loan currency.
     * @param amount The amount of tokens lent.
     * @param fee The additional amount of tokens to repay.
     * @param data Arbitrary data structure, intended to contain user-defined parameters.
     * @return The keccak256 hash of "ERC3156FlashBorrower.onFlashLoan"
     */
    function onFlashLoan(
        address initiator,
        address token,
        uint256 amount,
        uint256 fee,
        bytes calldata data
    ) external returns (bytes32);
}

pragma solidity ^0.8.0;

import "./interfaces/IERC20.sol";
import "./interfaces/IERC3156FlashBorrower.sol";
import "./interfaces/IERC3156FlashLender.sol";


contract FlashBorrower is IERC3156FlashBorrower {
    enum Action {NORMAL, OTHER}

    IERC3156FlashLender lender;

    constructor (
        IERC3156FlashLender lender_
    ) {
        lender = lender_;
    }

    /// @dev ERC-3156 Flash loan callback
    function onFlashLoan(
        address initiator,
        address token,
        uint256 amount,
        uint256 fee,
        bytes calldata data
    ) external override returns(bytes32) {
        require(
            msg.sender == address(lender),
            "FlashBorrower: Untrusted lender"
        );
        require(
            initiator == address(this),
            "FlashBorrower: Untrusted loan initiator"
        );
        (Action action) = abi.decode(data, (Action));
        if (action == Action.NORMAL) {
            // do one thing
        } else if (action == Action.OTHER) {
            // do another
        }
        return keccak256("ERC3156FlashBorrower.onFlashLoan");
    }

    /// @dev Initiate a flash loan
    function flashBorrow(
        address token,
        uint256 amount
    ) public {
        bytes memory data = abi.encode(Action.NORMAL);
        uint256 _allowance = IERC20(token).allowance(address(this), address(lender));
        uint256 _fee = lender.flashFee(token, amount);
        uint256 _repayment = amount + _fee;
        IERC20(token).approve(address(lender), _allowance + _repayment);
        lender.flashLoan(this, token, amount, data);
    }
}

enum Action {NORMAL, OTHER}

IERC3156FlashLender lender;

function onFlashLoan(
    address initiator,
    address token,
    uint256 amount,
    uint256 fee,
    bytes calldata data
) external override returns(bytes32)

function flashBorrow(
    address token,
    uint256 amount
) public

pragma solidity ^0.8.0;

import "../ERC20.sol";
import "../interfaces/IERC20.sol";
import "../interfaces/IERC3156FlashBorrower.sol";
import "../interfaces/IERC3156FlashLender.sol";


/**
 * @author Alberto Cuesta Cañada
 * @dev Extension of {ERC20} that allows flash minting.
 */
contract FlashMinter is ERC20, IERC3156FlashLender {

    bytes32 public constant CALLBACK_SUCCESS = keccak256("ERC3156FlashBorrower.onFlashLoan");
    uint256 public fee; //  1 == 0.01 %.

    /**
     * @param fee_ The percentage of the loan `amount` that needs to be repaid, in addition to `amount`.
     */
    constructor (
        string memory name,
        string memory symbol,
        uint256 fee_
    ) ERC20(name, symbol) {
        fee = fee_;
    }

    /**
     * @dev The amount of currency available to be lent.
     * @param token The loan currency.
     * @return The amount of `token` that can be borrowed.
     */
    function maxFlashLoan(
        address token
    ) external view override returns (uint256) {
        return type(uint256).max - totalSupply();
    }

    /**
     * @dev The fee to be charged for a given loan.
     * @param token The loan currency. Must match the address of this contract.
     * @param amount The amount of tokens lent.
     * @return The amount of `token` to be charged for the loan, on top of the returned principal.
     */
    function flashFee(
        address token,
        uint256 amount
    ) external view override returns (uint256) {
        require(
            token == address(this),
            "FlashMinter: Unsupported currency"
        );
        return _flashFee(token, amount);
    }

    /**
     * @dev Loan `amount` tokens to `receiver`, and takes it back plus a `flashFee` after the ERC3156 callback.
     * @param receiver The contract receiving the tokens, needs to implement the `onFlashLoan(address user, uint256 amount, uint256 fee, bytes calldata)` interface.
     * @param token The loan currency. Must match the address of this contract.
     * @param amount The amount of tokens lent.
     * @param data A data parameter to be passed on to the `receiver` for any custom use.
     */
    function flashLoan(
        IERC3156FlashBorrower receiver,
        address token,
        uint256 amount,
        bytes calldata data
    ) external override returns (bool){
        require(
            token == address(this),
            "FlashMinter: Unsupported currency"
        );
        uint256 fee = _flashFee(token, amount);
        _mint(address(receiver), amount);
        require(
            receiver.onFlashLoan(msg.sender, token, amount, fee, data) == CALLBACK_SUCCESS,
            "FlashMinter: Callback failed"
        );
        uint256 _allowance = allowance(address(receiver), address(this));
        require(
            _allowance >= (amount + fee),
            "FlashMinter: Repay not approved"
        );
        _approve(address(receiver), address(this), _allowance - (amount + fee));
        _burn(address(receiver), amount + fee);
        return true;
    }

    /**
     * @dev The fee to be charged for a given loan. Internal function with no checks.
     * @param token The loan currency.
     * @param amount The amount of tokens lent.
     * @return The amount of `token` to be charged for the loan, on top of the returned principal.
     */
    function _flashFee(
        address token,
        uint256 amount
    ) internal view returns (uint256) {
        return amount * fee / 10000;
    }
}

bytes32 public constant CALLBACK_SUCCESS = keccak256("ERC3156FlashBorrower.onFlashLoan");

uint256 public fee; //  1 == 0.01 %.

function maxFlashLoan(
    address token
) external view override returns (uint256)

function flashFee(
    address token,
    uint256 amount
) external view override returns (uint256)

function flashLoan(
    IERC3156FlashBorrower receiver,
    address token,
    uint256 amount,
    bytes calldata data
) external override returns (bool)

function _flashFee(
    address token,
    uint256 amount
) internal view returns (uint256)

pragma solidity ^0.8.0;

import "../interfaces/IERC20.sol";
import "../interfaces/IERC3156FlashBorrower.sol";
import "../interfaces/IERC3156FlashLender.sol";


/**
 * @author Alberto Cuesta Cañada
 * @dev Extension of {ERC20} that allows flash lending.
 */
contract FlashLender is IERC3156FlashLender {

    bytes32 public constant CALLBACK_SUCCESS = keccak256("ERC3156FlashBorrower.onFlashLoan");
    mapping(address => bool) public supportedTokens;
    uint256 public fee; //  1 == 0.01 %.


    /**
     * @param supportedTokens_ Token contracts supported for flash lending.
     * @param fee_ The percentage of the loan `amount` that needs to be repaid, in addition to `amount`.
     */
    constructor(
        address[] memory supportedTokens_,
        uint256 fee_
    ) {
        for (uint256 i = 0; i < supportedTokens_.length; i++) {
            supportedTokens[supportedTokens_[i]] = true;
        }
        fee = fee_;
    }

    /**
     * @dev Loan `amount` tokens to `receiver`, and takes it back plus a `flashFee` after the callback.
     * @param receiver The contract receiving the tokens, needs to implement the `onFlashLoan(address user, uint256 amount, uint256 fee, bytes calldata)` interface.
     * @param token The loan currency.
     * @param amount The amount of tokens lent.
     * @param data A data parameter to be passed on to the `receiver` for any custom use.
     */
    function flashLoan(
        IERC3156FlashBorrower receiver,
        address token,
        uint256 amount,
        bytes calldata data
    ) external override returns(bool) {
        require(
            supportedTokens[token],
            "FlashLender: Unsupported currency"
        );
        uint256 fee = _flashFee(token, amount);
        require(
            IERC20(token).transfer(address(receiver), amount),
            "FlashLender: Transfer failed"
        );
        require(
            receiver.onFlashLoan(msg.sender, token, amount, fee, data) == CALLBACK_SUCCESS,
            "FlashLender: Callback failed"
        );
        require(
            IERC20(token).transferFrom(address(receiver), address(this), amount + fee),
            "FlashLender: Repay failed"
        );
        return true;
    }

    /**
     * @dev The fee to be charged for a given loan.
     * @param token The loan currency.
     * @param amount The amount of tokens lent.
     * @return The amount of `token` to be charged for the loan, on top of the returned principal.
     */
    function flashFee(
        address token,
        uint256 amount
    ) external view override returns (uint256) {
        require(
            supportedTokens[token],
            "FlashLender: Unsupported currency"
        );
        return _flashFee(token, amount);
    }

    /**
     * @dev The fee to be charged for a given loan. Internal function with no checks.
     * @param token The loan currency.
     * @param amount The amount of tokens lent.
     * @return The amount of `token` to be charged for the loan, on top of the returned principal.
     */
    function _flashFee(
        address token,
        uint256 amount
    ) internal view returns (uint256) {
        return amount * fee / 10000;
    }

    /**
     * @dev The amount of currency available to be lent.
     * @param token The loan currency.
     * @return The amount of `token` that can be borrowed.
     */
    function maxFlashLoan(
        address token
    ) external view override returns (uint256) {
        return supportedTokens[token] ? IERC20(token).balanceOf(address(this)) : 0;
    }
}

bytes32 public constant CALLBACK_SUCCESS = keccak256("ERC3156FlashBorrower.onFlashLoan");

mapping(address => bool) public supportedTokens;

uint256 public fee; //  1 == 0.01 %.

function flashLoan(
    IERC3156FlashBorrower receiver,
    address token,
    uint256 amount,
    bytes calldata data
) external override returns(bool)

function flashFee(
    address token,
    uint256 amount
) external view override returns (uint256)

function _flashFee(
    address token,
    uint256 amount
) internal view returns (uint256)

function maxFlashLoan(
    address token
) external view override returns (uint256)