[ERC4931] ERC20トークンを新たな別のトークンに変換する仕組みを理解しよう！

interface IEIP4931 {

/// @dev A getter to determine the contract that is being upgraded from ("source contract")
/// @return The address of the source token contract
function upgradeSource() external view returns(address)

/// @dev A getter to determine the contract that is being upgraded to ("destination contract")
/// @return The address of the destination token contract
function upgradeDestination() external view returns(address)

/// @dev The method will return true when the contract is serving upgrades and otherwise false
/// @return The status of the upgrade as a boolean
function isUpgradeActive() external view returns(bool)

/// @dev The method will return true when the contract is serving downgrades and otherwise false
/// @return The status of the downgrade as a boolean
function isDowngradeActive() external view returns(bool)

/// @dev A getter for the ratio of destination tokens to source tokens received when conducting an upgrade
/// @return Two uint256, the first represents the numerator while the second represents
/// the denominator of the ratio of destination tokens to source tokens allotted during the upgrade
function ratio() external view returns(uint256, uint256)

/// @dev A getter for the total amount of source tokens that have been upgraded to destination tokens.
/// The value may not be strictly increasing if the downgrade Optional Ext. is implemented.
/// @return The number of source tokens that have been upgraded to destination tokens
function totalUpgraded() external view returns(uint256)

/// @dev A method to mock the upgrade call determining the amount of destination tokens received from an upgrade
/// as well as the amount of source tokens that are left over as remainder
/// @param sourceAmount The amount of source tokens that will be upgraded
/// @return destinationAmount A uint256 representing the amount of destination tokens received if upgrade is called
/// @return sourceRemainder A uint256 representing the amount of source tokens left over as remainder if upgrade is called
function computeUpgrade(uint256 sourceAmount) external view
        returns (uint256 destinationAmount, uint256 sourceRemainder)

/// @dev A method to mock the downgrade call determining the amount of source tokens received from a downgrade
/// as well as the amount of destination tokens that are left over as remainder
/// @param destinationAmount The amount of destination tokens that will be downgraded
/// @return sourceAmount A uint256 representing the amount of source tokens received if downgrade is called
/// @return destinationRemainder A uint256 representing the amount of destination tokens left over as remainder if upgrade is called
function computeDowngrade(uint256 destinationAmount) external view
        returns (uint256 sourceAmount, uint256 destinationRemainder)

/// @dev A method to conduct an upgrade from source token to destination token.
/// The call will fail if upgrade status is not true, if approve has not been called
/// on the source contract, or if sourceAmount is larger than the amount of source tokens at the msg.sender address.
/// If the ratio would cause an amount of tokens to be destroyed by rounding/truncation, the upgrade call will
/// only upgrade the nearest whole amount of source tokens returning the excess to the msg.sender address. 
/// Emits the Upgrade event
/// @param _to The address the destination tokens will be sent to upon completion of the upgrade
/// @param sourceAmount The amount of source tokens that will be upgraded 
function upgrade(address _to, uint256 sourceAmount) external

/// @dev A method to conduct a downgrade from destination token to source token.
/// The call will fail if downgrade status is not true, if approve has not been called
/// on the destination contract, or if destinationAmount is larger than the amount of destination tokens at the msg.sender address.
/// If the ratio would cause an amount of tokens to be destroyed by rounding/truncation, the downgrade call will only downgrade
/// the nearest whole amount of destination tokens returning the excess to the msg.sender address. 
///  Emits the Downgrade event
/// @param _to The address the source tokens will be sent to upon completion of the downgrade
/// @param destinationAmount The amount of destination tokens that will be downgraded 
function downgrade(address _to, uint256 destinationAmount) external

//SPDX-License-Identifier: Apache-2.0
pragma solidity 0.8.9;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
import "./IEIP4931.sol";

contract SourceUpgrade is  IEIP4931 {
	using SafeERC20  for IERC20;

	uint256 constant RATIO_SCALE = 10**18;
    
	IERC20 private source;
	IERC20 private destination;
	bool private upgradeStatus;
	bool private downgradeStatus;
	uint256 private numeratorRatio;
	uint256 private denominatorRatio;
	uint256 private sourceUpgradedTotal;

	mapping(address => uint256) public upgradedBalance;

	constructor(address _source, address _destination, bool _upgradeStatus, bool _downgradeStatus, uint256 _numeratorRatio, uint256 _denominatorRatio) {
		require(_source != _destination, "SourceUpgrade: source and destination addresses are the same");
		require(_source != address(0), "SourceUpgrade: source address cannot be zero address");
		require(_destination != address(0), "SourceUpgrade: destination address cannot be zero address");
		require(_numeratorRatio > 0, "SourceUpgrade: numerator of ratio cannot be zero");
		require(_denominatorRatio > 0, "SourceUpgrade: denominator of ratio cannot be zero");

		source = IERC20(_source);
		destination = IERC20(_destination);
		upgradeStatus = _upgradeStatus;
		downgradeStatus = _downgradeStatus;
		numeratorRatio = _numeratorRatio;
		denominatorRatio = _denominatorRatio;
	}

	/// @dev A getter to determine the contract that is being upgraded from ("source contract")
	/// @return The address of the source token contract
	function upgradeSource() external view returns(address) {
		return address(source);
	}

	/// @dev A getter to determine the contract that is being upgraded to ("destination contract")
	/// @return The address of the destination token contract
	function upgradeDestination() external view returns(address) {
		return address(destination);
	}

	/// @dev The method will return true when the contract is serving upgrades and otherwise false
	/// @return The status of the upgrade as a boolean
	function isUpgradeActive() external view returns(bool) {
		return upgradeStatus;
	}

	/// @dev The method will return true when the contract is serving downgrades and otherwise false
	/// @return The status of the downgrade as a boolean
	function isDowngradeActive() external view returns(bool) {
		return downgradeStatus;
	}

	/// @dev A getter for the ratio of destination tokens to source tokens received when conducting an upgrade
	/// @return Two uint256, the first represents the numerator while the second represents
	/// the denominator of the ratio of destination tokens to source tokens allotted during the upgrade
	function ratio() external view returns(uint256, uint256) {
		return (numeratorRatio, denominatorRatio);
	}

	/// @dev A getter for the total amount of source tokens that have been upgraded to destination tokens.
	/// The value may not be strictly increasing if the downgrade Optional Ext. is implemented.
	/// @return The number of source tokens that have been upgraded to destination tokens
	function totalUpgraded() external view returns(uint256) {
		return sourceUpgradedTotal;
	}

	/// @dev A method to mock the upgrade call determining the amount of destination tokens received from an upgrade
	/// as well as the amount of source tokens that are left over as remainder
	/// @param sourceAmount The amount of source tokens that will be upgraded
	/// @return destinationAmount A uint256 representing the amount of destination tokens received if upgrade is called
	/// @return sourceRemainder A uint256 representing the amount of source tokens left over as remainder if upgrade is called
	function computeUpgrade(uint256 sourceAmount)
		public
		view
		returns (uint256 destinationAmount, uint256 sourceRemainder)
	{
		sourceRemainder = sourceAmount % (numeratorRatio / denominatorRatio);
		uint256 upgradeableAmount = sourceAmount - (sourceRemainder * RATIO_SCALE);
		destinationAmount = upgradeableAmount * (numeratorRatio / denominatorRatio);
	}

	/// @dev A method to mock the downgrade call determining the amount of source tokens received from a downgrade
	/// as well as the amount of destination tokens that are left over as remainder
	/// @param destinationAmount The amount of destination tokens that will be downgraded
	/// @return sourceAmount A uint256 representing the amount of source tokens received if downgrade is called
	/// @return destinationRemainder A uint256 representing the amount of destination tokens left over as remainder if upgrade is called
	function computeDowngrade(uint256 destinationAmount)
		public
		view
		returns (uint256 sourceAmount, uint256 destinationRemainder)
	{
		destinationRemainder = destinationAmount % (denominatorRatio / numeratorRatio);
		uint256 upgradeableAmount = destinationAmount - (destinationRemainder * RATIO_SCALE);
		sourceAmount = upgradeableAmount / (denominatorRatio / numeratorRatio);
	}

	/// @dev A method to conduct an upgrade from source token to destination token.
	/// The call will fail if upgrade status is not true, if approve has not been called
	/// on the source contract, or if sourceAmount is larger than the amount of source tokens at the msg.sender address.
	/// If the ratio would cause an amount of tokens to be destroyed by rounding/truncation, the upgrade call will
	/// only upgrade the nearest whole amount of source tokens returning the excess to the msg.sender address.
	/// Emits the Upgrade event
	/// @param _to The address the destination tokens will be sent to upon completion of the upgrade
	/// @param sourceAmount The amount of source tokens that will be upgraded
	function upgrade(address _to, uint256 sourceAmount) external {
		require(upgradeStatus == true, "SourceUpgrade: upgrade status is not active");
		(uint256 destinationAmount, uint256 sourceRemainder) = computeUpgrade(sourceAmount);
		sourceAmount -= sourceRemainder;
		require(sourceAmount > 0, "SourceUpgrade: disallow conversions of zero value");

		upgradedBalance[msg.sender] += sourceAmount;
		source.safeTransferFrom(
			msg.sender,
			address(this),
			sourceAmount
			);
		destination.safeTransfer(_to, destinationAmount);
		sourceUpgradedTotal += sourceAmount;
		emit Upgrade(msg.sender, _to, sourceAmount, destinationAmount);
	}

	/// @dev A method to conduct a downgrade from destination token to source token.
	/// The call will fail if downgrade status is not true, if approve has not been called
	/// on the destination contract, or if destinationAmount is larger than the amount of destination tokens at the msg.sender address.
	/// If the ratio would cause an amount of tokens to be destroyed by rounding/truncation, the downgrade call will only downgrade
	/// the nearest whole amount of destination tokens returning the excess to the msg.sender address.
	///  Emits the Downgrade event
	/// @param _to The address the source tokens will be sent to upon completion of the downgrade
	/// @param destinationAmount The amount of destination tokens that will be downgraded
	function downgrade(address _to, uint256 destinationAmount) external {
		require(upgradeStatus == true, "SourceUpgrade: upgrade status is not active");
		(uint256 sourceAmount, uint256 destinationRemainder) = computeDowngrade(destinationAmount);
		destinationAmount -= destinationRemainder;
		require(destinationAmount > 0, "SourceUpgrade: disallow conversions of zero value");
		require(upgradedBalance[msg.sender] >= sourceAmount,
			"SourceUpgrade: can not downgrade more than previously upgraded"
			);

		upgradedBalance[msg.sender] -= sourceAmount;
		destination.safeTransferFrom(
			msg.sender,
			address(this),
			destinationAmount
			);
		source.safeTransfer(_to, sourceAmount);
		sourceUpgradedTotal -= sourceAmount;
		emit Downgrade(msg.sender, _to, sourceAmount, destinationAmount);
	}
}