// File: @openzeppelin/contracts/access/Ownable.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (access/Ownable.sol)

pragma solidity ^0.8.0;

import "../utils/Context.sol";

/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
abstract contract Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor() {
        _transferOwnership(_msgSender());
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view virtual returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(owner() == _msgSender(), "Ownable: caller is not the owner");
        _;
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions anymore. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby removing any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        _transferOwnership(address(0));
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Can only be called by the current owner.
     */
    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        _transferOwnership(newOwner);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Internal function without access restriction.
     */
    function _transferOwnership(address newOwner) internal virtual {
        address oldOwner = _owner;
        _owner = newOwner;
        emit OwnershipTransferred(oldOwner, newOwner);
    }
}

// File: @openzeppelin/contracts/security/ReentrancyGuard.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (security/ReentrancyGuard.sol)

pragma solidity ^0.8.0;

/**
 * @dev Contract module that helps prevent reentrant calls to a function.
 *
 * Inheriting from `ReentrancyGuard` will make the {nonReentrant} modifier
 * available, which can be applied to functions to make sure there are no nested
 * (reentrant) calls to them.
 *
 * Note that because there is a single `nonReentrant` guard, functions marked as
 * `nonReentrant` may not call one another. This can be worked around by making
 * those functions `private`, and then adding `external` `nonReentrant` entry
 * points to them.
 *
 * TIP: If you would like to learn more about reentrancy and alternative ways
 * to protect against it, check out our blog post
 * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
 */
abstract contract ReentrancyGuard {
    // Booleans are more expensive than uint256 or any type that takes up a full
    // word because each write operation emits an extra SLOAD to first read the
    // slot's contents, replace the bits taken up by the boolean, and then write
    // back. This is the compiler's defense against contract upgrades and
    // pointer aliasing, and it cannot be disabled.

    // The values being non-zero value makes deployment a bit more expensive,
    // but in exchange the refund on every call to nonReentrant will be lower in
    // amount. Since refunds are capped to a percentage of the total
    // transaction's gas, it is best to keep them low in cases like this one, to
    // increase the likelihood of the full refund coming into effect.
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    constructor() {
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and making it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        // On the first call to nonReentrant, _notEntered will be true
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }
}

// File: @openzeppelin/contracts/token/ERC20/IERC20.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.6.0) (token/ERC20/IERC20.sol)

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);

    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `to`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address to, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `from` to `to` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(
        address from,
        address to,
        uint256 amount
    ) external returns (bool);
}

// File: @openzeppelin/contracts/utils/Context.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/Context.sol)

pragma solidity ^0.8.0;

/**
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes calldata) {
        return msg.data;
    }
}

// File: contracts/BaseBurner.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "./interfaces/IBurner.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

abstract contract BaseBurner is IBurner, Ownable {
    /**
     * @notice The receiver address that will receive the targetToken after burn function is run
     */
    address public receiver;

    /**
     * @notice Burnable tokens mapped to targetTokens
     */
    mapping(address => address) burnableTokens;

    /**
     * @notice Emitted when the receiver is set
     */
    event receiverSet(address oldReceiver, address newReceiver);
    /**
     * @notice Emitted when a token's state in whitelistedToken mapping is set
     */
    event addedBurnableToken(address burnableToken, address targetToken);

    /**
     * @notice Emitted when token is withdrawn from this contract
     */
    event tokenWithdrawn(address token, address to, uint256 amount);

    modifier onlyBurnableToken(address token) {
        require(
            burnableTokens[token] != address(0),
            "token is not whitelisted, please call addBurnableTokens"
        );
        _;
    }

    constructor(address _receiver) {
        receiver = _receiver;
        emit receiverSet(address(0), receiver);
    }

    /* Admin functions */

    /*
     * @notice withdraw tokens from this address to `to` address
     * @param token The token to be withdrawn
     * @param to The receiver of this token withdrawal
     */
    function withdraw(address token, address to) external onlyOwner {
        uint256 balance = IERC20(token).balanceOf(address(this));
        IERC20(token).transfer(to, balance);
        emit tokenWithdrawn(token, to, balance);
    }

    /*
     * @notice set the receiver of targetToken for this contract
     * @param _receiver The receiver address
     */
    function setReceiver(address _receiver) external onlyOwner {
        address oldReceiver = receiver;
        receiver = _receiver;
        emit receiverSet(oldReceiver, _receiver);
    }

    /*
     * @notice set the burnableTokens of this contract, burnableTokens will be burned for the mapping result
     * @notice set the mapping result as address(0) to unset a token as burnable
     * @param burnableTokens An array of token addresses that are allowed to be burned by this contract
     * @param targetTokens An array of token addresses that are the resultant token received after burning the burnableToken
     */
    function addBurnableTokens(
        address[] calldata _burnableTokens,
        address[] calldata _targetTokens
    ) external virtual {
        require(
            _burnableTokens.length == _targetTokens.length,
            "array length mismatch"
        );
        for (uint256 i = 0; i < _burnableTokens.length; i++) {
            burnableTokens[_burnableTokens[i]] = _targetTokens[i];
            emit addedBurnableToken(_burnableTokens[i], _targetTokens[i]);
        }
    }
}

// File: contracts/SynthetixBurner.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "./BaseBurner.sol";
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";

interface ISNX {
    function exchange(
        bytes32 sourceCurrencyKey,
        uint256 sourceAmount,
        bytes32 destinationCurrencyKey
    ) external returns (uint256 amountReceived);

    function settle(bytes32 currencyKey) external returns (uint256[3] calldata);
}

interface ISynth {
    function currencyKey() external returns (bytes32);

    function transferAndSettle(address to, uint256 amount)
        external
        returns (bool);
}

contract SynthetixBurner is BaseBurner, ReentrancyGuard {
    address public SNX = 0xC011a73ee8576Fb46F5E1c5751cA3B9Fe0af2a6F;

    constructor(address _receiver, address _snx) BaseBurner(_receiver) {
        SNX = _snx;
    }

    /* User functions */
    function burn(address token)
        external
        onlyBurnableToken(token)
        nonReentrant
        returns (uint256)
    {
        require(receiver != address(0), "receiver not set");
        address targetToken = burnableTokens[token];
        uint256 initialTargetTokenBalance = IERC20(targetToken).balanceOf(
            address(this)
        );
        if (initialTargetTokenBalance > 0) {
            // Synthetix imposes a waiting period on any action of exchange(10 min)
            // https://blog.synthetix.io/how-fee-reclamation-rebates-work/
            ISynth(targetToken).transferAndSettle(
                receiver,
                initialTargetTokenBalance
            );
        }
        bytes32 burnableTokenCurrencyKey = ISynth(token).currencyKey();
        uint256 msgSenderBalance = IERC20(token).balanceOf(msg.sender);
        if (msgSenderBalance != 0) {
            IERC20(token).transferFrom(
                msg.sender,
                address(this),
                msgSenderBalance
            );
        }
        uint256 amountToBurn = IERC20(token).balanceOf(address(this));
        if (amountToBurn != 0) {
            // Due to Synthetix's waiting period, Do nothing after exchanging for target token;
            ISNX(SNX).exchange(
                burnableTokenCurrencyKey,
                amountToBurn,
                ISynth(targetToken).currencyKey()
            );
        }
        return msgSenderBalance;
    }
}

// File: contracts/interfaces/IBurner.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IBurner {
    function burn(address token) external returns (uint256);

    function withdraw(address token, address to) external;

    function setReceiver(address receiver) external;

    function addBurnableTokens(
        address[] calldata burnableTokens,
        address[] calldata targetTokens
    ) external;
}