// File: /wsl-files/vcs/work/bridge/Blockchains-bridge-contracts/contracts/ECDSAOffsetRecovery.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.7.0;

contract ECDSAOffsetRecovery 
{
    function getHashPacked(address user, uint256 amountWithFee, bytes32 originalTxHash) public pure returns (bytes32)
    {
        return keccak256(abi.encodePacked(user, amountWithFee, originalTxHash));
    }

    function toEthSignedMessageHash(bytes32 hash) public pure returns (bytes32) {
        // 32 is the length in bytes of hash,
        // enforced by the type signature above
        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
    }

    function ecOffsetRecover(bytes32 hash, bytes memory signature, uint256 offset)
        public
        pure
        returns (address)
    {
        bytes32 r;
        bytes32 s;
        uint8 v;

        // Divide the signature in r, s and v variables with inline assembly.
        assembly {
            r := mload(add(signature, add(offset, 0x20)))
            s := mload(add(signature, add(offset, 0x40)))
            v := byte(0, mload(add(signature, add(offset, 0x60))))
        }

        // Version of signature should be 27 or 28, but 0 and 1 are also possible versions
        if (v < 27) {
            v += 27;
        }

        // If the version is correct return the signer address
        if (v != 27 && v != 28) {
            return (address(0));
        }

        // bytes memory prefix = "\x19Ethereum Signed Message:\n32";
        // hash = keccak256(abi.encodePacked(prefix, hash));
        // solium-disable-next-line arg-overflow
        return ecrecover(toEthSignedMessageHash(hash), v, r, s);
    }
}
// File: /wsl-files/vcs/work/bridge/Blockchains-bridge-contracts/contracts/swapContract.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.7.0;

import "openzeppelin-solidity/contracts/token/ERC20/IERC20.sol";
//import "openzeppelin-solidity/contracts/access/Ownable.sol";
import "openzeppelin-solidity/contracts/access/AccessControl.sol";
import "openzeppelin-solidity/contracts/math/SafeMath.sol";
import "openzeppelin-solidity/contracts/utils/Pausable.sol";
import "./ECDSAOffsetRecovery.sol";


/// @title Swap contract for multisignature bridge
contract swapContract is AccessControl, Pausable, ECDSAOffsetRecovery
{
    using SafeMath for uint256;

    bytes32 public constant OWNER_ROLE = keccak256("OWNER_ROLE");
    bytes32 public constant MANAGER_ROLE = keccak256("MANAGER_ROLE");
    bytes32 public constant RELAYER_ROLE = keccak256("RELAYER_ROLE");
    bytes32 public constant VALIDATOR_ROLE = keccak256("VALIDATOR_ROLE");

    IERC20 public tokenAddress;
    address public feeAddress;

    uint128 public numOfThisBlockchain;
    mapping(uint128 => bool) public existingOtherBlockchain;
    mapping(uint128 => uint128) public feeAmountOfBlockchain;

    uint256 public constant SIGNATURE_LENGTH = 65;
    mapping(bytes32 => bytes32) public processedTransactions;
    uint256 public minConfirmationSignatures;

    uint256 public minTokenAmount;
    uint256 public maxGasPrice;
    uint256 public minConfirmationBlocks;

    event TransferFromOtherBlockchain(address user, uint256 amount, uint256 amountWithoutFee, bytes32 originalTxHash);
    event TransferToOtherBlockchain(uint128 blockchain, address user, uint256 amount, string newAddress);
    

    /** 
      * @dev throws if transaction sender is not in owner role
      */
    modifier onlyOwner() {
        require(
            hasRole(OWNER_ROLE, _msgSender()),
            "Caller is not in owner role"
        );
        _;
    }

    /** 
      * @dev throws if transaction sender is not in owner or manager role
      */
    modifier onlyOwnerAndManager() {
        require(
            hasRole(OWNER_ROLE, _msgSender()) || hasRole(MANAGER_ROLE, _msgSender()),
            "Caller is not in owner or manager role"
        );
        _;
    }

    /** 
      * @dev throws if transaction sender is not in relayer role
      */
    modifier onlyRelayer() {
        require(
            hasRole(RELAYER_ROLE, _msgSender()),
            "swapContract: Caller is not in relayer role"
        );
        _;
    }

    /** 
      * @dev Constructor of contract
      * @param _tokenAddress address Address of token contract
      * @param _feeAddress Address to receive deducted fees
      * @param _numOfThisBlockchain Number of blockchain where contract is deployed
      * @param _numsOfOtherBlockchains List of blockchain number that is supported by bridge
      * @param _minConfirmationSignatures Number of required signatures for token swap
      * @param _minTokenAmount Minimal amount of tokens required for token swap
      * @param _maxGasPrice Maximum gas price on which relayer nodes will operate
      * @param _minConfirmationBlocks Minimal amount of blocks for confirmation on validator nodes
      */
    constructor(
        IERC20 _tokenAddress,
        address _feeAddress,
        uint128 _numOfThisBlockchain,
        uint128 [] memory _numsOfOtherBlockchains,
        uint128 _minConfirmationSignatures,
        uint256 _minTokenAmount,
        uint256 _maxGasPrice,
        uint256 _minConfirmationBlocks
    )
    {
        tokenAddress = _tokenAddress;
        feeAddress = _feeAddress;
        for (uint i = 0; i < _numsOfOtherBlockchains.length; i++ ) {
            require(
                _numsOfOtherBlockchains[i] != _numOfThisBlockchain,
                "swapContract: Number of this blockchain is in array of other blockchains"
            );
            existingOtherBlockchain[_numsOfOtherBlockchains[i]] = true;
        }

        require(_maxGasPrice > 0, "swapContract: Gas price cannot be zero");
        
        numOfThisBlockchain = _numOfThisBlockchain;
        minConfirmationSignatures = _minConfirmationSignatures;
        minTokenAmount = _minTokenAmount;
        maxGasPrice = _maxGasPrice;
        minConfirmationBlocks = _minConfirmationBlocks;
        _setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
        _setupRole(OWNER_ROLE, _msgSender());
    }

    /** 
      * @dev Returns true if blockchain of passed id is registered to swap
      * @param blockchain number of blockchain
      */
    function getOtherBlockchainAvailableByNum(uint128 blockchain) external view returns (bool)
    {
        return existingOtherBlockchain[blockchain];
    }

    /** 
      * @dev Transfers tokens from sender to the contract. 
      * User calls this function when he wants to transfer tokens to another blockchain.
      * @param blockchain Number of blockchain
      * @param amount Amount of tokens
      * @param newAddress Address in the blockchain to which the user wants to transfer
      */
    function transferToOtherBlockchain(uint128 blockchain, uint256 amount, string memory newAddress) external whenNotPaused
    {
        require(
            bytes(newAddress).length > 0,
            "swapContract: No destination address provided"
        );
        require(
            existingOtherBlockchain[blockchain] && blockchain != numOfThisBlockchain,
            "swapContract: Wrong choose of blockchain"
        );
        require(
            amount >= feeAmountOfBlockchain[blockchain],
            "swapContract: Not enough amount of tokens"
        );
        address sender = _msgSender();
        require(
            tokenAddress.balanceOf(sender) >= amount,
            "swapContract: Not enough balance"
        );
        tokenAddress.transferFrom(sender, address(this), amount);
        emit TransferToOtherBlockchain(blockchain, sender, amount, newAddress);
    }
    /** 
      * @dev Transfers tokens to end user in current blockchain 
      * @param user User address
      * @param amountWithFee Amount of tokens with included fees
      * @param originalTxHash Hash of transaction from other network, on which swap was called
      * @param concatSignatures Concatenated string of signature bytes for verification of transaction
      */
    function transferToUserWithFee(
        address user,
        uint256 amountWithFee,
        bytes32 originalTxHash,
        bytes memory concatSignatures
    ) 
        external
        onlyRelayer
        whenNotPaused
    {
        require(
            amountWithFee >= minTokenAmount,
            "swapContract: Amount must be greater than minimum"
        );
        require(
            user != address(0),
            "swapContract: Address cannot be zero address"   // TODO: check this requirement
        );
        require(
            concatSignatures.length.mod(SIGNATURE_LENGTH) == 0,
            "swapContract: Signatures lengths must be divisible by 65"
        );
        require(
            concatSignatures.length.div(SIGNATURE_LENGTH) >= minConfirmationSignatures,
            "swapContract: Not enough signatures passed"
        );

        bytes32 hashedParams = getHashPacked(user, amountWithFee, originalTxHash);
        (bool processed, bytes32 savedHash) = isProcessedTransaction(originalTxHash);
        require(!processed && savedHash != hashedParams, "swapContract: Transaction already processed");
        
        uint256 signaturesCount = concatSignatures.length.div(uint256(SIGNATURE_LENGTH));
        address[] memory validatorAddresses = new address[](signaturesCount);
        for (uint256 i = 0; i < signaturesCount; i++) {
            address validatorAddress = ecOffsetRecover(hashedParams, concatSignatures, i * SIGNATURE_LENGTH);
            require(isValidator(validatorAddress), "swapContract: Validator address not in whitelist");
            for (uint256 j = 0; j < i; j++) {
                require(validatorAddress != validatorAddresses[j], "swapContract: Validator address is duplicated");
            }
            validatorAddresses[i] = validatorAddress;
        }
        processedTransactions[originalTxHash] = hashedParams;

        uint256 fee = feeAmountOfBlockchain[numOfThisBlockchain];
        uint256 amountWithoutFee = amountWithFee.sub(fee);
        tokenAddress.transfer(user, amountWithoutFee);
        tokenAddress.transfer(feeAddress, fee);
        emit TransferFromOtherBlockchain(user, amountWithFee, amountWithoutFee, originalTxHash);
    }

    // OTHER BLOCKCHAIN MANAGEMENT
    /** 
      * @dev Registers another blockchain for availability to swap
      * @param numOfOtherBlockchain number of blockchain
      */
    function addOtherBlockchain(
        uint128 numOfOtherBlockchain
    )
        external
        onlyOwner
    {
        require(
            numOfOtherBlockchain != numOfThisBlockchain,
            "swapContract: Cannot add this blockchain to array of other blockchains"
        );
        require(
            !existingOtherBlockchain[numOfOtherBlockchain],
            "swapContract: This blockchain is already added"
        );
        existingOtherBlockchain[numOfOtherBlockchain] = true;
    }

    /** 
      * @dev Unregisters another blockchain for availability to swap
      * @param numOfOtherBlockchain number of blockchain
      */
    function removeOtherBlockchain(
        uint128 numOfOtherBlockchain
    )
        external
        onlyOwner
    {
        require(
            existingOtherBlockchain[numOfOtherBlockchain],
            "swapContract: This blockchain was not added"
        );
        existingOtherBlockchain[numOfOtherBlockchain] = false;
    }

    /** 
      * @dev Change existing blockchain id
      * @param oldNumOfOtherBlockchain number of existing blockchain
      * @param newNumOfOtherBlockchain number of new blockchain
      */
    function changeOtherBlockchain(
        uint128 oldNumOfOtherBlockchain,
        uint128 newNumOfOtherBlockchain
    )
        external
        onlyOwner
    {
        require(
            oldNumOfOtherBlockchain != newNumOfOtherBlockchain,
            "swapContract: Cannot change blockchains with same number"
        );
        require(
            newNumOfOtherBlockchain != numOfThisBlockchain,
            "swapContract: Cannot add this blockchain to array of other blockchains"
        );
        require(
            existingOtherBlockchain[oldNumOfOtherBlockchain],
            "swapContract: This blockchain was not added"
        );
        require(
            !existingOtherBlockchain[newNumOfOtherBlockchain],
            "swapContract: This blockchain is already added"
        );
        
        existingOtherBlockchain[oldNumOfOtherBlockchain] = false;
        existingOtherBlockchain[newNumOfOtherBlockchain] = true;
    }


    // FEE MANAGEMENT

    /** 
      * @dev Changes address which receives fees from transfers
      * @param newFeeAddress New address for fees
      */
    function changeFeeAddress(address newFeeAddress) external onlyOwnerAndManager
    {
        feeAddress = newFeeAddress;
    }

    /** 
      * @dev Changes fee values for blockchains in feeAmountOfBlockchain variables
      * @param blockchainNum Existing number of blockchain
      * @param feeAmount Fee amount to substruct from transfer amount
      */
    function setFeeAmountOfBlockchain(uint128 blockchainNum, uint128 feeAmount) external onlyOwnerAndManager
    {
        feeAmountOfBlockchain[blockchainNum] = feeAmount;
    }

    // VALIDATOR CONFIRMATIONS MANAGEMENT

    /** 
      * @dev Changes requirement for minimal amount of signatures to validate on transfer
      * @param _minConfirmationSignatures Number of signatures to verify
      */
    function setMinConfirmationSignatures(uint256 _minConfirmationSignatures) external onlyOwner {
        require(_minConfirmationSignatures > 0, "swapContract: At least 1 confirmation can be set");
        minConfirmationSignatures = _minConfirmationSignatures;
    }

    /** 
      * @dev Changes requirement for minimal token amount on transfers
      * @param _minTokenAmount Amount of tokens
      */
    function setMinTokenAmount(uint256 _minTokenAmount) external onlyOwnerAndManager {
        minTokenAmount = _minTokenAmount;
    }

    /** 
      * @dev Changes parameter of maximum gas price on which relayer nodes will operate
      * @param _maxGasPrice Price of gas in wei
      */
    function setMaxGasPrice(uint256 _maxGasPrice) external onlyOwnerAndManager {
        require(_maxGasPrice > 0, "swapContract: Gas price cannot be zero");
        maxGasPrice = _maxGasPrice;
    }

    /** 
      * @dev Changes requirement for minimal amount of block to consider tx confirmed on validator
      * @param _minConfirmationBlocks Amount of blocks
      */

    function setMinConfirmationBlocks(uint256 _minConfirmationBlocks) external onlyOwnerAndManager {
        minConfirmationBlocks = _minConfirmationBlocks;
    }

    /** 
      * @dev Transfers permissions of contract ownership. 
      * Will setup new owner and one manager on contract.
      * Main purpose of this function is to transfer ownership from deployer account ot real owner
      * @param newOwner Address of new owner
      * @param newManager Address of new manager
      */
    function transferOwnerAndSetManager(address newOwner, address newManager) external onlyOwner {
        require(newOwner != _msgSender(), "swapContract: New owner must be different than current");
        require(newOwner != address(0x0), "swapContract: Owner cannot be zero address");
        require(newManager != address(0x0), "swapContract: Owner cannot be zero address");
        _setupRole(DEFAULT_ADMIN_ROLE, newOwner);
        _setupRole(OWNER_ROLE, newOwner);
        _setupRole(MANAGER_ROLE, newManager);
        renounceRole(OWNER_ROLE, _msgSender());
        renounceRole(DEFAULT_ADMIN_ROLE, _msgSender());
    }

    /** 
      * @dev Pauses transfers of tokens on contract
      */
    function pauseExecution() external onlyOwner {
        _pause();
    }

    /** 
      * @dev Resumes transfers of tokens on contract
      */
    function continueExecution() external onlyOwner {
        _unpause();
    }

    /** 
      * @dev Function to check if address is belongs to owner role
      * @param account Address to check
      */
    function isOwner(address account) public view returns (bool) {
        return hasRole(OWNER_ROLE, account);
    }

    /** 
      * @dev Function to check if address is belongs to manager role
      * @param account Address to check
      */
    function isManager(address account) public view returns (bool) {
        return hasRole(MANAGER_ROLE, account);
    }

    /** 
      * @dev Function to check if address is belongs to relayer role
      * @param account Address to check
      */
    function isRelayer(address account) public view returns (bool) {
        return hasRole(RELAYER_ROLE, account);
    }

    /** 
      * @dev Function to check if address is belongs to validator role
      * @param account Address to check
      * 
      */
    function isValidator(address account) public view returns (bool) {
        return hasRole(VALIDATOR_ROLE, account);
    }

    /** 
      * @dev Function to check if transfer of tokens on previous
      * transaction from other blockchain was executed
      * @param originalTxHash Transaction hash to check
      */
    function isProcessedTransaction(bytes32 originalTxHash) public view returns (bool processed, bytes32 hashedParams) {
        hashedParams = processedTransactions[originalTxHash];
        processed = hashedParams != bytes32(0);
    }
}
// File: openzeppelin-solidity/contracts/GSN/Context.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <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 GSN 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 payable) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

// File: openzeppelin-solidity/contracts/access/AccessControl.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "../utils/EnumerableSet.sol";
import "../utils/Address.sol";
import "../GSN/Context.sol";

/**
 * @dev Contract module that allows children to implement role-based access
 * control mechanisms.
 *
 * Roles are referred to by their `bytes32` identifier. These should be exposed
 * in the external API and be unique. The best way to achieve this is by
 * using `public constant` hash digests:
 *
 * ```
 * bytes32 public constant MY_ROLE = keccak256("MY_ROLE");
 * ```
 *
 * Roles can be used to represent a set of permissions. To restrict access to a
 * function call, use {hasRole}:
 *
 * ```
 * function foo() public {
 *     require(hasRole(MY_ROLE, msg.sender));
 *     ...
 * }
 * ```
 *
 * Roles can be granted and revoked dynamically via the {grantRole} and
 * {revokeRole} functions. Each role has an associated admin role, and only
 * accounts that have a role's admin role can call {grantRole} and {revokeRole}.
 *
 * By default, the admin role for all roles is `DEFAULT_ADMIN_ROLE`, which means
 * that only accounts with this role will be able to grant or revoke other
 * roles. More complex role relationships can be created by using
 * {_setRoleAdmin}.
 *
 * WARNING: The `DEFAULT_ADMIN_ROLE` is also its own admin: it has permission to
 * grant and revoke this role. Extra precautions should be taken to secure
 * accounts that have been granted it.
 */
abstract contract AccessControl is Context {
    using EnumerableSet for EnumerableSet.AddressSet;
    using Address for address;

    struct RoleData {
        EnumerableSet.AddressSet members;
        bytes32 adminRole;
    }

    mapping (bytes32 => RoleData) private _roles;

    bytes32 public constant DEFAULT_ADMIN_ROLE = 0x00;

    /**
     * @dev Emitted when `newAdminRole` is set as ``role``'s admin role, replacing `previousAdminRole`
     *
     * `DEFAULT_ADMIN_ROLE` is the starting admin for all roles, despite
     * {RoleAdminChanged} not being emitted signaling this.
     *
     * _Available since v3.1._
     */
    event RoleAdminChanged(bytes32 indexed role, bytes32 indexed previousAdminRole, bytes32 indexed newAdminRole);

    /**
     * @dev Emitted when `account` is granted `role`.
     *
     * `sender` is the account that originated the contract call, an admin role
     * bearer except when using {_setupRole}.
     */
    event RoleGranted(bytes32 indexed role, address indexed account, address indexed sender);

    /**
     * @dev Emitted when `account` is revoked `role`.
     *
     * `sender` is the account that originated the contract call:
     *   - if using `revokeRole`, it is the admin role bearer
     *   - if using `renounceRole`, it is the role bearer (i.e. `account`)
     */
    event RoleRevoked(bytes32 indexed role, address indexed account, address indexed sender);

    /**
     * @dev Returns `true` if `account` has been granted `role`.
     */
    function hasRole(bytes32 role, address account) public view returns (bool) {
        return _roles[role].members.contains(account);
    }

    /**
     * @dev Returns the number of accounts that have `role`. Can be used
     * together with {getRoleMember} to enumerate all bearers of a role.
     */
    function getRoleMemberCount(bytes32 role) public view returns (uint256) {
        return _roles[role].members.length();
    }

    /**
     * @dev Returns one of the accounts that have `role`. `index` must be a
     * value between 0 and {getRoleMemberCount}, non-inclusive.
     *
     * Role bearers are not sorted in any particular way, and their ordering may
     * change at any point.
     *
     * WARNING: When using {getRoleMember} and {getRoleMemberCount}, make sure
     * you perform all queries on the same block. See the following
     * https://forum.openzeppelin.com/t/iterating-over-elements-on-enumerableset-in-openzeppelin-contracts/2296[forum post]
     * for more information.
     */
    function getRoleMember(bytes32 role, uint256 index) public view returns (address) {
        return _roles[role].members.at(index);
    }

    /**
     * @dev Returns the admin role that controls `role`. See {grantRole} and
     * {revokeRole}.
     *
     * To change a role's admin, use {_setRoleAdmin}.
     */
    function getRoleAdmin(bytes32 role) public view returns (bytes32) {
        return _roles[role].adminRole;
    }

    /**
     * @dev Grants `role` to `account`.
     *
     * If `account` had not been already granted `role`, emits a {RoleGranted}
     * event.
     *
     * Requirements:
     *
     * - the caller must have ``role``'s admin role.
     */
    function grantRole(bytes32 role, address account) public virtual {
        require(hasRole(_roles[role].adminRole, _msgSender()), "AccessControl: sender must be an admin to grant");

        _grantRole(role, account);
    }

    /**
     * @dev Revokes `role` from `account`.
     *
     * If `account` had been granted `role`, emits a {RoleRevoked} event.
     *
     * Requirements:
     *
     * - the caller must have ``role``'s admin role.
     */
    function revokeRole(bytes32 role, address account) public virtual {
        require(hasRole(_roles[role].adminRole, _msgSender()), "AccessControl: sender must be an admin to revoke");

        _revokeRole(role, account);
    }

    /**
     * @dev Revokes `role` from the calling account.
     *
     * Roles are often managed via {grantRole} and {revokeRole}: this function's
     * purpose is to provide a mechanism for accounts to lose their privileges
     * if they are compromised (such as when a trusted device is misplaced).
     *
     * If the calling account had been granted `role`, emits a {RoleRevoked}
     * event.
     *
     * Requirements:
     *
     * - the caller must be `account`.
     */
    function renounceRole(bytes32 role, address account) public virtual {
        require(account == _msgSender(), "AccessControl: can only renounce roles for self");

        _revokeRole(role, account);
    }

    /**
     * @dev Grants `role` to `account`.
     *
     * If `account` had not been already granted `role`, emits a {RoleGranted}
     * event. Note that unlike {grantRole}, this function doesn't perform any
     * checks on the calling account.
     *
     * [WARNING]
     * ====
     * This function should only be called from the constructor when setting
     * up the initial roles for the system.
     *
     * Using this function in any other way is effectively circumventing the admin
     * system imposed by {AccessControl}.
     * ====
     */
    function _setupRole(bytes32 role, address account) internal virtual {
        _grantRole(role, account);
    }

    /**
     * @dev Sets `adminRole` as ``role``'s admin role.
     *
     * Emits a {RoleAdminChanged} event.
     */
    function _setRoleAdmin(bytes32 role, bytes32 adminRole) internal virtual {
        emit RoleAdminChanged(role, _roles[role].adminRole, adminRole);
        _roles[role].adminRole = adminRole;
    }

    function _grantRole(bytes32 role, address account) private {
        if (_roles[role].members.add(account)) {
            emit RoleGranted(role, account, _msgSender());
        }
    }

    function _revokeRole(bytes32 role, address account) private {
        if (_roles[role].members.remove(account)) {
            emit RoleRevoked(role, account, _msgSender());
        }
    }
}

// File: openzeppelin-solidity/contracts/math/SafeMath.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

// File: openzeppelin-solidity/contracts/token/ERC20/IERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @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 `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, 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 `sender` to `recipient` 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 sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @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);
}

// File: openzeppelin-solidity/contracts/utils/Address.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.2 <0.8.0;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
        (bool success, ) = recipient.call{ value: amount }("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain`call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        require(isContract(target), "Address: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.call{ value: value }(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data, string memory errorMessage) internal view returns (bytes memory) {
        require(isContract(target), "Address: static call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.staticcall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    function _verifyCallResult(bool success, bytes memory returndata, string memory errorMessage) private pure returns(bytes memory) {
        if (success) {
            return returndata;
        } else {
            // Look for revert reason and bubble it up if present
            if (returndata.length > 0) {
                // The easiest way to bubble the revert reason is using memory via assembly

                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

// File: openzeppelin-solidity/contracts/utils/EnumerableSet.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Library for managing
 * https://en.wikipedia.org/wiki/Set_(abstract_data_type)[sets] of primitive
 * types.
 *
 * Sets have the following properties:
 *
 * - Elements are added, removed, and checked for existence in constant time
 * (O(1)).
 * - Elements are enumerated in O(n). No guarantees are made on the ordering.
 *
 * ```
 * contract Example {
 *     // Add the library methods
 *     using EnumerableSet for EnumerableSet.AddressSet;
 *
 *     // Declare a set state variable
 *     EnumerableSet.AddressSet private mySet;
 * }
 * ```
 *
 * As of v3.3.0, sets of type `bytes32` (`Bytes32Set`), `address` (`AddressSet`)
 * and `uint256` (`UintSet`) are supported.
 */
library EnumerableSet {
    // To implement this library for multiple types with as little code
    // repetition as possible, we write it in terms of a generic Set type with
    // bytes32 values.
    // The Set implementation uses private functions, and user-facing
    // implementations (such as AddressSet) are just wrappers around the
    // underlying Set.
    // This means that we can only create new EnumerableSets for types that fit
    // in bytes32.

    struct Set {
        // Storage of set values
        bytes32[] _values;

        // Position of the value in the `values` array, plus 1 because index 0
        // means a value is not in the set.
        mapping (bytes32 => uint256) _indexes;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function _add(Set storage set, bytes32 value) private returns (bool) {
        if (!_contains(set, value)) {
            set._values.push(value);
            // The value is stored at length-1, but we add 1 to all indexes
            // and use 0 as a sentinel value
            set._indexes[value] = set._values.length;
            return true;
        } else {
            return false;
        }
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function _remove(Set storage set, bytes32 value) private returns (bool) {
        // We read and store the value's index to prevent multiple reads from the same storage slot
        uint256 valueIndex = set._indexes[value];

        if (valueIndex != 0) { // Equivalent to contains(set, value)
            // To delete an element from the _values array in O(1), we swap the element to delete with the last one in
            // the array, and then remove the last element (sometimes called as 'swap and pop').
            // This modifies the order of the array, as noted in {at}.

            uint256 toDeleteIndex = valueIndex - 1;
            uint256 lastIndex = set._values.length - 1;

            // When the value to delete is the last one, the swap operation is unnecessary. However, since this occurs
            // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.

            bytes32 lastvalue = set._values[lastIndex];

            // Move the last value to the index where the value to delete is
            set._values[toDeleteIndex] = lastvalue;
            // Update the index for the moved value
            set._indexes[lastvalue] = toDeleteIndex + 1; // All indexes are 1-based

            // Delete the slot where the moved value was stored
            set._values.pop();

            // Delete the index for the deleted slot
            delete set._indexes[value];

            return true;
        } else {
            return false;
        }
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function _contains(Set storage set, bytes32 value) private view returns (bool) {
        return set._indexes[value] != 0;
    }

    /**
     * @dev Returns the number of values on the set. O(1).
     */
    function _length(Set storage set) private view returns (uint256) {
        return set._values.length;
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function _at(Set storage set, uint256 index) private view returns (bytes32) {
        require(set._values.length > index, "EnumerableSet: index out of bounds");
        return set._values[index];
    }

    // Bytes32Set

    struct Bytes32Set {
        Set _inner;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(Bytes32Set storage set, bytes32 value) internal returns (bool) {
        return _add(set._inner, value);
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(Bytes32Set storage set, bytes32 value) internal returns (bool) {
        return _remove(set._inner, value);
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(Bytes32Set storage set, bytes32 value) internal view returns (bool) {
        return _contains(set._inner, value);
    }

    /**
     * @dev Returns the number of values in the set. O(1).
     */
    function length(Bytes32Set storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(Bytes32Set storage set, uint256 index) internal view returns (bytes32) {
        return _at(set._inner, index);
    }

    // AddressSet

    struct AddressSet {
        Set _inner;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(AddressSet storage set, address value) internal returns (bool) {
        return _add(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(AddressSet storage set, address value) internal returns (bool) {
        return _remove(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(AddressSet storage set, address value) internal view returns (bool) {
        return _contains(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Returns the number of values in the set. O(1).
     */
    function length(AddressSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(AddressSet storage set, uint256 index) internal view returns (address) {
        return address(uint256(_at(set._inner, index)));
    }


    // UintSet

    struct UintSet {
        Set _inner;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(UintSet storage set, uint256 value) internal returns (bool) {
        return _add(set._inner, bytes32(value));
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(UintSet storage set, uint256 value) internal returns (bool) {
        return _remove(set._inner, bytes32(value));
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(UintSet storage set, uint256 value) internal view returns (bool) {
        return _contains(set._inner, bytes32(value));
    }

    /**
     * @dev Returns the number of values on the set. O(1).
     */
    function length(UintSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(UintSet storage set, uint256 index) internal view returns (uint256) {
        return uint256(_at(set._inner, index));
    }
}

// File: openzeppelin-solidity/contracts/utils/Pausable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "../GSN/Context.sol";

/**
 * @dev Contract module which allows children to implement an emergency stop
 * mechanism that can be triggered by an authorized account.
 *
 * This module is used through inheritance. It will make available the
 * modifiers `whenNotPaused` and `whenPaused`, which can be applied to
 * the functions of your contract. Note that they will not be pausable by
 * simply including this module, only once the modifiers are put in place.
 */
abstract contract Pausable is Context {
    /**
     * @dev Emitted when the pause is triggered by `account`.
     */
    event Paused(address account);

    /**
     * @dev Emitted when the pause is lifted by `account`.
     */
    event Unpaused(address account);

    bool private _paused;

    /**
     * @dev Initializes the contract in unpaused state.
     */
    constructor () internal {
        _paused = false;
    }

    /**
     * @dev Returns true if the contract is paused, and false otherwise.
     */
    function paused() public view returns (bool) {
        return _paused;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    modifier whenNotPaused() {
        require(!_paused, "Pausable: paused");
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    modifier whenPaused() {
        require(_paused, "Pausable: not paused");
        _;
    }

    /**
     * @dev Triggers stopped state.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    function _pause() internal virtual whenNotPaused {
        _paused = true;
        emit Paused(_msgSender());
    }

    /**
     * @dev Returns to normal state.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    function _unpause() internal virtual whenPaused {
        _paused = false;
        emit Unpaused(_msgSender());
    }
}