interface IRelayHub {
    // Relay management

    /**
     * @dev Adds stake to a relay and sets its `unstakeDelay`. If the relay does not exist, it is created, and the caller
     * of this function becomes its owner. If the relay already exists, only the owner can call this function. A relay
     * cannot be its own owner.
     *
     * All Ether in this function call will be added to the relay's stake.
     * Its unstake delay will be assigned to `unstakeDelay`, but the new value must be greater or equal to the current one.
     *
     * Emits a {Staked} event.
     */
    function stake(address relayaddr, uint256 unstakeDelay) external payable;

    /**
     * @dev Emitted when a relay's stake or unstakeDelay are increased
     */
    event Staked(address indexed relay, uint256 stake, uint256 unstakeDelay);

    /**
     * @dev Registers the caller as a relay.
     * The relay must be staked for, and not be a contract (i.e. this function must be called directly from an EOA).
     *
     * This function can be called multiple times, emitting new {RelayAdded} events. Note that the received
     * `transactionFee` is not enforced by {relayCall}.
     *
     * Emits a {RelayAdded} event.
     */
    function registerRelay(uint256 transactionFee, string calldata url) external;

    /**
     * @dev Emitted when a relay is registered or re-registerd. Looking at these events (and filtering out
     * {RelayRemoved} events) lets a client discover the list of available relays.
     */
    event RelayAdded(address indexed relay, address indexed owner, uint256 transactionFee, uint256 stake, uint256 unstakeDelay, string url);

    /**
     * @dev Removes (deregisters) a relay. Unregistered (but staked for) relays can also be removed.
     *
     * Can only be called by the owner of the relay. After the relay's `unstakeDelay` has elapsed, {unstake} will be
     * callable.
     *
     * Emits a {RelayRemoved} event.
     */
    function removeRelayByOwner(address relay) external;

    /**
     * @dev Emitted when a relay is removed (deregistered). `unstakeTime` is the time when unstake will be callable.
     */
    event RelayRemoved(address indexed relay, uint256 unstakeTime);

    /** Deletes the relay from the system, and gives back its stake to the owner.
     *
     * Can only be called by the relay owner, after `unstakeDelay` has elapsed since {removeRelayByOwner} was called.
     *
     * Emits an {Unstaked} event.
     */
    function unstake(address relay) external;

    /**
     * @dev Emitted when a relay is unstaked for, including the returned stake.
     */
    event Unstaked(address indexed relay, uint256 stake);

    // States a relay can be in
    enum RelayState {
        Unknown, // The relay is unknown to the system: it has never been staked for
        Staked, // The relay has been staked for, but it is not yet active
        Registered, // The relay has registered itself, and is active (can relay calls)
        Removed    // The relay has been removed by its owner and can no longer relay calls. It must wait for its unstakeDelay to elapse before it can unstake
    }

    /**
     * @dev Returns a relay's status. Note that relays can be deleted when unstaked or penalized, causing this function
     * to return an empty entry.
     */
    function getRelay(address relay) external view returns (uint256 totalStake, uint256 unstakeDelay, uint256 unstakeTime, address payable owner, RelayState state);

    // Balance management

    /**
     * @dev Deposits Ether for a contract, so that it can receive (and pay for) relayed transactions.
     *
     * Unused balance can only be withdrawn by the contract itself, by calling {withdraw}.
     *
     * Emits a {Deposited} event.
     */
    function depositFor(address target) external payable;

    /**
     * @dev Emitted when {depositFor} is called, including the amount and account that was funded.
     */
    event Deposited(address indexed recipient, address indexed from, uint256 amount);

    /**
     * @dev Returns an account's deposits. These can be either a contracts's funds, or a relay owner's revenue.
     */
    function balanceOf(address target) external view returns (uint256);

    /**
     * Withdraws from an account's balance, sending it back to it. Relay owners call this to retrieve their revenue, and
     * contracts can use it to reduce their funding.
     *
     * Emits a {Withdrawn} event.
     */
    function withdraw(uint256 amount, address payable dest) external;

    /**
     * @dev Emitted when an account withdraws funds from `RelayHub`.
     */
    event Withdrawn(address indexed account, address indexed dest, uint256 amount);

    // Relaying

    /**
     * @dev Checks if the `RelayHub` will accept a relayed operation.
     * Multiple things must be true for this to happen:
     *  - all arguments must be signed for by the sender (`from`)
     *  - the sender's nonce must be the current one
     *  - the recipient must accept this transaction (via {acceptRelayedCall})
     *
     * Returns a `PreconditionCheck` value (`OK` when the transaction can be relayed), or a recipient-specific error
     * code if it returns one in {acceptRelayedCall}.
     */
    function canRelay(
        address relay,
        address from,
        address to,
        bytes calldata encodedFunction,
        uint256 transactionFee,
        uint256 gasPrice,
        uint256 gasLimit,
        uint256 nonce,
        bytes calldata signature,
        bytes calldata approvalData
    ) external view returns (uint256 status, bytes memory recipientContext);

    // Preconditions for relaying, checked by canRelay and returned as the corresponding numeric values.
    enum PreconditionCheck {
        OK,                         // All checks passed, the call can be relayed
        WrongSignature,             // The transaction to relay is not signed by requested sender
        WrongNonce,                 // The provided nonce has already been used by the sender
        AcceptRelayedCallReverted,  // The recipient rejected this call via acceptRelayedCall
        InvalidRecipientStatusCode  // The recipient returned an invalid (reserved) status code
    }

    /**
     * @dev Relays a transaction.
     *
     * For this to succeed, multiple conditions must be met:
     *  - {canRelay} must `return PreconditionCheck.OK`
     *  - the sender must be a registered relay
     *  - the transaction's gas price must be larger or equal to the one that was requested by the sender
     *  - the transaction must have enough gas to not run out of gas if all internal transactions (calls to the
     * recipient) use all gas available to them
     *  - the recipient must have enough balance to pay the relay for the worst-case scenario (i.e. when all gas is
     * spent)
     *
     * If all conditions are met, the call will be relayed and the recipient charged. {preRelayedCall}, the encoded
     * function and {postRelayedCall} will be called in that order.
     *
     * Parameters:
     *  - `from`: the client originating the request
     *  - `to`: the target {IRelayRecipient} contract
     *  - `encodedFunction`: the function call to relay, including data
     *  - `transactionFee`: fee (%) the relay takes over actual gas cost
     *  - `gasPrice`: gas price the client is willing to pay
     *  - `gasLimit`: gas to forward when calling the encoded function
     *  - `nonce`: client's nonce
     *  - `signature`: client's signature over all previous params, plus the relay and RelayHub addresses
     *  - `approvalData`: dapp-specific data forwared to {acceptRelayedCall}. This value is *not* verified by the
     * `RelayHub`, but it still can be used for e.g. a signature.
     *
     * Emits a {TransactionRelayed} event.
     */
    function relayCall(
        address from,
        address to,
        bytes calldata encodedFunction,
        uint256 transactionFee,
        uint256 gasPrice,
        uint256 gasLimit,
        uint256 nonce,
        bytes calldata signature,
        bytes calldata approvalData
    ) external;

    /**
     * @dev Emitted when an attempt to relay a call failed.
     *
     * This can happen due to incorrect {relayCall} arguments, or the recipient not accepting the relayed call. The
     * actual relayed call was not executed, and the recipient not charged.
     *
     * The `reason` parameter contains an error code: values 1-10 correspond to `PreconditionCheck` entries, and values
     * over 10 are custom recipient error codes returned from {acceptRelayedCall}.
     */
    event CanRelayFailed(address indexed relay, address indexed from, address indexed to, bytes4 selector, uint256 reason);

    /**
     * @dev Emitted when a transaction is relayed.
     * Useful when monitoring a relay's operation and relayed calls to a contract
     *
     * Note that the actual encoded function might be reverted: this is indicated in the `status` parameter.
     *
     * `charge` is the Ether value deducted from the recipient's balance, paid to the relay's owner.
     */
    event TransactionRelayed(address indexed relay, address indexed from, address indexed to, bytes4 selector, RelayCallStatus status, uint256 charge);

    // Reason error codes for the TransactionRelayed event
    enum RelayCallStatus {
        OK,                      // The transaction was successfully relayed and execution successful - never included in the event
        RelayedCallFailed,       // The transaction was relayed, but the relayed call failed
        PreRelayedFailed,        // The transaction was not relayed due to preRelatedCall reverting
        PostRelayedFailed,       // The transaction was relayed and reverted due to postRelatedCall reverting
        RecipientBalanceChanged  // The transaction was relayed and reverted due to the recipient's balance changing
    }

    /**
     * @dev Returns how much gas should be forwarded to a call to {relayCall}, in order to relay a transaction that will
     * spend up to `relayedCallStipend` gas.
     */
    function requiredGas(uint256 relayedCallStipend) external view returns (uint256);

    /**
     * @dev Returns the maximum recipient charge, given the amount of gas forwarded, gas price and relay fee.
     */
    function maxPossibleCharge(uint256 relayedCallStipend, uint256 gasPrice, uint256 transactionFee) external view returns (uint256);

     // Relay penalization.
     // Any account can penalize relays, removing them from the system immediately, and rewarding the
    // reporter with half of the relay's stake. The other half is burned so that, even if the relay penalizes itself, it
    // still loses half of its stake.

    /**
     * @dev Penalize a relay that signed two transactions using the same nonce (making only the first one valid) and
     * different data (gas price, gas limit, etc. may be different).
     *
     * The (unsigned) transaction data and signature for both transactions must be provided.
     */
    function penalizeRepeatedNonce(bytes calldata unsignedTx1, bytes calldata signature1, bytes calldata unsignedTx2, bytes calldata signature2) external;

    /**
     * @dev Penalize a relay that sent a transaction that didn't target `RelayHub`'s {registerRelay} or {relayCall}.
     */
    function penalizeIllegalTransaction(bytes calldata unsignedTx, bytes calldata signature) external;

    /**
     * @dev Emitted when a relay is penalized.
     */
    event Penalized(address indexed relay, address sender, uint256 amount);

    /**
     * @dev Returns an account's nonce in `RelayHub`.
     */
    function getNonce(address from) external view returns (uint256);
}
function relayCall(
    address from,
    address to,
    bytes calldata encodedFunction,
    uint256 transactionFee,
    uint256 gasPrice,
    uint256 gasLimit,
    uint256 nonce,
    bytes calldata signature,
    bytes calldata approvalData
) external;

/**
 * @dev Relays a transaction.
 *
 * For this to succeed, multiple conditions must be met:
 *  - {canRelay} must `return PreconditionCheck.OK`
 *  - the sender must be a registered relay
 *  - the transaction's gas price must be larger or equal to the one that was requested by the sender
 *  - the transaction must have enough gas to not run out of gas if all internal transactions (calls to the
 * recipient) use all gas available to them
 *  - the recipient must have enough balance to pay the relay for the worst-case scenario (i.e. when all gas is
 * spent)
 *
 * If all conditions are met, the call will be relayed and the recipient charged. {preRelayedCall}, the encoded
 * function and {postRelayedCall} will be called in that order.
 *
 * Parameters:
 *  - `from`: the client originating the request
 *  - `to`: the target {IRelayRecipient} contract
 *  - `encodedFunction`: the function call to relay, including data
 *  - `transactionFee`: fee (%) the relay takes over actual gas cost
 *  - `gasPrice`: gas price the client is willing to pay
 *  - `gasLimit`: gas to forward when calling the encoded function
 *  - `nonce`: client's nonce
 *  - `signature`: client's signature over all previous params, plus the relay and RelayHub addresses
 *  - `approvalData`: dapp-specific data forwared to {acceptRelayedCall}. This value is *not* verified by the
 * `RelayHub`, but it still can be used for e.g. a signature.
 *
 * Emits a {TransactionRelayed} event.
 */