[ERC3135] オンチェーン処理を最小限にしたマイクロペイメントの仕組みを理解しよう！

sign(keccak256(abi_encode(
    "\x19Ethereum Signed Message:\n32", 
    keccak256(abi_encode(
        token_address,
        payer_address,
        token_issuer,
        token_consumption,
        epoch
    ))
)), private_key)

/// @return Image url of this token or descriptive resources
function iconUrl() external view returns (string memory);

/// @return Issuer of this token. Only issuer can execute claim function
function issuer() external view returns (address);

/**
 *  @notice   Remove consumption from payer's deposite
 *  @dev      Check if msg.sender == issuer
 *  @param    from          Payer's address
 *  @param    consumption   How many token is consumed in this epoch, specified
 *  @param    epoch         Epoch increased by 1 after claim or withdraw, at the beginning of each epoch, consumption goes back to 0
 *  @param    signature     Signature of payment message signed by payer
*/
function claim(address from, uint256 consumption, uint256 epoch, bytes calldata signature) external;

function transferIssuer(address newIssuer) external;

/// @notice   Move amount from payer's token balance to deposite balance to ensure payment is sufficient
function deposit(uint256 amount) external;

/**
 *  @notice   Give remaining deposite balance back to "to" account, act as "refund" function
 *  @dev      In prepayment module, withdraw is executed from issuer account
 *            In lock-release module, withdraw is executed from user account
 *  @param    to            the account receiving remaining deposite
 *  @param    amount        how many token is returned
*/
function withdraw(address to, uint256 amount) external;

function depositBalanceOf(address user) external view returns(uint256 depositBalance, uint256 epoch);

event Deposit(
    address indexed from,
    uint256 amount
);

event Withdraw(
    address indexed to,
    uint256 amount
);
    
event TransferIssuer(
    address indexed oldIssuer,
    address indexed newIssuer
);

event Claim(
    address indexed from,
    address indexed to,
    uint256 epoch,
    uint256 consumption
);

function iconUrl() external view returns (string memory);

function issuer() external view returns (address);

function claim(address from, uint256 consumption, uint256 epoch, bytes calldata signature) external;

function transferIssuer(address newIssuer) external;

function deposit(uint256 amount) external;

function withdraw(address to, uint256 amount) external;

function depositBalanceOf(address user) external view returns(uint256 depositBalance, uint256 epoch);

event Deposit(address indexed from, uint256 amount);

event Withdraw(address indexed to, uint256 amount);

event TransferIssuer(address indexed oldIssuer, address indexed newIssuer);

event Claim(address indexed from, address indexed to, uint256 epoch, uint256 consumption);

mapping (address => StampBalance) private _depositBalance;
    
struct StampBalance{
    uint256 balance;
    uint256 epoch;
}
    
function deposit(uint256 value) override external{
    require(value <= _balances[msg.sender]);
    _balances[msg.sender] = _balances[msg.sender].sub(value);
    _depositBalance[msg.sender].balance = _depositBalance[msg.sender].balance.add(value);
    emit Deposit(msg.sender, value);
}

function withdraw(address to, uint256 value) override onlyIssuer external{
    require(value <= _depositBalance[to].balance);
    _depositBalance[to].balance = _depositBalance[to].balance.sub(value);
    _depositBalance[to].epoch += 1;
    _balances[to] = _balances[to].add(value);
    emit Withdraw(to, value);
}
    
function depositBalanceOf(address user) override public view returns(uint256 depositBalance, uint256 epoch){
    return (_depositBalance[user].balance, _depositBalance[user].epoch);
}

// prepayment model
function claim(address from, uint credit, uint epoch, bytes memory signature) override onlyIssuer external{
    require(credit > 0);
    require(_depositBalance[from].epoch + 1 == epoch);
    require(_depositBalance[from].balance >= credit);
    bytes32 message = keccak256(abi.encode(this, from, _issuer, credit, epoch));
    bytes32 msgHash = prefixed(message);
    require(recoverSigner(msgHash, signature) == from);
    _depositBalance[from].balance = _depositBalance[from].balance.sub(credit);
    _balances[_issuer] = _balances[_issuer].add(credit);
    _depositBalance[from].epoch += 1;
    emit Claim(from, msg.sender, credit, epoch);
}

function prefixed(bytes32 hash) internal pure returns (bytes32) {
    return keccak256(abi.encode("\x19Ethereum Signed Message:\n32", hash));
}

function recoverSigner(bytes32 message, bytes memory sig) internal pure  returns (address) {
    (uint8 v, bytes32 r, bytes32 s) = splitSignature(sig);
    return ecrecover(message, v, r, s);
}

function splitSignature(bytes memory sig) internal pure returns (uint8 v, bytes32 r, bytes32 s) {
    require(sig.length == 65);
    assembly {
        r := mload(add(sig, 32))
        s := mload(add(sig, 64))
        v := byte(0, mload(add(sig, 96)))
    }
    return (v, r, s);
}