leaseGenerator.sol

function payLease() public payable {
    Lease storage lease = tenantLease[msg.sender];
    require(lease.leaseDepositPaid, "Lease deposit must be paid before making lease payments");
    require(!lease.leaseFullyPaid, "Lease has already been fully paid");
    require(lease.leasePaymentWindowEnd >= now, "Lease payment must fit into payment window");

    tenantAddress = msg.sender;
    tenantPayment = msg.value;
    workingState = State.payingLease;
    fetchUsdRate();
}

function _payLease() internal {
    workingState = State.idle;
    Lease storage lease = tenantLease[tenantAddress];
    uint amountSentUsd = tenantPayment.mul(ETHUSD).div(1e18);

    require(
        amountSentUsd >= lease.monthlyAmountUsd - 5,
        "Lease payment must be greater than or equal to the monthly amount with a maximum offset of $5");

    uint monthsPaid = uint256(lease.monthsPaid).add(amountSentUsd.add(10).div(uint256(lease.monthlyAmountUsd)));
    lease.monthsPaid = uint8(monthsPaid);
    leaseBalanceWei = leaseBalanceWei.add(tenantPayment);

    if (monthsPaid == lease.numberOfMonths) {
        lease.leaseFullyPaid = true;
        lease.leasePaymentWindowEnd = 0;

        emit leaseFullyPaid(
            tenantAddress,
            lease.numberOfMonths,
            monthsPaid
        );
    } else {
        lease.leasePaymentWindowEnd = lease.leasePaymentWindowEnd + lease.leasePaymentWindowSeconds;

        emit leasePaymentPaid(
            tenantAddress,
            amountSentUsd
        );
    }
}