// Vulnerability label: external
pragma solidity ^0.4.11;

contract Base {

    function max(uint a, uint b) returns (uint) { return a >= b ? a : b; }
    function min(uint a, uint b) returns (uint) { return a <= b ? a : b; }

    modifier only(address allowed) {
        if (msg.sender != allowed) throw;
        _;
    }


    ///@return True if `_addr` is a contract
    function isContract(address _addr) constant internal returns (bool) {
        if (_addr == 0) return false;
        uint size;
        assembly {
            size := extcodesize(_addr)
        }
        return (size > 0);
    }

    // *************************************************
    // *          reentrancy handling                  *
    // *************************************************

    //@dev predefined locks (up to uint bit length, i.e. 256 possible)
    uint constant internal L00 = 2 ** 0;
    uint constant internal L01 = 2 ** 1;
    uint constant internal L02 = 2 ** 2;
    uint constant internal L03 = 2 ** 3;
    uint constant internal L04 = 2 ** 4;
    uint constant internal L05 = 2 ** 5;

    //prevents reentrancy attacs: specific locks
    uint private bitlocks = 0;
    modifier noReentrancy(uint m) {
        var _locks = bitlocks;
        if (_locks & m > 0) throw;
        bitlocks |= m;
        _;
        bitlocks = _locks;
    }

    modifier noAnyReentrancy {
        var _locks = bitlocks;
        if (_locks > 0) throw;
        bitlocks = uint(-1);
        _;
        bitlocks = _locks;
    }

    ///@dev empty marking modifier signaling to user of the marked function , that it can cause an reentrant call.
    ///     developer should make the caller function reentrant-safe if it use a reentrant function.
    modifier reentrant { _; }

}

contract Owned is Base {

    address public owner;
    address public newOwner;

    function Owned() {
        owner = msg.sender;
    }

    function transferOwnership(address _newOwner) only(owner) {
        newOwner = _newOwner;
    }

    function acceptOwnership() only(newOwner) {
        OwnershipTransferred(owner, newOwner);
        owner = newOwner;
    }

    event OwnershipTransferred(address indexed _from, address indexed _to);

}


contract ERC20 is Owned {

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    function transfer(address _to, uint256 _value) isStartedOnly returns (bool success) {
        if (balances[msg.sender] >= _value && balances[_to] + _value > balances[_to]) {
            balances[msg.sender] -= _value;
            balances[_to] += _value;
            Transfer(msg.sender, _to, _value);
            return true;
        } else { return false; }
    }

    function transferFrom(address _from, address _to, uint256 _value) isStartedOnly returns (bool success) {
        if (balances[_from] >= _value && allowed[_from][msg.sender] >= _value && balances[_to] + _value > balances[_to]) {
            balances[_to] += _value;
            balances[_from] -= _value;
            allowed[_from][msg.sender] -= _value;
            Transfer(_from, _to, _value);
            return true;
        } else { return false; }
    }

    function balanceOf(address _owner) constant returns (uint256 balance) {
        return balances[_owner];
    }

    function approve(address _spender, uint256 _value) isStartedOnly returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }

    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
        return allowed[_owner][_spender];
    }

    mapping (address => uint256) balances;
    mapping (address => mapping (address => uint256)) allowed;

    uint256 public totalSupply;
    bool    public isStarted = false;

    modifier onlyHolder(address holder) {
        if (balanceOf(holder) == 0) throw;
        _;
    }

    modifier isStartedOnly() {
        if (!isStarted) throw;
        _;
    }

}

//Decision made.
// 1 - Provider is solely responsible to consider failed sub charge as an error and stop the service,
//    therefore there is no separate error state or counter for that in this Token Contract.
//
// 2 - A call originated from the user (isContract(msg.sender)==false) should throw an exception on error,
//     but it should return "false" on error if called from other contract (isContract(msg.sender)==true).
//     Reason: thrown exception are easier to see in wallets, returned boolean values are easier to evaluate in the code of the calling contract.
//
// 3 - Service providers are responsible for firing events in case of offer changes;
//     it is theirs decision to inform DApps about offer changes or not.
//
//ToDo:
// 8 - validate linking modules and deployment process: attachToken(address token) public


///@dev an base class to implement by Service Provider contract to be notified about subscription changes (in-Tx notification).
///     Additionally it contains standard events to be fired by service provider on offer changes.
///     see alse EVM events logged by subscription module.
//
contract ServiceProvider {

    ///@dev get human readable descriptor (or url) for this Service provider
    //
    function info() constant public returns(string);

    ///@dev called to post-approve/reject incoming single payment.
    ///@return `false` causes an exception and reverts the payment.
    //
    function onPayment(address _from, uint _value, bytes _paymentData) public returns (bool);

    ///@dev called to post-approve/reject subscription charge.
    ///@return `false` causes an exception and reverts the operation.
    //
    function onSubExecuted(uint subId) public returns (bool);

    ///@dev called to post-approve/reject a creation of the subscription.
    ///@return `false` causes an exception and reverts the operation.
    //
    function onSubNew(uint newSubId, uint offerId) public returns (bool);

    ///@dev called to notify service provider about subscription cancellation.
    ///     Provider is not able to prevent the cancellation.
    ///@return <<reserved for future implementation>>
    //
    function onSubCanceled(uint subId, address caller) public returns (bool);

    ///@dev called to notify service provider about subscription got hold/unhold.
    ///@return `false` causes an exception and reverts the operation.
    //
    function onSubUnHold(uint subId, address caller, bool isOnHold) public returns (bool);


    ///@dev following events should be used by ServiceProvider contract to notify DApps about offer changes.
    ///     SubscriptionModule do not this notification and expects it from Service Provider if desired.
    ///
    ///@dev to be fired by ServiceProvider on new Offer created in a platform.
    event OfferCreated(uint offerId,  bytes descriptor, address provider);

    ///@dev to be fired by ServiceProvider on Offer updated.
    event OfferUpdated(uint offerId,  bytes descriptor, uint oldExecCounter, address provider);

    ///@dev to be fired by ServiceProvider on Offer canceled.
    event OfferCanceled(uint offerId, bytes descriptor, address provider);

    ///@dev to be fired by ServiceProvider on Offer hold/unhold status changed.
    event OfferUnHold(uint offerId,   bytes descriptor, bool isOnHoldNow, address provider);
} //ServiceProvider

///@notice XRateProvider is an external service providing an exchange rate from external currency to SAN token.
/// it used for subscriptions priced in other currency than SAN (even calculated and paid formally in SAN).
/// if non-default XRateProvider is set for some subscription, then the amount in SAN for every periodic payment
/// will be recalculated using provided exchange rate.
///
/// Please note, that the exchange rate fraction is (uint32,uint32) number. It should be enough to express
/// any real exchange rate volatility. Nevertheless you are advised to avoid too big numbers in the fraction.
/// Possiibly you could implement the ratio of multiple token per SAN in order to keep the average ratio around 1:1.
///
/// The default XRateProvider (with id==0) defines exchange rate 1:1 and represents exchange rate of SAN token to itself.
/// this provider is set by defalult and thus the subscription becomes nominated in SAN.
//
contract XRateProvider {

    //@dev returns current exchange rate (in form of a simple fraction) from other currency to SAN (f.e. ETH:SAN).
    //@dev fraction numbers are restricted to uint16 to prevent overflow in calculations;
    function getRate() public returns (uint32 /*nominator*/, uint32 /*denominator*/);

    //@dev provides a code for another currency, f.e. "ETH" or "USD"
    function getCode() public returns (string);
}


//@dev data structure for SubscriptionModule
contract SubscriptionBase {

    enum SubState   {NOT_EXIST, BEFORE_START, PAID, CHARGEABLE, ON_HOLD, CANCELED, EXPIRED, FINALIZED}
    enum OfferState {NOT_EXIST, BEFORE_START, ACTIVE, SOLD_OUT, ON_HOLD, EXPIRED}

    string[] internal SUB_STATES   = ["NOT_EXIST", "BEFORE_START", "PAID", "CHARGEABLE", "ON_HOLD", "CANCELED", "EXPIRED", "FINALIZED" ];
    string[] internal OFFER_STATES = ["NOT_EXIST", "BEFORE_START", "ACTIVE", "SOLD_OUT", "ON_HOLD", "EXPIRED"];

    //@dev subscription and subscription offer use the same structure. Offer is technically a template for subscription.
    struct Subscription {
        address transferFrom;   // customer (unset in subscription offer)
        address transferTo;     // service provider
        uint pricePerHour;      // price in SAN per hour (possibly recalculated using exchange rate)
        uint32 initialXrate_n;  // nominator
        uint32 initialXrate_d;  // denominator
        uint16 xrateProviderId; // id of a registered exchange rate provider
        uint paidUntil;         // subscription is paid until time
        uint chargePeriod;      // subscription can't be charged more often than this period
        uint depositAmount;     // upfront deposit on creating subscription (possibly recalculated using exchange rate)

        uint startOn;           // for offer: can't be accepted before  <startOn> ; for subscription: can't be charged before <startOn>
        uint expireOn;          // for offer: can't be accepted after  <expireOn> ; for subscription: can't be charged after  <expireOn>
        uint execCounter;       // for offer: max num of subscriptions available  ; for subscription: num of charges made.
        bytes descriptor;       // subscription payload (subject): evaluated by service provider.
        uint onHoldSince;       // subscription: on-hold since time or 0 if not onHold. offer: unused: //ToDo: to be implemented
    }

    struct Deposit {
        uint value;         // value on deposit
        address owner;      // usually a customer
        bytes descriptor;   // service related descriptor to be evaluated by service provider
    }

    event NewSubscription(address customer, address service, uint offerId, uint subId);
    event NewDeposit(uint depositId, uint value, address sender);
    event NewXRateProvider(address addr, uint16 xRateProviderId, address sender);
    event DepositReturned(uint depositId, address returnedTo);
    event SubscriptionDepositReturned(uint subId, uint amount, address returnedTo, address sender);
    event OfferOnHold(uint offerId, bool onHold, address sender);
    event OfferCanceled(uint offerId, address sender);
    event SubOnHold(uint offerId, bool onHold, address sender);
    event SubCanceled(uint subId, address sender);

}

///@dev an Interface for SubscriptionModule.
///     extracted here for better overview.
///     see detailed documentation in implementation module.
contract SubscriptionModule is SubscriptionBase, Base {

    ///@dev ***** module configuration *****
    function attachToken(address token) public;

    ///@dev ***** single payment handling *****
    function paymentTo(uint _value, bytes _paymentData, ServiceProvider _to) public reentrant returns (bool success);
    function paymentFrom(uint _value, bytes _paymentData, address _from, ServiceProvider _to) public reentrant returns (bool success);

    ///@dev ***** subscription handling *****
    ///@dev some functions are marked as reentrant, even theirs implementation is marked with noReentrancy(LOCK).
    ///     This is intentionally because these noReentrancy(LOCK) restrictions can be lifted in the future.
    //      Functions would become reentrant.
    function createSubscription(uint _offerId, uint _expireOn, uint _startOn) public reentrant returns (uint newSubId);
    function cancelSubscription(uint subId) reentrant public;
    function cancelSubscription(uint subId, uint gasReserve) reentrant public;
    function holdSubscription(uint subId) public reentrant returns (bool success);
    function unholdSubscription(uint subId) public reentrant returns (bool success);
    function executeSubscription(uint subId) public reentrant returns (bool success);
    function postponeDueDate(uint subId, uint newDueDate) public returns (bool success);
    function returnSubscriptionDesposit(uint subId) public;
    function claimSubscriptionDeposit(uint subId) public;
    function state(uint subId) public constant returns(string state);
    function stateCode(uint subId) public constant returns(uint stateCode);

    ///@dev ***** subscription offer handling *****
    function createSubscriptionOffer(uint _price, uint16 _xrateProviderId, uint _chargePeriod, uint _expireOn, uint _offerLimit, uint _depositValue, uint _startOn, bytes _descriptor) public reentrant returns (uint subId);
    function updateSubscriptionOffer(uint offerId, uint _offerLimit) public;
    function holdSubscriptionOffer(uint offerId) public returns (bool success);
    function unholdSubscriptionOffer(uint offerId) public returns (bool success);
    function cancelSubscriptionOffer(uint offerId) public returns (bool);

    ///@dev ***** simple deposit handling *****
    function createDeposit(uint _value, bytes _descriptor) public returns (uint subId);
    function claimDeposit(uint depositId) public;

    ///@dev ***** ExchangeRate provider *****
    function registerXRateProvider(XRateProvider addr) public returns (uint16 xrateProviderId);

    ///@dev ***** Service provider (payment receiver) *****
    function enableServiceProvider(ServiceProvider addr, bytes moreInfo) public;
    function disableServiceProvider(ServiceProvider addr, bytes moreInfo) public;


    ///@dev ***** convenience subscription getter *****
    function subscriptionDetails(uint subId) public constant returns(
        address transferFrom,
        address transferTo,
        uint pricePerHour,
        uint32 initialXrate_n, //nominator
        uint32 initialXrate_d, //denominator
        uint16 xrateProviderId,
        uint chargePeriod,
        uint startOn,
        bytes descriptor
    );

    function subscriptionStatus(uint subId) public constant returns(
        uint depositAmount,
        uint expireOn,
        uint execCounter,
        uint paidUntil,
        uint onHoldSince
    );

    enum PaymentStatus {OK, BALANCE_ERROR, APPROVAL_ERROR}
    event Payment(address _from, address _to, uint _value, uint _fee, address sender, PaymentStatus status, uint subId);
    event ServiceProviderEnabled(address addr, bytes moreInfo);
    event ServiceProviderDisabled(address addr, bytes moreInfo);

} //SubscriptionModule

contract ERC20ModuleSupport {
    function _fulfillPreapprovedPayment(address _from, address _to, uint _value, address msg_sender) public returns(bool success);
    function _fulfillPayment(address _from, address _to, uint _value, uint subId, address msg_sender) public returns (bool success);
    function _mintFromDeposit(address owner, uint amount) public;
    function _burnForDeposit(address owner, uint amount) public returns(bool success);
}

//@dev implementation
contract SubscriptionModuleImpl is SubscriptionModule, Owned  {

    string public constant VERSION = "0.1.0";

    // *************************************************
    // *              contract states                  *
    // *************************************************

    ///@dev list of all registered service provider contracts implemented as a map for better lookup.
    mapping (address=>bool) public providerRegistry;

    ///@dev all subscriptions and offers (incl. FINALIZED).
    mapping (uint => Subscription) public subscriptions;

    ///@dev all active simple deposits gived by depositId.
    mapping (uint => Deposit) public deposits;

    ///@dev addresses of registered exchange rate providers.
    XRateProvider[] public xrateProviders;

    ///@dev ongoing counter for subscription ids starting from 1.
    ///     Current value represents an id of last created subscription.
    uint public subscriptionCounter = 0;

    ///@dev ongoing counter for simple deposit ids starting from 1.
    ///     Current value represents an id of last created deposit.
    uint public depositCounter = 0;

    ///@dev Token contract with ERC20ModuleSupport addon.
    ///     Subscription Module operates on its balances via ERC20ModuleSupport interface as trusted module.
    ERC20ModuleSupport public san;



    // *************************************************
    // *     reject all ether sent to this contract    *
    // *************************************************
    function () {
        throw;
    }



    // *************************************************
    // *            setup and configuration            *
    // *************************************************

    ///@dev constructor
    function SubscriptionModuleImpl() {
        owner = msg.sender;
        xrateProviders.push(XRateProvider(this)); //this is a default SAN:SAN (1:1) provider with default id == 0
    }


    ///@dev attach SAN token to work with; can be done only once.
    function attachToken(address token) public {
        assert(address(san) == 0); //only in new deployed state
        san = ERC20ModuleSupport(token);
    }


    ///@dev register a new service provider to the platform.
    function enableServiceProvider(ServiceProvider addr, bytes moreInfo) public only(owner) {
        providerRegistry[addr] = true;
        ServiceProviderEnabled(addr, moreInfo);
    }


    ///@dev de-register the service provider with given `addr`.
    function disableServiceProvider(ServiceProvider addr, bytes moreInfo) public only(owner) {
        delete providerRegistry[addr];
        ServiceProviderDisabled(addr, moreInfo);
    }


    ///@dev register new exchange rate provider.
    ///     XRateProvider can't be de-registered, because they could be still in use by some subscription.
    function registerXRateProvider(XRateProvider addr) public only(owner) returns (uint16 xrateProviderId) {
        xrateProviderId = uint16(xrateProviders.length);
        xrateProviders.push(addr);
        NewXRateProvider(addr, xrateProviderId, msg.sender);
    }


    ///@dev xrateProviders length accessor.
    function getXRateProviderLength() public constant returns (uint) {
        return xrateProviders.length;
    }


    // *************************************************
    // *           single payment methods              *
    // *************************************************

    ///@notice makes single payment to service provider.
    ///@param _value - amount of SAN token to sent
    ///@param _paymentData - 'payment purpose' code usually issued by service provider to customer before payment.
    ///@param _to - service provider contract
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function paymentTo(uint _value, bytes _paymentData, ServiceProvider _to) public reentrant returns (bool success) {
        if (san._fulfillPayment(msg.sender, _to, _value, 0, msg.sender)) {
            // a ServiceProvider (a ServiceProvider) has here an opportunity verify and reject the payment
            assert (ServiceProvider(_to).onPayment(msg.sender, _value, _paymentData));                      // <=== possible reentrancy
            return true;
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }


    ///@notice makes single preapproved payment to service provider. An amount must be already preapproved by payment sender to recepient.
    ///@param _value - amount of SAN token to sent
    ///@param _paymentData - 'payment purpose' code usually issued by service provider to customer before payment.
    ///@param _from - sender of the payment (other than msg.sender)
    ///@param _to - service provider contract
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function paymentFrom(uint _value, bytes _paymentData, address _from, ServiceProvider _to) public reentrant returns (bool success) {
        if (san._fulfillPreapprovedPayment(_from, _to, _value, msg.sender)) {
            // a ServiceProvider (a ServiceProvider) has here an opportunity verify and reject the payment
            assert (ServiceProvider(_to).onPayment(_from, _value, _paymentData));                           // <=== possible reentrancy
            return true;
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }


    // *************************************************
    // *            subscription handling              *
    // *************************************************

    ///@dev convenience getter for some subscription fields
    function subscriptionDetails(uint subId) public constant returns (
        address transferFrom,
        address transferTo,
        uint pricePerHour,
        uint32 initialXrate_n, //nominator
        uint32 initialXrate_d, //denominator
        uint16 xrateProviderId,
        uint chargePeriod,
        uint startOn,
        bytes descriptor
    ) {
        Subscription sub = subscriptions[subId];
        return (sub.transferFrom, sub.transferTo, sub.pricePerHour, sub.initialXrate_n, sub.initialXrate_d, sub.xrateProviderId, sub.chargePeriod, sub.startOn, sub.descriptor);
    }


    ///@dev convenience getter for some subscription fields
    ///     a caller must know, that the subscription with given id exists, because all these fields can be 0 even the subscription with given id exists.
    function subscriptionStatus(uint subId) public constant returns(
        uint depositAmount,
        uint expireOn,
        uint execCounter,
        uint paidUntil,
        uint onHoldSince
    ) {
        Subscription sub = subscriptions[subId];
        return (sub.depositAmount, sub.expireOn, sub.execCounter, sub.paidUntil, sub.onHoldSince);
    }


    ///@notice execute periodic subscription payment.
    ///        Any of customer, service provider and platform owner can execute this function.
    ///        This ensures, that the subscription charge doesn't become delayed.
    ///        At least the platform owner has an incentive to get fee and thus can trigger the function.
    ///        An execution fails if subscription is not in status `CHARGEABLE`.
    ///@param subId - subscription to be charged.
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function executeSubscription(uint subId) public noReentrancy(L00) returns (bool) {
        Subscription storage sub = subscriptions[subId];
        assert (msg.sender == sub.transferFrom || msg.sender == sub.transferTo || msg.sender == owner);
        if (_subscriptionState(sub)==SubState.CHARGEABLE) {
            var _from = sub.transferFrom;
            var _to = sub.transferTo;
            var _value = _amountToCharge(sub);
            if (san._fulfillPayment(_from, _to, _value, subId, msg.sender)) {
                sub.paidUntil  = max(sub.paidUntil, sub.startOn) + sub.chargePeriod;
                ++sub.execCounter;
                // a ServiceProvider (a ServiceProvider) has here an opportunity to verify and reject the payment
                assert (ServiceProvider(_to).onSubExecuted(subId));
                return true;
            }
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }


    ///@notice move `paidUntil` forward to given `newDueDate`. It waives payments for given time.
    ///        This function can be used by service provider to `give away` some service time for free.
    ///@param subId - id of subscription to be postponed.
    ///@param newDueDate - new `paidUntil` datetime; require `newDueDate > paidUntil`.
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function postponeDueDate(uint subId, uint newDueDate) public returns (bool success){
        Subscription storage sub = subscriptions[subId];
        assert (_isSubscription(sub));
        assert (sub.transferTo == msg.sender); //only Service Provider is allowed to postpone the DueDate
        if (sub.paidUntil < newDueDate) {
            sub.paidUntil = newDueDate;
            return true;
        } else if (isContract(msg.sender)) { return false; }
          else { throw; }
    }


    ///@dev return current status as a name of a subscription (or an offer) with given id;
    function state(uint subOrOfferId) public constant returns(string state) {
        Subscription subOrOffer = subscriptions[subOrOfferId];
        return _isOffer(subOrOffer)
              ? OFFER_STATES[uint(_offerState(subOrOffer))]
              : SUB_STATES[uint(_subscriptionState(subOrOffer))];
    }


    ///@dev return current status as a code of a subscription (or an offer) with given id;
    function stateCode(uint subOrOfferId) public constant returns(uint stateCode) {
        Subscription subOrOffer = subscriptions[subOrOfferId];
        return _isOffer(subOrOffer)
              ? uint(_offerState(subOrOffer))
              : uint(_subscriptionState(subOrOffer));
    }


    function _offerState(Subscription storage sub) internal constant returns(OfferState status) {
        if (!_isOffer(sub)) {
            return OfferState.NOT_EXIST;
        } else if (sub.startOn > now) {
            return OfferState.BEFORE_START;
        } else if (sub.onHoldSince > 0) {
            return OfferState.ON_HOLD;
        } else if (now <= sub.expireOn) {
            return sub.execCounter > 0
                ? OfferState.ACTIVE
                : OfferState.SOLD_OUT;
        } else {
            return OfferState.EXPIRED;
        }
    }

    function _subscriptionState(Subscription storage sub) internal constant returns(SubState status) {
        if (!_isSubscription(sub)) {
            return SubState.NOT_EXIST;
        } else if (sub.startOn > now) {
            return SubState.BEFORE_START;
        } else if (sub.onHoldSince > 0) {
            return SubState.ON_HOLD;
        } else if (sub.paidUntil >= sub.expireOn) {
            return now < sub.expireOn
                ? SubState.CANCELED
                : sub.depositAmount > 0
                    ? SubState.EXPIRED
                    : SubState.FINALIZED;
        } else if (sub.paidUntil <= now) {
            return SubState.CHARGEABLE;
        } else {
            return SubState.PAID;
        }
    }


    ///@notice create a new subscription offer.
    ///@dev only registered service provider is allowed to create offers.
    ///@dev subscription uses SAN token for payment, but an exact amount to be paid or deposit is calculated using exchange rate from external xrateProvider (previosly registered on platform).
    ///    This allows to create a subscription bound to another token or even fiat currency.
    ///@param _pricePerHour - subscription price per hour in SAN
    ///@param _xrateProviderId - id of external exchange rate provider from subscription currency to SAN; "0" means subscription is priced in SAN natively.
    ///@param _chargePeriod - time period to charge; subscription can't be charged more often than this period. Time units are native ethereum time, returning by `now`, i.e. seconds.
    ///@param _expireOn - offer can't be accepted after this time.
    ///@param _offerLimit - how many subscription are available to created from this offer; there is no magic number for unlimited offer -- use big number instead.
    ///@param _depositAmount - upfront deposit required for creating a subscription; this deposit becomes fully returned on subscription is over.
    ///       currently this deposit is not subject of platform fees and will be refunded in full. Next versions of this module can use deposit in case of outstanding payments.
    ///@param _startOn - a subscription from this offer can't be created before this time. Time units are native ethereum time, returning by `now`, i.e. seconds.
    ///@param _descriptor - arbitrary bytes as an offer descriptor. This descriptor is copied into subscription and then service provider becomes it passed in notifications.
    //
    function createSubscriptionOffer(uint _pricePerHour, uint16 _xrateProviderId, uint _chargePeriod, uint _expireOn, uint _offerLimit, uint _depositAmount, uint _startOn, bytes _descriptor)
    public
    noReentrancy(L01)
    onlyRegisteredProvider
    returns (uint subId) {
        assert (_startOn < _expireOn);
        assert (_chargePeriod <= 10 years); //sanity check
        var (_xrate_n, _xrate_d) = _xrateProviderId == 0
                                 ? (1,1)
                                 : XRateProvider(xrateProviders[_xrateProviderId]).getRate(); // <=== possible reentrancy
        assert (_xrate_n > 0 && _xrate_d > 0);
        subscriptions[++subscriptionCounter] = Subscription ({
            transferFrom    : 0,                  // empty transferFrom field means we have an offer, not a subscription
            transferTo      : msg.sender,         // service provider is a beneficiary of subscripton payments
            pricePerHour    : _pricePerHour,      // price per hour in SAN (recalculated from base currency if needed)
            xrateProviderId : _xrateProviderId,   // id of registered exchange rate provider or zero if an offer is nominated in SAN.
            initialXrate_n  : _xrate_n,           // fraction nominator of the initial exchange rate
            initialXrate_d  : _xrate_d,           // fraction denominator of the initial exchange rate
            paidUntil       : 0,                  // service is considered to be paid until this time; no charge is possible while subscription is paid for now.
            chargePeriod    : _chargePeriod,      // period in seconds (ethereum block time unit) to charge.
            depositAmount   : _depositAmount,     // deposit required for subscription accept.
            startOn         : _startOn,
            expireOn        : _expireOn,
            execCounter     : _offerLimit,
            descriptor      : _descriptor,
            onHoldSince     : 0                   // offer is not on hold by default.
        });
        return subscriptionCounter;               // returns an id of the new offer.
    }


    ///@notice updates currently available number of subscription for this offer.
    ///        Other offer's parameter can't be updated because they are considered to be a public offer reviewed by customers.
    ///        The service provider should recreate the offer as a new one in case of other changes.
    //
    function updateSubscriptionOffer(uint _offerId, uint _offerLimit) public {
        Subscription storage offer = subscriptions[_offerId];
        assert (_isOffer(offer));
        assert (offer.transferTo == msg.sender); //only Provider is allowed to update the offer.
        offer.execCounter = _offerLimit;
    }


    ///@notice accept given offer and create a new subscription on the base of it.
    ///
    ///@dev the service provider (offer.`transferTo`) becomes notified about new subscription by call `onSubNew(newSubId, _offerId)`.
    ///     It is provider's responsibility to retrieve and store any necessary information about offer and this new subscription. Some of info is only available at this point.
    ///     The Service Provider can also reject the new subscription by throwing an exception or returning `false` from `onSubNew(newSubId, _offerId)` event handler.
    ///@param _offerId   - id of the offer to be accepted
    ///@param _expireOn  - subscription expiration time; no charges are possible behind this time.
    ///@param _startOn   - subscription start time; no charges are possible before this time.
    ///                    If the `_startOn` is in the past or is zero, it means start the subscription ASAP.
    //
    function createSubscription(uint _offerId, uint _expireOn, uint _startOn) public noReentrancy(L02) returns (uint newSubId) {
        assert (_startOn < _expireOn);
        Subscription storage offer = subscriptions[_offerId];
        assert (_isOffer(offer));
        assert (offer.startOn == 0     || offer.startOn  <= now);
        assert (offer.expireOn == 0    || offer.expireOn >= now);
        assert (offer.onHoldSince == 0);
        assert (offer.execCounter > 0);
        --offer.execCounter;
        newSubId = ++subscriptionCounter;
        //create a clone of the offer...
        Subscription storage newSub = subscriptions[newSubId] = offer;
        //... and adjust some fields specific to subscription
        newSub.transferFrom = msg.sender;
        newSub.execCounter = 0;
        newSub.paidUntil = newSub.startOn = max(_startOn, now);     //no debts before actual creation time!
        newSub.expireOn = _expireOn;
        newSub.depositAmount = _applyXchangeRate(newSub.depositAmount, newSub);                    // <=== possible reentrancy
        //depositAmount is now stored in the sub, so burn the same amount from customer's account.
        assert (san._burnForDeposit(msg.sender, newSub.depositAmount));
        assert (ServiceProvider(newSub.transferTo).onSubNew(newSubId, _offerId));                  // <=== possible reentrancy; service provider can still reject the new subscription here

        NewSubscription(newSub.transferFrom, newSub.transferTo, _offerId, newSubId);
        return newSubId;
    }


    ///@notice cancel an offer given by `offerId`.
    ///@dev sets offer.`expireOn` to `expireOn`.
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function cancelSubscriptionOffer(uint offerId) public returns (bool) {
        Subscription storage offer = subscriptions[offerId];
        assert (_isOffer(offer));
        assert (offer.transferTo == msg.sender || owner == msg.sender); //only service provider or platform owner is allowed to cancel the offer
        if (offer.expireOn>now){
            offer.expireOn = now;
            OfferCanceled(offerId, msg.sender);
            return true;
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }


    ///@notice cancel an subscription given by `subId` (a graceful version).
    ///@notice IMPORTANT: a malicious service provider can consume all gas and preventing subscription from cancellation.
    ///        If so, use `cancelSubscription(uint subId, uint gasReserve)` as the forced version.
    ///         see `cancelSubscription(uint subId, uint gasReserve)` for more documentation.
    //
    function cancelSubscription(uint subId) public {
        return cancelSubscription(subId, 0);
    }


    ///@notice cancel an subscription given by `subId` (a forced version).
    ///        Cancellation means no further charges to this subscription are possible. The provided subscription deposit can be withdrawn only `paidUntil` period is over.
    ///        Depending on nature of the service provided, the service provider can allow an immediate deposit withdrawal by `returnSubscriptionDesposit(uint subId)` call, but its on his own.
    ///        In some business cases a deposit must remain locked until `paidUntil` period is over even, the subscription is already canceled.
    ///@notice gasReserve is a gas amount reserved for contract execution AFTER service provider becomes `onSubCanceled(uint256,address)` notification.
    ///        It guarantees, that cancellation becomes executed even a (malicious) service provider consumes all gas provided.
    ///        If so, use `cancelSubscription(uint subId, uint gasReserve)` as the forced version.
    ///        This difference is because the customer must always have a possibility to cancel his contract even the service provider disagree on cancellation.
    ///@param subId - subscription to be cancelled
    ///@param gasReserve - gas reserved for call finalization (minimum reservation is 10000 gas)
    //
    function cancelSubscription(uint subId, uint gasReserve) public noReentrancy(L03) {
        Subscription storage sub = subscriptions[subId];
        assert (sub.transferFrom == msg.sender || owner == msg.sender); //only subscription owner or platform owner is allowed to cancel it
        assert (_isSubscription(sub));
        var _to = sub.transferTo;
        sub.expireOn = max(now, sub.paidUntil);
        if (msg.sender != _to) {
            //supress re-throwing of exceptions; reserve enough gas to finish this function
            gasReserve = max(gasReserve,10000);  //reserve minimum 10000 gas
            assert (msg.gas > gasReserve);       //sanity check
            if (_to.call.gas(msg.gas-gasReserve)(bytes4(sha3("onSubCanceled(uint256,address)")), subId, msg.sender)) {     // <=== possible reentrancy
                //do nothing. it is notification only.
                //Later: is it possible to evaluate return value here? If is better to return the subscription deposit here.
            }
        }
        SubCanceled(subId, msg.sender);
    }


    ///@notice place an active offer on hold; it means no subscriptions can be created from this offer.
    ///        Only service provider (or platform owner) is allowed to hold/unhold a subscription offer.
    ///@param offerId - id of the offer to be placed on hold.
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function holdSubscriptionOffer(uint offerId) public returns (bool success) {
        Subscription storage offer = subscriptions[offerId];
        assert (_isOffer(offer));
        require (msg.sender == offer.transferTo || msg.sender == owner); //only owner or service provider can place the offer on hold.
        if (offer.onHoldSince == 0) {
            offer.onHoldSince = now;
            OfferOnHold(offerId, true, msg.sender);
            return true;
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }


    ///@notice resume on-hold offer; subscriptions can be created from this offer again (if other conditions are met).
    ///        Only service provider (or platform owner) is allowed to hold/unhold a subscription offer.
    ///@param offerId - id of the offer to be resumed.
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function unholdSubscriptionOffer(uint offerId) public returns (bool success) {
        Subscription storage offer = subscriptions[offerId];
        assert (_isOffer(offer));
        require (msg.sender == offer.transferTo || msg.sender == owner); //only owner or service provider can reactivate the offer.
        if (offer.onHoldSince > 0) {
            offer.onHoldSince = 0;
            OfferOnHold(offerId, false, msg.sender);
            return true;
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }


    ///@notice called by customer or service provider to place a subscription on hold.
    ///        If call is originated by customer the service provider can reject the request.
    ///        A subscription on hold will not be charged. The service is usually not provided as well.
    ///        During hold time a subscription preserve remaining paid time period, which becomes available after unhold.
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function holdSubscription(uint subId) public noReentrancy(L04) returns (bool success) {
        Subscription storage sub = subscriptions[subId];
        assert (_isSubscription(sub));
        var _to = sub.transferTo;
        require (msg.sender == _to || msg.sender == sub.transferFrom); //only customer or provider can place the subscription on hold.
        if (sub.onHoldSince == 0) {
            if (msg.sender == _to || ServiceProvider(_to).onSubUnHold(subId, msg.sender, true)) {          // <=== possible reentrancy
                sub.onHoldSince = now;
                SubOnHold(subId, true, msg.sender);
                return true;
            }
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }


    ///@notice called by customer or service provider to unhold subscription.
    ///        If call is originated by customer the service provider can reject the request.
    ///        A subscription on hold will not be charged. The service is usually not provided as well.
    ///        During hold time a subscription preserve remaining paid time period, which becomes available after unhold.
    ///@return `true` on success; `false` of failure (if caller is a contract) or throw an exception (if caller is not a contract)
    //
    function unholdSubscription(uint subId) public noReentrancy(L05) returns (bool success) {
        Subscription storage sub = subscriptions[subId];
        assert (_isSubscription(sub));
        var _to = sub.transferTo;
        require (msg.sender == _to || msg.sender == sub.transferFrom); //only customer or provider can place the subscription on hold.
        if (sub.onHoldSince > 0) {
            if (msg.sender == _to || ServiceProvider(_to).onSubUnHold(subId, msg.sender, false)) {         // <=== possible reentrancy
                sub.paidUntil += now - sub.onHoldSince;
                sub.onHoldSince = 0;
                SubOnHold(subId, false, msg.sender);
                return true;
            }
        }
        if (isContract(msg.sender)) { return false; }
        else { throw; }
    }



    // *************************************************
    // *              deposit handling                 *
    // *************************************************

    ///@notice can be called by provider on CANCELED subscription to return a subscription deposit to customer immediately.
    ///        Customer can anyway collect his deposit after `paidUntil` period is over.
    ///@param subId - subscription holding the deposit
    //
    function returnSubscriptionDesposit(uint subId) public {
        Subscription storage sub = subscriptions[subId];
        assert (_subscriptionState(sub) == SubState.CANCELED);
        assert (sub.depositAmount > 0); //sanity check
        assert (sub.transferTo == msg.sender || owner == msg.sender); //only subscription owner or platform owner is allowed to release deposit.
        sub.expireOn = now;
        _returnSubscriptionDesposit(subId, sub);
    }


    ///@notice called by customer on EXPIRED subscription (`paidUntil` period is over) to collect a subscription deposit.
    ///        Customer can anyway collect his deposit after `paidUntil` period is over.
    ///@param subId - subscription holding the deposit
    //
    function claimSubscriptionDeposit(uint subId) public {
        Subscription storage sub = subscriptions[subId];
        assert (_subscriptionState(sub) == SubState.EXPIRED);
        assert (sub.transferFrom == msg.sender);
        assert (sub.depositAmount > 0);
        _returnSubscriptionDesposit(subId, sub);
    }


    //@dev returns subscription deposit to customer
    function _returnSubscriptionDesposit(uint subId, Subscription storage sub) internal {
        uint depositAmount = sub.depositAmount;
        sub.depositAmount = 0;
        san._mintFromDeposit(sub.transferFrom, depositAmount);
        SubscriptionDepositReturned(subId, depositAmount, sub.transferFrom, msg.sender);
    }


    ///@notice create simple unlocked deposit, required by some services. It can be considered as prove of customer's stake.
    ///        This desposit can be claimed back by the customer at anytime.
    ///        The service provider is responsible to check the deposit before providing the service.
    ///@param _value - non zero deposit amount.
    ///@param _descriptor - is a uniq key, usually given by service provider to the customer in order to make this deposit unique.
    ///        Service Provider should reject deposit with unknown descriptor, because most probably it is in use for some another service.
    ///@return depositId - a handle to claim back the deposit later.
    //
    function createDeposit(uint _value, bytes _descriptor) public returns (uint depositId) {
        require (_value > 0);
        assert (san._burnForDeposit(msg.sender,_value));
        deposits[++depositCounter] = Deposit ({
            owner : msg.sender,
            value : _value,
            descriptor : _descriptor
        });
        NewDeposit(depositCounter, _value, msg.sender);
        return depositCounter;
    }


    ///@notice return previously created deposit to the user. User can collect only own deposit.
    ///        The service provider is responsible to check the deposit before providing the service.
    ///@param _depositId - an id of the deposit to be collected.
    //
    function claimDeposit(uint _depositId) public {
        var deposit = deposits[_depositId];
        require (deposit.owner == msg.sender);
        var value = deposits[_depositId].value;
        delete deposits[_depositId];
        san._mintFromDeposit(msg.sender, value);
        DepositReturned(_depositId, msg.sender);
    }



    // *************************************************
    // *            some internal functions            *
    // *************************************************

    function _amountToCharge(Subscription storage sub) internal reentrant returns (uint) {
        return _applyXchangeRate(sub.pricePerHour * sub.chargePeriod, sub) / 1 hours;       // <==== reentrant function usage
    }

    function _applyXchangeRate(uint amount, Subscription storage sub) internal reentrant returns (uint) {  // <== actually called from reentrancy guarded context only (i.e. externally secured)
        if (sub.xrateProviderId > 0) {
            // xrate_n: nominator
            // xrate_d: denominator of the exchange rate fraction.
            var (xrate_n, xrate_d) = XRateProvider(xrateProviders[sub.xrateProviderId]).getRate();        // <=== possible reentrancy
            amount = amount * sub.initialXrate_n * xrate_d / sub.initialXrate_d / xrate_n;
        }
        return amount;
    }

    function _isOffer(Subscription storage sub) internal constant returns (bool){
        return sub.transferFrom == 0 && sub.transferTo != 0;
    }

    function _isSubscription(Subscription storage sub) internal constant returns (bool){
        return sub.transferFrom != 0 && sub.transferTo != 0;
    }

    function _exists(Subscription storage sub) internal constant returns (bool){
        return sub.transferTo != 0;   //existing subscription or offer has always transferTo set.
    }

    modifier onlyRegisteredProvider(){
        if (!providerRegistry[msg.sender]) throw;
        _;
    }

} //SubscriptionModuleImpl