// File: Address.sol



// SPDX-License-Identifier: MIT

/*
Copyright (c) 2016-2020 zOS Global Limited

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

pragma solidity ^0.7.0;

/**
 * @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);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        require(isContract(target), "Address: delegate call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.delegatecall(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: IERC20.sol



// SPDX-License-Identifier: MIT

/*
Copyright (c) 2016-2020 zOS Global Limited

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

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: SafeMath.sol



// SPDX-License-Identifier: MIT

/*
Copyright (c) 2016-2020 zOS Global Limited

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

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: YieldFarm.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.7.6;

import "./SafeMath.sol";
import "./IERC20.sol";

interface IStaking {
    function getEpochId(uint timestamp) external view returns (uint); // get epoch id
    function getEpochUserBalance(address user, address token, uint128 epoch) external view returns(uint);
    function getEpochPoolSize(address token, uint128 epoch) external view returns (uint);
    function epoch1Start() external view returns (uint);
    function epochDuration() external view returns (uint);
    function hasReferrer(address addr) external view returns(bool);
    function referrals(address addr) external view returns(address);
    function firstReferrerRewardPercentage() external view returns(uint256);
    function secondReferrerRewardPercentage() external view returns(uint256);
}

interface TokenInterface is IERC20 {
    function mintSupply(address _investorAddress, uint256 _amount) external;
}

contract YieldFarm {
    // lib
    using SafeMath for uint;
    using SafeMath for uint128;
    using SafeMath for uint256;
    
    // constants
    uint public constant NR_OF_EPOCHS = 24;

    // addreses
    address public _usdc;
    address public _usdt;
    address public _dai;
    // contracts
    TokenInterface private _swapp;
    IStaking private _staking;
    
    address public _owner;

    // fixed size array holdings total number of epochs + 1 (epoch 0 doesn't count)
    uint[] private epochs = new uint[](NR_OF_EPOCHS + 1);
    // pre-computed variable for optimization. total amount of swapp tokens to be distributed on each epoch
    // uint private _totalAmountPerEpoch;

    // id of last init epoch, for optimization purposes moved from struct to a single id.
    uint128 public lastInitializedEpoch;

    // state of user harvest epoch
    mapping(address => uint128) private lastEpochIdHarvested;
    uint public epochDuration; // init from staking contract
    uint public epochStart; // init from staking contract
    mapping(uint128 => uint256) public epochAmounts;

    // events
    event MassHarvest(address indexed user, uint256 epochsHarvested, uint256 totalValue);
    event Harvest(address indexed user, uint128 indexed epochId, uint256 amount);
    event ReferrerRewardCollected(address indexed staker, address indexed referrer, uint256 rewardAmount);
    event Referrer2RewardCollected(address indexed staker, address indexed referrer, address indexed referrer2, uint256 rewardAmount);

    // constructor
    constructor() {
        _swapp = TokenInterface(0x8CB924583681cbFE487A62140a994A49F833c244);
        _staking = IStaking(0x245a551ee0F55005e510B239c917fA34b41B3461);

        _usdc = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;
        _usdt = 0xdAC17F958D2ee523a2206206994597C13D831ec7;
        _dai = 0x6B175474E89094C44Da98b954EedeAC495271d0F;

        epochStart = _staking.epoch1Start();
        epochDuration = _staking.epochDuration();
        
        _owner = msg.sender;
        
        for (uint128 i = 1; i <= 24; i++) {
            // i = epochId
            epochAmounts[i] = 5000000;
        }
    }
    
    function setEpochAmount(uint128 epochId, uint256 amount) external {
        require(msg.sender == _owner, "Only owner can update epoch amount");
        require(epochId > 0 && epochId <= NR_OF_EPOCHS, "Minimum epoch number is 1 and Maximum number of epochs is 24");
        require(epochId > _getEpochId(), "Only future epoch can be updated");
        epochAmounts[epochId] = amount;
    }
    
    function getTotalAmountPerEpoch() public view returns (uint) {
        uint128 currentEpoch = _getEpochId();
        if (currentEpoch > NR_OF_EPOCHS) {
            currentEpoch = 25;
        }

        return epochAmounts[currentEpoch-1].mul(10**18);
    }
    
    function getCurrentEpochAmount() public view returns (uint) {
        uint128 currentEpoch = _getEpochId();
        if (currentEpoch <= 0 || currentEpoch > NR_OF_EPOCHS) {
            return 0;
        }

        return epochAmounts[currentEpoch];
    }

    function getTotalDistributedAmount() external view returns(uint256) {
        uint256 totalDistributed;
        for (uint128 i = 1; i <= NR_OF_EPOCHS; i++) {
            totalDistributed += epochAmounts[i];
        }
        return totalDistributed;
    }

    // public methods
    // public method to harvest all the unharvested epochs until current epoch - 1
    function massHarvest() external returns (uint){
        uint totalDistributedValue;
        uint epochId = _getEpochId().sub(1); // fails in epoch 0
        // force max number of epochs
        if (epochId > NR_OF_EPOCHS) {
            epochId = NR_OF_EPOCHS;
        }

        for (uint128 i = lastEpochIdHarvested[msg.sender] + 1; i <= epochId; i++) {
            // i = epochId
            // compute distributed Value and do one single transfer at the end
            totalDistributedValue += _harvest(i);
        }

        emit MassHarvest(msg.sender, epochId.sub(lastEpochIdHarvested[msg.sender]), totalDistributedValue);

        if (totalDistributedValue > 0) {
            _swapp.mintSupply(msg.sender, totalDistributedValue);
            //Referrer reward
            distributeReferrerReward(totalDistributedValue);
        }

        return totalDistributedValue;
    }

    function harvest (uint128 epochId) external returns (uint){
        // checks for requested epoch
        require (_getEpochId() > epochId, "This epoch is in the future");
        require(epochId <= NR_OF_EPOCHS, "Maximum number of epochs is 24");
        require (lastEpochIdHarvested[msg.sender].add(1) == epochId, "Harvest in order");
        uint userReward = _harvest(epochId);
        if (userReward > 0) {
            _swapp.mintSupply(msg.sender, userReward);
            //Referrer reward
            distributeReferrerReward(userReward);
        }
        emit Harvest(msg.sender, epochId, userReward);
        return userReward;
    }
    
    function distributeReferrerReward(uint256 stakerReward) internal {
        if (_staking.hasReferrer(msg.sender)) {
            address referrer = _staking.referrals(msg.sender);
            uint256 ref1Reward = stakerReward.mul(_staking.firstReferrerRewardPercentage()).div(10000);
            _swapp.mintSupply(referrer, ref1Reward);
            emit ReferrerRewardCollected(msg.sender, referrer, ref1Reward);
            
            // second step referrer
            if (_staking.hasReferrer(referrer)) {
                address referrer2 = _staking.referrals(referrer);
                uint256 ref2Reward = stakerReward.mul(_staking.secondReferrerRewardPercentage()).div(10000);
                _swapp.mintSupply(referrer2, ref2Reward);
                emit Referrer2RewardCollected(msg.sender, referrer, referrer2, ref2Reward);
            }
        }
    }

    // views
    // calls to the staking smart contract to retrieve the epoch total pool size
    function getPoolSize(uint128 epochId) external view returns (uint) {
        return _getPoolSize(epochId);
    }

    function getCurrentEpoch() external view returns (uint) {
        return _getEpochId();
    }

    // calls to the staking smart contract to retrieve user balance for an epoch
    function getEpochStake(address userAddress, uint128 epochId) external view returns (uint) {
        return _getUserBalancePerEpoch(userAddress, epochId);
    }

    function userLastEpochIdHarvested() external view returns (uint){
        return lastEpochIdHarvested[msg.sender];
    }

    // internal methods

    function _initEpoch(uint128 epochId) internal {
        require(lastInitializedEpoch.add(1) == epochId, "Epoch can be init only in order");
        lastInitializedEpoch = epochId;
        // call the staking smart contract to init the epoch
        epochs[epochId] = _getPoolSize(epochId);
    }

    function _harvest (uint128 epochId) internal returns (uint) {
        // try to initialize an epoch. if it can't it fails
        // if it fails either user either a Swapp account will init not init epochs
        if (lastInitializedEpoch < epochId) {
            _initEpoch(epochId);
        }
        // Set user last harvested epoch
        lastEpochIdHarvested[msg.sender] = epochId;
        // compute and return user total reward. For optimization reasons the transfer have been moved to an upper layer (i.e. massHarvest needs to do a single transfer)

        // exit if there is no stake on the epoch
        if (epochs[epochId] == 0) {
            return 0;
        }

        return getTotalAmountPerEpoch()
        .mul(_getUserBalancePerEpoch(msg.sender, epochId))
        .div(epochs[epochId]);
    }

    function _getPoolSize(uint128 epochId) internal view returns (uint) {
        // retrieve stable coins total staked in epoch
        uint valueUsdc = _staking.getEpochPoolSize(_usdc, epochId).mul(10 ** 12); // for usdc which has 6 decimals add a 10**12 to get to a common ground
        uint valueUsdt = _staking.getEpochPoolSize(_usdt, epochId).mul(10 ** 12); // for usdt which has 6 decimals add a 10**12 to get to a common ground
        uint valueDai = _staking.getEpochPoolSize(_dai, epochId);
        return valueUsdc.add(valueUsdt).add(valueDai);
    }

    function _getUserBalancePerEpoch(address userAddress, uint128 epochId) internal view returns (uint){
        // retrieve stable coins total staked per user in epoch
        uint valueUsdc = _staking.getEpochUserBalance(userAddress, _usdc, epochId).mul(10 ** 12); // for usdc which has 6 decimals add a 10**12 to get to a common ground
        uint valueUsdt = _staking.getEpochUserBalance(userAddress, _usdt, epochId).mul(10 ** 12); // for usdt which has 6 decimals add a 10**12 to get to a common ground
        uint valueDai = _staking.getEpochUserBalance(userAddress, _dai, epochId);
        return valueUsdc.add(valueUsdt).add(valueDai);
    }

    // compute epoch id from blocktimestamp and epochstart date
    function _getEpochId() internal view returns (uint128 epochId) {
        if (block.timestamp < epochStart) {
            return 0;
        }
        epochId = uint128(block.timestamp.sub(epochStart).div(epochDuration).add(1));
    }
}