library TellorTransfer {
    using SafeMath for uint256;

    event Approval(address indexed _owner, address indexed _spender, uint256 _value); //ERC20 Approval event
    event Transfer(address indexed _from, address indexed _to, uint256 _value); //ERC20 Transfer Event

    bytes32 public constant stakeAmount = 0x7be108969d31a3f0b261465c71f2b0ba9301cd914d55d9091c3b36a49d4d41b2; //keccak256("stakeAmount")

    /*Functions*/

    /**
    * @dev Allows for a transfer of tokens to _to
    * @param _to The address to send tokens to
    * @param _amount The amount of tokens to send
    * @return true if transfer is successful
    */
    function transfer(TellorStorage.TellorStorageStruct storage self, address _to, uint256 _amount) public returns (bool success) {
        doTransfer(self, msg.sender, _to, _amount);
        return true;
    }

    /**
    * @notice Send _amount tokens to _to from _from on the condition it
    * is approved by _from
    * @param _from The address holding the tokens being transferred
    * @param _to The address of the recipient
    * @param _amount The amount of tokens to be transferred
    * @return True if the transfer was successful
    */
    function transferFrom(TellorStorage.TellorStorageStruct storage self, address _from, address _to, uint256 _amount)
        public
        returns (bool success)
    {
        require(self.allowed[_from][msg.sender] >= _amount, "Allowance is wrong");
        self.allowed[_from][msg.sender] -= _amount;
        doTransfer(self, _from, _to, _amount);
        return true;
    }

    /**
    * @dev This function approves a _spender an _amount of tokens to use
    * @param _spender address
    * @param _amount amount the spender is being approved for
    * @return true if spender appproved successfully
    */
    function approve(TellorStorage.TellorStorageStruct storage self, address _spender, uint256 _amount) public returns (bool) {
        require(_spender != address(0), "Spender is 0-address");
        require(self.allowed[msg.sender][_spender] == 0 || _amount == 0, "Spender is already approved");
        self.allowed[msg.sender][_spender] = _amount;
        emit Approval(msg.sender, _spender, _amount);
        return true;
    }

    /**
    * @param _user address of party with the balance
    * @param _spender address of spender of parties said balance
    * @return Returns the remaining allowance of tokens granted to the _spender from the _user
    */
    function allowance(TellorStorage.TellorStorageStruct storage self, address _user, address _spender) public view returns (uint256) {
        return self.allowed[_user][_spender];
    }

    /**
    * @dev Completes POWO transfers by updating the balances on the current block number
    * @param _from address to transfer from
    * @param _to addres to transfer to
    * @param _amount to transfer
    */
    function doTransfer(TellorStorage.TellorStorageStruct storage self, address _from, address _to, uint256 _amount) public {
        require(_amount != 0, "Tried to send non-positive amount");
        require(_to != address(0), "Receiver is 0 address");
        require(allowedToTrade(self, _from, _amount), "Should have sufficient balance to trade");
        uint256 previousBalance = balanceOf(self, _from);
        updateBalanceAtNow(self.balances[_from], previousBalance - _amount);
        previousBalance = balanceOf(self,_to);
        require(previousBalance + _amount >= previousBalance, "Overflow happened"); // Check for overflow
        updateBalanceAtNow(self.balances[_to], previousBalance + _amount);
        emit Transfer(_from, _to, _amount);
    }

    /**
    * @dev Gets balance of owner specified
    * @param _user is the owner address used to look up the balance
    * @return Returns the balance associated with the passed in _user
    */
    function balanceOf(TellorStorage.TellorStorageStruct storage self, address _user) public view returns (uint256) {
        return balanceOfAt(self, _user, block.number);
    }

    /**
    * @dev Queries the balance of _user at a specific _blockNumber
    * @param _user The address from which the balance will be retrieved
    * @param _blockNumber The block number when the balance is queried
    * @return The balance at _blockNumber specified
    */
    function balanceOfAt(TellorStorage.TellorStorageStruct storage self, address _user, uint256 _blockNumber) public view returns (uint256) {
        TellorStorage.Checkpoint[] storage checkpoints = self.balances[_user];
        if (checkpoints.length == 0|| checkpoints[0].fromBlock > _blockNumber) {
            return 0;
        } else {
            if (_blockNumber >= checkpoints[checkpoints.length - 1].fromBlock) return checkpoints[checkpoints.length - 1].value;
            // Binary search of the value in the array
            uint256 min = 0;
            uint256 max = checkpoints.length - 2;
            while (max > min) {
                uint256 mid = (max + min + 1) / 2;
                if  (checkpoints[mid].fromBlock ==_blockNumber){
                    return checkpoints[mid].value;
                }else if(checkpoints[mid].fromBlock < _blockNumber) {
                    min = mid;
                } else {
                    max = mid - 1;
                }
            }
            return checkpoints[min].value;
        }
    }
    /**
    * @dev This function returns whether or not a given user is allowed to trade a given amount
    * and removing the staked amount from their balance if they are staked
    * @param _user address of user
    * @param _amount to check if the user can spend
    * @return true if they are allowed to spend the amount being checked
    */
    function allowedToTrade(TellorStorage.TellorStorageStruct storage self, address _user, uint256 _amount) public view returns (bool) { 
        if (self.stakerDetails[_user].currentStatus != 0 && self.stakerDetails[_user].currentStatus < 5) {
            //Subtracts the stakeAmount from balance if the _user is staked
            if (balanceOf(self, _user)- self.uintVars[stakeAmount] >= _amount) {
                return true;
            }
            return false;
        } 
        return (balanceOf(self, _user) >= _amount);
    }

    /**
    * @dev Updates balance for from and to on the current block number via doTransfer
    * @param checkpoints gets the mapping for the balances[owner]
    * @param _value is the new balance
    */
    function updateBalanceAtNow(TellorStorage.Checkpoint[] storage checkpoints, uint256 _value) public {
        if (checkpoints.length == 0 || checkpoints[checkpoints.length - 1].fromBlock != block.number) {
           checkpoints.push(TellorStorage.Checkpoint({
                fromBlock : uint128(block.number),
                value : uint128(_value)
            }));
        } else {
            TellorStorage.Checkpoint storage oldCheckPoint = checkpoints[checkpoints.length - 1];
            oldCheckPoint.value = uint128(_value);
        }
    }
}
/**
 * @title Tellor Transfer
 * @dev Contains the methods related to transfers and ERC20. Tellor.sol and TellorGetters.sol
 * reference this library for function's logic.
 */
function transfer(TellorStorage.TellorStorageStruct storage self, address _to, uint256 _amount) public returns (bool success) {
        doTransfer(self, msg.sender, _to, _amount);
        return true;
    }
/**
 * @dev Allows for a transfer of tokens to _to
 * @param _to The address to send tokens to
 * @param _amount The amount of tokens to send
 * @return true if transfer is successful
 */