/*

SafeERC20 by daostack.
The code is based on a fix by SECBIT Team.

USE WITH CAUTION & NO WARRANTY

REFERENCE & RELATED READING
- https://github.com/ethereum/solidity/issues/4116
- https://medium.com/@chris_77367/explaining-unexpected-reverts-starting-with-solidity-0-4-22-3ada6e82308c
- https://medium.com/coinmonks/missing-return-value-bug-at-least-130-tokens-affected-d67bf08521ca
- https://gist.github.com/BrendanChou/88a2eeb80947ff00bcf58ffdafeaeb61

*/
pragma solidity 0.5.17;

import "openzeppelin-solidity/contracts/utils/Address.sol";
import "openzeppelin-solidity/contracts/token/ERC20/IERC20.sol";

library SafeERC20 {
    using Address for address;

    bytes4 constant private TRANSFER_SELECTOR = bytes4(keccak256(bytes("transfer(address,uint256)")));
    bytes4 constant private TRANSFERFROM_SELECTOR = bytes4(keccak256(bytes("transferFrom(address,address,uint256)")));
    bytes4 constant private APPROVE_SELECTOR = bytes4(keccak256(bytes("approve(address,uint256)")));

    function safeTransfer(address _erc20Addr, address _to, uint256 _value) internal {

        // Must be a contract addr first!
        require(_erc20Addr.isContract());

        (bool success, bytes memory returnValue) =
        // solhint-disable-next-line avoid-low-level-calls
        _erc20Addr.call(abi.encodeWithSelector(TRANSFER_SELECTOR, _to, _value));
        // call return false when something wrong
        require(success);
        //check return value
        require(returnValue.length == 0 || (returnValue.length == 32 && (returnValue[31] != 0)));
    }

    function safeTransferFrom(address _erc20Addr, address _from, address _to, uint256 _value) internal {

        // Must be a contract addr first!
        require(_erc20Addr.isContract());

        (bool success, bytes memory returnValue) =
        // solhint-disable-next-line avoid-low-level-calls
        _erc20Addr.call(abi.encodeWithSelector(TRANSFERFROM_SELECTOR, _from, _to, _value));
        // call return false when something wrong
        require(success);
        //check return value
        require(returnValue.length == 0 || (returnValue.length == 32 && (returnValue[31] != 0)));
    }

    function safeApprove(address _erc20Addr, address _spender, uint256 _value) internal {

        // Must be a contract addr first!
        require(_erc20Addr.isContract());

        // safeApprove should only be called when setting an initial allowance,
        // or when resetting it to zero.
        require((_value == 0) || (IERC20(_erc20Addr).allowance(address(this), _spender) == 0));

        (bool success, bytes memory returnValue) =
        // solhint-disable-next-line avoid-low-level-calls
        _erc20Addr.call(abi.encodeWithSelector(APPROVE_SELECTOR, _spender, _value));
        // call return false when something wrong
        require(success);
        //check return value
        require(returnValue.length == 0 || (returnValue.length == 32 && (returnValue[31] != 0)));
    }
}