// SPDX-License-Identifier: agpl-3.0
pragma solidity ^0.6.12;

// NOTE: this interface lacks return values for transfer/transferFrom/approve on purpose,
// as we use the SafeERC20 library to check the return value
interface GeneralERC20 {
	function transfer(address to, uint256 amount) external;
	function transferFrom(address from, address to, uint256 amount) external;
	function approve(address spender, uint256 amount) external;
	function balanceOf(address spender) external view returns (uint);
	function allowance(address owner, address spender) external view returns (uint);
}

library SafeERC20 {
	function checkSuccess()
		private
		pure
		returns (bool)
	{
		uint256 returnValue = 0;

		assembly {
			// check number of bytes returned from last function call
			switch returndatasize()

			// no bytes returned: assume success
			case 0x0 {
				returnValue := 1
			}

			// 32 bytes returned: check if non-zero
			case 0x20 {
				// copy 32 bytes into scratch space
				returndatacopy(0x0, 0x0, 0x20)

				// load those bytes into returnValue
				returnValue := mload(0x0)
			}

			// not sure what was returned: don't mark as success
			default { }
		}

		return returnValue != 0;
	}

	function transfer(address token, address to, uint256 amount) internal {
		GeneralERC20(token).transfer(to, amount);
		require(checkSuccess());
	}

	function transferFrom(address token, address from, address to, uint256 amount) internal {
		GeneralERC20(token).transferFrom(from, to, amount);
		require(checkSuccess());
	}

	function approve(address token, address spender, uint256 amount) internal {
		GeneralERC20(token).approve(spender, amount);
		require(checkSuccess());
	}
}