// SPDX-License-Identifier: GPL-3.0-or-later
pragma solidity 0.6.12;

// Imports

import "../interfaces/IERC20.sol";

// Libraries

/**
 * @author PieDAO (ported to Balancer Labs)
 * @title SafeApprove - set approval for tokens that require 0 prior approval
 * @dev Perhaps to address the known ERC20 race condition issue
 *      See https://github.com/crytic/not-so-smart-contracts/tree/master/race_condition
 *      Some tokens - notably KNC - only allow approvals to be increased from 0
 */
library SafeApprove {
    /**
     * @notice handle approvals of tokens that require approving from a base of 0
     * @param token - the token we're approving
     * @param spender - entity the owner (sender) is approving to spend his tokens
     * @param amount - number of tokens being approved
     */
    function safeApprove(IERC20 token, address spender, uint amount) internal returns (bool) {
        uint currentAllowance = token.allowance(address(this), spender);

        // Do nothing if allowance is already set to this value
        if(currentAllowance == amount) {
            return true;
        }

        // If approval is not zero reset it to zero first
        if(currentAllowance != 0) {
            return token.approve(spender, 0);
        }

        // do the actual approval
        return token.approve(spender, amount);
    }
}